fix(watcher): pick an unfinished backlog back up without restarting the bot

backend/src/watcher/registry.py

@@ -36,6 +36,7 @@ class RunningBot:
     client: PyroClient
     account_id: int
     tasks: list[asyncio.Task]
+    backlog: asyncio.Task | None = None
 
 
 async def _cancel(task: asyncio.Task) -> None:
@@ -105,6 +106,8 @@ class BotRegistry:
                 await self._stop(bot_id)
             for bot_id in enabled - set(self._running):
                 await self._start(bot_id)
+            for bot_id in enabled & set(self._running):
+                await self._resume_backlog(bot_id)
             for bot_id in await bots.removed_bot_ids(self._pool):
                 tokens.drop(bot_id)
 
@@ -177,14 +180,24 @@ class BotRegistry:
         await accounts.notify_accounts_changed(self._pool)
 
         consumer = JobConsumer(client, self._pool, account_id)
-        tasks = [asyncio.create_task(consumer.run())]
-        if await bots.needs_backfill(self._pool, bot_id):
-            tasks.append(asyncio.create_task(self._backlog(client, bot_id, account_id)))
-        self._running[bot_id] = RunningBot(client, account_id, tasks)
+        self._running[bot_id] = RunningBot(
+            client, account_id, [asyncio.create_task(consumer.run())]
+        )
+        await self._resume_backlog(bot_id)
         logger.info(
             f"[green]Bot account {account_id} watched:[/] @{me.username} ({bot_id})"
         )
 
+    async def _resume_backlog(self, bot_id: int) -> None:
+        running = self._running[bot_id]
+        if running.backlog is not None and not running.backlog.done():
+            return
+        if not await bots.needs_backfill(self._pool, bot_id):
+            return
+        running.backlog = asyncio.create_task(
+            self._backlog(running.client, bot_id, running.account_id)
+        )
+
     async def _backlog(self, client: PyroClient, bot_id: int, account_id: int) -> None:
         async def save(at: DrainCursor) -> None:
             await bots.save_backlog_cursor(self._pool, bot_id, at.pts, at.qts, at.date)
@@ -217,8 +230,9 @@ class BotRegistry:
         running = self._running.pop(bot_id, None)
         if running is None:
             return
-        for task in running.tasks:
-            await _cancel(task)
+        for task in (*running.tasks, running.backlog):
+            if task is not None:
+                await _cancel(task)
         with contextlib.suppress(Exception):
             await running.client.stop()
         logger.info(f"[yellow]Bot unwatched:[/] {bot_id}")