net/rpc: avoid racy use of closing flag.

It's accessed without mutex protection
in a different goroutine from the one that
sets it.

Also make sure that Client.Call after Client.Close
will reliably return ErrShutdown, and that clients
see ErrShutdown rather than io.EOF when appropriate.

Suggestions welcome for a way to reliably test
the mutex issue.

R=r, iant
CC=golang-dev
https://golang.org/cl/7338045

src/pkg/net/rpc/client.go

@@ -71,7 +71,7 @@ func (client *Client) send(call *Call) {
 
 	// Register this call.
 	client.mutex.Lock()
-	if client.shutdown {
+	if client.shutdown || client.closing {
 		call.Error = ErrShutdown
 		client.mutex.Unlock()
 		call.done()
@@ -105,9 +105,6 @@ func (client *Client) input() {
 		response = Response{}
 		err = client.codec.ReadResponseHeader(&response)
 		if err != nil {
-			if err == io.EOF && !client.closing {
-				err = io.ErrUnexpectedEOF
-			}
 			break
 		}
 		seq := response.Seq
@@ -150,6 +147,13 @@ func (client *Client) input() {
 	client.mutex.Lock()
 	client.shutdown = true
 	closing := client.closing
+	if err == io.EOF {
+		if closing {
+			err = ErrShutdown
+		} else {
+			err = io.ErrUnexpectedEOF
+		}
+	}
 	for _, call := range client.pending {
 		call.Error = err
 		call.done()

src/pkg/net/rpc/server_test.go

@@ -524,6 +524,23 @@ func TestTCPClose(t *testing.T) {
 	}
 }
 
+func TestErrorAfterClientClose(t *testing.T) {
+	once.Do(startServer)
+
+	client, err := dialHTTP()
+	if err != nil {
+		t.Fatalf("dialing: %v", err)
+	}
+	err = client.Close()
+	if err != nil {
+		t.Fatal("close error:", err)
+	}
+	err = client.Call("Arith.Add", &Args{7, 9}, new(Reply))
+	if err != ErrShutdown {
+		t.Errorf("Forever: expected ErrShutdown got %v", err)
+	}
+}
+
 func benchmarkEndToEnd(dial func() (*Client, error), b *testing.B) {
 	b.StopTimer()
 	once.Do(startServer)