I think it would be nice to have a straightforward way to do this in the standard library, just as proposed.

There are already some building blocks in stdlib that can do roughly this in a more roundabout way:

func LocalPath(s string) bool {
	return fs.ValidPath(path.Clean(s))
}

fs.ValidPath is essentially an extra-strict version of "IsLocal" which also prohibits ../ and ./ segments. path.Clean removes any ../ or ./ segments that are redundant, but leaves behind those that would traverse upwards, therefore making fs.ValidPath return false for those.

The path package doesn't operate on the filesystem, this seems in conflict with the first point of filepath.IsLocal: "is within the subtree rooted at the directory in which path is evaluated"

The path package doesn't operate on the filesystem, this seems in conflict with the first point of filepath.IsLocal: "is within the subtree rooted at the directory in which path is evaluated"

It’s a purely lexical consideration though. It’s not looking at CWD to decide this. I think path is an appropriate package.

fs.ValidPath is essentially an extra-strict version of "IsLocal" which also prohibits ../ and ./ segments. path.Clean removes any ../ or ./ segments that are redundant, but leaves behind those that would traverse upwards, therefore making fs.ValidPath return false for those.

If you take a possible path and clean it then test that it’s local, it will pass fs.ValidPath.

(CC @neild)

If you take a possible path and clean it then test that it’s local, it will pass fs.ValidPath.

fs.ValidPath also checks for a trailing slash, so it's not totally equivalent after all.

fs.ValidPath also checks for a trailing slash, so it's not totally equivalent after all.

path.Clean removes trailing slashes, though.

I personally wouldn't object to path.Local, for consistency with filepath if nothing else. However, it's much less necessary; filepath.Local exists because it's difficult to answer the question "does this path refer to a file in the current directory" in a general sense. (Mostly because of Windows, which has a variety of surprising path constructs, although Plan 9 does have its # prefix.)

I believe path.IsLocal would be equivalent to:

func IsLocal(p string) bool {
  p = path.Clean()
  return !strings.HasPrefix(p, "/") && !strings.HasPrefix(p, "../")
}

fs.ValidPath(path.Clean(p)) will reject paths containing invalid UTF-8, while the above will not.

There is now filepath.Localize #57151, so I guess this issue is obsolete? You could do filepath.IsLocal(filepath.Localize(s)) instead.