testing/fstest: TestFS: relax check ReadDir based on perms

[dolmen]

TL;DR

Drop one false positive case in testing/fstest.TestFS: it is ok for ReadDir to return ErrPermission if the fs.FileMode of the file says some users are not allowed to read the directory.

Context

With io/fs.FS, there is no way to select which permission bits (fs.FileMode) apply when reading a file. So fstest.TestFS has no way to enforce that given a FileMode's file/directory based on user/group/others permission bits, Open, ReadFile or ReadDir should trigger or not an fs.ErrPermission.

Currently any error (including ErrPermission) on ReadDir is always considered an error. However returning fs.ErrPermission is correct behaviour if the filesystem enforces fs.FileMode and the directory has read permissions bits off.

Proposal

I propose that TestFS to ignore ErrPermission on ReadDir if at least one read permission bit is not set: ignore ErrPermission (and silently skip other ReadDir checks) if mode & 0444 != 0444.

Impact

This change will only drop some returned errors and will never raise more errors, so backward compatibility is preserved.

[dolmen]

Cc: @bcmills

[neild]

Permission bits aren't universally supported; if TestFS should ignore a permission error opening a file with no Unix read bits, shouldn't it ignore a permission error opening a Windows file with an ACL forbidding access?

[dolmen]

@josharian May I ask your opinion on this (as you were the author of #46776 as well as of various io/fs.FS implementations)?

[dolmen]

@neild The documentation for fs.FileMode mentions:

The nine least-significant bits are the standard Unix rwxrwxrwx permissions. The values of these bits should be considered part of the public API and may be used in wire protocols or disk representations