fix(tdler): only apply EOF guard when size is known

Photos pass size 0 (unknown, not available in InputPhotoFileLocation);
the guard then answered the first request at offset 0 with an empty
chunk, silently saving every photo as a 0-byte file.

Fixes #238

common/tdler/dler.go

@@ -28,7 +28,7 @@ type eofAwareClient struct {
 }
 
 func (c eofAwareClient) UploadGetFile(ctx context.Context, req *tg.UploadGetFileRequest) (tg.UploadFileClass, error) {
-	if req.Offset >= c.size {
+	if c.size > 0 && req.Offset >= c.size {
 		return &tg.UploadFile{}, nil
 	}
 	return c.Client.UploadGetFile(ctx, req)

common/tdler/dler_test.go

@@ -110,3 +110,31 @@ func TestDownloadServerLikeEOF(t *testing.T) {
 		})
 	}
 }
+
+// Photos have no size in InputPhotoFileLocation, so TGFile.Size() is 0
+// ("unknown"). The EOF guard must not fire on the very first request at
+// offset 0, or the download silently yields a 0-byte file.
+func TestDownloadPhotoUnknownSize(t *testing.T) {
+	data := make([]byte, 89708)
+	for i := range data {
+		data[i] = byte(i % 251)
+	}
+	client := &serverLikeClient{data: data}
+	file := tfile.NewTGFile(
+		&tg.InputPhotoFileLocation{ID: 1, AccessHash: 2},
+		client, 0, "photo.png",
+	)
+
+	dl := NewDownloader(file)
+	buf := make([]byte, len(data))
+	_, err := dl.WithThreads(1).Parallel(context.Background(), &memWriterAt{b: buf})
+	if err != nil {
+		t.Fatalf("download failed: %v", err)
+	}
+	if !bytes.Equal(buf, data) {
+		t.Fatalf("downloaded %d bytes, want %d matching bytes", len(buf), len(data))
+	}
+	if client.maxOffset >= int64(len(data)) {
+		t.Fatalf("requested offset %d at or past EOF (size %d)", client.maxOffset, len(data))
+	}
+}