[issue31842] pathlib: "Incorrect function" during resolve()

[Eryk Sun]

Eryk Sun <eryksun at gmail.com> added the comment:

> perfectly valid redirected paths (winfsp ram drives for example)

I mounted a WinFsp MEMFS filesystem on a directory, which set a mountpoint that targets the root path of a volume device in the native "\Device" object directory. It didn't create a volume GUID name, which means the mountpoint manager isn't supported in this configuration.

The error code ERROR_UNRECOGNIZED_VOLUME (1005) is meaningless in this case. The mountpoint manager queries a volume with IOCTLs such as IOCTL_MOUNTDEV_QUERY_DEVICE_NAME, which the WinFsp virtual volume (in the above configuration) doesn't support. Weirdly, it returns the error code STATUS_UNRECOGNIZED_VOLUME instead of STATUS_INVALID_DEVICE_REQUEST. It does this as a lazy workaround for various IOCTLs it receives from filesystem drivers while the volume is in the process of being mounted [1][2]. The side effect is that it returns STATUS_UNRECOGNIZED_VOLUME for unhandled IOCTLs even when it's not getting mounted. This behavior should have been restricted to when the volume parameter block (VPB) is unmounted. Otherwise it should return the expected error code STATUS_INVALID_DEVICE_REQUEST (i.e. ERROR_INVALID_FUNCTION) instead of confusing users with a meaningless error.

WinFsp does support the mountpoint manager, in a restricted fashion. The mount target has to be a drive device name in the form "\\.\X:". This gets registered with the mountpoint manager as the canonical DOS name of the volume. Since it's a global name, administrator access is required. It also creates a GUID volume name. Run mountvol.exe without arguments to find the volume name that's associated with the drive letter. Then run it again as `mountvol <path> <volume_name>`, where <path> is an empty directory on which to mount the volume. Note that Python's os.path.realpath() will resolve the volume to the canonical drive name, even if the path traverses a directory mountpoint for the volume.

A new issue should be created to ignore ERROR_UNRECOGNIZED_VOLUME in 3.10+, for which Path.resolve() was updated to call os.path.realpath(). For 3.9, fixing Path.resolve() is still possible. There are 3 remaining bug releases planned: 3.9.9: (2022-01-03), 3.9.10 (2022-02-28), and 3.9.11 (2022-05-02).

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[1] https://github.com/billziss-gh/winfsp/blob/v1.9/src/sys/devctl.c#L49
[2] https://github.com/billziss-gh/winfsp/issues/177

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<https://bugs.python.org/issue31842>
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