cache: fix race when clearning a cache (#261)

When clearing a cache while other goroutines are actively Wait()-ing for it, the waiting goroutines can become blocked indefinitely if one of the cache items is flushed by the Clear() code instead of by the background worker, because the Clear() code is not marking sync.WaitGroups as done.

cache.go

@@ -348,6 +348,10 @@ loop:
 	for {
 		select {
 		case i := <-c.setBuf:
+			if i.wg != nil {
+				i.wg.Done()
+				continue
+			}
 			if i.flag != itemUpdate {
 				// In itemUpdate, the value is already set in the store.  So, no need to call
 				// onEvict here.

cache_test.go

@@ -715,6 +715,37 @@ func init() {
 	bucketDurationSecs = 1
 }
 
+func TestBlockOnClear(t *testing.T) {
+	c, err := NewCache(&Config{
+		NumCounters: 100,
+		MaxCost:     10,
+		BufferItems: 64,
+		Metrics:     false,
+	})
+	require.NoError(t, err)
+	defer c.Close()
+
+	done := make(chan struct{})
+
+	go func() {
+		for i := 0; i < 10; i++ {
+			c.Wait()
+		}
+		close(done)
+	}()
+
+	for i := 0; i < 10; i++ {
+		c.Clear()
+	}
+
+	select {
+	case <-done:
+		// We're OK
+	case <-time.After(1 * time.Second):
+		t.Fatalf("timed out while waiting on cache")
+	}
+}
+
 // Regression test for bug https://github.com/dgraph-io/ristretto/issues/167
 func TestDropUpdates(t *testing.T) {
 	originalSetBugSize := setBufSize