Fix data race in unix.BoundEndpoint.UnidirectionalConnect.

Data race is:

Read:
(*connectionlessEndpoint).UnidirectionalConnect:
writeQueue: e.receiver.(*queueReceiver).readQueue,

Write:
(*connectionlessEndpoint).Close:
e.receiver = nil

The problem is that (*connectionlessEndpoint).UnidirectionalConnect assumed
that baseEndpoint.receiver is immutable which is explicitly not the case.
Fixing this required two changes:
1. Add synchronization around access of baseEndpoint.receiver in
   (*connectionlessEndpoint).UnidirectionalConnect.
2. Check for baseEndpoint.receiver being nil in
   (*connectionlessEndpoint).UnidirectionalConnect.

PiperOrigin-RevId: 207984402
Change-Id: Icddeeb43805e777fa3ef874329fa704891d14181

pkg/tcpip/transport/unix/connectionless.go

@@ -77,9 +77,15 @@ func (e *connectionlessEndpoint) BidirectionalConnect(ce ConnectingEndpoint, ret
 
 // UnidirectionalConnect implements BoundEndpoint.UnidirectionalConnect.
 func (e *connectionlessEndpoint) UnidirectionalConnect() (ConnectedEndpoint, *tcpip.Error) {
+	e.Lock()
+	r := e.receiver
+	e.Unlock()
+	if r == nil {
+		return nil, tcpip.ErrConnectionRefused
+	}
 	return &connectedEndpoint{
 		endpoint:   e,
-		writeQueue: e.receiver.(*queueReceiver).readQueue,
+		writeQueue: r.(*queueReceiver).readQueue,
 	}, nil
 }
 

pkg/tcpip/transport/unix/unix.go

@@ -224,7 +224,11 @@ type BoundEndpoint interface {
 	// type that isn't SockStream or SockSeqpacket.
 	BidirectionalConnect(ep ConnectingEndpoint, returnConnect func(Receiver, ConnectedEndpoint)) *tcpip.Error
 
-	// UnidirectionalConnect establishes a write-only connection to a unix endpoint.
+	// UnidirectionalConnect establishes a write-only connection to a unix
+	// endpoint.
+	//
+	// An endpoint which calls UnidirectionalConnect and supports it itself must
+	// not hold its own lock when calling UnidirectionalConnect.
 	//
 	// This method will return tcpip.ErrConnectionRefused on a non-SockDgram
 	// endpoint.