src/cmd/go/internal/web/url_windows.go

// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package web

import (
	"errors"
	"path/filepath"
	"strings"
)

func convertFileURLPath(host, path string) (string, error) {
	if len(path) == 0 || path[0] != '/' {
		return "", errNotAbsolute
	}

	path = filepath.FromSlash(path)

	// We interpret Windows file URLs per the description in
	// https://blogs.msdn.microsoft.com/ie/2006/12/06/file-uris-in-windows/.

	// The host part of a file URL (if any) is the UNC volume name,
	// but RFC 8089 reserves the authority "localhost" for the local machine.
	if host != "" && host != "localhost" {
		// A common "legacy" format omits the leading slash before a drive letter,
		// encoding the drive letter as the host instead of part of the path.
		// (See https://blogs.msdn.microsoft.com/freeassociations/2005/05/19/the-bizarre-and-unhappy-story-of-file-urls/.)
		// We do not support that format, but we should at least emit a more
		// helpful error message for it.
		if filepath.VolumeName(host) != "" {
			return "", errors.New("file URL encodes volume in host field: too few slashes?")
		}
		return `\\` + host + path, nil
	}

	// If host is empty, path must contain an initial slash followed by a
	// drive letter and path. Remove the slash and verify that the path is valid.
	if vol := filepath.VolumeName(path[1:]); vol == "" || strings.HasPrefix(vol, `\\`) {
		return "", errors.New("file URL missing drive letter")
	}
	return path[1:], nil
}