Fix rendering notifications from the state at event time (#32401)

util/param.go

@@ -31,15 +31,9 @@ type ParamCache struct {
 	val map[string]Param
 }
 
-// flush is the value type used as parameter for flushing the cache.
-// Flushing is implemented by closing the channel. At this time, it is guaranteed
-// that the cache has catched up processing all pending messages.
-type flush chan struct{}
-
-// Flusher returns a new flush channel
-func Flusher() flush {
-	return make(flush)
-}
+// Snapshot requests a copy of the cache state at the parameter's position in
+// the stream. It runs on the cache's goroutine and must not block for long.
+type Snapshot func([]Param)
 
 // NewCache creates cache
 func NewParamCache() *ParamCache {
@@ -51,8 +45,8 @@ func NewParamCache() *ParamCache {
 // Run adds input channel's values to cache
 func (c *ParamCache) Run(in <-chan Param) {
 	for p := range in {
-		if flushC, ok := p.Val.(flush); ok {
-			close(flushC)
+		if snapshot, ok := p.Val.(Snapshot); ok {
+			snapshot(c.All())
 			continue
 		}
 

util/param_test.go

@@ -21,3 +21,20 @@ func TestParam(t *testing.T) {
 func TestParamCache(t *testing.T) {
 	NewParamCache().Add("foo", Param{})
 }
+
+func TestParamCacheSnapshot(t *testing.T) {
+	in := make(chan Param)
+	go NewParamCache().Run(in)
+
+	in <- Param{Key: "before", Val: 1}
+
+	res := make(chan []Param, 1)
+	in <- Param{Val: Snapshot(func(state []Param) { res <- state })}
+
+	// published after the snapshot request, must not be included
+	in <- Param{Key: "after", Val: 2}
+
+	state := <-res
+	assert.Len(t, state, 1)
+	assert.Equal(t, "before", state[0].Key)
+}