chunker: fix panic listing a directory after an interrupted upload with meta_format none

With meta_format = none, listing a directory which contained the
temporary chunks of an interrupted upload, but no completed chunks for
that file, panicked with "invalid chunked object".

The listing made a placeholder object for the file on seeing a
temporary chunk, which then had no chunks in it when it was validated.
Only make the placeholder on seeing a data chunk, so files which have
nothing but temporary chunks are ignored as they are when metadata is
in use.

backend/chunker/chunker.go

@@ -801,9 +801,12 @@ func (f *Fs) processEntries(ctx context.Context, origEntries fs.DirEntries, dirP
 				fs.Debugf(f, "skip orphan data chunk %q", remote)
 				break
 			}
-			if mainObject == nil && !f.useMeta {
+			if mainObject == nil && !f.useMeta && !isSpecial {
 				// this is the "nometa" case
 				// create dummy chunked object without metadata
+				//
+				// not for temporary or control chunks as they may be all
+				// that is left of an interrupted upload
 				mainObject = f.newObject(mainRemote, nil, nil)
 				byRemote[mainRemote] = mainObject
 				if !badEntry[mainRemote] {

backend/chunker/chunker_internal_test.go

@@ -627,6 +627,22 @@ func testChunkNumberOverflow(t *testing.T, f *Fs) {
 	_ = file.Remove(ctx)
 }
 
+// testOrphanTempChunks checks that temporary chunks left behind by an
+// interrupted upload are not listed and don't break the listing
+func testOrphanTempChunks(t *testing.T, f *Fs) {
+	const dir = "orphan"
+	ctx := context.Background()
+	defer func() {
+		_ = operations.Purge(ctx, f.base, dir)
+	}()
+
+	modTime := fstest.Time("2001-02-03T04:05:06.499999999Z")
+	item := fstest.Item{Path: f.makeChunkName(path.Join(dir, "interrupted"), 0, "", "abc123"), ModTime: modTime}
+	fstests.PutTestContents(ctx, t, f.base, &item, "data", true)
+
+	fstest.CheckListingWithRoot(t, f, dir, nil, nil, f.Precision())
+}
+
 func testMetadataInput(t *testing.T, f *Fs) {
 	const minChunkForTest = 50
 	if f.opt.ChunkSize < minChunkForTest {
@@ -928,6 +944,9 @@ func (f *Fs) InternalTest(t *testing.T) {
 	t.Run("BackwardsCompatibility", func(t *testing.T) {
 		testBackwardsCompatibility(t, f)
 	})
+	t.Run("OrphanTempChunks", func(t *testing.T) {
+		testOrphanTempChunks(t, f)
+	})
 	t.Run("ChunkerServerSideMove", func(t *testing.T) {
 		testChunkerServerSideMove(t, f)
 	})