<filesystem>: directory_iterator(...) and is_empty(...) not working with empty volumes

[MartinKuschnik]

I'm woking with virtual disks and have some trouble if a new volume does not contain any files or directories.

First we have to ceate and attch a new virtual disk:

New-VHD -Path "$env:TEMP\test_volume.vhdx" -SizeBytes 100MB | Mount-VHD -Passthru -NoDriveLetter |Initialize-Disk -Passthru | New-Partition -UseMaximumSize | Format-Volume -FileSystem NTFS -Confirm:$false -Force | Format-List -Property Path

The command should return the volume name

Path : \\?\Volume{069de63e-8072-42fe-9adf-5a9b9cec201d}\

that can be used to access it:

#include <filesystem> 
#include <iostream> 

int wmain(int argc)
{
	constexpr const char* volume = "\\\\?\\Volume{069de63e-8072-42fe-9adf-5a9b9cec201d}\\";

	std::cout 
		<< std::format("std::filesystem::exists('{0}')) : {1}", volume, std::filesystem::exists(volume)) 
		<< std::endl;

	try
	{
		std::cout 
			<< std::format("std::filesystem::is_empty('{0}')) : {1}", volume, std::filesystem::is_empty(volume)) 
			<< std::endl;
	}
	catch (const std::exception& ex)
	{
		std::cout 
			<< std::format("std::filesystem::is_empty('{0}')) failed: {1}", volume, ex.what()) 
			<< std::endl;
	}

	try
	{
		const auto it = std::filesystem::directory_iterator(volume);

		std::cout 
			<< std::format("std::filesystem::directory_iterator('{0}')) succeeded.", volume) 
			<< std::endl;
	}
	catch (const std::exception& ex)
	{
		std::cout 
			<< std::format("std::filesystem::directory_iterator('{0}')) failed: {1}", volume, ex.what()) 
			<< std::endl;
	}
}

This code will produce the following output:

std::filesystem::exists('\\?\Volume{069de63e-8072-42fe-9adf-5a9b9cec201d}\')) : true
std::filesystem::is_empty('\\?\Volume{069de63e-8072-42fe-9adf-5a9b9cec201d}\')) failed: is_empty: The system cannot find the file specified.: "\\?\Volume{069de63e-8072-42fe-9adf-5a9b9cec201d}\"
std::filesystem::directory_iterator('\\?\Volume{069de63e-8072-42fe-9adf-5a9b9cec201d}\')) failed: directory_iterator::directory_iterator: The system cannot find the file specified.: "\\?\Volume{069de63e-8072-42fe-9adf-5a9b9cec201d}\"

Expected behavior

• std::filesystem::is_empty(...) should not throw any exception for an empty volume.
• std::filesystem::directory_iterator(...) should not throw any exception for an empty volume.

STL version

Microsoft Visual Studio Community 2022 (64-bit) - Current
Version 17.8.3

Additional context

After asigning a drive letter or creating a file or directory the code workes like expacted.

[Fulgen301]

is_empty constructs a directory_iterator, which calls FindFirstFileExW. which ultimately calls NtQueryDirectoryEx with a file handle to the volume directory, QueryFlags = SL_RETURN_SINGLE_ENTRY and FileName = "*", which therefore returns the first directory entry. A "regular" empty directory (e.g. one created by mkdir) isn't actually empty, as far as that function is concerned - it returns an entry with the file name . However, your volume path does not have such an entry - it is truly empty, so NtQueryDirectoryEx fails with STATUS_NO_SUCH_FILE. This behavior isn't explicitely documented in the syscall documentation, but the Server Message Block protocol specification before Revision 51.0 documented that error code with the same behavior:

If there are no entries to return and this was the initial query (Open.EnumerationLocation was zero
before querying the object store), the server MUST fail the request with STATUS_NO_SUCH_FILE.

The error code then bubbles up and is returned as ERROR_FILE_NOT_FOUND by FindFirstFileExW.

As calling FindFirstFileExW with an inexistent path results in ERROR_PATH_NOT_FOUND,

STL/stl/inc/filesystem

Lines 2572 to 2581 in a888880

	_Creator(const path& _Path_arg, const directory_options _Options) : _Path(_Path_arg) {
	const auto _Error = _Open_dir(_Path, _Options, _Dir, _Find_data);
	if (_Error == __std_win_error::_Success) {
	_Status = {true, __std_win_error::_Success};
	} else if (_Error == __std_win_error::_No_more_files) {
	_Status = {false, __std_win_error::_Success};
	} else {
	_Status = {false, _Error};
	}
	}

could maybe be extended to interpret ERROR_FILE_NOT_FOUND as "no more files"?