internal/repository/index: Simplify MasterIndex concurrency

internal/repository/index/master_index.go

@@ -459,27 +459,24 @@ func (mi *MasterIndex) Rewrite(ctx context.Context, repo restic.Unpacked[restic.
 		return nil
 	})
 
-	// a worker receives an index from ch, and saves the index
-	worker := func() error {
-		for idx := range saveCh {
-			idx.Finalize()
-			if len(idx.packs) == 0 {
-				continue
-			}
-			if _, err := idx.SaveIndex(wgCtx, repo); err != nil {
-				return err
-			}
-		}
-		return nil
-	}
-
+	var savers errgroup.Group
 	// encoding an index can take quite some time such that this can be CPU- or IO-bound
 	// do not add repo.Connections() here as there are already the loader goroutines.
-	workerCount := runtime.GOMAXPROCS(0)
-	// run workers on ch
-	for i := 0; i < workerCount; i++ {
-		wg.Go(worker)
+	savers.SetLimit(runtime.GOMAXPROCS(0))
+
+	for idx := range saveCh {
+		savers.Go(func() error {
+			idx.Finalize()
+			if len(idx.packs) == 0 {
+				return nil
+			}
+			_, err := idx.SaveIndex(wgCtx, repo)
+			return err
+		})
 	}
+
+	wg.Go(savers.Wait)
+
 	err := wg.Wait()
 	p.Done()
 	if err != nil {
@@ -514,6 +511,8 @@ func (mi *MasterIndex) SaveFallback(ctx context.Context, repo restic.SaverRemove
 
 	obsolete := restic.NewIDSet()
 	wg, wgCtx := errgroup.WithContext(ctx)
+	// keep concurrency bounded as we're on a fallback path
+	wg.SetLimit(1 + int(repo.Connections()))
 
 	ch := make(chan *Index)
 	wg.Go(func() error {
@@ -553,23 +552,14 @@ func (mi *MasterIndex) SaveFallback(ctx context.Context, repo restic.SaverRemove
 		return nil
 	})
 
-	// a worker receives an index from ch, and saves the index
-	worker := func() error {
-		for idx := range ch {
+	for idx := range ch {
+		wg.Go(func() error {
 			idx.Finalize()
-			if _, err := idx.SaveIndex(wgCtx, repo); err != nil {
-				return err
-			}
-		}
-		return nil
+			_, err := idx.SaveIndex(wgCtx, repo)
+			return err
+		})
 	}
 
-	// keep concurrency bounded as we're on a fallback path
-	workerCount := int(repo.Connections())
-	// run workers on ch
-	for i := 0; i < workerCount; i++ {
-		wg.Go(worker)
-	}
 	err := wg.Wait()
 	p.Done()
 	// the index no longer matches to stored state