Fix GH-10548: copy() fails on cifs mounts because of incorrect length (cfr_max) specified in streams.c:1584 copy_file_range()

[nielsdos]

On some filesystems, the copy operation fails if we specify a size
larger than the file size. We use a stat call to clamp the size to copy
to the actual filesize. This stat call shouldn't impact performance
notably because stat calls can be cached. In some cases (like for /proc
files), the returned size is 0, so we should avoid problems by not using
copy_file_range in those cases (copy_file_range wouldn't work anyway on
this particular example because the syscall is not supported for /proc).

I will do some cleanups and improvements on copy_file_range (like the loop I described in #10440 in a follow-up PR for the master branch).

/cc @knowsshit It would be great if you could test this patch out and let us know if this works for you :)
/cc @arnaud-lb since he reviewed the PR I referenced in here.

Edit: CI fail in x32 is unrelated

[knowsshit]

Thank you! It should work if strace shows that the length argument that is passed to copy_file_range() is not greater than the source file size. I have tested that copy_file_range() works fine in the environment when this is true.

The environment where copy() fails for me on the cifs mount is an Azure Web App, so not sure if I am able to test the patch there.

[nielsdos]

Thank you! It should work if strace shows that the length argument that is passed to copy_file_range() is not greater than the source file size. I have tested that copy_file_range() works fine in the environment when this is true.

Thanks, I verified that is the case indeed so it should work. At least it worked in my testing.

The environment where copy() fails for me on the cifs mount is an Azure Web App, so not sure if I am able to test the patch there.

No problem :)

[nielsdos]

Rebased on upstream 8.2 since the fix on which this depends got merged.

[nielsdos]

@arnaud-lb Updated such that on EIO it will fall back to the mmap or classic copy method.

[arnaud-lb]

Thank you!