net/url: add URL.Redacted to return a password scrubbed string

Returning an URL.String() without the password is very useful for
situations where the URL is supposed to be logged and the password is
not useful to be shown.

This method re-uses URL.String() but with the password scrubbed and
substituted for a "xxxxx" in order to make it obvious that there was a
password. If the URL had no password then no "xxxxx" will be shown.

Fixes #34855

Change-Id: I7f17d81aa09a7963d2731d16fe15c6ae8e2285fc
GitHub-Last-Rev: 46d06dbc4f9e30a57667bb8d0627bc1abed83bdc
GitHub-Pull-Request: golang/go#35578
Reviewed-on: https://go-review.googlesource.com/c/go/+/207082
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/net/url/url.go

@@ -821,6 +821,20 @@ func (u *URL) String() string {
 	return buf.String()
 }
 
+// Redacted is like String but replaces any password with "xxxxx".
+// Only the password in u.URL is redacted.
+func (u *URL) Redacted() string {
+	if u == nil {
+		return ""
+	}
+
+	ru := *u
+	if _, has := ru.User.Password(); has {
+		ru.User = UserPassword(ru.User.Username(), "xxxxx")
+	}
+	return ru.String()
+}
+
 // Values maps a string key to a list of values.
 // It is typically used for query parameters and form values.
 // Unlike in the http.Header map, the keys in a Values map

src/net/url/url_test.go

@@ -765,6 +765,73 @@ func TestURLString(t *testing.T) {
 	}
 }
 
+func TestURLRedacted(t *testing.T) {
+	cases := []struct {
+		name string
+		url  *URL
+		want string
+	}{
+		{
+			name: "non-blank Password",
+			url: &URL{
+				Scheme: "http",
+				Host:   "host.tld",
+				Path:   "this:that",
+				User:   UserPassword("user", "password"),
+			},
+			want: "http://user:xxxxx@host.tld/this:that",
+		},
+		{
+			name: "blank Password",
+			url: &URL{
+				Scheme: "http",
+				Host:   "host.tld",
+				Path:   "this:that",
+				User:   User("user"),
+			},
+			want: "http://user@host.tld/this:that",
+		},
+		{
+			name: "nil User",
+			url: &URL{
+				Scheme: "http",
+				Host:   "host.tld",
+				Path:   "this:that",
+				User:   UserPassword("", "password"),
+			},
+			want: "http://:xxxxx@host.tld/this:that",
+		},
+		{
+			name: "blank Username, blank Password",
+			url: &URL{
+				Scheme: "http",
+				Host:   "host.tld",
+				Path:   "this:that",
+			},
+			want: "http://host.tld/this:that",
+		},
+		{
+			name: "empty URL",
+			url:  &URL{},
+			want: "",
+		},
+		{
+			name: "nil URL",
+			url:  nil,
+			want: "",
+		},
+	}
+
+	for _, tt := range cases {
+		t := t
+		t.Run(tt.name, func(t *testing.T) {
+			if g, w := tt.url.Redacted(), tt.want; g != w {
+				t.Fatalf("got: %q\nwant: %q", g, w)
+			}
+		})
+	}
+}
+
 type EscapeTest struct {
 	in  string
 	out string