lib/encoding/zstd: switch back from atomic.Pointer to atomic.Value for map[...]...

The map[...]... is already a pointer type, so atomic.Pointer[map[...]...] results in double pointer.

This is a follow-up for 140e7b6b74

lib/encoding/zstd/zstd_pure.go

@@ -15,13 +15,13 @@ var (
 	decoder *zstd.Decoder
 
 	mu sync.Mutex
-	av atomic.Pointer[registry]
+
+	// do not use atomic.Pointer, since the stored map there is already a pointer type.
+	av atomic.Value
 )
 
-type registry map[int]*zstd.Encoder
-
 func init() {
-	r := make(registry)
+	r := make(map[int]*zstd.Encoder)
 	av.Store(r)
 
 	var err error
@@ -45,7 +45,7 @@ func CompressLevel(dst, src []byte, compressionLevel int) []byte {
 }
 
 func getEncoder(compressionLevel int) *zstd.Encoder {
-	r := av.Load()
+	r := av.Load().(map[int]*zstd.Encoder)
 	e := r[compressionLevel]
 	if e != nil {
 		return e
@@ -54,10 +54,10 @@ func getEncoder(compressionLevel int) *zstd.Encoder {
 	mu.Lock()
 	// Create the encoder under lock in order to prevent from wasted work
 	// when concurrent goroutines create encoder for the same compressionLevel.
-	r1 := av.Load()
+	r1 := av.Load().(map[int]*zstd.Encoder)
 	if e = r1[compressionLevel]; e == nil {
 		e = newEncoder(compressionLevel)
-		r2 := make(registry)
+		r2 := make(map[int]*zstd.Encoder)
 		for k, v := range r1 {
 			r2[k] = v
 		}

lib/tenantmetrics/counter_map.go

@@ -18,6 +18,8 @@ type TenantID struct {
 // CounterMap is a map of counters keyed by tenant.
 type CounterMap struct {
 	metric string
+
+	// do not use atomic.Pointer, since the stored map there is already a pointer type.
 	m atomic.Value
 }