Fix concurrent downloading

src/app/services/downloader.py

@@ -81,7 +81,15 @@ async def download_file_to_file(link: str, output: BytesIO) -> bool:
 
 
 @broker.task()
-async def download(task_id: str, book_id: int, file_type: str) -> str | None:
+async def download(
+    task_id: str, book_id: int, file_type: str, prev_task_id: str | None = None
+) -> str | None:
+    if prev_task_id:
+        prev_task = AsyncTaskiqTask(prev_task_id, result_backend)
+
+        while not (await prev_task.is_ready()):
+            await asyncio.sleep(0.1)
+
     try:
         with tempfile.SpooledTemporaryFile() as temp_file:
             data = await _download_to_tmpfile(book_id, file_type, temp_file)

src/app/services/task_creator.py

@@ -60,11 +60,16 @@ class TaskCreator:
 
         task_ids: list[str] = []
 
+        prev_task_id = None
+
         for book in books:
             if file_format not in book.available_types:
                 continue
 
-            task = await download.kiq(str(task_id), book.id, file_format)
+            task = await download.kiq(
+                str(task_id), book.id, file_format, prev_task_id=prev_task_id
+            )
+            prev_task_id = task.task_id
             task_ids.append(task.task_id)
 
         if len(task_ids) == 0: