net/url: add OmitHost bool to url.URL

Previously, myscheme:/path and myscheme:///path were treated as the same URL
although materially different. The distinction made clear by RFC 3986 sec. 5.3 where
a different recomposition behavior is expected when a URI reference has an undefined
host(authority) as in myscheme:/path vs. one with an empty host(authority)
as in myscheme:///path.

This change fixes the Parse/String roundtrip limitation for URLs with an undefined
host and a single slash.

Fixes #46059

Change-Id: I1b8d6042135513616374ff8c8dfb1cdb640f8efe
Reviewed-on: https://go-review.googlesource.com/c/go/+/391294
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/net/url/url.go

@@ -363,6 +363,7 @@ type URL struct {
 	Host        string    // host or host:port
 	Path        string    // path (relative paths may omit leading slash)
 	RawPath     string    // encoded path hint (see EscapedPath method)
+	OmitHost    bool      // do not emit empty host (authority)
 	ForceQuery  bool      // append a query ('?') even if RawQuery is empty
 	RawQuery    string    // encoded query values, without '?'
 	Fragment    string    // fragment for references, without '#'
@@ -556,7 +557,12 @@ func parse(rawURL string, viaRequest bool) (*URL, error) {
 		if err != nil {
 			return nil, err
 		}
+	} else if url.Scheme != "" && strings.HasPrefix(rest, "/") {
+		// OmitHost is set to true when rawURL has an empty host (authority).
+		// See golang.org/issue/46059.
+		url.OmitHost = true
 	}
+
 	// Set Path and, optionally, RawPath.
 	// RawPath is a hint of the encoding of Path. We don't want to set it if
 	// the default escaping of Path is equivalent, to help make sure that people
@@ -806,15 +812,19 @@ func (u *URL) String() string {
 		buf.WriteString(u.Opaque)
 	} else {
 		if u.Scheme != "" || u.Host != "" || u.User != nil {
-			if u.Host != "" || u.Path != "" || u.User != nil {
-				buf.WriteString("//")
-			}
-			if ui := u.User; ui != nil {
-				buf.WriteString(ui.String())
-				buf.WriteByte('@')
-			}
-			if h := u.Host; h != "" {
-				buf.WriteString(escape(h, encodeHost))
+			if u.OmitHost && u.Host == "" && u.User == nil {
+				// omit empty host
+			} else {
+				if u.Host != "" || u.Path != "" || u.User != nil {
+					buf.WriteString("//")
+				}
+				if ui := u.User; ui != nil {
+					buf.WriteString(ui.String())
+					buf.WriteByte('@')
+				}
+				if h := u.Host; h != "" {
+					buf.WriteString(escape(h, encodeHost))
+				}
 			}
 		}
 		path := u.EscapedPath()

src/net/url/url_test.go

@@ -163,14 +163,15 @@ var urltests = []URLTest{
 		},
 		"http:%2f%2fwww.google.com/?q=go+language",
 	},
-	// non-authority with path
+	// non-authority with path; see golang.org/issue/46059
 	{
 		"mailto:/webmaster@golang.org",
 		&URL{
-			Scheme: "mailto",
-			Path:   "/webmaster@golang.org",
+			Scheme:   "mailto",
+			Path:     "/webmaster@golang.org",
+			OmitHost: true,
 		},
-		"mailto:///webmaster@golang.org", // unfortunate compromise
+		"",
 	},
 	// non-authority
 	{
@@ -625,8 +626,8 @@ func ufmt(u *URL) string {
 			pass = p
 		}
 	}
-	return fmt.Sprintf("opaque=%q, scheme=%q, user=%#v, pass=%#v, host=%q, path=%q, rawpath=%q, rawq=%q, frag=%q, rawfrag=%q, forcequery=%v",
-		u.Opaque, u.Scheme, user, pass, u.Host, u.Path, u.RawPath, u.RawQuery, u.Fragment, u.RawFragment, u.ForceQuery)
+	return fmt.Sprintf("opaque=%q, scheme=%q, user=%#v, pass=%#v, host=%q, path=%q, rawpath=%q, rawq=%q, frag=%q, rawfrag=%q, forcequery=%v, omithost=%t",
+		u.Opaque, u.Scheme, user, pass, u.Host, u.Path, u.RawPath, u.RawQuery, u.Fragment, u.RawFragment, u.ForceQuery, u.OmitHost)
 }
 
 func BenchmarkString(b *testing.B) {