data: rework StreamTrees to use synchronous callbacks

The tree.Nodes will be replaced by an iterator to loads and serializes
tree node ondemand. Thus, the processing moves from StreamTrees into the
callback. Schedule them onto the workers used by StreamTrees for proper
load distribution.

internal/data/find.go

@@ -6,7 +6,6 @@ import (
 
 	"github.com/restic/restic/internal/restic"
 	"github.com/restic/restic/internal/ui/progress"
-	"golang.org/x/sync/errgroup"
 )
 
 // FindUsedBlobs traverses the tree ID and adds all seen blobs (trees and data
@@ -14,8 +13,7 @@ import (
 func FindUsedBlobs(ctx context.Context, repo restic.Loader, treeIDs restic.IDs, blobs restic.FindBlobSet, p *progress.Counter) error {
 	var lock sync.Mutex
 
-	wg, ctx := errgroup.WithContext(ctx)
-	treeStream := StreamTrees(ctx, wg, repo, treeIDs, func(treeID restic.ID) bool {
+	return StreamTrees(ctx, repo, treeIDs, p, func(treeID restic.ID) bool {
 		// locking is necessary the goroutine below concurrently adds data blobs
 		lock.Lock()
 		h := restic.BlobHandle{ID: treeID, Type: restic.TreeBlob}
@@ -24,26 +22,21 @@ func FindUsedBlobs(ctx context.Context, repo restic.Loader, treeIDs restic.IDs,
 		blobs.Insert(h)
 		lock.Unlock()
 		return blobReferenced
-	}, p)
-
-	wg.Go(func() error {
-		for tree := range treeStream {
-			if tree.Error != nil {
-				return tree.Error
-			}
+	}, func(_ restic.ID, err error, tree *Tree) error {
+		if err != nil {
+			return err
+		}
 
+		for _, node := range tree.Nodes {
 			lock.Lock()
-			for _, node := range tree.Nodes {
-				switch node.Type {
-				case NodeTypeFile:
-					for _, blob := range node.Content {
-						blobs.Insert(restic.BlobHandle{ID: blob, Type: restic.DataBlob})
-					}
+			switch node.Type {
+			case NodeTypeFile:
+				for _, blob := range node.Content {
+					blobs.Insert(restic.BlobHandle{ID: blob, Type: restic.DataBlob})
 				}
 			}
 			lock.Unlock()
 		}
 		return nil
 	})
-	return wg.Wait()
 }