crypto/tls: fix data race on conn.err

Fixes #3862.

There were many areas where conn.err was being accessed
outside the mutex. This proposal moves the err value to
an embedded struct to make it more obvious when the error
value is being accessed.

As there are no Benchmark tests in this package I cannot
feel confident of the impact of this additional locking,
although most will be uncontended.

R=dvyukov, agl
CC=golang-dev
https://golang.org/cl/6497070

conn.go

@@ -44,8 +44,7 @@ type Conn struct {
 	clientProtocolFallback bool
 
 	// first permanent error
-	errMutex sync.Mutex
-	err      error
+	connErr
 
 	// input/output
 	in, out  halfConn     // in.Mutex < out.Mutex
@@ -56,21 +55,25 @@ type Conn struct {
 	tmp [16]byte
 }
 
-func (c *Conn) setError(err error) error {
-	c.errMutex.Lock()
-	defer c.errMutex.Unlock()
+type connErr struct {
+	mu    sync.Mutex
+	value error
+}
 
-	if c.err == nil {
-		c.err = err
+func (e *connErr) setError(err error) error {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+
+	if e.value == nil {
+		e.value = err
 	}
 	return err
 }
 
-func (c *Conn) error() error {
-	c.errMutex.Lock()
-	defer c.errMutex.Unlock()
-
-	return c.err
+func (e *connErr) error() error {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+	return e.value
 }
 
 // Access to net.Conn methods.
@@ -660,8 +663,7 @@ func (c *Conn) writeRecord(typ recordType, data []byte) (n int, err error) {
 			c.tmp[0] = alertLevelError
 			c.tmp[1] = byte(err.(alert))
 			c.writeRecord(recordTypeAlert, c.tmp[0:2])
-			c.err = &net.OpError{Op: "local error", Err: err}
-			return n, c.err
+			return n, c.setError(&net.OpError{Op: "local error", Err: err})
 		}
 	}
 	return
@@ -672,8 +674,8 @@ func (c *Conn) writeRecord(typ recordType, data []byte) (n int, err error) {
 // c.in.Mutex < L; c.out.Mutex < L.
 func (c *Conn) readHandshake() (interface{}, error) {
 	for c.hand.Len() < 4 {
-		if c.err != nil {
-			return nil, c.err
+		if err := c.error(); err != nil {
+			return nil, err
 		}
 		if err := c.readRecord(recordTypeHandshake); err != nil {
 			return nil, err
@@ -684,11 +686,11 @@ func (c *Conn) readHandshake() (interface{}, error) {
 	n := int(data[1])<<16 | int(data[2])<<8 | int(data[3])
 	if n > maxHandshake {
 		c.sendAlert(alertInternalError)
-		return nil, c.err
+		return nil, c.error()
 	}
 	for c.hand.Len() < 4+n {
-		if c.err != nil {
-			return nil, c.err
+		if err := c.error(); err != nil {
+			return nil, err
 		}
 		if err := c.readRecord(recordTypeHandshake); err != nil {
 			return nil, err
@@ -738,12 +740,12 @@ func (c *Conn) readHandshake() (interface{}, error) {
 
 // Write writes data to the connection.
 func (c *Conn) Write(b []byte) (int, error) {
-	if c.err != nil {
-		return 0, c.err
+	if err := c.error(); err != nil {
+		return 0, err
 	}
 
-	if c.err = c.Handshake(); c.err != nil {
-		return 0, c.err
+	if err := c.Handshake(); err != nil {
+		return 0, c.setError(err)
 	}
 
 	c.out.Lock()
@@ -753,9 +755,8 @@ func (c *Conn) Write(b []byte) (int, error) {
 		return 0, alertInternalError
 	}
 
-	var n int
-	n, c.err = c.writeRecord(recordTypeApplicationData, b)
-	return n, c.err
+	n, err := c.writeRecord(recordTypeApplicationData, b)
+	return n, c.setError(err)
 }
 
 // Read can be made to time out and return a net.Error with Timeout() == true
@@ -768,14 +769,14 @@ func (c *Conn) Read(b []byte) (n int, err error) {
 	c.in.Lock()
 	defer c.in.Unlock()
 
-	for c.input == nil && c.err == nil {
+	for c.input == nil && c.error() == nil {
 		if err := c.readRecord(recordTypeApplicationData); err != nil {
 			// Soft error, like EAGAIN
 			return 0, err
 		}
 	}
-	if c.err != nil {
-		return 0, c.err
+	if err := c.error(); err != nil {
+		return 0, err
 	}
 	n, err = c.input.Read(b)
 	if c.input.off >= len(c.input.data) {

handshake_client.go

@@ -306,8 +306,8 @@ func (c *Conn) clientHandshake() error {
 	serverHash := suite.mac(c.vers, serverMAC)
 	c.in.prepareCipherSpec(c.vers, serverCipher, serverHash)
 	c.readRecord(recordTypeChangeCipherSpec)
-	if c.err != nil {
-		return c.err
+	if err := c.error(); err != nil {
+		return err
 	}
 
 	msg, err = c.readHandshake()