net/rpc: do not leak client socket on closed connection

Fixes #6897.

LGTM=bradfitz
R=golang-codereviews, bradfitz, r, rsc
CC=golang-codereviews
https://golang.org/cl/91230045

src/pkg/net/rpc/client.go

@@ -39,14 +39,16 @@ type Call struct {
 // with a single Client, and a Client may be used by
 // multiple goroutines simultaneously.
 type Client struct {
-	mutex    sync.Mutex // protects pending, seq, request
-	sending  sync.Mutex
+	codec ClientCodec
+
+	sending sync.Mutex
+
+	mutex    sync.Mutex // protects following
 	request  Request
 	seq      uint64
-	codec    ClientCodec
 	pending  map[uint64]*Call
-	closing  bool
-	shutdown bool
+	closing  bool // user has called Close
+	shutdown bool // server has told us to stop
 }
 
 // A ClientCodec implements writing of RPC requests and
@@ -274,7 +276,7 @@ func Dial(network, address string) (*Client, error) {
 
 func (client *Client) Close() error {
 	client.mutex.Lock()
-	if client.shutdown || client.closing {
+	if client.closing {
 		client.mutex.Unlock()
 		return ErrShutdown
 	}

src/pkg/net/rpc/client_test.go

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package rpc
+
+import (
+	"errors"
+	"testing"
+)
+
+type shutdownCodec struct {
+	responded chan int
+	closed    bool
+}
+
+func (c *shutdownCodec) WriteRequest(*Request, interface{}) error { return nil }
+func (c *shutdownCodec) ReadResponseBody(interface{}) error       { return nil }
+func (c *shutdownCodec) ReadResponseHeader(*Response) error {
+	c.responded <- 1
+	return errors.New("shutdownCodec ReadResponseHeader")
+}
+func (c *shutdownCodec) Close() error {
+	c.closed = true
+	return nil
+}
+
+func TestCloseCodec(t *testing.T) {
+	codec := &shutdownCodec{responded: make(chan int)}
+	client := NewClientWithCodec(codec)
+	<-codec.responded
+	client.Close()
+	if !codec.closed {
+		t.Error("client.Close did not close codec")
+	}
+}