Fix random "ERROR_FILE_NOT_FOUND" when unmounting with absolute path

[chemwolf6922]

Summary of the Pull Request

Fix this random issue where sometimes unmount with the absolute path results in ERROR_FILE_NOT_FOUND

PS D:\tmp> wsl --mount "D:\tmp\test.vhdx" --vhd --bare
wsl: The .wslconfig setting 'wsl2.kernelCommandLine' is disabled by the computer policy.
The operation completed successfully.
PS D:\tmp> wsl --unmount  "D:\tmp\test.vhdx"
The system cannot find the file specified.
Error code: Wsl/Service/DetachDisk/ERROR_FILE_NOT_FOUND
PS D:\tmp> wsl --unmount  "\\?\D:\tmp\test.vhdx"
The operation completed successfully.

PR Checklist

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

There is already a test case MountTests::MountTests::AbsolutePathVhdUnmount for this. But since this issue is not deterministic, it does not catch the problem reliably.
Instead, I did this loop for 20 times for before and after the fix:

wsl --shutdown
wsl --mount "D:\tmp\test.vhdx" --vhd --bare
wsl --unmount  "D:\tmp\test.vhdx"
# And wsl --unmount  "\\?\D:\tmp\test.vhdx" if it fails.

Before:
6 / 20 hits.
After:
0 / 20 hits.

[Copilot]

Pull request overview

This PR addresses intermittent ERROR_FILE_NOT_FOUND failures when unmounting a VHD by an absolute path without the \\?\ prefix, by retrying the detach using a normalized path representation.

Changes:

• Retry DetachDisk with a normalized/full path on ERROR_FILE_NOT_FOUND (not only for relative paths), to handle \\?\ prefix mismatches.
• Update the inline comment to reflect the broadened retry behavior.

Comments suppressed due to low confidence (1)

src/windows/common/WslClient.cpp:1226

• The unconditional retry normalizes the disk argument via filesystem::GetFullPath(), which only works for real filesystem paths. If the user passes a non-filesystem disk identifier (e.g. passthrough paths like "\.\PHYSICALDRIVE…") and DetachDisk returns ERROR_FILE_NOT_FOUND, GetFullPath will throw (often ERROR_INVALID_NAME), changing the user-visible failure from ERROR_FILE_NOT_FOUND to a different error. Consider making normalization best-effort (catch/ignore GetFullPath failures and fall back to the original HRESULT), or gate the retry to VHD/filesystem-style paths only.

    // Retry with the normalized path to handle relative paths and \\?\ prefix mismatches.
    if (result == HRESULT_FROM_WIN32(ERROR_FILE_NOT_FOUND))
    {
        // retry dismounting with the absolute path
        const auto absoluteDisk = wsl::windows::common::filesystem::GetFullPath(filesystem::UnquotePath(disk).c_str());
        value = service.DetachDisk(absoluteDisk.c_str());

[benhillis]

Did you figure out why the result is not deterministic? That’s very odd.