check: extract parallel index loading

internal/repository/index_parallel.go

@@ -0,0 +1,77 @@
+package repository
+
+import (
+	"context"
+	"sync"
+
+	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/restic"
+	"golang.org/x/sync/errgroup"
+)
+
+const loadIndexParallelism = 5
+
+// ForAllIndexes loads all index files in parallel and calls the given callback.
+// It is guaranteed that the function is not run concurrently. If the callback
+// returns an error, this function is cancelled and also returns that error.
+func ForAllIndexes(ctx context.Context, repo restic.Repository,
+	fn func(id restic.ID, index *Index, oldFormat bool, err error) error) error {
+
+	debug.Log("Start")
+
+	type FileInfo struct {
+		restic.ID
+		Size int64
+	}
+
+	var m sync.Mutex
+
+	// 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 FileInfo)
+	// send list of index files through ch, which is closed afterwards
+	wg.Go(func() error {
+		defer close(ch)
+		return repo.List(ctx, restic.IndexFile, func(id restic.ID, size int64) error {
+			select {
+			case <-ctx.Done():
+				return nil
+			case ch <- FileInfo{id, size}:
+			}
+			return nil
+		})
+	})
+
+	// a worker receives an index ID from ch, loads the index, and sends it to indexCh
+	worker := func() error {
+		var buf []byte
+		for fi := range ch {
+			debug.Log("worker got file %v", fi.ID.Str())
+			var err error
+			var idx *Index
+			oldFormat := false
+
+			buf, err = repo.LoadAndDecrypt(ctx, buf[:0], restic.IndexFile, fi.ID)
+			if err == nil {
+				idx, oldFormat, err = DecodeIndex(buf, fi.ID)
+			}
+
+			m.Lock()
+			err = fn(fi.ID, idx, oldFormat, err)
+			m.Unlock()
+			if err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+
+	// run workers on ch
+	wg.Go(func() error {
+		return RunWorkers(loadIndexParallelism, worker)
+	})
+
+	return wg.Wait()
+}