cmd/go/internal/modfetch: workaround file URL parsing on Windows

Remove the leading slash from GOPROXY file:// URLs.

Updates #31675

Change-Id: Id45af2a806afc3c216181c13f6bc73713b925693
Reviewed-on: https://go-review.googlesource.com/c/go/+/173499
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/modfetch/proxy.go

@@ -14,9 +14,11 @@ import (
 	"os"
 	pathpkg "path"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"sync"
 	"time"
+	"unicode"
 
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
@@ -219,6 +221,11 @@ func (p *proxyRepo) getBody(path string) (io.ReadCloser, error) {
 		if err != nil {
 			return nil, err
 		}
+		if runtime.GOOS == "windows" && len(rawPath) >= 4 && rawPath[0] == '/' && unicode.IsLetter(rune(rawPath[1])) && rawPath[2] == ':' {
+			// On Windows, file URLs look like "file:///C:/foo/bar". url.Path will
+			// start with a slash which must be removed. See golang.org/issue/6027.
+			rawPath = rawPath[1:]
+		}
 		return os.Open(filepath.FromSlash(rawPath))
 	}