Add OnConnectionDown callback in autopaho

Provides callback that is called when an established conneciton is dropped. Presents an opportunity to prevent future attempts to connect.

autopaho/auto.go

@@ -87,8 +87,9 @@ type ClientConfig struct {
 	// To fix, use packets.NewThreadSafeConn wrapper or extend the custom net.Conn struct with sync.Locker.
 	AttemptConnection func(context.Context, ClientConfig, *url.URL) (net.Conn, error)
 
-	OnConnectionUp func(*ConnectionManager, *paho.Connack) // Called when a connection is made (including reconnection). Connection Manager passed to simplify subscriptions. Supplied function must not block.
-	OnConnectError func(error)                             // Called (within a goroutine) whenever a connection attempt fails. Will wrap autopaho.ConnackError on server deny.
+	OnConnectionUp   func(*ConnectionManager, *paho.Connack) // Called when a connection is made (including reconnection). Connection Manager passed to simplify subscriptions. Supplied function must not block.
+	OnConnectionDown func() bool                             // Only called after the connection that resulted in OnConnectionUp is dropped. Returning false will cause autopaho to cease attempting to connect. Supplied function must not block.
+	OnConnectError   func(error)                             // Called (within a goroutine) whenever a connection attempt fails. Will wrap autopaho.ConnackError on server deny.
 
 	Debug      log.Logger // By default set to NOOPLogger{},set to a logger for debugging info
 	Errors     log.Logger // By default set to NOOPLogger{},set to a logger for errors
@@ -359,6 +360,11 @@ func NewConnection(ctx context.Context, cfg ClientConfig) (*ConnectionManager, e
 			close(c.connDown)
 			c.connUp = make(chan struct{})
 			c.mu.Unlock()
+
+			if cfg.OnConnectionDown != nil && !cfg.OnConnectionDown() {
+				cfg.Debug.Printf("mainLoop: connection to server lost (%s); OnConnectionDown aborts reconnect\n", err)
+				break mainLoop
+			}
 			cfg.Debug.Printf("mainLoop: connection to server lost (%s); will reconnect\n", err)
 		}
 		cfg.Debug.Println("mainLoop: connection manager has terminated")