net/rpc: fix data race on Call.Error

+eliminates a possibility of sending a call to Done several times.
+fixes memory leak in case of temporal Write errors.
+fixes data race on Client.shutdown.
+fixes data race on Client.closing.
+fixes comments.
Fixes #2780.

R=r, rsc
CC=golang-dev, mpimenov
https://golang.org/cl/5571063

src/pkg/net/rpc/client.go

@@ -31,8 +31,7 @@ type Call struct {
 	Args          interface{} // The argument to the function (*struct).
 	Reply         interface{} // The reply from the function (*struct).
 	Error         error       // After completion, the error status.
-	Done          chan *Call  // Strobes when call is complete; value is the error status.
-	seq           uint64
+	Done          chan *Call  // Strobes when call is complete.
 }
 
 // Client represents an RPC Client.
@@ -65,28 +64,33 @@ type ClientCodec interface {
 	Close() error
 }
 
-func (client *Client) send(c *Call) {
+func (client *Client) send(call *Call) {
+	client.sending.Lock()
+	defer client.sending.Unlock()
+
 	// Register this call.
 	client.mutex.Lock()
 	if client.shutdown {
-		c.Error = ErrShutdown
+		call.Error = ErrShutdown
 		client.mutex.Unlock()
-		c.done()
+		call.done()
 		return
 	}
-	c.seq = client.seq
+	seq := client.seq
 	client.seq++
-	client.pending[c.seq] = c
+	client.pending[seq] = call
 	client.mutex.Unlock()
 
 	// Encode and send the request.
-	client.sending.Lock()
-	defer client.sending.Unlock()
-	client.request.Seq = c.seq
-	client.request.ServiceMethod = c.ServiceMethod
-	if err := client.codec.WriteRequest(&client.request, c.Args); err != nil {
-		c.Error = err
-		c.done()
+	client.request.Seq = seq
+	client.request.ServiceMethod = call.ServiceMethod
+	err := client.codec.WriteRequest(&client.request, call.Args)
+	if err != nil {
+		client.mutex.Lock()
+		delete(client.pending, seq)
+		client.mutex.Unlock()
+		call.Error = err
+		call.done()
 	}
 }
 
@@ -104,36 +108,39 @@ func (client *Client) input() {
 		}
 		seq := response.Seq
 		client.mutex.Lock()
-		c := client.pending[seq]
+		call := client.pending[seq]
 		delete(client.pending, seq)
 		client.mutex.Unlock()
 
 		if response.Error == "" {
-			err = client.codec.ReadResponseBody(c.Reply)
+			err = client.codec.ReadResponseBody(call.Reply)
 			if err != nil {
-				c.Error = errors.New("reading body " + err.Error())
+				call.Error = errors.New("reading body " + err.Error())
 			}
 		} else {
 			// We've got an error response. Give this to the request;
 			// any subsequent requests will get the ReadResponseBody
 			// error if there is one.
-			c.Error = ServerError(response.Error)
+			call.Error = ServerError(response.Error)
 			err = client.codec.ReadResponseBody(nil)
 			if err != nil {
 				err = errors.New("reading error body: " + err.Error())
 			}
 		}
-		c.done()
+		call.done()
 	}
 	// Terminate pending calls.
+	client.sending.Lock()
 	client.mutex.Lock()
 	client.shutdown = true
+	closing := client.closing
 	for _, call := range client.pending {
 		call.Error = err
 		call.done()
 	}
 	client.mutex.Unlock()
-	if err != io.EOF || !client.closing {
+	client.sending.Unlock()
+	if err != io.EOF || !closing {
 		log.Println("rpc: client protocol error:", err)
 	}
 }
@@ -269,20 +276,12 @@ func (client *Client) Go(serviceMethod string, args interface{}, reply interface
 		}
 	}
 	call.Done = done
-	if client.shutdown {
-		call.Error = ErrShutdown
-		call.done()
-		return call
-	}
 	client.send(call)
 	return call
 }
 
 // Call invokes the named function, waits for it to complete, and returns its error status.
 func (client *Client) Call(serviceMethod string, args interface{}, reply interface{}) error {
-	if client.shutdown {
-		return ErrShutdown
-	}
 	call := <-client.Go(serviceMethod, args, reply, make(chan *Call, 1)).Done
 	return call.Error
 }