check: Use PackBlobIterator instead of StreamPack

To only stream the content of a pack file once, check used StreamPack
with a custom pack load function. This combination was always brittle
and complicates using StreamPack everywhere else. Now that StreamPack
internally uses PackBlobIterator use that primitive instead, which is a
much better fit for what the check command requires.

internal/checker/checker.go

@@ -10,6 +10,7 @@ import (
 	"sort"
 	"sync"
 
+	"github.com/klauspost/compress/zstd"
 	"github.com/minio/sha256-simd"
 	"github.com/restic/restic/internal/backend"
 	"github.com/restic/restic/internal/backend/s3"
@@ -526,7 +527,7 @@ func (c *Checker) GetPacks() map[restic.ID]int64 {
 }
 
 // checkPack reads a pack and checks the integrity of all blobs.
-func checkPack(ctx context.Context, r restic.Repository, id restic.ID, blobs []restic.Blob, size int64, bufRd *bufio.Reader) error {
+func checkPack(ctx context.Context, r restic.Repository, id restic.ID, blobs []restic.Blob, size int64, bufRd *bufio.Reader, dec *zstd.Decoder) error {
 	debug.Log("checking pack %v", id.String())
 
 	if len(blobs) == 0 {
@@ -557,49 +558,44 @@ func checkPack(ctx context.Context, r restic.Repository, id restic.ID, blobs []r
 	// calculate hash on-the-fly while reading the pack and capture pack header
 	var hash restic.ID
 	var hdrBuf []byte
-	hashingLoader := func(ctx context.Context, h backend.Handle, length int, offset int64, fn func(rd io.Reader) error) error {
-		return r.Backend().Load(ctx, h, int(size), 0, func(rd io.Reader) error {
-			hrd := hashing.NewReader(rd, sha256.New())
-			bufRd.Reset(hrd)
+	h := backend.Handle{Type: backend.PackFile, Name: id.String()}
+	err := r.Backend().Load(ctx, h, int(size), 0, func(rd io.Reader) error {
+		hrd := hashing.NewReader(rd, sha256.New())
+		bufRd.Reset(hrd)
 
-			// skip to start of first blob, offset == 0 for correct pack files
-			_, err := bufRd.Discard(int(offset))
-			if err != nil {
+		it := repository.NewPackBlobIterator(id, bufRd, 0, blobs, r.Key(), dec)
+		for {
+			val, err := it.Next()
+			if err == repository.ErrPackEOF {
+				break
+			} else if err != nil {
 				return err
 			}
-
-			err = fn(bufRd)
-			if err != nil {
-				return err
+			debug.Log("  check blob %v: %v", val.Handle.ID, val.Handle)
+			if val.Err != nil {
+				debug.Log("  error verifying blob %v: %v", val.Handle.ID, err)
+				errs = append(errs, errors.Errorf("blob %v: %v", val.Handle.ID, err))
 			}
-
-			// skip enough bytes until we reach the possible header start
-			curPos := length + int(offset)
-			minHdrStart := int(size) - pack.MaxHeaderSize
-			if minHdrStart > curPos {
-				_, err := bufRd.Discard(minHdrStart - curPos)
-				if err != nil {
-					return err
-				}
-			}
-
-			// read remainder, which should be the pack header
-			hdrBuf, err = io.ReadAll(bufRd)
-			if err != nil {
-				return err
-			}
-
-			hash = restic.IDFromHash(hrd.Sum(nil))
-			return nil
-		})
-	}
-
-	err := repository.StreamPack(ctx, hashingLoader, r.Key(), id, blobs, func(blob restic.BlobHandle, buf []byte, err error) error {
-		debug.Log("  check blob %v: %v", blob.ID, blob)
-		if err != nil {
-			debug.Log("  error verifying blob %v: %v", blob.ID, err)
-			errs = append(errs, errors.Errorf("blob %v: %v", blob.ID, err))
 		}
+
+		// skip enough bytes until we reach the possible header start
+		curPos := lastBlobEnd
+		minHdrStart := int(size) - pack.MaxHeaderSize
+		if minHdrStart > curPos {
+			_, err := bufRd.Discard(minHdrStart - curPos)
+			if err != nil {
+				return err
+			}
+		}
+
+		// read remainder, which should be the pack header
+		var err error
+		hdrBuf, err = io.ReadAll(bufRd)
+		if err != nil {
+			return err
+		}
+
+		hash = restic.IDFromHash(hrd.Sum(nil))
 		return nil
 	})
 	if err != nil {
@@ -670,6 +666,11 @@ func (c *Checker) ReadPacks(ctx context.Context, packs map[restic.ID]int64, p *p
 			// create a buffer that is large enough to be reused by repository.StreamPack
 			// this ensures that we can read the pack header later on
 			bufRd := bufio.NewReaderSize(nil, repository.MaxStreamBufferSize)
+			dec, err := zstd.NewReader(nil)
+			if err != nil {
+				panic(dec)
+			}
+			defer dec.Close()
 			for {
 				var ps checkTask
 				var ok bool
@@ -683,7 +684,7 @@ func (c *Checker) ReadPacks(ctx context.Context, packs map[restic.ID]int64, p *p
 					}
 				}
 
-				err := checkPack(ctx, c.repo, ps.id, ps.blobs, ps.size, bufRd)
+				err := checkPack(ctx, c.repo, ps.id, ps.blobs, ps.size, bufRd, dec)
 				p.Add(1)
 				if err == nil {
 					continue