Add agent initialization wait method in controller

Agent initialization wait method is added to make sure callers for
controller methods which depend on agent initialization to be complete
can wait on it.

Signed-off-by: Jana Radhakrishnan <mrjana@docker.com>

libnetwork/controller.go

@@ -114,6 +114,9 @@ type NetworkController interface {
 
 	// SetClusterProvider sets cluster provider
 	SetClusterProvider(provider cluster.Provider)
+
+	// Wait for agent initialization complete in libnetwork controller
+	AgentInitWait()
 }
 
 // NetworkWalker is a client provided function which will be used to walk the Networks.
@@ -143,6 +146,7 @@ type controller struct {
 	ingressSandbox  *sandbox
 	sboxOnce        sync.Once
 	agent           *agent
+	agentInitDone   chan struct{}
 	sync.Mutex
 }
 
@@ -159,6 +163,7 @@ func New(cfgOptions ...config.Option) (NetworkController, error) {
 		sandboxes:       sandboxTable{},
 		svcRecords:      make(map[string]svcInfo),
 		serviceBindings: make(map[string]*service),
+		agentInitDone:   make(chan struct{}),
 	}
 
 	if err := c.initStores(); err != nil {
@@ -248,6 +253,10 @@ func (c *controller) clusterAgentInit() {
 							}
 							return false
 						})
+
+						if c.agent != nil {
+							close(c.agentInitDone)
+						}
 					}
 				}
 				if remoteAddr != "" {
@@ -262,6 +271,12 @@ func (c *controller) clusterAgentInit() {
 	}
 }
 
+// AgentInitWait waits for agent initialization to be completed in the
+// controller.
+func (c *controller) AgentInitWait() {
+	<-c.agentInitDone
+}
+
 func (c *controller) makeDriverConfig(ntype string) map[string]interface{} {
 	if c.cfg == nil {
 		return nil