net/http: refine trailing-slash redirect logic

Do not add a trailing slash and redirect if the path already
ends in a slash.

Also, and unrelatedly, add a test for cleanPath.

Fixes #65624.

Change-Id: Ifcf9edc929d2eb6db88132c09d2bade85c5dda3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/562557
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/net/http/serve_test.go

@@ -597,6 +597,22 @@ func TestServeWithSlashRedirectForHostPatterns(t *testing.T) {
 	}
 }
 
+// Test that we don't attempt trailing-slash redirect on a path that already has
+// a trailing slash.
+// See issue #65624.
+func TestMuxNoSlashRedirectWithTrailingSlash(t *testing.T) {
+	mux := NewServeMux()
+	mux.HandleFunc("/{x}/", func(w ResponseWriter, r *Request) {
+		fmt.Fprintln(w, "ok")
+	})
+	w := httptest.NewRecorder()
+	req, _ := NewRequest("GET", "/", nil)
+	mux.ServeHTTP(w, req)
+	if g, w := w.Code, 404; g != w {
+		t.Errorf("got %d, want %d", g, w)
+	}
+}
+
 func TestShouldRedirectConcurrency(t *testing.T) { run(t, testShouldRedirectConcurrency) }
 func testShouldRedirectConcurrency(t *testing.T, mode testMode) {
 	mux := NewServeMux()

src/net/http/server.go

@@ -2577,8 +2577,8 @@ func (mux *ServeMux) matchOrRedirect(host, method, path string, u *url.URL) (_ *
 
 	n, matches := mux.tree.match(host, method, path)
 	// If we have an exact match, or we were asked not to try trailing-slash redirection,
-	// then we're done.
-	if !exactMatch(n, path) && u != nil {
+	// or the URL already has a trailing slash, then we're done.
+	if !exactMatch(n, path) && u != nil && !strings.HasSuffix(path, "/") {
 		// If there is an exact match with a trailing slash, then redirect.
 		path += "/"
 		n2, _ := mux.tree.match(host, method, path)

src/net/http/server_test.go

@@ -250,6 +250,24 @@ func TestEscapedPathsAndPatterns(t *testing.T) {
 	t.Run("1.21", func(t *testing.T) { run(t, true) })
 }
 
+func TestCleanPath(t *testing.T) {
+	for _, test := range []struct {
+		in, want string
+	}{
+		{"//", "/"},
+		{"/x", "/x"},
+		{"//x", "/x"},
+		{"x//", "/x/"},
+		{"a//b/////c", "/a/b/c"},
+		{"/foo/../bar/./..//baz", "/baz"},
+	} {
+		got := cleanPath(test.in)
+		if got != test.want {
+			t.Errorf("%s: got %q, want %q", test.in, got, test.want)
+		}
+	}
+}
+
 func BenchmarkServerMatch(b *testing.B) {
 	fn := func(w ResponseWriter, r *Request) {
 		fmt.Fprintf(w, "OK")