[BUGFIX] TSDB: Apply fixes from loadWAL to loadWBL

Move a couple of variables inside the scope of a goroutine, to avoid
data races.

Use `zeropool` to reduce garbage and avoid some lint warnings.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

tsdb/head_wal.go

@@ -656,32 +656,15 @@ func (h *Head) loadWBL(r *wlog.Reader, syms *labels.SymbolTable, multiRef map[ch
 		concurrency = h.opts.WALReplayConcurrency
 		processors  = make([]wblSubsetProcessor, concurrency)
 
-		dec             record.Decoder
 		shards          = make([][]record.RefSample, concurrency)
 		histogramShards = make([][]histogramRecord, concurrency)
 
-		decodedCh   = make(chan interface{}, 10)
-		decodeErr   error
-		samplesPool = sync.Pool{
-			New: func() interface{} {
-				return []record.RefSample{}
-			},
-		}
-		markersPool = sync.Pool{
-			New: func() interface{} {
-				return []record.RefMmapMarker{}
-			},
-		}
-		histogramSamplesPool = sync.Pool{
-			New: func() interface{} {
-				return []record.RefHistogramSample{}
-			},
-		}
-		floatHistogramSamplesPool = sync.Pool{
-			New: func() interface{} {
-				return []record.RefFloatHistogramSample{}
-			},
-		}
+		decodedCh                 = make(chan interface{}, 10)
+		decodeErr                 error
+		samplesPool               zeropool.Pool[[]record.RefSample]
+		markersPool               zeropool.Pool[[]record.RefMmapMarker]
+		histogramSamplesPool      zeropool.Pool[[]record.RefHistogramSample]
+		floatHistogramSamplesPool zeropool.Pool[[]record.RefFloatHistogramSample]
 	)
 
 	defer func() {
@@ -711,11 +694,13 @@ func (h *Head) loadWBL(r *wlog.Reader, syms *labels.SymbolTable, multiRef map[ch
 
 	go func() {
 		defer close(decodedCh)
+		var err error
+		dec := record.NewDecoder(syms)
 		for r.Next() {
 			rec := r.Record()
 			switch dec.Type(rec) {
 			case record.Samples:
-				samples := samplesPool.Get().([]record.RefSample)[:0]
+				samples := samplesPool.Get()[:0]
 				samples, err = dec.Samples(rec, samples)
 				if err != nil {
 					decodeErr = &wlog.CorruptionErr{
@@ -727,7 +712,7 @@ func (h *Head) loadWBL(r *wlog.Reader, syms *labels.SymbolTable, multiRef map[ch
 				}
 				decodedCh <- samples
 			case record.MmapMarkers:
-				markers := markersPool.Get().([]record.RefMmapMarker)[:0]
+				markers := markersPool.Get()[:0]
 				markers, err = dec.MmapMarkers(rec, markers)
 				if err != nil {
 					decodeErr = &wlog.CorruptionErr{
@@ -739,7 +724,7 @@ func (h *Head) loadWBL(r *wlog.Reader, syms *labels.SymbolTable, multiRef map[ch
 				}
 				decodedCh <- markers
 			case record.HistogramSamples:
-				hists := histogramSamplesPool.Get().([]record.RefHistogramSample)[:0]
+				hists := histogramSamplesPool.Get()[:0]
 				hists, err = dec.HistogramSamples(rec, hists)
 				if err != nil {
 					decodeErr = &wlog.CorruptionErr{
@@ -751,7 +736,7 @@ func (h *Head) loadWBL(r *wlog.Reader, syms *labels.SymbolTable, multiRef map[ch
 				}
 				decodedCh <- hists
 			case record.FloatHistogramSamples:
-				hists := floatHistogramSamplesPool.Get().([]record.RefFloatHistogramSample)[:0]
+				hists := floatHistogramSamplesPool.Get()[:0]
 				hists, err = dec.FloatHistogramSamples(rec, hists)
 				if err != nil {
 					decodeErr = &wlog.CorruptionErr{
@@ -802,7 +787,7 @@ func (h *Head) loadWBL(r *wlog.Reader, syms *labels.SymbolTable, multiRef map[ch
 				}
 				samples = samples[m:]
 			}
-			samplesPool.Put(d)
+			samplesPool.Put(v)
 		case []record.RefMmapMarker:
 			markers := v
 			for _, rm := range markers {
@@ -857,7 +842,7 @@ func (h *Head) loadWBL(r *wlog.Reader, syms *labels.SymbolTable, multiRef map[ch
 				}
 				samples = samples[m:]
 			}
-			histogramSamplesPool.Put(v) //nolint:staticcheck
+			histogramSamplesPool.Put(v)
 		case []record.RefFloatHistogramSample:
 			samples := v
 			// We split up the samples into chunks of 5000 samples or less.
@@ -889,7 +874,7 @@ func (h *Head) loadWBL(r *wlog.Reader, syms *labels.SymbolTable, multiRef map[ch
 				}
 				samples = samples[m:]
 			}
-			floatHistogramSamplesPool.Put(v) //nolint:staticcheck
+			floatHistogramSamplesPool.Put(v)
 		default:
 			panic(fmt.Errorf("unexpected decodedCh type: %T", d))
 		}