crypto/tls: buffer handshake messages.

This change causes TLS handshake messages to be buffered and written in
a single Write to the underlying net.Conn.

There are two reasons to want to do this:

Firstly, it's slightly preferable to do this in order to save sending
several, small packets over the network where a single one will do.

Secondly, since 37c28759ca46cf381a466e32168a793165d9c9e9 errors from
Write have been returned from a handshake. This means that, if a peer
closes the connection during a handshake, a “broken pipe” error may
result from tls.Conn.Handshake(). This can mask any, more detailed,
fatal alerts that the peer may have sent because a read will never
happen.

Buffering handshake messages means that the peer will not receive, and
possibly reject, any of a flow while it's still being written.

Fixes #15709

Change-Id: I38dcff1abecc06e52b2de647ea98713ce0fb9a21
Reviewed-on: https://go-review.googlesource.com/23609
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

handshake_client_test.go

@@ -983,7 +983,7 @@ func (b *brokenConn) Write(data []byte) (int, error) {
 
 func TestFailedWrite(t *testing.T) {
 	// Test that a write error during the handshake is returned.
-	for _, breakAfter := range []int{0, 1, 2, 3} {
+	for _, breakAfter := range []int{0, 1} {
 		c, s := net.Pipe()
 		done := make(chan bool)
 
@@ -1003,3 +1003,45 @@ func TestFailedWrite(t *testing.T) {
 		<-done
 	}
 }
+
+// writeCountingConn wraps a net.Conn and counts the number of Write calls.
+type writeCountingConn struct {
+	net.Conn
+
+	// numWrites is the number of writes that have been done.
+	numWrites int
+}
+
+func (wcc *writeCountingConn) Write(data []byte) (int, error) {
+	wcc.numWrites++
+	return wcc.Conn.Write(data)
+}
+
+func TestBuffering(t *testing.T) {
+	c, s := net.Pipe()
+	done := make(chan bool)
+
+	clientWCC := &writeCountingConn{Conn: c}
+	serverWCC := &writeCountingConn{Conn: s}
+
+	go func() {
+		Server(serverWCC, testConfig).Handshake()
+		serverWCC.Close()
+		done <- true
+	}()
+
+	err := Client(clientWCC, testConfig).Handshake()
+	if err != nil {
+		t.Fatal(err)
+	}
+	clientWCC.Close()
+	<-done
+
+	if n := clientWCC.numWrites; n != 2 {
+		t.Errorf("expected client handshake to complete with only two writes, but saw %d", n)
+	}
+
+	if n := serverWCC.numWrites; n != 2 {
+		t.Errorf("expected server handshake to complete with only two writes, but saw %d", n)
+	}
+}