chore: events: implement event observer deregister method

chain/events/events_test.go

@@ -1469,3 +1469,35 @@ func TestReconnect(t *testing.T) {
 	fcs.advance(0, 5, 2, nil, 0, 1, 3)
 	require.True(t, fcs.callNumber["ChainGetPath"] == 4)
 }
+
+func TestUnregister(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	fcs := newFakeCS(t)
+
+	events, err := NewEvents(ctx, fcs)
+	require.NoError(t, err)
+
+	tsObs := &testObserver{t: t}
+	events.Observe(tsObs)
+
+	// observer receives heads as the chain advances
+	fcs.advance(0, 1, 0, nil)
+	headBeforeDeregister := events.lastTs
+	require.Equal(t, tsObs.head, headBeforeDeregister)
+
+	// observer unregistered successfully
+	found := events.Unregister(tsObs)
+	require.True(t, found)
+
+	// observer stops receiving heads as the chain advances
+	fcs.advance(0, 1, 0, nil)
+	require.Equal(t, tsObs.head, headBeforeDeregister)
+	require.NotEqual(t, tsObs.head, events.lastTs)
+
+	// unregistering an invalid observer returns false
+	dneObs := &testObserver{t: t}
+	found = events.Unregister(dneObs)
+	require.False(t, found)
+}

chain/events/observer.go

@@ -253,3 +253,27 @@ func (o *observer) Observe(obs TipSetObserver) *types.TipSet {
 	o.observers = append(o.observers, obs)
 	return o.head
 }
+
+// Unregister unregisters an observer. Returns true if we successfully removed the observer.
+//
+// NOTE: The observer _may_ be called after being removed. Observers MUST handle this case
+// internally.
+func (o *observer) Unregister(obs TipSetObserver) (found bool) {
+	o.lk.Lock()
+	defer o.lk.Unlock()
+	// We _copy_ the observers list because we may be concurrently reading it from a headChange
+	// handler.
+	//
+	// This should happen infrequently, so it's fine if we spend a bit of time here.
+	newObservers := make([]TipSetObserver, 0, len(o.observers))
+	for _, existingObs := range o.observers {
+		if existingObs == obs {
+			found = true
+			continue
+		}
+		newObservers = append(newObservers, existingObs)
+	}
+
+	o.observers = newObservers
+	return found
+}