net/http: fix Server.Close double Lock

Fixes #17878

Change-Id: I062ac514239068c58175c9ee7964b3590f956a82
Reviewed-on: https://go-review.googlesource.com/33026
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/serve_test.go

@@ -4988,3 +4988,10 @@ func testServerShutdown(t *testing.T, h2 bool) {
 		t.Fatal("second request should fail. server should be shut down")
 	}
 }
+
+// Issue 17878: tests that we can call Close twice.
+func TestServerCloseDeadlock(t *testing.T) {
+	var s Server
+	s.Close()
+	s.Close()
+}

src/net/http/server.go

@@ -2362,7 +2362,7 @@ func (s *Server) closeDoneChanLocked() {
 // regardless of their state. For a graceful shutdown, use Shutdown.
 func (s *Server) Close() error {
 	s.mu.Lock()
-	defer s.mu.Lock()
+	defer s.mu.Unlock()
 	s.closeDoneChanLocked()
 	err := s.closeListenersLocked()
 	for c := range s.activeConn {