File.Copy: Clone file when possible on MacOS

[hamarb123]

For File.Copy on macOS we're currently doing the following:

runtime/src/native/libs/System.Native/pal_io.c

Line 1257 in d320fd9

return fcopyfile(inFd, outFd, NULL, COPYFILE_ALL) == 0 ? 0 : -1;

If we instead use copyfile() we can use the COPYFILE_CLONE option (with the existing COPYFILE_ALL obviously, which I think the clone option may include all of the flags of anyway), which allows the file to be instantly copied on supported filesystems, and falls back to exactly the same implementation when it's not available (note - this behaves identically (except with symlinks (ie. this copies the symlink itself I think) and when the destination exists, which we can special case), it just allows APFS to immediately set the data location to the existing file, and when it's updated it will be create a copy since APFS is a copy-on-write filesystem). This is obviously much faster (I tested it on a hard drive with a 30GB file and it copied instantly).

We can implement that in this file:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/FileSystem.Unix.cs

Line 31 in 21e9913

public static void CopyFile(string sourceFullPath, string destFullPath, bool overwrite)

by splitting the OSX-like platforms code, like we have with the code that sets the creation date.

We should be able to implement it with effectively the same behaviour as whatever the current behaviour is with relative ease. Note: apps like finder clone files in this way.

I'd like to implement this in a PR if approved (ideally by .NET 8).

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

Issue Details

---

For File.Copy on macOS we're currently doing the following:

runtime/src/native/libs/System.Native/pal_io.c

Line 1257 in d320fd9

return fcopyfile(inFd, outFd, NULL, COPYFILE_ALL) == 0 ? 0 : -1;

If we instead use copyfile() we can use the COPYFILE_CLONE option (with the existing COPYFILE_ALL obviously), which allows the file to be instantly copied on supported filesystems, and falls back to exactly the same implementation when it's not available (note - this behaves identically (except with symlinks (ie. this copies the symlink itself I think) and when the destination exists, which we can special case), it just allows APFS to immediately set the data location to the existing file, and when it's updated it will be create a copy since APFS is a copy-on-write filesystem). This is obviously much faster (I tested it on a hard drive with a 30GB file and it copied instantly).

We can implement that in this file:

runtime/src/libraries/System.Private.CoreLib/src/System/IO/FileSystem.Unix.cs

Line 31 in 21e9913

public static void CopyFile(string sourceFullPath, string destFullPath, bool overwrite)

by splitting the OSX-like platforms code, like we have with the code that sets the creation date.

We should be able to implement it with exactly the same behaviour as whatever the current behaviour is with relative ease.

I'd like to implement this in a PR if approved (ideally by .NET 8).

Author:	hamarb123
Assignees:	-
Labels:	area-System.IO
Milestone:	-

[hamarb123]

Can someone also tag this with the performance improvement tag (tenet I think).

[adamsitnik]

It was discussed in the past: #26713

This comment is most interesting: #26713 (comment)

Note that we had to remove COPYFILE_CLONE and switch to calling clonefile() API instead since the former had some unintended sideeffects: mono/mono#10020

cc @filipnavara who contributed dotnet/corefx#37583

[filipnavara]

There's more than one closed PR that attempted to do this. The work can be resurrected. The main problem was duplicating logic between the managed side of the code and the native side of the code that dealt with the file sharing emulation.

[hamarb123]

The behaviour discovered in mono/mono#10020 is now documented in the man pages:

 COPYFILE_CLONE Try to clone the file instead. This is a best try flag i.e. if cloning fails, fallback to copying the file. This flag is equivalent to (COPYFILE_EXCL | COPYFILE_ACL | COPYFILE_STAT |
 COPYFILE_XATTR | COPYFILE_DATA | COPYFILE_NOFOLLOW_SRC). Note that if cloning is successful, progress callbacks will not be invoked. Note also that there is no support for cloning directories: if
 a directory is provided as the source and COPYFILE_CLONE_FORCE is not passed, this will instead copy the directory. Since this flag implies COPYFILE_NOFOLLOW_SRC, symbolic links themselves will be
 cloned instead of their targets. Recursive copying however is supported, see below for more information. (This is only applicable for the copyfile() function.)

[filipnavara]

I think the last attempt was #33159.

[hamarb123]

If you could find links to all of the previous attempts that'd be great (if they all link to each other in the description that is fine as well) so I can see what was there and what went wrong with it.

If you're happy for me to have a go, can you please 'assign' this issue to me, thanks.

[hamarb123]

The main problem was duplicating logic between the managed side of the code and the native side of the code that dealt with the file sharing emulation.

Can you please explain to me what the issue with file locking is specifically related to this feature. I read through the issue you commented with and some other linked ones (including the ones about file locking itself), and I don't understand what it has to do with File.Copy.

The solution that I plan to implement is (with appropriate platform checks and error handling):

1. Attempt to do a COPYFILE_CLONE_FORCE with copyfile() as the very first thing we do (ie. top line in FileSystem.Unix.cs's CopyFile function I linked in an earlier comment)
2. If it fails, use the existing code since it seems to work fine for copying a file
3. If it succeeds, check if we made a symlink and delete it and fall back to existing code. This should be quite fast as copying a symlink should be much, much quicker than copying the average file anyway, so the additional check should end up faster for most cases than what we currently have.
4. If overwrite = false, that already works with COPYFILE_CLONE_FORCE since it won't overwrite without errors
5. If overwrite = true, we need to check at some point if the file exists, COPYFILE_CLONE_FORCE should create the EEXIST error in the case the file already exists, and we can deal with it then by deleting it and retrying, this saves us the extra syscall for when the file doesn't exist, and isn't that expensive imo when it does (we could also just check if it does exist, but I think this way is better).

COPYFILE_CLONE_FORCE:

 COPYFILE_CLONE_FORCE Clone the file instead. This is a force flag i.e. if cloning fails, an error is returned. This flag is equivalent to (COPYFILE_EXCL | COPYFILE_ACL | COPYFILE_STAT | COPYFILE_XATTR | COPYFILE_DATA
 | COPYFILE_NOFOLLOW_SRC). Note that if cloning is successful, progress callbacks will not be invoked. Note also that there is no support for cloning directories: if a directory is provided as the
 source, an error will be returned. Since this flag implies COPYFILE_NOFOLLOW_SRC, symbolic links themselves will be cloned instead of their targets. (This is only applicable for the copyfile()
 function.)

[filipnavara]

.NET uses the POSIX advisory locks to implement the FileShare option of FileStream on Linux/macOS. File.Copy respects these locks on both the source and target file. The current implementation uses managed code to get file handles for both the source and target file, which happens to do all the necessary advisory lock checks.

If clonefile is used you work with paths instead of handles. Moreover, the destination file handle cannot be open and the file cannot exists. Depending on some conditions you may need to fallback to the regular file copy which needs the handles. That results in a tricky situation that doesn't fit well with the current code structure. The previous PRs handled this by duplicating some of the managed logic inside the SystemNative_CopyFile native method. It worked and passed the unit tests but it was not pretty. Alternative option would be to have a new C shim function specifically for clonefile and have all the advisory lock logic, destination file checks, and fallbacks in the managed code.