net/http: lock the read-only mutex in shouldRedirect

Since that method uses 'mux.m', we need to lock the mutex to avoid data races.

Change-Id: I998448a6e482b5d6a1b24f3354bb824906e23172
GitHub-Last-Rev: 163a7d4942e793b328e05a7eb91f6d3fdc4ba12b
GitHub-Pull-Request: golang/go#23994
Reviewed-on: https://go-review.googlesource.com/96575Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/serve_test.go

@@ -581,6 +581,16 @@ func TestServeWithSlashRedirectForHostPatterns(t *testing.T) {
 	}
 }
 
+func TestShouldRedirectConcurrency(t *testing.T) {
+	setParallel(t)
+	defer afterTest(t)
+
+	mux := NewServeMux()
+	ts := httptest.NewServer(mux)
+	defer ts.Close()
+	mux.HandleFunc("/", func(w ResponseWriter, r *Request) {})
+}
+
 func BenchmarkServeMux(b *testing.B) {
 
 	type test struct {

src/net/http/server.go

@@ -2223,6 +2223,9 @@ func (mux *ServeMux) redirectToPathSlash(host, path string, u *url.URL) (*url.UR
 // path+"/". This should happen if a handler is registered for path+"/" but
 // not path -- see comments at ServeMux.
 func (mux *ServeMux) shouldRedirect(host, path string) bool {
+	mux.mu.RLock()
+	defer mux.mu.RUnlock()
+
 	p := []string{path, host + path}
 
 	for _, c := range p {