Only refresh certificates if the fsnotify event is relevant

[roberthbailey]

Is your feature request related to a problem? Please describe.
Right now the certificates for the certs directory and client-ca are refreshed on any event fired from the fsnotify watcher: https://github.com/googleforgames/agones/blob/master/cmd/allocator/main.go#L147-L156

The watcher fires multiple events when a new k8s secret is installed into the directory. From the logs I see:

Create
Chmod
Rename
Remove
Remove
Remove

all being logged within 100ms for a single update to the cert.

The current code takes a mutex, re-reads the file system, re-parses the certs, and drops the mutex for each event. It would make it easier to tell what is happening in the system if we only did the work to refresh certificates when necessary.

Describe alternatives you've considered
Leave it alone; the refresh is currently working.

[roberthbailey]

The code for refreshing certs was added in #1145 and #1777.

[zmerlynn]

Just noting the range from the initial comment:

agones/cmd/allocator/main.go

Lines 147 to 156 in 23b60be

	case event := <-watcher.Events:
	h.certMutex.Lock()
	caCertPool, err := getCACertPool(certDir)
	if err != nil {
	logger.WithError(err).Error("could not load CA certs; keeping old ones")
	} else {
	h.caCertPool = caCertPool
	}
	logger.Infof("Certificate directory change event %v", event)
	h.certMutex.Unlock()

, currently at:

agones/cmd/allocator/main.go

Lines 274 to 283 in dc3eb37

	case event := <-watcher.Events:
	h.certMutex.Lock()
	caCertPool, err := getCACertPool(certDir)
	if err != nil {
	logger.WithError(err).Error("could not load CA certs; keeping old ones")
	} else {
	h.caCertPool = caCertPool
	}
	logger.Infof("Certificate directory change event %v", event)
	h.certMutex.Unlock()