lib/protocol: Ensure starting & closing a connection are exclusive (f…

…ixes syncthing#9102) (syncthing#9103)

In principle a connection can close while it's in progress with
starting, and then it's undefined if we wait for goroutines to exit etc.
With this change, we will wait for start to complete before starting to
stop everything.

lib/protocol/protocol.go

@@ -208,6 +208,7 @@ type rawConnection struct {
 	closeOnce             sync.Once
 	sendCloseOnce         sync.Once
 	compression           Compression
+	startStopMut          sync.Mutex // start and stop must be serialized
 
 	loopWG sync.WaitGroup // Need to ensure no leftover routines in testing
 }
@@ -295,6 +296,8 @@ func newRawConnection(deviceID DeviceID, reader io.Reader, writer io.Writer, clo
 // Start creates the goroutines for sending and receiving of messages. It must
 // be called exactly once after creating a connection.
 func (c *rawConnection) Start() {
+	c.startStopMut.Lock()
+	defer c.startStopMut.Unlock()
 	c.loopWG.Add(5)
 	go func() {
 		c.readerLoop()
@@ -963,6 +966,8 @@ func (c *rawConnection) Close(err error) {
 
 // internalClose is called if there is an unexpected error during normal operation.
 func (c *rawConnection) internalClose(err error) {
+	c.startStopMut.Lock()
+	defer c.startStopMut.Unlock()
 	c.closeOnce.Do(func() {
 		l.Debugf("close connection to %s at %s due to %v", c.deviceID.Short(), c.ConnectionInfo, err)
 		if cerr := c.closer.Close(); cerr != nil {