refactor: add typed sync.Pool wrapper to eliminate any

Introduce util/pools.Pool[T] as a generic typed wrapper
around sync.Pool. Migrate existing pool usages in
contenthash, converter, and overlay packages to use the
new wrapper, removing unchecked type assertions at each
call site.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

cache/contenthash/checksum.go

@@ -19,6 +19,7 @@ import (
 	"github.com/moby/buildkit/session"
 	"github.com/moby/buildkit/snapshot"
 	"github.com/moby/buildkit/util/cachedigest"
+	"github.com/moby/buildkit/util/pools"
 	"github.com/moby/locker"
 	"github.com/moby/patternmatcher"
 	digest "github.com/opencontainers/go-digest"
@@ -1261,15 +1262,13 @@ func ensureOriginMetadata(md cache.RefMetadata) cache.RefMetadata {
 	return em
 }
 
-var pool32K = sync.Pool{
-	New: func() any {
-		buf := make([]byte, 32*1024) // 32K
-		return &buf
-	},
-}
+var pool32K = pools.New(func() *[]byte {
+	buf := make([]byte, 32*1024) // 32K
+	return &buf
+})
 
 func poolsCopy(dst io.Writer, src io.Reader) (written int64, err error) {
-	buf := pool32K.Get().(*[]byte)
+	buf := pool32K.Get()
 	written, err = io.CopyBuffer(dst, src, *buf)
 	pool32K.Put(buf)
 	return

util/converter/converter.go

@@ -19,6 +19,7 @@ import (
 	"github.com/moby/buildkit/util/compression"
 	"github.com/moby/buildkit/util/converter/tarconverter"
 	"github.com/moby/buildkit/util/iohelper"
+	"github.com/moby/buildkit/util/pools"
 	digest "github.com/opencontainers/go-digest"
 	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
@@ -68,11 +69,9 @@ type conversion struct {
 	immDiffIDs       map[digest.Digest]struct{} // diffIDs of immutable layers
 }
 
-var bufioPool = sync.Pool{
-	New: func() any {
-		return nil
-	},
-}
+var bufioPool = pools.New(func() *bufio.Writer {
+	return nil
+})
 
 func rewriteTimestampInTarHeader(epoch time.Time) tarconverter.HeaderConverter {
 	return func(hdr *tar.Header) {
@@ -104,7 +103,7 @@ func (c *conversion) convert(ctx context.Context, cs content.Store, desc ocispec
 
 	var bufW *bufio.Writer
 	if pooledW := bufioPool.Get(); pooledW != nil {
-		bufW = pooledW.(*bufio.Writer)
+		bufW = pooledW
 		bufW.Reset(w)
 	} else {
 		bufW = bufio.NewWriterSize(w, 128*1024)

util/overlay/overlay_linux.go

@@ -10,7 +10,6 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
-	"sync"
 	"syscall"
 
 	"github.com/containerd/containerd/v2/core/mount"
@@ -18,6 +17,7 @@ import (
 	"github.com/containerd/continuity/devices"
 	"github.com/containerd/continuity/fs"
 	"github.com/containerd/continuity/sysx"
+	"github.com/moby/buildkit/util/pools"
 	"github.com/pkg/errors"
 	"golang.org/x/sys/unix"
 )
@@ -404,12 +404,10 @@ func compareSymlinkTarget(p1, p2 string) (bool, error) {
 	return t1 == t2, nil
 }
 
-var bufPool = sync.Pool{
-	New: func() any {
-		b := make([]byte, 32*1024)
-		return &b
-	},
-}
+var bufPool = pools.New(func() *[]byte {
+	b := make([]byte, 32*1024)
+	return &b
+})
 
 // Ported from continuity project
 // https://github.com/containerd/continuity/blob/bce1c3f9669b6f3e7f6656ee715b0b4d75fa64a6/fs/path.go#L151
@@ -437,9 +435,9 @@ func compareFileContent(p1, p2 string) (bool, error) {
 		return false, errors.Errorf("%s is not a regular file", p2)
 	}
 
-	b1 := bufPool.Get().(*[]byte)
+	b1 := bufPool.Get()
 	defer bufPool.Put(b1)
-	b2 := bufPool.Get().(*[]byte)
+	b2 := bufPool.Get()
 	defer bufPool.Put(b2)
 	for {
 		n1, err1 := io.ReadFull(f1, *b1)

util/pools/pools.go

@@ -0,0 +1,29 @@
+package pools
+
+import "sync"
+
+// Pool provides a typed wrapper around sync.Pool.
+type Pool[T any] struct {
+	pool sync.Pool
+}
+
+// New returns a typed pool backed by sync.Pool.
+func New[T any](newFn func() T) *Pool[T] {
+	return &Pool[T]{
+		pool: sync.Pool{
+			New: func() any {
+				return newFn()
+			},
+		},
+	}
+}
+
+// Get returns a pooled value.
+func (p *Pool[T]) Get() T {
+	return p.pool.Get().(T)
+}
+
+// Put returns a value to the pool.
+func (p *Pool[T]) Put(v T) {
+	p.pool.Put(v)
+}