[wasi] File.Copy fails with "Function not implemented" when copying file metadata

[lewing]

Description

On WASI, File.Copy and FileInfo.CopyTo always fail after copying the data:

System.IO.IOException : Function not implemented
   at Interop.ThrowExceptionForIoErrno(ErrorInfo errorInfo, String path, Boolean isDirError)
   at System.IO.FileSystem.CopyFile(String sourceFullPath, String destFullPath, Boolean overwrite)
   at System.IO.File.Copy(String sourceFileName, String destFileName, Boolean overwrite)

SystemNative_CopyFile (src/native/libs/System.Native/pal_io.c) copies the contents with its read/write fallback (no sendfile or fcopyfile on WASI), then copies metadata. The WASI build has HAVE_FCHMOD set, but WASI has no permission bits. fchmod very likely fails with ENOSYS, and the code only tolerates EPERM there, so the whole copy reports failure. futimens is also called and may fail the same way.

In the CoreCLR WASI library test runs on #134813 this fails 78 test cases in System.IO.Tests (the File.Copy/FileInfo.CopyTo paths, including the GB18030 file tests) and 3 in Microsoft.VisualBasic.Core.Tests. Microsoft.VisualBasic.FileIO.FileSystem copy APIs are affected too.

Proposed fix

On WASI, treat metadata copying as best effort: tolerate ENOSYS (and ENOTSUP) from fchmod and futimens in SystemNative_CopyFile the same way EPERM is tolerated, or skip the permission copy under TARGET_WASI. Confirm with a WASI run that fchmod is the call returning ENOSYS.

Note

This issue was drafted with the help of GitHub Copilot.

[dotnet-policy-service]

Tagging subscribers to 'arch-wasm': @lewing, @pavelsavara
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

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