net/http: fix server connection leak on Handler's panic(nil)

If a handler did a panic(nil), the connection was never closed.

Fixes #4050

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/6971049

src/pkg/net/http/serve_test.go

@@ -918,15 +918,19 @@ func TestZeroLengthPostAndResponse(t *testing.T) {
 	}
 }
 
+func TestHandlerPanicNil(t *testing.T) {
+	testHandlerPanic(t, false, nil)
+}
+
 func TestHandlerPanic(t *testing.T) {
-	testHandlerPanic(t, false)
+	testHandlerPanic(t, false, "intentional death for testing")
 }
 
 func TestHandlerPanicWithHijack(t *testing.T) {
-	testHandlerPanic(t, true)
+	testHandlerPanic(t, true, "intentional death for testing")
 }
 
-func testHandlerPanic(t *testing.T, withHijack bool) {
+func testHandlerPanic(t *testing.T, withHijack bool, panicValue interface{}) {
 	// Unlike the other tests that set the log output to ioutil.Discard
 	// to quiet the output, this test uses a pipe.  The pipe serves three
 	// purposes:
@@ -955,7 +959,7 @@ func testHandlerPanic(t *testing.T, withHijack bool) {
 			}
 			defer rwc.Close()
 		}
-		panic("intentional death for testing")
+		panic(panicValue)
 	}))
 	defer ts.Close()
 
@@ -968,7 +972,7 @@ func testHandlerPanic(t *testing.T, withHijack bool) {
 		_, err := pr.Read(buf)
 		pr.Close()
 		if err != nil {
-			t.Fatal(err)
+			t.Error(err)
 		}
 		done <- true
 	}()
@@ -978,6 +982,10 @@ func testHandlerPanic(t *testing.T, withHijack bool) {
 		t.Logf("expected an error")
 	}
 
+	if panicValue == nil {
+		return
+	}
+
 	select {
 	case <-done:
 		return

src/pkg/net/http/server.go

@@ -716,6 +716,7 @@ func (c *conn) serve() {
 			c.rwc.Close()
 		}
 	}()
+	defer c.close()
 
 	if tlsConn, ok := c.rwc.(*tls.Conn); ok {
 		if err := tlsConn.Handshake(); err != nil {
@@ -791,7 +792,6 @@ func (c *conn) serve() {
 			break
 		}
 	}
-	c.close()
 }
 
 func (w *response) sendExpectationFailed() {