net/url: generate correct Path when hostname empty

Parse("file:///foo") previously returned a URL with Scheme "file"
and Path "///foo". Now it returns a URL with Path "/foo",
such that
        &URL{Scheme: "file", Path: "/foo"}.String() == "file:///foo"

This means that parsing and stringifying the URL "file:/foo"
returns "file:///foo", technically a regression but one that only
affects a corner case.

Fixes #4189.

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/7135051

src/pkg/net/url/url.go

@@ -386,7 +386,7 @@ func parse(rawurl string, viaRequest bool) (url *URL, err error) {
 		}
 	}
 
-	if (url.Scheme != "" || !viaRequest) && strings.HasPrefix(rest, "//") && !strings.HasPrefix(rest, "///") {
+	if (url.Scheme != "" || !viaRequest && !strings.HasPrefix(rest, "///")) && strings.HasPrefix(rest, "//") {
 		var authority string
 		authority, rest = split(rest[2:], '/', false)
 		url.User, url.Host, err = parseAuthority(authority)
@@ -442,12 +442,14 @@ func (u *URL) String() string {
 	if u.Opaque != "" {
 		result += u.Opaque
 	} else {
-		if u.Host != "" || u.User != nil {
+		if u.Scheme != "" || u.Host != "" || u.User != nil {
 			result += "//"
 			if u := u.User; u != nil {
 				result += u.String() + "@"
 			}
-			result += u.Host
+			if h := u.Host; h != "" {
+				result += u.Host
+			}
 		}
 		result += escape(u.Path, encodePath)
 	}

src/pkg/net/url/url_test.go

@@ -122,14 +122,14 @@ var urltests = []URLTest{
 		},
 		"http:%2f%2fwww.google.com/?q=go+language",
 	},
-	// non-authority
+	// non-authority with path
 	{
 		"mailto:/webmaster@golang.org",
 		&URL{
 			Scheme: "mailto",
 			Path:   "/webmaster@golang.org",
 		},
-		"",
+		"mailto:///webmaster@golang.org", // unfortunate compromise
 	},
 	// non-authority
 	{
@@ -242,6 +242,15 @@ var urltests = []URLTest{
 		},
 		"http://www.google.com/?q=go+language#foo&bar",
 	},
+	{
+		"file:///home/adg/rabbits",
+		&URL{
+			Scheme: "file",
+			Host:   "",
+			Path:   "/home/adg/rabbits",
+		},
+		"file:///home/adg/rabbits",
+	},
 }
 
 // more useful string for debugging than fmt's struct printer