Enable File.Copy to EXFAT volumes when not running as root

[danmoseley]

Fixes #45730

In File.Copy on Linux, after successfully copying the file we copy over the access and modified time, and copy the access permissions. See #16366 for why this was done: it's because on Windows, CopyFileEx preserves them, and some programs took a dependency on this e.g., for up to dateness checks, and arguably it's always been documented that File.Copy copies 'attributes'.

The problem on exfat volumes is that this file system does not store POSIX ownership; when viewed from Linux, all files appear to be owned by root. When we copy to such a volume, the copy succeeds if we have write permissions, but subsequently setting the timestamps and access permissions fails because (unless running as root) we need to own the file, and only root owns it. This manifests as System.IO.IOException: Operation not permitted. even though the file is successfully copied.

The workaround if you want to copy to an exfat volume on Linux is to either implement File.Copy yourself, or run as root.

The fix is to ignore EPERM when setting the time stamps and permissions. That gives a reasonable outcome, the file is copied in the best way that we can achieve given the filesystem limitations.

Note that "touch" does succeed against files on exfat if you do not supply a time -- the man page for utimensat notes that to set current time only requires write access, to set a specific time requires ownership rights or sufficient privileges (eg., root) -- and on exfat, only root ever has ownership rights. Conceivably we could at least set the current time, therefore, but it's not clear that's an improvement.

I don't think there's a good way to write a test but I tested manually on an exfat volume.

[danmoseley]

Test failure is #44657

[danmoseley]

This also impacts FileInfo.CopyTo

[danmoseley]

@jozkee any chance you could take a quick look?

[jozkee]

Sounds reasonable to me. Thanks!

I don't think there's a good way to write a test but I tested manually on an exfat volume.

Not required for this PR but if you think is valuable to have the manual test you could consider adding it to https://github.com/dotnet/runtime/blob/master/src/libraries/System.IO.FileSystem/tests/ManualTests/ManualTests.cs

[jozkee]

@danmosemsft forgot to ask, is there a problem if you run the same scenario on Windows? I wonder if this might cause a platform inconsistency.

[danmoseley]

@jozkee that should not be a problem, as on Windows we don't copy the attributes -- Windows itself does, inside CopyFileEx. That's what our Unix implementation is trying to mimic.

[danmoseley]

added manual test

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