Recreate listener if error is occured

[clyang82]

recreate the listener when the notify is nil or there is an error for Ping()
here is the document for notify is nil

A notification channel will remain open until Unlisten is called, though connection loss might
result in some notifications being lost. To solve this problem, Listener sends
a nil pointer over the Notify channel any time the connection is re-established
following a connection loss. The application can get information about the
state of the underlying connection by setting an event callback in the call to
NewListener.

[skeeey]

how about new the listener in the goroutine waitForNotification? we can maintain the listener in one place, may like

func waitForNotification() {
   listener := newListener(ctx, f.config, channel)
   for {
     		// ...	
		// recreate the listener
		listener = newListener(ctx, dbConfig, channel)
        }
    }
}

[clyang82]

if encapsulate the listener into waitForNotification, we cannot get the listener in the integration test. without the listener we cannot close the listener to generate an errListenerClosed to test the listener re-creation.

[skeeey]

ok ...

may add some comments, the returned listener only for test purpose, it cannot be shared by others?

[clyang82]

/retest

[qiujian16]

will this be a problem? it seems we are changing a shared variable without lock. What happened when another is using the listener, and it is changed here?

[clyang82]

if reaching here, it means the listener has problem. it cannot be used anymore.

[qiujian16]

what happens when a caller calls the listener at the same time, will it panic?

[clyang82]

the listener is initialized when the maestro server is starting.

maestro/cmd/maestro/server/controllers.go

Line 61 in e28d043

go env().Database.SessionFactory.NewListener(ctx, "events", s.KindControllerManager.AddEvent)

so there is no new caller to call the listener. we leverage LISTEN/NOTIFY mechanism to sends a notification event together with an optional “payload” string to listener that has previously executed LISTEN channel.