Parallelize MasterIndex.Save()

internal/repository/master_index.go

@@ -7,6 +7,7 @@ import (
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/restic"
 	"github.com/restic/restic/internal/ui/progress"
+	"golang.org/x/sync/errgroup"
 )
 
 // MasterIndex is a collection of indexes and IDs of chunks that are in the process of being saved.
@@ -261,10 +262,12 @@ func (mi *MasterIndex) MergeFinalIndexes() {
 	mi.idx = newIdx
 }
 
+const saveIndexParallelism = 4
+
 // Save saves all known indexes to index files, leaving out any
-// packs whose ID is contained in packBlacklist. The new index contains the IDs
-// of all known indexes in the "supersedes" field. The IDs are also returned in
-// the IDSet obsolete
+// packs whose ID is contained in packBlacklist from finalized indexes.
+// The new index contains the IDs of all known indexes in the "supersedes"
+// field. The IDs are also returned in the IDSet obsolete.
 // After calling this function, you should remove the obsolete index files.
 func (mi *MasterIndex) Save(ctx context.Context, repo restic.Repository, packBlacklist restic.IDSet, extraObsolete restic.IDs, p *progress.Counter) (obsolete restic.IDSet, err error) {
 	mi.idxMutex.Lock()
@@ -275,19 +278,27 @@ func (mi *MasterIndex) Save(ctx context.Context, repo restic.Repository, packBla
 	newIndex := NewIndex()
 	obsolete = restic.NewIDSet()
 
+	// track spawned goroutines using wg, create a new context which is
+	// cancelled as soon as an error occurs.
+	wg, ctx := errgroup.WithContext(ctx)
+
+	ch := make(chan *Index)
+
+	wg.Go(func() error {
+		defer close(ch)
 		for i, idx := range mi.idx {
 			if idx.Final() {
 				ids, err := idx.IDs()
 				if err != nil {
 					debug.Log("index %d does not have an ID: %v", err)
-				return nil, err
+					return err
 				}
 
 				debug.Log("adding index ids %v to supersedes field", ids)
 
 				err = newIndex.AddToSupersedes(ids...)
 				if err != nil {
-				return nil, err
+					return err
 				}
 				obsolete.Merge(restic.NewIDSet(ids...))
 			} else {
@@ -300,9 +311,10 @@ func (mi *MasterIndex) Save(ctx context.Context, repo restic.Repository, packBla
 				newIndex.StorePack(pbs.packID, pbs.blobs)
 				p.Add(1)
 				if IndexFull(newIndex) {
-				newIndex.Finalize()
-				if _, err := SaveIndex(ctx, repo, newIndex); err != nil {
-					return nil, err
+					select {
+					case ch <- newIndex:
+					case <-ctx.Done():
+						return nil
 					}
 					newIndex = NewIndex()
 				}
@@ -311,14 +323,34 @@ func (mi *MasterIndex) Save(ctx context.Context, repo restic.Repository, packBla
 
 		err = newIndex.AddToSupersedes(extraObsolete...)
 		if err != nil {
-		return nil, err
+			return err
 		}
 		obsolete.Merge(restic.NewIDSet(extraObsolete...))
 
-	newIndex.Finalize()
-	if _, err := SaveIndex(ctx, repo, newIndex); err != nil {
-		return nil, err
+		select {
+		case ch <- newIndex:
+		case <-ctx.Done():
+		}
+		return nil
+	})
+
+	// a worker receives an index from ch, and saves the index
+	worker := func() error {
+		for idx := range ch {
+			idx.Finalize()
+			if _, err := SaveIndex(ctx, repo, idx); err != nil {
+				return err
+			}
+		}
+		return nil
 	}
 
-	return
+	// run workers on ch
+	wg.Go(func() error {
+		return RunWorkers(saveIndexParallelism, worker)
+	})
+
+	err = wg.Wait()
+
+	return obsolete, err
 }