Fix: PostingCache promise should fetch data only once

pkg/storage/tsdb/expanded_postings_cache.go

@@ -197,10 +197,17 @@ func (c *BlocksPostingsForMatchersCache) fetchPostings(blockID ulid.ULID, ix tsd
 	return c.result(promise)
 }
 
-func (c *BlocksPostingsForMatchersCache) result(promise *cacheEntryPromise[[]storage.SeriesRef]) func(ctx context.Context) (index.Postings, error) {
+func (c *BlocksPostingsForMatchersCache) result(ce *cacheEntryPromise[[]storage.SeriesRef]) func(ctx context.Context) (index.Postings, error) {
 	return func(ctx context.Context) (index.Postings, error) {
-		ids, err := promise.result(ctx)
-		return index.NewListPostings(ids), err
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		case <-ce.done:
+			if ctx.Err() != nil {
+				return nil, ctx.Err()
+			}
+			return index.NewListPostings(ce.v), ce.err
+		}
 	}
 }
 
@@ -327,9 +334,12 @@ func (c *fifoCache[V]) getPromiseForKey(k string, fetch func() (V, int64, error)
 		c.expire()
 	}
 
-	// If is cached but is expired, lets try to replace the cache value
-	if ok && loaded.(*cacheEntryPromise[V]).isExpired(c.cfg.Ttl, c.timeNow()) {
-		if c.cachedValues.CompareAndSwap(k, loaded, r) {
+	if ok {
+		// If the promise is already in the cache, lets wait it to fetch the data.
+		<-loaded.(*cacheEntryPromise[V]).done
+
+		// If is cached but is expired, lets try to replace the cache value.
+		if loaded.(*cacheEntryPromise[V]).isExpired(c.cfg.Ttl, c.timeNow()) && c.cachedValues.CompareAndSwap(k, loaded, r) {
 			r.v, r.sizeBytes, r.err = fetch()
 			r.sizeBytes += int64(len(k))
 			c.updateSize(loaded.(*cacheEntryPromise[V]).sizeBytes, r.sizeBytes)
@@ -404,19 +414,6 @@ type cacheEntryPromise[V any] struct {
 	err  error
 }
 
-func (ce *cacheEntryPromise[V]) result(ctx context.Context) (V, error) {
-	select {
-	case <-ctx.Done():
-		return ce.v, ctx.Err()
-	case <-ce.done:
-		if ctx.Err() != nil {
-			return ce.v, ctx.Err()
-		}
-
-		return ce.v, ce.err
-	}
-}
-
 func (ce *cacheEntryPromise[V]) isExpired(ttl time.Duration, now time.Time) bool {
 	ts := ce.ts
 	r := now.Sub(ts)

pkg/storage/tsdb/expanded_postings_cache_test.go

@@ -1,16 +1,48 @@
 package tsdb
 
 import (
-	"context"
 	"fmt"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/stretchr/testify/require"
+	"go.uber.org/atomic"
 )
 
+func Test_ShouldFetchPromiseOnlyOnce(t *testing.T) {
+	cfg := PostingsCacheConfig{
+		Enabled:  true,
+		Ttl:      time.Hour,
+		MaxBytes: 10 << 20,
+	}
+	m := NewPostingCacheMetrics(prometheus.DefaultRegisterer)
+	cache := newFifoCache[int](cfg, "test", m, time.Now)
+	calls := atomic.Int64{}
+	concurrency := 100
+	wg := sync.WaitGroup{}
+	wg.Add(concurrency)
+
+	fetchFunc := func() (int, int64, error) {
+		calls.Inc()
+		time.Sleep(100 * time.Millisecond)
+		return 0, 0, nil
+	}
+
+	for i := 0; i < 100; i++ {
+		go func() {
+			defer wg.Done()
+			cache.getPromiseForKey("key1", fetchFunc)
+		}()
+	}
+
+	wg.Wait()
+	require.Equal(t, int64(1), calls.Load())
+
+}
+
 func TestFifoCacheDisabled(t *testing.T) {
 	cfg := PostingsCacheConfig{}
 	cfg.Enabled = false
@@ -21,9 +53,7 @@ func TestFifoCacheDisabled(t *testing.T) {
 		return 1, 0, nil
 	})
 	require.False(t, loaded)
-	v, err := old.result(context.Background())
-	require.NoError(t, err)
-	require.Equal(t, 1, v)
+	require.Equal(t, 1, old.v)
 	require.False(t, cache.contains("key1"))
 }
 
@@ -68,17 +98,13 @@ func TestFifoCacheExpire(t *testing.T) {
 					return 1, 8, nil
 				})
 				require.False(t, loaded)
-				v, err := p.result(context.Background())
-				require.NoError(t, err)
-				require.Equal(t, 1, v)
+				require.Equal(t, 1, p.v)
 				require.True(t, cache.contains(key))
 				p, loaded = cache.getPromiseForKey(key, func() (int, int64, error) {
 					return 1, 0, nil
 				})
 				require.True(t, loaded)
-				v, err = p.result(context.Background())
-				require.NoError(t, err)
-				require.Equal(t, 1, v)
+				require.Equal(t, 1, p.v)
 			}
 
 			totalCacheSize := 0
@@ -104,10 +130,8 @@ func TestFifoCacheExpire(t *testing.T) {
 						return 2, 18, nil
 					})
 					require.False(t, loaded)
-					v, err := p.result(context.Background())
-					require.NoError(t, err)
 					// New value
-					require.Equal(t, 2, v)
+					require.Equal(t, 2, p.v)
 					// Total Size Updated
 					require.Equal(t, originalSize+10, cache.cachedBytes)
 				}