os: (*File).Readdirnames returns an error that does not include the path

[bcmills]

In general, when *os.File methods fail they return a PathError referring to the failing operation and the name of the file:

go/src/os/stat_unix.go

Lines 20 to 23 in a38a917

	err := f.pfd.Fstat(&fs.sys)
	if err != nil {
	return nil, &PathError{"stat", f.name, err}
	}

Unfortunately, on Unix and Darwin, the Readdir and Readdirnames methods do not: instead, they returns a *os.SyscallError that does not include the path:

go/src/os/dir_unix.go

Lines 52 to 54 in a38a917

	if errno != nil {
	return names, wrapSyscallError("readdirent", errno)
	}

go/src/os/dir_darwin.go

Lines 30 to 32 in a38a917

	if errno != nil {
	return nil, wrapSyscallError(call, errno)
	}

This not only violates the general expectations for error descriptiveness in the standard library, but also makes the behavior inconsistent across platforms: Windows and Plan 9 both already return a *PathError from these methods.

CC @ianlancetaylor @bradfitz

[bcmills]

I think this is likely too invasive to fix in the 1.15 cycle, but I plan to address it in 1.16.

[gopherbot]

Change https://golang.org/cl/233917 mentions this issue: os: return a *PathError from Readdirnames and Readdir on POSIX platforms