colblk: remove BlockDecoder allocation

```
goos: darwin
goarch: arm64
pkg: github.com/cockroachdb/pebble/sstable/colblk
cpu: Apple M1 Pro
                        │   old.txt   │               new.txt               │
                        │   sec/op    │   sec/op     vs base                │
DataBlockDecoderInit-10   94.53n ± 0%   82.12n ± 0%  -13.13% (p=0.000 n=25)

                        │  old.txt   │              new.txt               │
                        │    B/op    │   B/op     vs base                 │
DataBlockDecoderInit-10   48.00 ± 0%   0.00 ± 0%  -100.00% (p=0.000 n=25)

                        │  old.txt   │               new.txt               │
                        │ allocs/op  │ allocs/op   vs base                 │
DataBlockDecoderInit-10   1.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=25)
```

Fix #5339.

Author: jbowens

cockroachkvs/cockroachkvs.go

@@ -498,7 +498,7 @@ var KeySchema = colblk.KeySchema{
 	NewKeyWriter: func() colblk.KeyWriter {
 		return makeCockroachKeyWriter()
 	},
-	InitKeySeekerMetadata: func(meta *colblk.KeySeekerMetadata, d *colblk.DataBlockDecoder, bd *colblk.BlockDecoder) {
+	InitKeySeekerMetadata: func(meta *colblk.KeySeekerMetadata, d *colblk.DataBlockDecoder, bd colblk.BlockDecoder) {
 		ks := (*cockroachKeySeeker)(unsafe.Pointer(meta))
 		ks.init(d, bd)
 	},
@@ -747,7 +747,7 @@ var _ uint = colblk.KeySeekerMetadataSize - uint(unsafe.Sizeof(cockroachKeySeeke
 
 var _ colblk.KeySeeker = (*cockroachKeySeeker)(nil)
 
-func (ks *cockroachKeySeeker) init(d *colblk.DataBlockDecoder, bd *colblk.BlockDecoder) {
+func (ks *cockroachKeySeeker) init(d *colblk.DataBlockDecoder, bd colblk.BlockDecoder) {
 	ks.roachKeys = bd.PrefixBytes(cockroachColRoachKey)
 	ks.roachKeyChanged = d.PrefixChanged()
 	ks.mvccWallTimes = bd.Uints(cockroachColMVCCWallTime)

cockroachkvs/cockroachkvs_test.go

@@ -204,7 +204,7 @@ func TestKeySchema_KeySeeker(t *testing.T) {
 	var buf bytes.Buffer
 	var enc colblk.DataBlockEncoder
 	var dec colblk.DataBlockDecoder
-	var bd *colblk.BlockDecoder
+	var bd colblk.BlockDecoder
 	var ks colblk.KeySeeker
 	var maxKeyLen int
 	enc.Init(&KeySchema)

sstable/colblk/data_block.go

@@ -44,7 +44,7 @@ type KeySchema struct {
 	// InitKeySeekerMetadata initializes the provided KeySeekerMetadata. This
 	// happens once when a block enters the block cache and can be used to save
 	// computation in NewKeySeeker.
-	InitKeySeekerMetadata func(meta *KeySeekerMetadata, d *DataBlockDecoder, bd *BlockDecoder)
+	InitKeySeekerMetadata func(meta *KeySeekerMetadata, d *DataBlockDecoder, bd BlockDecoder)
 
 	// KeySeeker returns a KeySeeker using metadata that was previously
 	// initialized with InitKeySeekerMetadata. The returned key seeker can be an
@@ -215,10 +215,10 @@ func DefaultKeySchema(comparer *base.Comparer, prefixBundleSize int) KeySchema {
 			kw.suffixes.Init()
 			return kw
 		},
-		InitKeySeekerMetadata: func(meta *KeySeekerMetadata, d *DataBlockDecoder, bd *BlockDecoder) {
+		InitKeySeekerMetadata: func(meta *KeySeekerMetadata, d *DataBlockDecoder, bd BlockDecoder) {
 			ks := (*defaultKeySeeker)(unsafe.Pointer(&meta[0]))
 			ks.comparer = comparer
-			ks.init(d, bd)
+			ks.init(d, &bd)
 		},
 		KeySeeker: func(meta *KeySeekerMetadata) KeySeeker {
 			ks := (*defaultKeySeeker)(unsafe.Pointer(&meta[0]))
@@ -892,7 +892,7 @@ func (d *DataBlockDecoder) PrefixChanged() Bitmap {
 }
 
 // Init initializes the data block reader with the given serialized data block.
-func (d *DataBlockDecoder) Init(schema *KeySchema, data []byte) *BlockDecoder {
+func (d *DataBlockDecoder) Init(schema *KeySchema, data []byte) BlockDecoder {
 	if uintptr(unsafe.Pointer(unsafe.SliceData(data)))&7 != 0 {
 		panic("data buffer not 8-byte aligned")
 	}
@@ -904,12 +904,12 @@ func (d *DataBlockDecoder) Init(schema *KeySchema, data []byte) *BlockDecoder {
 	d.isValueExternal = bd.Bitmap(len(schema.ColumnTypes) + dataBlockColumnIsValueExternal)
 	d.isObsolete = bd.Bitmap(len(schema.ColumnTypes) + dataBlockColumnIsObsolete)
 	d.maximumKeyLength = binary.LittleEndian.Uint32(data[schema.HeaderSize:])
-	return &bd
+	return bd
 }
 
 // Describe descirbes the binary format of the data block, assuming f.Offset()
 // is positioned at the beginning of the same data block described by d.
-func (d *DataBlockDecoder) Describe(f *binfmt.Formatter, tp treeprinter.Node, bd *BlockDecoder) {
+func (d *DataBlockDecoder) Describe(f *binfmt.Formatter, tp treeprinter.Node, bd BlockDecoder) {
 	// Set the relative offset. When loaded into memory, the beginning of blocks
 	// are aligned. Padding that ensures alignment is done relative to the
 	// current offset. Setting the relative offset ensures that if we're
@@ -1051,7 +1051,7 @@ func (i *DataBlockIter) InitOnce(
 // Init initializes the data block iterator, configuring it to read from the
 // provided decoder.
 func (i *DataBlockIter) Init(
-	d *DataBlockDecoder, bd *BlockDecoder, transforms blockiter.Transforms,
+	d *DataBlockDecoder, bd BlockDecoder, transforms blockiter.Transforms,
 ) error {
 	i.d = d
 	// Leave i.h unchanged.

sstable/colblk/data_block_test.go

@@ -29,7 +29,7 @@ func TestDataBlock(t *testing.T) {
 	var buf bytes.Buffer
 	var w DataBlockEncoder
 	var r DataBlockDecoder
-	var bd *BlockDecoder
+	var bd BlockDecoder
 	var v DataBlockValidator
 	var it DataBlockIter
 	rw := NewDataBlockRewriter(&testKeysSchema, testkeys.Comparer.EnsureDefaults())