checker: Optimize checker.Packs()

Use result of single repository.List() to find both missing and
orphaned data packs. For 500GB repository this eliminates ~100K
repository.Test() calls and improves check time by >30M in my
environment (~45min before this change and ~7min after).

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>

changelog/0.8.2/issue-1541

@@ -0,0 +1,6 @@
+Enhancement: Reduce number of remote requests during repository check
+
+This change eliminates redundant remote repository calls and significantly
+improves repository check time.
+
+https://github.com/restic/restic/issues/1541

internal/checker/checker.go

@@ -31,7 +31,6 @@ type Checker struct {
 		M map[restic.ID]uint
 	}
 	indexes map[restic.ID]*repository.Index
-	orphanedPacks restic.IDs
 
 	masterIndex *repository.MasterIndex
 
@@ -183,38 +182,6 @@ func (e PackError) Error() string {
 	return "pack " + e.ID.String() + ": " + e.Err.Error()
 }
 
-func packIDTester(ctx context.Context, repo restic.Repository, inChan <-chan restic.ID, errChan chan<- error, wg *sync.WaitGroup) {
-	debug.Log("worker start")
-	defer debug.Log("worker done")
-
-	defer wg.Done()
-
-	for id := range inChan {
-		h := restic.Handle{Type: restic.DataFile, Name: id.String()}
-		ok, err := repo.Backend().Test(ctx, h)
-		if err != nil {
-			err = PackError{ID: id, Err: err}
-		} else {
-			if !ok {
-				err = PackError{ID: id, Err: errors.New("does not exist")}
-			}
-		}
-
-		if err != nil {
-			debug.Log("error checking for pack %s: %v", id.Str(), err)
-			select {
-			case <-ctx.Done():
-				return
-			case errChan <- err:
-			}
-
-			continue
-		}
-
-		debug.Log("pack %s exists", id.Str())
-	}
-}
-
 // Packs checks that all packs referenced in the index are still available and
 // there are no packs that aren't in an index. errChan is closed after all
 // packs have been checked.
@@ -222,35 +189,33 @@ func (c *Checker) Packs(ctx context.Context, errChan chan<- error) {
 	defer close(errChan)
 
 	debug.Log("checking for %d packs", len(c.packs))
-	seenPacks := restic.NewIDSet()
-
-	var workerWG sync.WaitGroup
-
-	IDChan := make(chan restic.ID)
-	for i := 0; i < defaultParallelism; i++ {
-		workerWG.Add(1)
-		go packIDTester(ctx, c.repo, IDChan, errChan, &workerWG)
-	}
-
-	for id := range c.packs {
-		seenPacks.Insert(id)
-		IDChan <- id
-	}
-	close(IDChan)
-
-	debug.Log("waiting for %d workers to terminate", defaultParallelism)
-	workerWG.Wait()
-	debug.Log("workers terminated")
 
+	debug.Log("listing repository packs")
+	repoPacks := restic.NewIDSet()
 	for id := range c.repo.List(ctx, restic.DataFile) {
-		debug.Log("check data blob %v", id.Str())
-		if !seenPacks.Has(id) {
-			c.orphanedPacks = append(c.orphanedPacks, id)
 		select {
 		case <-ctx.Done():
 			return
-			case errChan <- PackError{ID: id, Orphaned: true, Err: errors.New("not referenced in any index")}:
+		default:
 		}
+		repoPacks.Insert(id)
+	}
+
+	// orphaned: present in the repo but not in c.packs
+	for orphanID := range repoPacks.Sub(c.packs) {
+		select {
+		case <-ctx.Done():
+			return
+		case errChan <- PackError{ID: orphanID, Orphaned: true, Err: errors.New("not referenced in any index")}:
+		}
+	}
+
+	// missing: present in c.packs but not in the repo
+	for missingID := range c.packs.Sub(repoPacks) {
+		select {
+		case <-ctx.Done():
+			return
+		case errChan <- PackError{ID: missingID, Err: errors.New("does not exist")}:
 		}
 	}
 }