net/url: allow *User functions to work on a nil receiver.

Fixes #20924

Change-Id: If89f31da63cbea38d7e615a428b7b07629770a45
Reviewed-on: https://go-review.googlesource.com/47851
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tim Cooper <tim.cooper@layeh.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/url/url.go

@@ -372,17 +372,26 @@ type Userinfo struct {
 
 // Username returns the username.
 func (u *Userinfo) Username() string {
+	if u == nil {
+		return ""
+	}
 	return u.username
 }
 
 // Password returns the password in case it is set, and whether it is set.
 func (u *Userinfo) Password() (string, bool) {
+	if u == nil {
+		return "", false
+	}
 	return u.password, u.passwordSet
 }
 
 // String returns the encoded userinfo information in the standard form
 // of "username[:password]".
 func (u *Userinfo) String() string {
+	if u == nil {
+		return ""
+	}
 	s := escape(u.username, encodeUserPassword)
 	if u.passwordSet {
 		s += ":" + escape(u.password, encodeUserPassword)

src/net/url/url_test.go

@@ -1709,3 +1709,29 @@ func TestGob(t *testing.T) {
 		t.Errorf("json decoded to: %s\nwant: %s\n", u1, u)
 	}
 }
+
+func TestNilUser(t *testing.T) {
+	defer func() {
+		if v := recover(); v != nil {
+			t.Fatalf("unexpected panic: %v", v)
+		}
+	}()
+
+	u, err := Parse("http://foo.com/")
+
+	if err != nil {
+		t.Fatalf("parse err: %v", err)
+	}
+
+	if v := u.User.Username(); v != "" {
+		t.Fatalf("expected empty username, got %s", v)
+	}
+
+	if v, ok := u.User.Password(); v != "" || ok {
+		t.Fatalf("expected empty password, got %s (%v)", v, ok)
+	}
+
+	if v := u.User.String(); v != "" {
+		t.Fatalf("expected empty string, got %s", v)
+	}
+}