tsdb: faster CRC check by avoiding allocations (#10789)

Instead of creating a new hashing object every time, call `crc32.Checksum`
which computes the answer without allocations.

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

tsdb/chunks/chunks.go

@@ -15,7 +15,6 @@ package chunks
 
 import (
 	"bufio"
-	"bytes"
 	"encoding/binary"
 	"fmt"
 	"hash"
@@ -166,6 +165,17 @@ func newCRC32() hash.Hash32 {
 	return crc32.New(castagnoliTable)
 }
 
+// Check if the CRC of data matches that stored in sum, computed when the chunk was stored.
+func checkCRC32(data, sum []byte) error {
+	got := crc32.Checksum(data, castagnoliTable)
+	// This combination of shifts is the inverse of digest.Sum() in go/src/hash/crc32.
+	want := uint32(sum[0])<<24 + uint32(sum[1])<<16 + uint32(sum[2])<<8 + uint32(sum[3])
+	if got != want {
+		return errors.Errorf("checksum mismatch expected:%x, actual:%x", want, got)
+	}
+	return nil
+}
+
 // Writer implements the ChunkWriter interface for the standard
 // serialization format.
 type Writer struct {
@@ -547,7 +557,6 @@ func (s *Reader) Size() int64 {
 // Chunk returns a chunk from a given reference.
 func (s *Reader) Chunk(ref ChunkRef) (chunkenc.Chunk, error) {
 	sgmIndex, chkStart := BlockChunkRef(ref).Unpack()
-	chkCRC32 := newCRC32()
 
 	if sgmIndex >= len(s.bs) {
 		return nil, errors.Errorf("segment index %d out of range", sgmIndex)
@@ -576,14 +585,10 @@ func (s *Reader) Chunk(ref ChunkRef) (chunkenc.Chunk, error) {
 	}
 
 	sum := sgmBytes.Range(chkDataEnd, chkEnd)
-	if _, err := chkCRC32.Write(sgmBytes.Range(chkEncStart, chkDataEnd)); err != nil {
+	if err := checkCRC32(sgmBytes.Range(chkEncStart, chkDataEnd), sum); err != nil {
 		return nil, err
 	}
 
-	if act := chkCRC32.Sum(nil); !bytes.Equal(act, sum) {
-		return nil, errors.Errorf("checksum mismatch expected:%x, actual:%x", sum, act)
-	}
-
 	chkData := sgmBytes.Range(chkDataStart, chkDataEnd)
 	chkEnc := sgmBytes.Range(chkEncStart, chkEncStart+ChunkEncodingSize)[0]
 	return s.pool.Get(chunkenc.Encoding(chkEnc), chkData)