From 163a7d4942e793b328e05a7eb91f6d3fdc4ba12b Mon Sep 17 00:00:00 2001
From: Damien Mathieu <42@dmathieu.com>
Date: Wed, 21 Feb 2018 11:32:23 +0100
Subject: [PATCH] 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.
---
 src/net/http/serve_test.go | 10 ++++++++++
 src/net/http/server.go     |  3 +++
 2 files changed, 13 insertions(+)

diff --git a/src/net/http/serve_test.go b/src/net/http/serve_test.go
index 5ff08b4036..2fa3bc664f 100644
--- a/src/net/http/serve_test.go
+++ b/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 {
diff --git a/src/net/http/server.go b/src/net/http/server.go
index c4377e8ce5..a7ba753bf5 100644
--- a/src/net/http/server.go
+++ b/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 {