crypto/tls: check errors from (*Conn).writeRecord

This promotes a connection hang during TLS handshake to a proper error.
This doesn't fully address #14539 because the error reported in that
case is a write-on-socket-not-connected error, which implies that an
earlier error during connection setup is not being checked, but it is
an improvement over the current behaviour.

Updates #14539.

Change-Id: I0571a752d32d5303db48149ab448226868b19495
Reviewed-on: https://go-review.googlesource.com/19990
Reviewed-by: Adam Langley <agl@golang.org>

handshake_client_test.go

@@ -12,6 +12,7 @@ import (
 	"encoding/base64"
 	"encoding/binary"
 	"encoding/pem"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -725,3 +726,51 @@ func TestServerSelectingUnconfiguredCipherSuite(t *testing.T) {
 		t.Fatalf("Expected error about unconfigured cipher suite but got %q", err)
 	}
 }
+
+// brokenConn wraps a net.Conn and causes all Writes after a certain number to
+// fail with brokenConnErr.
+type brokenConn struct {
+	net.Conn
+
+	// breakAfter is the number of successful writes that will be allowed
+	// before all subsequent writes fail.
+	breakAfter int
+
+	// numWrites is the number of writes that have been done.
+	numWrites int
+}
+
+// brokenConnErr is the error that brokenConn returns once exhausted.
+var brokenConnErr = errors.New("too many writes to brokenConn")
+
+func (b *brokenConn) Write(data []byte) (int, error) {
+	if b.numWrites >= b.breakAfter {
+		return 0, brokenConnErr
+	}
+
+	b.numWrites++
+	return b.Conn.Write(data)
+}
+
+func TestFailedWrite(t *testing.T) {
+	// Test that a write error during the handshake is returned.
+	for _, breakAfter := range []int{0, 1, 2, 3} {
+		c, s := net.Pipe()
+		done := make(chan bool)
+
+		go func() {
+			Server(s, testConfig).Handshake()
+			s.Close()
+			done <- true
+		}()
+
+		brokenC := &brokenConn{Conn: c, breakAfter: breakAfter}
+		err := Client(brokenC, testConfig).Handshake()
+		if err != brokenConnErr {
+			t.Errorf("#%d: expected error from brokenConn but got %q", breakAfter, err)
+		}
+		brokenC.Close()
+
+		<-done
+	}
+}