backend/local: skip entries removed concurrently with List() (#5297)

This change fixes the bug described below:
if a file is removed while the local backend List() runs,
the call will flag an accounting error.
The bug manifests itself if local backend is the Sync target
due to intrinsic concurrency.
The odds to hit this bug depend on --checkers and --transfers.
Chunker over local backend is affected even more because
updating a composite object with a smaller size content
translates into removing chunks on the underlying file system
and involves a number of List() calls.

backend/local/local.go

@@ -467,6 +467,10 @@ func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err e
 				for _, name := range names {
 					namepath := filepath.Join(fsDirPath, name)
 					fi, fierr := os.Lstat(namepath)
+					if os.IsNotExist(fierr) {
+						// skip entry removed by a concurrent goroutine
+						continue
+					}
 					if fierr != nil {
 						err = errors.Wrapf(err, "failed to read directory %q", namepath)
 						fs.Errorf(dir, "%v", fierr)

fs/sync/sync.go

@@ -624,9 +624,7 @@ func (s *syncCopyMove) srcParentDirCheck(entry fs.DirEntry) {
 	if parentDir == "." {
 		parentDir = ""
 	}
-	if _, ok := s.srcEmptyDirs[parentDir]; ok {
-		delete(s.srcEmptyDirs, parentDir)
-	}
+	delete(s.srcEmptyDirs, parentDir)
 }
 
 // parseTrackRenamesStrategy turns a config string into a trackRenamesStrategy

fs/sync/sync_test.go

@@ -1973,3 +1973,54 @@ func TestMaxTransfer(t *testing.T) {
 	t.Run("Soft", func(t *testing.T) { test(t, fs.CutoffModeSoft) })
 	t.Run("Cautious", func(t *testing.T) { test(t, fs.CutoffModeCautious) })
 }
+
+func testSyncConcurrent(t *testing.T, subtest string) {
+	const (
+		NFILES     = 20
+		NCHECKERS  = 4
+		NTRANSFERS = 4
+	)
+
+	ctx, ci := fs.AddConfig(context.Background())
+	ci.Checkers = NCHECKERS
+	ci.Transfers = NTRANSFERS
+
+	r := fstest.NewRun(t)
+	defer r.Finalise()
+	stats := accounting.GlobalStats()
+
+	itemsBefore := []fstest.Item{}
+	itemsAfter := []fstest.Item{}
+	for i := 0; i < NFILES; i++ {
+		nameBoth := fmt.Sprintf("both%d", i)
+		nameOnly := fmt.Sprintf("only%d", i)
+		switch subtest {
+		case "delete":
+			fileBoth := r.WriteBoth(ctx, nameBoth, "potato", t1)
+			fileOnly := r.WriteObject(ctx, nameOnly, "potato", t1)
+			itemsBefore = append(itemsBefore, fileBoth, fileOnly)
+			itemsAfter = append(itemsAfter, fileBoth)
+		case "truncate":
+			fileBoth := r.WriteBoth(ctx, nameBoth, "potato", t1)
+			fileFull := r.WriteObject(ctx, nameOnly, "potato", t1)
+			fileEmpty := r.WriteFile(nameOnly, "", t1)
+			itemsBefore = append(itemsBefore, fileBoth, fileFull)
+			itemsAfter = append(itemsAfter, fileBoth, fileEmpty)
+		}
+	}
+
+	fstest.CheckItems(t, r.Fremote, itemsBefore...)
+	stats.ResetErrors()
+	err := Sync(ctx, r.Fremote, r.Flocal, false)
+	assert.NoError(t, err, "Sync must not return a error")
+	assert.False(t, stats.Errored(), "Low level errors must not have happened")
+	fstest.CheckItems(t, r.Fremote, itemsAfter...)
+}
+
+func TestSyncConcurrentDelete(t *testing.T) {
+	testSyncConcurrent(t, "delete")
+}
+
+func TestSyncConcurrentTruncate(t *testing.T) {
+	testSyncConcurrent(t, "truncate")
+}