feat: add GetCached method to report cache hit status

groupcache.go

@@ -210,31 +210,44 @@ func (g *Group) initPeers() {
 }
 
 func (g *Group) Get(ctx context.Context, key string, dest Sink) error {
+	_, err := g.getCached(ctx, key, dest)
+	return err
+}
+
+// GetCached acts like Get but additionally reports whether the
+// value was served from the local cache. This is useful for
+// callers that want to distinguish cache hits from cache misses
+// without relying on the Stats counters.
+func (g *Group) GetCached(ctx context.Context, key string, dest Sink) (cached bool, err error) {
+	return g.getCached(ctx, key, dest)
+}
+
+func (g *Group) getCached(ctx context.Context, key string, dest Sink) (cached bool, err error) {
 	g.peersOnce.Do(g.initPeers)
 	g.Stats.Gets.Add(1)
 	if dest == nil {
-		return errors.New("groupcache: nil dest Sink")
+		return false, errors.New("groupcache: nil dest Sink")
 	}
 	value, cacheHit := g.lookupCache(key)
 
 	if cacheHit {
 		g.Stats.CacheHits.Add(1)
-		return setSinkView(dest, value)
+		return true, setSinkView(dest, value)
 	}
 
 	// Optimization to avoid double unmarshalling or copying: keep
 	// track of whether the dest was already populated. One caller
 	// (if local) will set this; the losers will not. The common
 	// case will likely be one caller.
 	destPopulated := false
-	value, destPopulated, err := g.load(ctx, key, dest)
+	value, destPopulated, err = g.load(ctx, key, dest)
 	if err != nil {
-		return err
+		return false, err
 	}
 	if destPopulated {
-		return nil
+		return false, nil
 	}
-	return setSinkView(dest, value)
+	return false, setSinkView(dest, value)
 }
 
 // load loads key either by invoking the getter locally or by sending it to another machine.

groupcache_test.go

@@ -453,5 +453,47 @@ func TestGroupStatsAlignment(t *testing.T) {
 	}
 }
 
+// TestGetCached tests that GetCached correctly reports cache hit/miss.
+func TestGetCached(t *testing.T) {
+	once.Do(testSetup)
+	g := newGroup("TestGetCached-group", 1<<20, GetterFunc(func(_ context.Context, key string, dest Sink) error {
+		return dest.SetString("val:" + key)
+	}), nil)
+
+	key := "cached-key"
+	var s string
+
+	// First call: cache miss.
+	cached, err := g.GetCached(dummyCtx, key, StringSink(&s))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cached {
+		t.Error("first GetCached returned cached=true; want false")
+	}
+	if s != "val:cached-key" {
+		t.Errorf("got %q; want %q", s, "val:cached-key")
+	}
+
+	// Second call: should be a cache hit.
+	s = ""
+	cached, err = g.GetCached(dummyCtx, key, StringSink(&s))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !cached {
+		t.Error("second GetCached returned cached=false; want true")
+	}
+	if s != "val:cached-key" {
+		t.Errorf("got %q; want %q", s, "val:cached-key")
+	}
+
+	// Nil dest should return error.
+	_, err = g.GetCached(dummyCtx, "any", nil)
+	if err == nil {
+		t.Error("GetCached with nil dest should return error")
+	}
+}
+
 // TODO(bradfitz): port the Google-internal full integration test into here,
 // using HTTP requests instead of our RPC system.