restic: parallelize lock file loading

internal/restic/lock.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/restic/restic/internal/errors"
+	"golang.org/x/sync/errgroup"
 
 	"github.com/restic/restic/internal/debug"
 )
@@ -138,12 +139,7 @@ func (l *Lock) fillUserInfo() error {
 // non-exclusive lock is to be created, an error is only returned when an
 // exclusive lock is found.
 func (l *Lock) checkForOtherLocks(ctx context.Context) error {
-	return l.repo.List(ctx, LockFile, func(id ID, size int64) error {
-		if l.lockID != nil && id.Equal(*l.lockID) {
-			return nil
-		}
-
-		lock, err := LoadLock(ctx, l.repo, id)
+	return ForAllLocks(ctx, l.repo, l.lockID, func(id ID, lock *Lock, err error) error {
 		if err != nil {
 			// ignore locks that cannot be loaded
 			debug.Log("ignore lock %v: %v", id, err)
@@ -275,8 +271,7 @@ func LoadLock(ctx context.Context, repo Repository, id ID) (*Lock, error) {
 
 // RemoveStaleLocks deletes all locks detected as stale from the repository.
 func RemoveStaleLocks(ctx context.Context, repo Repository) error {
-	return repo.List(ctx, LockFile, func(id ID, size int64) error {
-		lock, err := LoadLock(ctx, repo, id)
+	return ForAllLocks(ctx, repo, nil, func(id ID, lock *Lock, err error) error {
 		if err != nil {
 			// ignore locks that cannot be loaded
 			debug.Log("ignore lock %v: %v", id, err)
@@ -297,3 +292,59 @@ func RemoveAllLocks(ctx context.Context, repo Repository) error {
 		return repo.Backend().Remove(ctx, Handle{Type: LockFile, Name: id.String()})
 	})
 }
+
+const loadLockParallelism = 5
+
+// ForAllLocks reads all locks 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.
+// If a lock ID is passed via excludeID, it will be ignored.
+func ForAllLocks(ctx context.Context, repo Repository, excludeID *ID, fn func(ID, *Lock, error) error) error {
+	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 ID)
+
+	// send list of lock files through ch, which is closed afterwards
+	wg.Go(func() error {
+		defer close(ch)
+		return repo.List(ctx, LockFile, func(id ID, size int64) error {
+			if excludeID != nil && id.Equal(*excludeID) {
+				return nil
+			}
+
+			select {
+			case <-ctx.Done():
+				return nil
+			case ch <- id:
+			}
+			return nil
+		})
+	})
+
+	// a worker receives an snapshot ID from ch, loads the snapshot
+	// and runs fn with id, the snapshot and the error
+	worker := func() error {
+		for id := range ch {
+			debug.Log("load lock %v", id)
+			lock, err := LoadLock(ctx, repo, id)
+
+			m.Lock()
+			err = fn(id, lock, err)
+			m.Unlock()
+			if err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+
+	for i := 0; i < loadLockParallelism; i++ {
+		wg.Go(worker)
+	}
+
+	return wg.Wait()
+}