Allow concurrent calls to agentClose

- This fixes a panic in memberlist.Leave() because called
  after memberlist.shutdown = false
  It happens because of two interlocking calls to NetworkDB.clusterLeave()
  It is easily reproducible with two back-to-back calls
  to docker swarm init && docker swarm leave --force
  While the first clusterLeave() is waiting for sendNodeEvent(NodeEventTypeLeave)
  to timeout (5 sec) a second clusterLeave() is called. The second clusterLeave()
  will end up invoking memberlist.Leave() after the previous call already did
  the same, therefore after memberlist.shutdown was set false.
- The fix is to have agentClose() acquire the agent instance and reset the
  agent pointer right away under lock. Then execute the closing/leave functions
  on the agent instance.

Signed-off-by: Alessandro Boch <aboch@docker.com>

libnetwork/agent.go

@@ -328,22 +328,26 @@ func (c *controller) agentDriverNotify(d driverapi.Driver) {
 }
 
 func (c *controller) agentClose() {
-	if c.agent == nil {
+	// Acquire current agent instance and reset its pointer
+	// then run closing functions
+	c.Lock()
+	agent := c.agent
+	c.agent = nil
+	c.Unlock()
+
+	if agent == nil {
 		return
 	}
 
-	for _, cancelFuncs := range c.agent.driverCancelFuncs {
+	for _, cancelFuncs := range agent.driverCancelFuncs {
 		for _, cancel := range cancelFuncs {
 			cancel()
 		}
 	}
-	c.agent.epTblCancel()
 
-	c.agent.networkDB.Close()
+	agent.epTblCancel()
 
-	c.Lock()
-	c.agent = nil
-	c.Unlock()
+	agent.networkDB.Close()
 }
 
 func (n *network) isClusterEligible() bool {