feat: bound bulk notification fan-out (#145)

src/main.py

@@ -53,6 +53,8 @@ EVENT_STREAM_TIMEOUT_SECONDS = 5.0
 READINESS_TIMEOUT_SECONDS = 5.0
 REVIEW_DETAIL_TIMEOUT_SECONDS = 5.0
 NOTIFICATION_MUTATION_TIMEOUT_SECONDS = 5.0
+BULK_NOTIFICATION_CONCURRENCY = 5
+BULK_NOTIFICATION_DEADLINE_SECONDS = 6.0
 LIVE_SNAPSHOT_FRESHNESS_SECONDS = 8.0
 FRONTEND_DIR = Path(__file__).resolve().parent.parent / "frontend"
 _live_snapshot_task: asyncio.Task | None = None
@@ -444,13 +446,37 @@ async def _mark_notification_read_result(thread_id: int) -> tuple[int, bool]:
 @app.patch("/api/v1/notifications/read")
 async def read_notifications(batch: NotificationReadBatch) -> JSONResponse:
     thread_ids = list(dict.fromkeys(batch.ids))
-    results = await asyncio.gather(
-        *(_mark_notification_read_result(thread_id) for thread_id in thread_ids)
+
+    semaphore = asyncio.Semaphore(BULK_NOTIFICATION_CONCURRENCY)
+
+    async def mark_within_limit(thread_id: int) -> tuple[int, bool]:
+        async with semaphore:
+            return await _mark_notification_read_result(thread_id)
+
+    tasks = [asyncio.create_task(mark_within_limit(thread_id)) for thread_id in thread_ids]
+    done, pending = await asyncio.wait(
+        tasks, timeout=BULK_NOTIFICATION_DEADLINE_SECONDS
+    )
+    for task in pending:
+        task.cancel()
+    if pending:
+        await asyncio.gather(*pending, return_exceptions=True)
+
+    succeeded_ids = {
+        thread_id
+        for task in done
+        if not task.cancelled() and task.exception() is None
+        for thread_id, succeeded in [task.result()]
+        if succeeded
+    }
+    failed = [thread_id for thread_id in thread_ids if thread_id not in succeeded_ids]
+    return JSONResponse(
+        {
+            "marked": [thread_id for thread_id in thread_ids if thread_id in succeeded_ids],
+            "failed": failed,
+        },
+        headers={"Retry-After": "1"} if failed else None,
     )
-    return JSONResponse({
-        "marked": [thread_id for thread_id, succeeded in results if succeeded],
-        "failed": [thread_id for thread_id, succeeded in results if not succeeded],
-    })
 
 
 @app.patch("/api/v1/notifications/{thread_id}/read")