Fix a deadlock bug in the rpc client. The panic will trigger

regularly when client connections are flaky (probably another
issue).

(credits to jussi@tinkercad.com for finding the issue)

R=rsc, r
CC=golang-dev, jussi
https://golang.org/cl/2831042

src/pkg/rpc/client.go

@@ -69,12 +69,12 @@ func (client *Client) send(c *Call) {
 	// Encode and send the request.
 	request := new(Request)
 	client.sending.Lock()
+	defer client.sending.Unlock()
 	request.Seq = c.seq
 	request.ServiceMethod = c.ServiceMethod
 	if err := client.codec.WriteRequest(request, c.Args); err != nil {
 		panic("rpc: client encode error: " + err.String())
 	}
-	client.sending.Unlock()
 }
 
 func (client *Client) input() {

src/pkg/rpc/server_test.go

@@ -13,6 +13,7 @@ import (
 	"strings"
 	"sync"
 	"testing"
+	"time"
 )
 
 var (
@@ -332,3 +333,52 @@ func TestRegistrationError(t *testing.T) {
 		t.Errorf("expected error registering ReplyNotPublic")
 	}
 }
+
+type WriteFailCodec int
+
+func (WriteFailCodec) WriteRequest(*Request, interface{}) os.Error {
+	// the panic caused by this error used to not unlock a lock.
+	return os.NewError("fail")
+}
+
+func (WriteFailCodec) ReadResponseHeader(*Response) os.Error {
+	time.Sleep(60e9)
+	panic("unreachable")
+}
+
+func (WriteFailCodec) ReadResponseBody(interface{}) os.Error {
+	time.Sleep(60e9)
+	panic("unreachable")
+}
+
+func (WriteFailCodec) Close() os.Error {
+	return nil
+}
+
+func TestSendDeadlock(t *testing.T) {
+	client := NewClientWithCodec(WriteFailCodec(0))
+
+	done := make(chan bool)
+	go func() {
+		testSendDeadlock(client)
+		testSendDeadlock(client)
+		done <- true
+	}()
+	for i := 0; i < 50; i++ {
+		time.Sleep(100 * 1e6)
+		_, ok := <-done
+		if ok {
+			return
+		}
+	}
+	t.Fatal("deadlock")
+}
+
+func testSendDeadlock(client *Client) {
+	defer func() {
+		recover()
+	}()
+	args := &Args{7, 8}
+	reply := new(Reply)
+	client.Call("Arith.Add", args, reply)
+}