path/filepath: add Localize to safely convert a slash-separated path into an operating system path #57151
Description
The filepath.FromSlash function sounds like it converts a /-separated path into an operating system path. What it actually does is replace every / in its input with os.PathSeparator.
FromSlash can map an input path into semantically different paths on different operating systems. For example, FromSlash("a\b") returns the filename a\b on Unix and the file b in the directory a on Windows. FromSlash("C:/foo") returns a relative path on Unix and an absolute path on Windows.
#56694 involves failures in the standard library to safely convert a non-platform-specific /-separated path into a semantically equivalent operating system path. The fix (https://go.dev/cl/455716) introduces an internal function to perform this operation. We should have a public API for this.
The proposal:
// FromFS converts a slash-separated path into an operating system path.
//
// FromFS returns an error if the path cannot be represented by the operating system.
// For example, paths containing '\' and ':' characters are rejected on Windows.
func FromFS(path string) (string, error)
FromFS rejects empty paths ("") and, on Windows, reserved device names such as NUL.
FromFS and IsLocal (#56219) are similar in that both involve performing safety checks on a potentially-untrusted input path. They serve different roles, however:
FromFStakes a /-separated path in the form operated on by thepathpackage and safely converts it to a semantically-equivalent operating system path.IsLocaltakes an operating system path and reports whether it refers to something surprising.