Fix deterministic MountVolume test failures on ARM64 Helix machines

[danmoseley]

Note

This PR was created with Copilot assistance.

Fix deterministic MountVolume test failures on ARM64 Helix machines

Fixes#125295, fixes#125624, fixes#126627

Problem

Directory_Delete_MountVolume.RunTest and Directory_ReparsePoints_MountVolume.runTest fail deterministically (~100% of the time, ~750ms duration) on the Windows.11.Arm64.Open Helix machine pool. This is not timing-related and was not addressed by the delay/polling fixes in #125914 or the Unmount resilience fix in #125625 (those PRs fixed real timing issues -- pre-fix failures on other configurations have since expired from AzDO retention, so we can't verify directly, but there is no evidence they were ineffective for their intended purpose).

Root cause: The ARM64 Helix machines have an E:\ drive (likely an Azure resource/temp disk) that passes all DriveInfo checks -- DriveType=Fixed, DriveFormat=NTFS, IsReady=True -- but GetVolumeNameForVolumeMountPoint fails with ERROR_INVALID_PARAMETER (87). The drive has no volume GUID and doesn't support volume mount point operations. IOServices.GetNtfsDriveOtherThanCurrent() returns this drive, and the test crashes trying to use it.

Some ARM64 Helix machines have only C:\ and a CD-ROM (no second drive at all). On those machines, the cross-drive scenarios already skip gracefully and only same-drive scenarios 3.x run.

Evidence

Analyzed Helix console logs from 5 post-fix builds (all arm64-NativeAOT-Win11, same C:\ volume GUID). Every failure shows the identical pattern:

• Scenario 1: GetVolumeNameForVolumeMountPoint("E:\") -> error 87
• Scenario 2: SetVolumeMountPoint onto E:\ succeeds but path traversal through the mount point fails with DirectoryNotFoundException
• Scenarios 3.x (same-drive): Always pass

Reproduced locally by removing the real E: drive letter and creating SUBST E: which exhibits identical error 87 behavior.

Changes

1. IOServices.GetNtfsDriveOtherThan(): After the existing Fixed/Ready/NTFS checks, also verify the drive has a volume GUID via GetVolumeNameForVolumeMountPoint. Drives without one (SUBST drives, Azure resource disks) are skipped.

2. DumpDriveInformation diagnostic test: New Helix-only test (following the DescriptionNameTests.DumpRuntimeInformationToConsole pattern) that dumps all drives with their volume GUIDs to the console log. Makes future drive-related CI issues immediately diagnosable from the same Helix work item log.

3. GetVolumeNameForVolumeMountPoint P/Invoke in DllImports.cs: Uses char[] (not StringBuilder) because this file uses LibraryImport which does not support StringBuilder.

Local validation

Scenario	Before fix	After fix
SUBST E: (no volume GUID)	Error 87 / DirectoryNotFoundException	Pass (SUBST filtered, scenarios 3.x run)
Real NTFS E:	Pass (all scenarios)	Pass (all scenarios)
Single-drive machine	Scenarios 1/2 skip, 3.x pass	Same -- no change

[danmoseley]

/azp run runtime-ioslikesimulator

[danmoseley]

/azp run runtime-android

[danmoseley]

/azp run runtime-maccatalyst

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Addresses deterministic failures in Windows MountVolume-related filesystem tests on ARM64 Helix by improving second-drive selection and adding Helix diagnostics to make drive/volume issues easier to diagnose from logs.

Changes:

• Filter candidate “other NTFS drives” to only those that support volume mount point operations (via GetVolumeNameForVolumeMountPoint).
• Add a Helix-only diagnostic test that logs drive details and volume GUID availability.
• Improve Mount/Unmount error messages by using P/Invoke error messages instead of raw codes, and add a Helix guard for missing second-drive scenarios.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Adds the new DumpDriveInformation.cs test file to the project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Enhances NTFS drive selection by skipping drives without a volume GUID/mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke used to validate mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	New Helix-only diagnostic test that prints drive and volume GUID information to the console log.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs	Improves mount/unmount exception messages by using P/Invoke error message APIs.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs:260

• Marshal.GetLastPInvokeError() / Marshal.GetLastPInvokeErrorMessage() are not available on .NET Framework. Since this file is compiled for NETFRAMEWORK in some test projects, this change will break those builds unless guarded (e.g., keep GetLastWin32Error() on NETFRAMEWORK and format the message via Win32Exception).

 bool r = DeleteVolumeMountPoint(mountPoint);
if (!r)
{
int error = Marshal.GetLastPInvokeError();
// Ignore expected cleanup errors: 4390 (ERROR_NOT_A_REPARSE_POINT),
// 3 (ERROR_PATH_NOT_FOUND), 2 (ERROR_FILE_NOT_FOUND)
if (error != 4390 && error != 3 && error != 2)
throw new Exception(string.Format("DeleteVolumeMountPoint({0}) failed: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));
Console.WriteLine(string.Format("Ignoring expected error while unmounting {0}: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));

• Files reviewed: 7/7 changed files
• Comments generated: 4

[Copilot]

Pull request overview

This PR addresses deterministic failures in the System.IO mount-volume tests on Windows 11 ARM64 Helix machines by avoiding “fixed/NTFS/ready” drives that don’t actually support volume mount-point operations (no volume GUID), and by adding Helix-focused diagnostics/guards to keep cross-drive scenarios from silently becoming dead code.

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to also require a successful GetVolumeNameForVolumeMountPoint (volume GUID present) before selecting a “usable” NTFS drive.
• Add Helix-only hard-fail guards in both mount-volume tests when no suitable second NTFS drive is available.
• Add a Helix-only diagnostic test to dump drive and volume GUID information into Helix logs.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new DumpDriveInformation.cs test file in the test project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring a volume GUID (mount-point capable).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke via LibraryImport.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only diagnostic logging of drive details + selected “other NTFS drive”.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 3

[Copilot]

Pull request overview

This PR addresses deterministic failures in the MountVolume-related filesystem tests on Windows 11 ARM64 Helix machines by ensuring the “other NTFS drive” selection skips fixed/NTFS/ready drives that don’t actually support volume mount point operations (e.g., drives without a volume GUID).

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to additionally require that candidate drives have a volume GUID via GetVolumeNameForVolumeMountPoint.
• Add a GetVolumeNameForVolumeMountPointWLibraryImport to the test DllImports helpers.
• Add a Helix-only diagnostic test (DumpDriveInformation) and wire it into the test project to print drive/volume details to the Helix console log.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test source file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring successful GetVolumeNameForVolumeMountPoint (i.e., a volume GUID exists).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke for drive capability detection and diagnostics.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging test that dumps drive properties and (on Windows) volume GUID availability to aid CI diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2

[Copilot]

Pull request overview

This PR aims to address deterministic MountVolume-related test failures on Windows ARM64 Helix agents by ensuring the “other NTFS drive” selection excludes drives that don’t support volume mount point operations, and by adding Helix-only diagnostics to quickly identify problematic drives.

Changes:

• Update IOServices.GetNtfsDriveOtherThan() to require a successful GetVolumeNameForVolumeMountPoint call (filters out drives without a usable volume GUID).
• Add a new kernel32 P/Invoke (GetVolumeNameForVolumeMountPointW) to the test DllImports.
• Add a Helix-only diagnostic “test” that prints drive properties and volume GUID availability to the console, and include it in the test project.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring GetVolumeNameForVolumeMountPoint success.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds P/Invoke for GetVolumeNameForVolumeMountPointW.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging of drive info + volume GUID presence for future diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 1

[adamsitnik]

@copilot please address my feedback

[adamsitnik]

LGTM, thank you @danmoseley !

[danmoseley]

Note

This PR was created with Copilot assistance.

Fix deterministic MountVolume test failures on ARM64 Helix machines

Fixes#125295, fixes#125624, fixes#126627

Problem

Directory_Delete_MountVolume.RunTest and Directory_ReparsePoints_MountVolume.runTest fail deterministically (~100% of the time, ~750ms duration) on the Windows.11.Arm64.Open Helix machine pool. This is not timing-related and was not addressed by the delay/polling fixes in #125914 or the Unmount resilience fix in #125625 (those PRs fixed real timing issues -- pre-fix failures on other configurations have since expired from AzDO retention, so we can't verify directly, but there is no evidence they were ineffective for their intended purpose).

Root cause: The ARM64 Helix machines have an E:\ drive (likely an Azure resource/temp disk) that passes all DriveInfo checks -- DriveType=Fixed, DriveFormat=NTFS, IsReady=True -- but GetVolumeNameForVolumeMountPoint fails with ERROR_INVALID_PARAMETER (87). The drive has no volume GUID and doesn't support volume mount point operations. IOServices.GetNtfsDriveOtherThanCurrent() returns this drive, and the test crashes trying to use it.

Some ARM64 Helix machines have only C:\ and a CD-ROM (no second drive at all). On those machines, the cross-drive scenarios already skip gracefully and only same-drive scenarios 3.x run.

Evidence

Analyzed Helix console logs from 5 post-fix builds (all arm64-NativeAOT-Win11, same C:\ volume GUID). Every failure shows the identical pattern:

• Scenario 1: GetVolumeNameForVolumeMountPoint("E:\") -> error 87
• Scenario 2: SetVolumeMountPoint onto E:\ succeeds but path traversal through the mount point fails with DirectoryNotFoundException
• Scenarios 3.x (same-drive): Always pass

Reproduced locally by removing the real E: drive letter and creating SUBST E: which exhibits identical error 87 behavior.

Changes

1. IOServices.GetNtfsDriveOtherThan(): After the existing Fixed/Ready/NTFS checks, also verify the drive has a volume GUID via GetVolumeNameForVolumeMountPoint. Drives without one (SUBST drives, Azure resource disks) are skipped.

2. DumpDriveInformation diagnostic test: New Helix-only test (following the DescriptionNameTests.DumpRuntimeInformationToConsole pattern) that dumps all drives with their volume GUIDs to the console log. Makes future drive-related CI issues immediately diagnosable from the same Helix work item log.

3. GetVolumeNameForVolumeMountPoint P/Invoke in DllImports.cs: Uses char[] (not StringBuilder) because this file uses LibraryImport which does not support StringBuilder.

Local validation

Scenario	Before fix	After fix
SUBST E: (no volume GUID)	Error 87 / DirectoryNotFoundException	Pass (SUBST filtered, scenarios 3.x run)
Real NTFS E:	Pass (all scenarios)	Pass (all scenarios)
Single-drive machine	Scenarios 1/2 skip, 3.x pass	Same -- no change

[danmoseley]

/azp run runtime-ioslikesimulator

[danmoseley]

/azp run runtime-android

[danmoseley]

/azp run runtime-maccatalyst

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Addresses deterministic failures in Windows MountVolume-related filesystem tests on ARM64 Helix by improving second-drive selection and adding Helix diagnostics to make drive/volume issues easier to diagnose from logs.

Changes:

• Filter candidate “other NTFS drives” to only those that support volume mount point operations (via GetVolumeNameForVolumeMountPoint).
• Add a Helix-only diagnostic test that logs drive details and volume GUID availability.
• Improve Mount/Unmount error messages by using P/Invoke error messages instead of raw codes, and add a Helix guard for missing second-drive scenarios.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Adds the new DumpDriveInformation.cs test file to the project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Enhances NTFS drive selection by skipping drives without a volume GUID/mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke used to validate mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	New Helix-only diagnostic test that prints drive and volume GUID information to the console log.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs	Improves mount/unmount exception messages by using P/Invoke error message APIs.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs:260

• Marshal.GetLastPInvokeError() / Marshal.GetLastPInvokeErrorMessage() are not available on .NET Framework. Since this file is compiled for NETFRAMEWORK in some test projects, this change will break those builds unless guarded (e.g., keep GetLastWin32Error() on NETFRAMEWORK and format the message via Win32Exception).

 bool r = DeleteVolumeMountPoint(mountPoint);
if (!r)
{
int error = Marshal.GetLastPInvokeError();
// Ignore expected cleanup errors: 4390 (ERROR_NOT_A_REPARSE_POINT),
// 3 (ERROR_PATH_NOT_FOUND), 2 (ERROR_FILE_NOT_FOUND)
if (error != 4390 && error != 3 && error != 2)
throw new Exception(string.Format("DeleteVolumeMountPoint({0}) failed: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));
Console.WriteLine(string.Format("Ignoring expected error while unmounting {0}: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));

• Files reviewed: 7/7 changed files
• Comments generated: 4

[Copilot]

Pull request overview

This PR addresses deterministic failures in the System.IO mount-volume tests on Windows 11 ARM64 Helix machines by avoiding “fixed/NTFS/ready” drives that don’t actually support volume mount-point operations (no volume GUID), and by adding Helix-focused diagnostics/guards to keep cross-drive scenarios from silently becoming dead code.

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to also require a successful GetVolumeNameForVolumeMountPoint (volume GUID present) before selecting a “usable” NTFS drive.
• Add Helix-only hard-fail guards in both mount-volume tests when no suitable second NTFS drive is available.
• Add a Helix-only diagnostic test to dump drive and volume GUID information into Helix logs.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new DumpDriveInformation.cs test file in the test project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring a volume GUID (mount-point capable).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke via LibraryImport.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only diagnostic logging of drive details + selected “other NTFS drive”.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 3

[Copilot]

Pull request overview

This PR addresses deterministic failures in the MountVolume-related filesystem tests on Windows 11 ARM64 Helix machines by ensuring the “other NTFS drive” selection skips fixed/NTFS/ready drives that don’t actually support volume mount point operations (e.g., drives without a volume GUID).

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to additionally require that candidate drives have a volume GUID via GetVolumeNameForVolumeMountPoint.
• Add a GetVolumeNameForVolumeMountPointWLibraryImport to the test DllImports helpers.
• Add a Helix-only diagnostic test (DumpDriveInformation) and wire it into the test project to print drive/volume details to the Helix console log.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test source file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring successful GetVolumeNameForVolumeMountPoint (i.e., a volume GUID exists).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke for drive capability detection and diagnostics.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging test that dumps drive properties and (on Windows) volume GUID availability to aid CI diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2

[Copilot]

Pull request overview

This PR aims to address deterministic MountVolume-related test failures on Windows ARM64 Helix agents by ensuring the “other NTFS drive” selection excludes drives that don’t support volume mount point operations, and by adding Helix-only diagnostics to quickly identify problematic drives.

Changes:

• Update IOServices.GetNtfsDriveOtherThan() to require a successful GetVolumeNameForVolumeMountPoint call (filters out drives without a usable volume GUID).
• Add a new kernel32 P/Invoke (GetVolumeNameForVolumeMountPointW) to the test DllImports.
• Add a Helix-only diagnostic “test” that prints drive properties and volume GUID availability to the console, and include it in the test project.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring GetVolumeNameForVolumeMountPoint success.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds P/Invoke for GetVolumeNameForVolumeMountPointW.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging of drive info + volume GUID presence for future diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 1

[adamsitnik]

@copilot please address my feedback

[adamsitnik]

LGTM, thank you @danmoseley !

[danmoseley]

Note

This PR was created with Copilot assistance.

Fix deterministic MountVolume test failures on ARM64 Helix machines

Fixes#125295, fixes#125624, fixes#126627

Problem

Directory_Delete_MountVolume.RunTest and Directory_ReparsePoints_MountVolume.runTest fail deterministically (~100% of the time, ~750ms duration) on the Windows.11.Arm64.Open Helix machine pool. This is not timing-related and was not addressed by the delay/polling fixes in #125914 or the Unmount resilience fix in #125625 (those PRs fixed real timing issues -- pre-fix failures on other configurations have since expired from AzDO retention, so we can't verify directly, but there is no evidence they were ineffective for their intended purpose).

Root cause: The ARM64 Helix machines have an E:\ drive (likely an Azure resource/temp disk) that passes all DriveInfo checks -- DriveType=Fixed, DriveFormat=NTFS, IsReady=True -- but GetVolumeNameForVolumeMountPoint fails with ERROR_INVALID_PARAMETER (87). The drive has no volume GUID and doesn't support volume mount point operations. IOServices.GetNtfsDriveOtherThanCurrent() returns this drive, and the test crashes trying to use it.

Some ARM64 Helix machines have only C:\ and a CD-ROM (no second drive at all). On those machines, the cross-drive scenarios already skip gracefully and only same-drive scenarios 3.x run.

Evidence

Analyzed Helix console logs from 5 post-fix builds (all arm64-NativeAOT-Win11, same C:\ volume GUID). Every failure shows the identical pattern:

• Scenario 1: GetVolumeNameForVolumeMountPoint("E:\") -> error 87
• Scenario 2: SetVolumeMountPoint onto E:\ succeeds but path traversal through the mount point fails with DirectoryNotFoundException
• Scenarios 3.x (same-drive): Always pass

Reproduced locally by removing the real E: drive letter and creating SUBST E: which exhibits identical error 87 behavior.

Changes

1. IOServices.GetNtfsDriveOtherThan(): After the existing Fixed/Ready/NTFS checks, also verify the drive has a volume GUID via GetVolumeNameForVolumeMountPoint. Drives without one (SUBST drives, Azure resource disks) are skipped.

2. DumpDriveInformation diagnostic test: New Helix-only test (following the DescriptionNameTests.DumpRuntimeInformationToConsole pattern) that dumps all drives with their volume GUIDs to the console log. Makes future drive-related CI issues immediately diagnosable from the same Helix work item log.

3. GetVolumeNameForVolumeMountPoint P/Invoke in DllImports.cs: Uses char[] (not StringBuilder) because this file uses LibraryImport which does not support StringBuilder.

Local validation

Scenario	Before fix	After fix
SUBST E: (no volume GUID)	Error 87 / DirectoryNotFoundException	Pass (SUBST filtered, scenarios 3.x run)
Real NTFS E:	Pass (all scenarios)	Pass (all scenarios)
Single-drive machine	Scenarios 1/2 skip, 3.x pass	Same -- no change

[danmoseley]

/azp run runtime-ioslikesimulator

[danmoseley]

/azp run runtime-android

[danmoseley]

/azp run runtime-maccatalyst

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Addresses deterministic failures in Windows MountVolume-related filesystem tests on ARM64 Helix by improving second-drive selection and adding Helix diagnostics to make drive/volume issues easier to diagnose from logs.

Changes:

• Filter candidate “other NTFS drives” to only those that support volume mount point operations (via GetVolumeNameForVolumeMountPoint).
• Add a Helix-only diagnostic test that logs drive details and volume GUID availability.
• Improve Mount/Unmount error messages by using P/Invoke error messages instead of raw codes, and add a Helix guard for missing second-drive scenarios.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Adds the new DumpDriveInformation.cs test file to the project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Enhances NTFS drive selection by skipping drives without a volume GUID/mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke used to validate mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	New Helix-only diagnostic test that prints drive and volume GUID information to the console log.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs	Improves mount/unmount exception messages by using P/Invoke error message APIs.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs:260

• Marshal.GetLastPInvokeError() / Marshal.GetLastPInvokeErrorMessage() are not available on .NET Framework. Since this file is compiled for NETFRAMEWORK in some test projects, this change will break those builds unless guarded (e.g., keep GetLastWin32Error() on NETFRAMEWORK and format the message via Win32Exception).

 bool r = DeleteVolumeMountPoint(mountPoint);
if (!r)
{
int error = Marshal.GetLastPInvokeError();
// Ignore expected cleanup errors: 4390 (ERROR_NOT_A_REPARSE_POINT),
// 3 (ERROR_PATH_NOT_FOUND), 2 (ERROR_FILE_NOT_FOUND)
if (error != 4390 && error != 3 && error != 2)
throw new Exception(string.Format("DeleteVolumeMountPoint({0}) failed: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));
Console.WriteLine(string.Format("Ignoring expected error while unmounting {0}: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));

• Files reviewed: 7/7 changed files
• Comments generated: 4

[Copilot]

Pull request overview

This PR addresses deterministic failures in the System.IO mount-volume tests on Windows 11 ARM64 Helix machines by avoiding “fixed/NTFS/ready” drives that don’t actually support volume mount-point operations (no volume GUID), and by adding Helix-focused diagnostics/guards to keep cross-drive scenarios from silently becoming dead code.

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to also require a successful GetVolumeNameForVolumeMountPoint (volume GUID present) before selecting a “usable” NTFS drive.
• Add Helix-only hard-fail guards in both mount-volume tests when no suitable second NTFS drive is available.
• Add a Helix-only diagnostic test to dump drive and volume GUID information into Helix logs.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new DumpDriveInformation.cs test file in the test project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring a volume GUID (mount-point capable).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke via LibraryImport.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only diagnostic logging of drive details + selected “other NTFS drive”.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 3

[Copilot]

Pull request overview

This PR addresses deterministic failures in the MountVolume-related filesystem tests on Windows 11 ARM64 Helix machines by ensuring the “other NTFS drive” selection skips fixed/NTFS/ready drives that don’t actually support volume mount point operations (e.g., drives without a volume GUID).

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to additionally require that candidate drives have a volume GUID via GetVolumeNameForVolumeMountPoint.
• Add a GetVolumeNameForVolumeMountPointWLibraryImport to the test DllImports helpers.
• Add a Helix-only diagnostic test (DumpDriveInformation) and wire it into the test project to print drive/volume details to the Helix console log.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test source file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring successful GetVolumeNameForVolumeMountPoint (i.e., a volume GUID exists).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke for drive capability detection and diagnostics.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging test that dumps drive properties and (on Windows) volume GUID availability to aid CI diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2

[Copilot]

Pull request overview

This PR aims to address deterministic MountVolume-related test failures on Windows ARM64 Helix agents by ensuring the “other NTFS drive” selection excludes drives that don’t support volume mount point operations, and by adding Helix-only diagnostics to quickly identify problematic drives.

Changes:

• Update IOServices.GetNtfsDriveOtherThan() to require a successful GetVolumeNameForVolumeMountPoint call (filters out drives without a usable volume GUID).
• Add a new kernel32 P/Invoke (GetVolumeNameForVolumeMountPointW) to the test DllImports.
• Add a Helix-only diagnostic “test” that prints drive properties and volume GUID availability to the console, and include it in the test project.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring GetVolumeNameForVolumeMountPoint success.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds P/Invoke for GetVolumeNameForVolumeMountPointW.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging of drive info + volume GUID presence for future diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 1

[adamsitnik]

@copilot please address my feedback

[adamsitnik]

LGTM, thank you @danmoseley !

[danmoseley]

Note

This PR was created with Copilot assistance.

Fix deterministic MountVolume test failures on ARM64 Helix machines

Fixes#125295, fixes#125624, fixes#126627

Problem

Directory_Delete_MountVolume.RunTest and Directory_ReparsePoints_MountVolume.runTest fail deterministically (~100% of the time, ~750ms duration) on the Windows.11.Arm64.Open Helix machine pool. This is not timing-related and was not addressed by the delay/polling fixes in #125914 or the Unmount resilience fix in #125625 (those PRs fixed real timing issues -- pre-fix failures on other configurations have since expired from AzDO retention, so we can't verify directly, but there is no evidence they were ineffective for their intended purpose).

Root cause: The ARM64 Helix machines have an E:\ drive (likely an Azure resource/temp disk) that passes all DriveInfo checks -- DriveType=Fixed, DriveFormat=NTFS, IsReady=True -- but GetVolumeNameForVolumeMountPoint fails with ERROR_INVALID_PARAMETER (87). The drive has no volume GUID and doesn't support volume mount point operations. IOServices.GetNtfsDriveOtherThanCurrent() returns this drive, and the test crashes trying to use it.

Some ARM64 Helix machines have only C:\ and a CD-ROM (no second drive at all). On those machines, the cross-drive scenarios already skip gracefully and only same-drive scenarios 3.x run.

Evidence

Analyzed Helix console logs from 5 post-fix builds (all arm64-NativeAOT-Win11, same C:\ volume GUID). Every failure shows the identical pattern:

• Scenario 1: GetVolumeNameForVolumeMountPoint("E:\") -> error 87
• Scenario 2: SetVolumeMountPoint onto E:\ succeeds but path traversal through the mount point fails with DirectoryNotFoundException
• Scenarios 3.x (same-drive): Always pass

Reproduced locally by removing the real E: drive letter and creating SUBST E: which exhibits identical error 87 behavior.

Changes

1. IOServices.GetNtfsDriveOtherThan(): After the existing Fixed/Ready/NTFS checks, also verify the drive has a volume GUID via GetVolumeNameForVolumeMountPoint. Drives without one (SUBST drives, Azure resource disks) are skipped.

2. DumpDriveInformation diagnostic test: New Helix-only test (following the DescriptionNameTests.DumpRuntimeInformationToConsole pattern) that dumps all drives with their volume GUIDs to the console log. Makes future drive-related CI issues immediately diagnosable from the same Helix work item log.

3. GetVolumeNameForVolumeMountPoint P/Invoke in DllImports.cs: Uses char[] (not StringBuilder) because this file uses LibraryImport which does not support StringBuilder.

Local validation

Scenario	Before fix	After fix
SUBST E: (no volume GUID)	Error 87 / DirectoryNotFoundException	Pass (SUBST filtered, scenarios 3.x run)
Real NTFS E:	Pass (all scenarios)	Pass (all scenarios)
Single-drive machine	Scenarios 1/2 skip, 3.x pass	Same -- no change

[danmoseley]

/azp run runtime-ioslikesimulator

[danmoseley]

/azp run runtime-android

[danmoseley]

/azp run runtime-maccatalyst

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Addresses deterministic failures in Windows MountVolume-related filesystem tests on ARM64 Helix by improving second-drive selection and adding Helix diagnostics to make drive/volume issues easier to diagnose from logs.

Changes:

• Filter candidate “other NTFS drives” to only those that support volume mount point operations (via GetVolumeNameForVolumeMountPoint).
• Add a Helix-only diagnostic test that logs drive details and volume GUID availability.
• Improve Mount/Unmount error messages by using P/Invoke error messages instead of raw codes, and add a Helix guard for missing second-drive scenarios.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Adds the new DumpDriveInformation.cs test file to the project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Enhances NTFS drive selection by skipping drives without a volume GUID/mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke used to validate mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	New Helix-only diagnostic test that prints drive and volume GUID information to the console log.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs	Improves mount/unmount exception messages by using P/Invoke error message APIs.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs:260

• Marshal.GetLastPInvokeError() / Marshal.GetLastPInvokeErrorMessage() are not available on .NET Framework. Since this file is compiled for NETFRAMEWORK in some test projects, this change will break those builds unless guarded (e.g., keep GetLastWin32Error() on NETFRAMEWORK and format the message via Win32Exception).

 bool r = DeleteVolumeMountPoint(mountPoint);
if (!r)
{
int error = Marshal.GetLastPInvokeError();
// Ignore expected cleanup errors: 4390 (ERROR_NOT_A_REPARSE_POINT),
// 3 (ERROR_PATH_NOT_FOUND), 2 (ERROR_FILE_NOT_FOUND)
if (error != 4390 && error != 3 && error != 2)
throw new Exception(string.Format("DeleteVolumeMountPoint({0}) failed: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));
Console.WriteLine(string.Format("Ignoring expected error while unmounting {0}: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));

• Files reviewed: 7/7 changed files
• Comments generated: 4

[Copilot]

Pull request overview

This PR addresses deterministic failures in the System.IO mount-volume tests on Windows 11 ARM64 Helix machines by avoiding “fixed/NTFS/ready” drives that don’t actually support volume mount-point operations (no volume GUID), and by adding Helix-focused diagnostics/guards to keep cross-drive scenarios from silently becoming dead code.

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to also require a successful GetVolumeNameForVolumeMountPoint (volume GUID present) before selecting a “usable” NTFS drive.
• Add Helix-only hard-fail guards in both mount-volume tests when no suitable second NTFS drive is available.
• Add a Helix-only diagnostic test to dump drive and volume GUID information into Helix logs.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new DumpDriveInformation.cs test file in the test project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring a volume GUID (mount-point capable).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke via LibraryImport.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only diagnostic logging of drive details + selected “other NTFS drive”.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 3

[Copilot]

Pull request overview

This PR addresses deterministic failures in the MountVolume-related filesystem tests on Windows 11 ARM64 Helix machines by ensuring the “other NTFS drive” selection skips fixed/NTFS/ready drives that don’t actually support volume mount point operations (e.g., drives without a volume GUID).

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to additionally require that candidate drives have a volume GUID via GetVolumeNameForVolumeMountPoint.
• Add a GetVolumeNameForVolumeMountPointWLibraryImport to the test DllImports helpers.
• Add a Helix-only diagnostic test (DumpDriveInformation) and wire it into the test project to print drive/volume details to the Helix console log.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test source file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring successful GetVolumeNameForVolumeMountPoint (i.e., a volume GUID exists).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke for drive capability detection and diagnostics.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging test that dumps drive properties and (on Windows) volume GUID availability to aid CI diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2

[Copilot]

Pull request overview

This PR aims to address deterministic MountVolume-related test failures on Windows ARM64 Helix agents by ensuring the “other NTFS drive” selection excludes drives that don’t support volume mount point operations, and by adding Helix-only diagnostics to quickly identify problematic drives.

Changes:

• Update IOServices.GetNtfsDriveOtherThan() to require a successful GetVolumeNameForVolumeMountPoint call (filters out drives without a usable volume GUID).
• Add a new kernel32 P/Invoke (GetVolumeNameForVolumeMountPointW) to the test DllImports.
• Add a Helix-only diagnostic “test” that prints drive properties and volume GUID availability to the console, and include it in the test project.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring GetVolumeNameForVolumeMountPoint success.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds P/Invoke for GetVolumeNameForVolumeMountPointW.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging of drive info + volume GUID presence for future diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 1

[adamsitnik]

@copilot please address my feedback

[adamsitnik]

LGTM, thank you @danmoseley !

[danmoseley]

Note

This PR was created with Copilot assistance.

Fix deterministic MountVolume test failures on ARM64 Helix machines

Fixes#125295, fixes#125624, fixes#126627

Problem

Directory_Delete_MountVolume.RunTest and Directory_ReparsePoints_MountVolume.runTest fail deterministically (~100% of the time, ~750ms duration) on the Windows.11.Arm64.Open Helix machine pool. This is not timing-related and was not addressed by the delay/polling fixes in #125914 or the Unmount resilience fix in #125625 (those PRs fixed real timing issues -- pre-fix failures on other configurations have since expired from AzDO retention, so we can't verify directly, but there is no evidence they were ineffective for their intended purpose).

Root cause: The ARM64 Helix machines have an E:\ drive (likely an Azure resource/temp disk) that passes all DriveInfo checks -- DriveType=Fixed, DriveFormat=NTFS, IsReady=True -- but GetVolumeNameForVolumeMountPoint fails with ERROR_INVALID_PARAMETER (87). The drive has no volume GUID and doesn't support volume mount point operations. IOServices.GetNtfsDriveOtherThanCurrent() returns this drive, and the test crashes trying to use it.

Some ARM64 Helix machines have only C:\ and a CD-ROM (no second drive at all). On those machines, the cross-drive scenarios already skip gracefully and only same-drive scenarios 3.x run.

Evidence

Analyzed Helix console logs from 5 post-fix builds (all arm64-NativeAOT-Win11, same C:\ volume GUID). Every failure shows the identical pattern:

• Scenario 1: GetVolumeNameForVolumeMountPoint("E:\") -> error 87
• Scenario 2: SetVolumeMountPoint onto E:\ succeeds but path traversal through the mount point fails with DirectoryNotFoundException
• Scenarios 3.x (same-drive): Always pass

Reproduced locally by removing the real E: drive letter and creating SUBST E: which exhibits identical error 87 behavior.

Changes

1. IOServices.GetNtfsDriveOtherThan(): After the existing Fixed/Ready/NTFS checks, also verify the drive has a volume GUID via GetVolumeNameForVolumeMountPoint. Drives without one (SUBST drives, Azure resource disks) are skipped.

2. DumpDriveInformation diagnostic test: New Helix-only test (following the DescriptionNameTests.DumpRuntimeInformationToConsole pattern) that dumps all drives with their volume GUIDs to the console log. Makes future drive-related CI issues immediately diagnosable from the same Helix work item log.

3. GetVolumeNameForVolumeMountPoint P/Invoke in DllImports.cs: Uses char[] (not StringBuilder) because this file uses LibraryImport which does not support StringBuilder.

Local validation

Scenario	Before fix	After fix
SUBST E: (no volume GUID)	Error 87 / DirectoryNotFoundException	Pass (SUBST filtered, scenarios 3.x run)
Real NTFS E:	Pass (all scenarios)	Pass (all scenarios)
Single-drive machine	Scenarios 1/2 skip, 3.x pass	Same -- no change

[danmoseley]

/azp run runtime-ioslikesimulator

[danmoseley]

/azp run runtime-android

[danmoseley]

/azp run runtime-maccatalyst

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Addresses deterministic failures in Windows MountVolume-related filesystem tests on ARM64 Helix by improving second-drive selection and adding Helix diagnostics to make drive/volume issues easier to diagnose from logs.

Changes:

• Filter candidate “other NTFS drives” to only those that support volume mount point operations (via GetVolumeNameForVolumeMountPoint).
• Add a Helix-only diagnostic test that logs drive details and volume GUID availability.
• Improve Mount/Unmount error messages by using P/Invoke error messages instead of raw codes, and add a Helix guard for missing second-drive scenarios.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Adds the new DumpDriveInformation.cs test file to the project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Enhances NTFS drive selection by skipping drives without a volume GUID/mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke used to validate mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	New Helix-only diagnostic test that prints drive and volume GUID information to the console log.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs	Improves mount/unmount exception messages by using P/Invoke error message APIs.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs:260

• Marshal.GetLastPInvokeError() / Marshal.GetLastPInvokeErrorMessage() are not available on .NET Framework. Since this file is compiled for NETFRAMEWORK in some test projects, this change will break those builds unless guarded (e.g., keep GetLastWin32Error() on NETFRAMEWORK and format the message via Win32Exception).

 bool r = DeleteVolumeMountPoint(mountPoint);
if (!r)
{
int error = Marshal.GetLastPInvokeError();
// Ignore expected cleanup errors: 4390 (ERROR_NOT_A_REPARSE_POINT),
// 3 (ERROR_PATH_NOT_FOUND), 2 (ERROR_FILE_NOT_FOUND)
if (error != 4390 && error != 3 && error != 2)
throw new Exception(string.Format("DeleteVolumeMountPoint({0}) failed: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));
Console.WriteLine(string.Format("Ignoring expected error while unmounting {0}: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));

• Files reviewed: 7/7 changed files
• Comments generated: 4

[Copilot]

Pull request overview

This PR addresses deterministic failures in the System.IO mount-volume tests on Windows 11 ARM64 Helix machines by avoiding “fixed/NTFS/ready” drives that don’t actually support volume mount-point operations (no volume GUID), and by adding Helix-focused diagnostics/guards to keep cross-drive scenarios from silently becoming dead code.

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to also require a successful GetVolumeNameForVolumeMountPoint (volume GUID present) before selecting a “usable” NTFS drive.
• Add Helix-only hard-fail guards in both mount-volume tests when no suitable second NTFS drive is available.
• Add a Helix-only diagnostic test to dump drive and volume GUID information into Helix logs.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new DumpDriveInformation.cs test file in the test project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring a volume GUID (mount-point capable).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke via LibraryImport.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only diagnostic logging of drive details + selected “other NTFS drive”.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 3

[Copilot]

Pull request overview

This PR addresses deterministic failures in the MountVolume-related filesystem tests on Windows 11 ARM64 Helix machines by ensuring the “other NTFS drive” selection skips fixed/NTFS/ready drives that don’t actually support volume mount point operations (e.g., drives without a volume GUID).

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to additionally require that candidate drives have a volume GUID via GetVolumeNameForVolumeMountPoint.
• Add a GetVolumeNameForVolumeMountPointWLibraryImport to the test DllImports helpers.
• Add a Helix-only diagnostic test (DumpDriveInformation) and wire it into the test project to print drive/volume details to the Helix console log.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test source file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring successful GetVolumeNameForVolumeMountPoint (i.e., a volume GUID exists).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke for drive capability detection and diagnostics.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging test that dumps drive properties and (on Windows) volume GUID availability to aid CI diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2

[Copilot]

Pull request overview

This PR aims to address deterministic MountVolume-related test failures on Windows ARM64 Helix agents by ensuring the “other NTFS drive” selection excludes drives that don’t support volume mount point operations, and by adding Helix-only diagnostics to quickly identify problematic drives.

Changes:

• Update IOServices.GetNtfsDriveOtherThan() to require a successful GetVolumeNameForVolumeMountPoint call (filters out drives without a usable volume GUID).
• Add a new kernel32 P/Invoke (GetVolumeNameForVolumeMountPointW) to the test DllImports.
• Add a Helix-only diagnostic “test” that prints drive properties and volume GUID availability to the console, and include it in the test project.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring GetVolumeNameForVolumeMountPoint success.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds P/Invoke for GetVolumeNameForVolumeMountPointW.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging of drive info + volume GUID presence for future diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 1

[adamsitnik]

@copilot please address my feedback

[adamsitnik]

LGTM, thank you @danmoseley !

[danmoseley]

Note

This PR was created with Copilot assistance.

Fix deterministic MountVolume test failures on ARM64 Helix machines

Fixes#125295, fixes#125624, fixes#126627

Problem

Directory_Delete_MountVolume.RunTest and Directory_ReparsePoints_MountVolume.runTest fail deterministically (~100% of the time, ~750ms duration) on the Windows.11.Arm64.Open Helix machine pool. This is not timing-related and was not addressed by the delay/polling fixes in #125914 or the Unmount resilience fix in #125625 (those PRs fixed real timing issues -- pre-fix failures on other configurations have since expired from AzDO retention, so we can't verify directly, but there is no evidence they were ineffective for their intended purpose).

Root cause: The ARM64 Helix machines have an E:\ drive (likely an Azure resource/temp disk) that passes all DriveInfo checks -- DriveType=Fixed, DriveFormat=NTFS, IsReady=True -- but GetVolumeNameForVolumeMountPoint fails with ERROR_INVALID_PARAMETER (87). The drive has no volume GUID and doesn't support volume mount point operations. IOServices.GetNtfsDriveOtherThanCurrent() returns this drive, and the test crashes trying to use it.

Some ARM64 Helix machines have only C:\ and a CD-ROM (no second drive at all). On those machines, the cross-drive scenarios already skip gracefully and only same-drive scenarios 3.x run.

Evidence

Analyzed Helix console logs from 5 post-fix builds (all arm64-NativeAOT-Win11, same C:\ volume GUID). Every failure shows the identical pattern:

• Scenario 1: GetVolumeNameForVolumeMountPoint("E:\") -> error 87
• Scenario 2: SetVolumeMountPoint onto E:\ succeeds but path traversal through the mount point fails with DirectoryNotFoundException
• Scenarios 3.x (same-drive): Always pass

Reproduced locally by removing the real E: drive letter and creating SUBST E: which exhibits identical error 87 behavior.

Changes

1. IOServices.GetNtfsDriveOtherThan(): After the existing Fixed/Ready/NTFS checks, also verify the drive has a volume GUID via GetVolumeNameForVolumeMountPoint. Drives without one (SUBST drives, Azure resource disks) are skipped.

2. DumpDriveInformation diagnostic test: New Helix-only test (following the DescriptionNameTests.DumpRuntimeInformationToConsole pattern) that dumps all drives with their volume GUIDs to the console log. Makes future drive-related CI issues immediately diagnosable from the same Helix work item log.

3. GetVolumeNameForVolumeMountPoint P/Invoke in DllImports.cs: Uses char[] (not StringBuilder) because this file uses LibraryImport which does not support StringBuilder.

Local validation

Scenario	Before fix	After fix
SUBST E: (no volume GUID)	Error 87 / DirectoryNotFoundException	Pass (SUBST filtered, scenarios 3.x run)
Real NTFS E:	Pass (all scenarios)	Pass (all scenarios)
Single-drive machine	Scenarios 1/2 skip, 3.x pass	Same -- no change

[danmoseley]

/azp run runtime-ioslikesimulator

[danmoseley]

/azp run runtime-android

[danmoseley]

/azp run runtime-maccatalyst

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Addresses deterministic failures in Windows MountVolume-related filesystem tests on ARM64 Helix by improving second-drive selection and adding Helix diagnostics to make drive/volume issues easier to diagnose from logs.

Changes:

• Filter candidate “other NTFS drives” to only those that support volume mount point operations (via GetVolumeNameForVolumeMountPoint).
• Add a Helix-only diagnostic test that logs drive details and volume GUID availability.
• Improve Mount/Unmount error messages by using P/Invoke error messages instead of raw codes, and add a Helix guard for missing second-drive scenarios.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Adds the new DumpDriveInformation.cs test file to the project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Enhances NTFS drive selection by skipping drives without a volume GUID/mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke used to validate mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	New Helix-only diagnostic test that prints drive and volume GUID information to the console log.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs	Improves mount/unmount exception messages by using P/Invoke error message APIs.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs:260

• Marshal.GetLastPInvokeError() / Marshal.GetLastPInvokeErrorMessage() are not available on .NET Framework. Since this file is compiled for NETFRAMEWORK in some test projects, this change will break those builds unless guarded (e.g., keep GetLastWin32Error() on NETFRAMEWORK and format the message via Win32Exception).

 bool r = DeleteVolumeMountPoint(mountPoint);
if (!r)
{
int error = Marshal.GetLastPInvokeError();
// Ignore expected cleanup errors: 4390 (ERROR_NOT_A_REPARSE_POINT),
// 3 (ERROR_PATH_NOT_FOUND), 2 (ERROR_FILE_NOT_FOUND)
if (error != 4390 && error != 3 && error != 2)
throw new Exception(string.Format("DeleteVolumeMountPoint({0}) failed: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));
Console.WriteLine(string.Format("Ignoring expected error while unmounting {0}: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));

• Files reviewed: 7/7 changed files
• Comments generated: 4

[Copilot]

Pull request overview

This PR addresses deterministic failures in the System.IO mount-volume tests on Windows 11 ARM64 Helix machines by avoiding “fixed/NTFS/ready” drives that don’t actually support volume mount-point operations (no volume GUID), and by adding Helix-focused diagnostics/guards to keep cross-drive scenarios from silently becoming dead code.

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to also require a successful GetVolumeNameForVolumeMountPoint (volume GUID present) before selecting a “usable” NTFS drive.
• Add Helix-only hard-fail guards in both mount-volume tests when no suitable second NTFS drive is available.
• Add a Helix-only diagnostic test to dump drive and volume GUID information into Helix logs.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new DumpDriveInformation.cs test file in the test project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring a volume GUID (mount-point capable).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke via LibraryImport.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only diagnostic logging of drive details + selected “other NTFS drive”.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 3

[Copilot]

Pull request overview

This PR addresses deterministic failures in the MountVolume-related filesystem tests on Windows 11 ARM64 Helix machines by ensuring the “other NTFS drive” selection skips fixed/NTFS/ready drives that don’t actually support volume mount point operations (e.g., drives without a volume GUID).

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to additionally require that candidate drives have a volume GUID via GetVolumeNameForVolumeMountPoint.
• Add a GetVolumeNameForVolumeMountPointWLibraryImport to the test DllImports helpers.
• Add a Helix-only diagnostic test (DumpDriveInformation) and wire it into the test project to print drive/volume details to the Helix console log.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test source file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring successful GetVolumeNameForVolumeMountPoint (i.e., a volume GUID exists).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke for drive capability detection and diagnostics.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging test that dumps drive properties and (on Windows) volume GUID availability to aid CI diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2

[Copilot]

Pull request overview

This PR aims to address deterministic MountVolume-related test failures on Windows ARM64 Helix agents by ensuring the “other NTFS drive” selection excludes drives that don’t support volume mount point operations, and by adding Helix-only diagnostics to quickly identify problematic drives.

Changes:

• Update IOServices.GetNtfsDriveOtherThan() to require a successful GetVolumeNameForVolumeMountPoint call (filters out drives without a usable volume GUID).
• Add a new kernel32 P/Invoke (GetVolumeNameForVolumeMountPointW) to the test DllImports.
• Add a Helix-only diagnostic “test” that prints drive properties and volume GUID availability to the console, and include it in the test project.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring GetVolumeNameForVolumeMountPoint success.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds P/Invoke for GetVolumeNameForVolumeMountPointW.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging of drive info + volume GUID presence for future diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 1

[adamsitnik]

@copilot please address my feedback

[adamsitnik]

LGTM, thank you @danmoseley !

[danmoseley]

Note

This PR was created with Copilot assistance.

Fix deterministic MountVolume test failures on ARM64 Helix machines

Fixes#125295, fixes#125624, fixes#126627

Problem

Directory_Delete_MountVolume.RunTest and Directory_ReparsePoints_MountVolume.runTest fail deterministically (~100% of the time, ~750ms duration) on the Windows.11.Arm64.Open Helix machine pool. This is not timing-related and was not addressed by the delay/polling fixes in #125914 or the Unmount resilience fix in #125625 (those PRs fixed real timing issues -- pre-fix failures on other configurations have since expired from AzDO retention, so we can't verify directly, but there is no evidence they were ineffective for their intended purpose).

Root cause: The ARM64 Helix machines have an E:\ drive (likely an Azure resource/temp disk) that passes all DriveInfo checks -- DriveType=Fixed, DriveFormat=NTFS, IsReady=True -- but GetVolumeNameForVolumeMountPoint fails with ERROR_INVALID_PARAMETER (87). The drive has no volume GUID and doesn't support volume mount point operations. IOServices.GetNtfsDriveOtherThanCurrent() returns this drive, and the test crashes trying to use it.

Some ARM64 Helix machines have only C:\ and a CD-ROM (no second drive at all). On those machines, the cross-drive scenarios already skip gracefully and only same-drive scenarios 3.x run.

Evidence

Analyzed Helix console logs from 5 post-fix builds (all arm64-NativeAOT-Win11, same C:\ volume GUID). Every failure shows the identical pattern:

• Scenario 1: GetVolumeNameForVolumeMountPoint("E:\") -> error 87
• Scenario 2: SetVolumeMountPoint onto E:\ succeeds but path traversal through the mount point fails with DirectoryNotFoundException
• Scenarios 3.x (same-drive): Always pass

Reproduced locally by removing the real E: drive letter and creating SUBST E: which exhibits identical error 87 behavior.

Changes

1. IOServices.GetNtfsDriveOtherThan(): After the existing Fixed/Ready/NTFS checks, also verify the drive has a volume GUID via GetVolumeNameForVolumeMountPoint. Drives without one (SUBST drives, Azure resource disks) are skipped.

2. DumpDriveInformation diagnostic test: New Helix-only test (following the DescriptionNameTests.DumpRuntimeInformationToConsole pattern) that dumps all drives with their volume GUIDs to the console log. Makes future drive-related CI issues immediately diagnosable from the same Helix work item log.

3. GetVolumeNameForVolumeMountPoint P/Invoke in DllImports.cs: Uses char[] (not StringBuilder) because this file uses LibraryImport which does not support StringBuilder.

Local validation

Scenario	Before fix	After fix
SUBST E: (no volume GUID)	Error 87 / DirectoryNotFoundException	Pass (SUBST filtered, scenarios 3.x run)
Real NTFS E:	Pass (all scenarios)	Pass (all scenarios)
Single-drive machine	Scenarios 1/2 skip, 3.x pass	Same -- no change

[danmoseley]

/azp run runtime-ioslikesimulator

[danmoseley]

/azp run runtime-android

[danmoseley]

/azp run runtime-maccatalyst

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Addresses deterministic failures in Windows MountVolume-related filesystem tests on ARM64 Helix by improving second-drive selection and adding Helix diagnostics to make drive/volume issues easier to diagnose from logs.

Changes:

• Filter candidate “other NTFS drives” to only those that support volume mount point operations (via GetVolumeNameForVolumeMountPoint).
• Add a Helix-only diagnostic test that logs drive details and volume GUID availability.
• Improve Mount/Unmount error messages by using P/Invoke error messages instead of raw codes, and add a Helix guard for missing second-drive scenarios.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Adds the new DumpDriveInformation.cs test file to the project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Enhances NTFS drive selection by skipping drives without a volume GUID/mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke used to validate mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	New Helix-only diagnostic test that prints drive and volume GUID information to the console log.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs	Improves mount/unmount exception messages by using P/Invoke error message APIs.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs:260

• Marshal.GetLastPInvokeError() / Marshal.GetLastPInvokeErrorMessage() are not available on .NET Framework. Since this file is compiled for NETFRAMEWORK in some test projects, this change will break those builds unless guarded (e.g., keep GetLastWin32Error() on NETFRAMEWORK and format the message via Win32Exception).

 bool r = DeleteVolumeMountPoint(mountPoint);
if (!r)
{
int error = Marshal.GetLastPInvokeError();
// Ignore expected cleanup errors: 4390 (ERROR_NOT_A_REPARSE_POINT),
// 3 (ERROR_PATH_NOT_FOUND), 2 (ERROR_FILE_NOT_FOUND)
if (error != 4390 && error != 3 && error != 2)
throw new Exception(string.Format("DeleteVolumeMountPoint({0}) failed: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));
Console.WriteLine(string.Format("Ignoring expected error while unmounting {0}: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));

• Files reviewed: 7/7 changed files
• Comments generated: 4

[Copilot]

Pull request overview

This PR addresses deterministic failures in the System.IO mount-volume tests on Windows 11 ARM64 Helix machines by avoiding “fixed/NTFS/ready” drives that don’t actually support volume mount-point operations (no volume GUID), and by adding Helix-focused diagnostics/guards to keep cross-drive scenarios from silently becoming dead code.

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to also require a successful GetVolumeNameForVolumeMountPoint (volume GUID present) before selecting a “usable” NTFS drive.
• Add Helix-only hard-fail guards in both mount-volume tests when no suitable second NTFS drive is available.
• Add a Helix-only diagnostic test to dump drive and volume GUID information into Helix logs.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new DumpDriveInformation.cs test file in the test project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring a volume GUID (mount-point capable).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke via LibraryImport.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only diagnostic logging of drive details + selected “other NTFS drive”.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 3

[Copilot]

Pull request overview

This PR addresses deterministic failures in the MountVolume-related filesystem tests on Windows 11 ARM64 Helix machines by ensuring the “other NTFS drive” selection skips fixed/NTFS/ready drives that don’t actually support volume mount point operations (e.g., drives without a volume GUID).

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to additionally require that candidate drives have a volume GUID via GetVolumeNameForVolumeMountPoint.
• Add a GetVolumeNameForVolumeMountPointWLibraryImport to the test DllImports helpers.
• Add a Helix-only diagnostic test (DumpDriveInformation) and wire it into the test project to print drive/volume details to the Helix console log.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test source file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring successful GetVolumeNameForVolumeMountPoint (i.e., a volume GUID exists).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke for drive capability detection and diagnostics.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging test that dumps drive properties and (on Windows) volume GUID availability to aid CI diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2

[Copilot]

Pull request overview

This PR aims to address deterministic MountVolume-related test failures on Windows ARM64 Helix agents by ensuring the “other NTFS drive” selection excludes drives that don’t support volume mount point operations, and by adding Helix-only diagnostics to quickly identify problematic drives.

Changes:

• Update IOServices.GetNtfsDriveOtherThan() to require a successful GetVolumeNameForVolumeMountPoint call (filters out drives without a usable volume GUID).
• Add a new kernel32 P/Invoke (GetVolumeNameForVolumeMountPointW) to the test DllImports.
• Add a Helix-only diagnostic “test” that prints drive properties and volume GUID availability to the console, and include it in the test project.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring GetVolumeNameForVolumeMountPoint success.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds P/Invoke for GetVolumeNameForVolumeMountPointW.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging of drive info + volume GUID presence for future diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 1

[adamsitnik]

@copilot please address my feedback

[adamsitnik]

LGTM, thank you @danmoseley !

[danmoseley]

Note

This PR was created with Copilot assistance.

Fix deterministic MountVolume test failures on ARM64 Helix machines

Fixes#125295, fixes#125624, fixes#126627

Problem

Directory_Delete_MountVolume.RunTest and Directory_ReparsePoints_MountVolume.runTest fail deterministically (~100% of the time, ~750ms duration) on the Windows.11.Arm64.Open Helix machine pool. This is not timing-related and was not addressed by the delay/polling fixes in #125914 or the Unmount resilience fix in #125625 (those PRs fixed real timing issues -- pre-fix failures on other configurations have since expired from AzDO retention, so we can't verify directly, but there is no evidence they were ineffective for their intended purpose).

Root cause: The ARM64 Helix machines have an E:\ drive (likely an Azure resource/temp disk) that passes all DriveInfo checks -- DriveType=Fixed, DriveFormat=NTFS, IsReady=True -- but GetVolumeNameForVolumeMountPoint fails with ERROR_INVALID_PARAMETER (87). The drive has no volume GUID and doesn't support volume mount point operations. IOServices.GetNtfsDriveOtherThanCurrent() returns this drive, and the test crashes trying to use it.

Some ARM64 Helix machines have only C:\ and a CD-ROM (no second drive at all). On those machines, the cross-drive scenarios already skip gracefully and only same-drive scenarios 3.x run.

Evidence

Analyzed Helix console logs from 5 post-fix builds (all arm64-NativeAOT-Win11, same C:\ volume GUID). Every failure shows the identical pattern:

• Scenario 1: GetVolumeNameForVolumeMountPoint("E:\") -> error 87
• Scenario 2: SetVolumeMountPoint onto E:\ succeeds but path traversal through the mount point fails with DirectoryNotFoundException
• Scenarios 3.x (same-drive): Always pass

Reproduced locally by removing the real E: drive letter and creating SUBST E: which exhibits identical error 87 behavior.

Changes

1. IOServices.GetNtfsDriveOtherThan(): After the existing Fixed/Ready/NTFS checks, also verify the drive has a volume GUID via GetVolumeNameForVolumeMountPoint. Drives without one (SUBST drives, Azure resource disks) are skipped.

2. DumpDriveInformation diagnostic test: New Helix-only test (following the DescriptionNameTests.DumpRuntimeInformationToConsole pattern) that dumps all drives with their volume GUIDs to the console log. Makes future drive-related CI issues immediately diagnosable from the same Helix work item log.

3. GetVolumeNameForVolumeMountPoint P/Invoke in DllImports.cs: Uses char[] (not StringBuilder) because this file uses LibraryImport which does not support StringBuilder.

Local validation

Scenario	Before fix	After fix
SUBST E: (no volume GUID)	Error 87 / DirectoryNotFoundException	Pass (SUBST filtered, scenarios 3.x run)
Real NTFS E:	Pass (all scenarios)	Pass (all scenarios)
Single-drive machine	Scenarios 1/2 skip, 3.x pass	Same -- no change

[danmoseley]

/azp run runtime-ioslikesimulator

[danmoseley]

/azp run runtime-android

[danmoseley]

/azp run runtime-maccatalyst

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

Addresses deterministic failures in Windows MountVolume-related filesystem tests on ARM64 Helix by improving second-drive selection and adding Helix diagnostics to make drive/volume issues easier to diagnose from logs.

Changes:

• Filter candidate “other NTFS drives” to only those that support volume mount point operations (via GetVolumeNameForVolumeMountPoint).
• Add a Helix-only diagnostic test that logs drive details and volume GUID availability.
• Improve Mount/Unmount error messages by using P/Invoke error messages instead of raw codes, and add a Helix guard for missing second-drive scenarios.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Adds the new DumpDriveInformation.cs test file to the project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Enhances NTFS drive selection by skipping drives without a volume GUID/mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke used to validate mount-point support.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	New Helix-only diagnostic test that prints drive and volume GUID information to the console log.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds a Helix check to fail when a suitable second NTFS drive can’t be found.
src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs	Improves mount/unmount exception messages by using P/Invoke error message APIs.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/Common/tests/System/IO/ReparsePointUtilities.cs:260

• Marshal.GetLastPInvokeError() / Marshal.GetLastPInvokeErrorMessage() are not available on .NET Framework. Since this file is compiled for NETFRAMEWORK in some test projects, this change will break those builds unless guarded (e.g., keep GetLastWin32Error() on NETFRAMEWORK and format the message via Win32Exception).

 bool r = DeleteVolumeMountPoint(mountPoint);
if (!r)
{
int error = Marshal.GetLastPInvokeError();
// Ignore expected cleanup errors: 4390 (ERROR_NOT_A_REPARSE_POINT),
// 3 (ERROR_PATH_NOT_FOUND), 2 (ERROR_FILE_NOT_FOUND)
if (error != 4390 && error != 3 && error != 2)
throw new Exception(string.Format("DeleteVolumeMountPoint({0}) failed: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));
Console.WriteLine(string.Format("Ignoring expected error while unmounting {0}: {1}", mountPoint, Marshal.GetLastPInvokeErrorMessage()));

• Files reviewed: 7/7 changed files
• Comments generated: 4

[Copilot]

Pull request overview

This PR addresses deterministic failures in the System.IO mount-volume tests on Windows 11 ARM64 Helix machines by avoiding “fixed/NTFS/ready” drives that don’t actually support volume mount-point operations (no volume GUID), and by adding Helix-focused diagnostics/guards to keep cross-drive scenarios from silently becoming dead code.

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to also require a successful GetVolumeNameForVolumeMountPoint (volume GUID present) before selecting a “usable” NTFS drive.
• Add Helix-only hard-fail guards in both mount-volume tests when no suitable second NTFS drive is available.
• Add a Helix-only diagnostic test to dump drive and volume GUID information into Helix logs.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new DumpDriveInformation.cs test file in the test project.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring a volume GUID (mount-point capable).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke via LibraryImport.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only diagnostic logging of drive details + selected “other NTFS drive”.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/ReparsePoints_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/Delete_MountVolume.cs	Adds Helix guard that fails when no suitable second NTFS drive is found.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 3

[Copilot]

Pull request overview

This PR addresses deterministic failures in the MountVolume-related filesystem tests on Windows 11 ARM64 Helix machines by ensuring the “other NTFS drive” selection skips fixed/NTFS/ready drives that don’t actually support volume mount point operations (e.g., drives without a volume GUID).

Changes:

• Update IOServices.GetNtfsDriveOtherThan(...) to additionally require that candidate drives have a volume GUID via GetVolumeNameForVolumeMountPoint.
• Add a GetVolumeNameForVolumeMountPointWLibraryImport to the test DllImports helpers.
• Add a Helix-only diagnostic test (DumpDriveInformation) and wire it into the test project to print drive/volume details to the Helix console log.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test source file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring successful GetVolumeNameForVolumeMountPoint (i.e., a volume GUID exists).
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds GetVolumeNameForVolumeMountPointW P/Invoke for drive capability detection and diagnostics.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging test that dumps drive properties and (on Windows) volume GUID availability to aid CI diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2

[Copilot]

Pull request overview

This PR aims to address deterministic MountVolume-related test failures on Windows ARM64 Helix agents by ensuring the “other NTFS drive” selection excludes drives that don’t support volume mount point operations, and by adding Helix-only diagnostics to quickly identify problematic drives.

Changes:

• Update IOServices.GetNtfsDriveOtherThan() to require a successful GetVolumeNameForVolumeMountPoint call (filters out drives without a usable volume GUID).
• Add a new kernel32 P/Invoke (GetVolumeNameForVolumeMountPointW) to the test DllImports.
• Add a Helix-only diagnostic “test” that prints drive properties and volume GUID availability to the console, and include it in the test project.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj	Includes the new diagnostic test file in the test project compilation list.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/IOServices.cs	Filters candidate NTFS drives by requiring GetVolumeNameForVolumeMountPoint success.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/PortedCommon/DllImports.cs	Adds P/Invoke for GetVolumeNameForVolumeMountPointW.
src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/DumpDriveInformation.cs	Adds Helix-only logging of drive info + volume GUID presence for future diagnosis.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 1

[adamsitnik]

@copilot please address my feedback

[adamsitnik]

LGTM, thank you @danmoseley !