[PERF] Remote-write: re-use memory to read WAL data (prometheus#16197)

The `:=` causes new variables to be created, which means the outer
slice stays at nil, and new memory is allocated every time round the
loop.

Extracted from prometheus#16182
Credit to @bwplotka.

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

Author: bboreham

tsdb/wlog/watcher.go

@@ -491,12 +491,13 @@ func (w *Watcher) readSegment(r *LiveReader, segmentNum int, tail bool) error {
 		metadata              []record.RefMetadata
 	)
 	for r.Next() && !isClosed(w.quit) {
+		var err error
 		rec := r.Record()
 		w.recordsReadMetric.WithLabelValues(dec.Type(rec).String()).Inc()
 
 		switch dec.Type(rec) {
 		case record.Series:
-			series, err := dec.Series(rec, series[:0])
+			series, err = dec.Series(rec, series[:0])
 			if err != nil {
 				w.recordDecodeFailsMetric.Inc()
 				return err
@@ -509,7 +510,7 @@ func (w *Watcher) readSegment(r *LiveReader, segmentNum int, tail bool) error {
 			if !tail {
 				break
 			}
-			samples, err := dec.Samples(rec, samples[:0])
+			samples, err = dec.Samples(rec, samples[:0])
 			if err != nil {
 				w.recordDecodeFailsMetric.Inc()
 				return err
@@ -539,7 +540,7 @@ func (w *Watcher) readSegment(r *LiveReader, segmentNum int, tail bool) error {
 			if !tail {
 				break
 			}
-			exemplars, err := dec.Exemplars(rec, exemplars[:0])
+			exemplars, err = dec.Exemplars(rec, exemplars[:0])
 			if err != nil {
 				w.recordDecodeFailsMetric.Inc()
 				return err
@@ -554,7 +555,7 @@ func (w *Watcher) readSegment(r *LiveReader, segmentNum int, tail bool) error {
 			if !tail {
 				break
 			}
-			histograms, err := dec.HistogramSamples(rec, histograms[:0])
+			histograms, err = dec.HistogramSamples(rec, histograms[:0])
 			if err != nil {
 				w.recordDecodeFailsMetric.Inc()
 				return err
@@ -582,7 +583,7 @@ func (w *Watcher) readSegment(r *LiveReader, segmentNum int, tail bool) error {
 			if !tail {
 				break
 			}
-			floatHistograms, err := dec.FloatHistogramSamples(rec, floatHistograms[:0])
+			floatHistograms, err = dec.FloatHistogramSamples(rec, floatHistograms[:0])
 			if err != nil {
 				w.recordDecodeFailsMetric.Inc()
 				return err
@@ -606,12 +607,12 @@ func (w *Watcher) readSegment(r *LiveReader, segmentNum int, tail bool) error {
 			if !w.sendMetadata {
 				break
 			}
-			meta, err := dec.Metadata(rec, metadata[:0])
+			metadata, err = dec.Metadata(rec, metadata[:0])
 			if err != nil {
 				w.recordDecodeFailsMetric.Inc()
 				return err
 			}
-			w.writer.StoreMetadata(meta)
+			w.writer.StoreMetadata(metadata)
 
 		case record.Unknown:
 			// Could be corruption, or reading from a WAL from a newer Prometheus.