Add notification filtering by target media type

The Hub registry generates a large volume of notifications, many of
which are uninteresting based on target media type.  Discarding them
within the notification endpoint consumes considerable resources that
could be saved by discarding them within the registry.  To that end,
this change adds registry configuration options to restrict the
notifications sent to an endpoint based on target media type.

Signed-off-by: Noah Treuhaft <noah.treuhaft@docker.com>

notifications/sinks.go

@@ -210,6 +210,44 @@ func (eq *eventQueue) next() []Event {
 	return block
 }
 
+// ignoredMediaTypesSink discards events with ignored target media types and
+// passes the rest along.
+type ignoredMediaTypesSink struct {
+	Sink
+	ignored map[string]bool
+}
+
+func newIgnoredMediaTypesSink(sink Sink, ignored []string) Sink {
+	if len(ignored) == 0 {
+		return sink
+	}
+
+	ignoredMap := make(map[string]bool)
+	for _, mediaType := range ignored {
+		ignoredMap[mediaType] = true
+	}
+
+	return &ignoredMediaTypesSink{
+		Sink:    sink,
+		ignored: ignoredMap,
+	}
+}
+
+// Write discards events with ignored target media types and passes the rest
+// along.
+func (imts *ignoredMediaTypesSink) Write(events ...Event) error {
+	var kept []Event
+	for _, e := range events {
+		if !imts.ignored[e.Target.MediaType] {
+			kept = append(kept, e)
+		}
+	}
+	if len(kept) == 0 {
+		return nil
+	}
+	return imts.Sink.Write(kept...)
+}
+
 // retryingSink retries the write until success or an ErrSinkClosed is
 // returned. Underlying sink must have p > 0 of succeeding or the sink will
 // block. Internally, it is a circuit breaker retries to manage reset.