fix: universe build을 isolated thread로 분리하여 API 먹통 방지

BackgroundTask가 FastAPI event loop을 공유해서 deadlock 발생 →
별도 thread에서 새 asyncio event loop으로 실행하도록 변경.
- API event loop 완전 분리
- DB 커넥션 풀 독립적 사용 (per-batch factory session)
- 빌드 중 API 정상 응답 유지

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/api/v1/endpoints/universe.py

@@ -181,14 +181,25 @@ async def discover_tickers(
 async def _run_build_snapshots(
     tickers, start_date: str, end_date: str, force_rebuild: bool
 ):
-    """Background task wrapper for build_snapshots (needs its own DB session)."""
+    """Background task wrapper for build_snapshots.
+
+    Spawns a completely separate thread with its own asyncio event loop so
+    the long-running build never touches the API's event loop or DB connection
+    pool, keeping the API responsive throughout.
+    """
+    import asyncio as _asyncio
+    import threading
     from app.core.database import AsyncSessionLocal
 
-    async with AsyncSessionLocal() as db:
+    def _thread_main():
+        loop = _asyncio.new_event_loop()
+        _asyncio.set_event_loop(loop)
+
+        async def _build():
             svc = UniverseService()
             try:
                 result = await svc.build_snapshots(
-                db,
+                    AsyncSessionLocal,
                     tickers=tickers,
                     start_date=start_date,
                     end_date=end_date,
@@ -198,6 +209,15 @@ async def _run_build_snapshots(
             except Exception as e:
                 logger.error(f"Universe background build failed: {e}")
 
+        try:
+            loop.run_until_complete(_build())
+        finally:
+            loop.close()
+
+    t = threading.Thread(target=_thread_main, daemon=True, name="universe-build")
+    t.start()
+    logger.info(f"Universe build started in isolated thread (daemon)")
+
 
 @router.post(
     "/admin/build-snapshots",
@@ -231,7 +251,7 @@ async def build_snapshots(
         svc = UniverseService()
         try:
             result = await svc.build_snapshots(
-                db,
+                db,   # existing session is fine for small quick jobs
                 tickers=tickers,
                 start_date=body.start_date,
                 end_date=body.end_date,

app/services/universe_service.py

@@ -234,7 +234,7 @@ class UniverseService:
 
     async def build_snapshots(
         self,
-        db: AsyncSession,
+        db_or_factory,
         tickers: Optional[List[str]],
         start_date: str,
         end_date: str,
@@ -243,15 +243,33 @@ class UniverseService:
         """
         Build monthly market_cap snapshots for given tickers (or all registry).
 
-        For each ticker:
+        db_or_factory: Either an AsyncSession (for small sync calls ≤20 tickers)
-          1. Fetch shares_outstanding history from SEC EDGAR companyfacts
+                       or an AsyncSessionLocal factory (for large background jobs).
-          2. Fetch monthly close prices via yfinance bulk download
+                       Using a factory means each batch opens/closes its own DB
-          3. Compute market_cap = close × shares per month
+                       connection, preventing the long-running job from monopolising
-          4. Upsert into universe_snapshot
+                       the connection pool and blocking the API.
 
         Returns: { tickers_processed, tickers_failed, snapshots_created }
         """
-        # Resolve ticker list
+        from app.core.database import AsyncSessionLocal
+
+        # Normalise: wrap a plain session in a no-op factory so the rest of the
+        # code can always call  `async with _db_ctx() as db:`
+        from contextlib import asynccontextmanager
+
+        if callable(db_or_factory) and not isinstance(db_or_factory, AsyncSession):
+            # It's a factory (e.g. AsyncSessionLocal)
+            _db_ctx = db_or_factory
+        else:
+            # It's an existing session — wrap it so we never close it here
+            _session = db_or_factory
+
+            @asynccontextmanager
+            async def _db_ctx():
+                yield _session
+
+        # ---- Resolve ticker list ----
+        async with _db_ctx() as db:
             if tickers:
                 ticker_list = [t.upper() for t in tickers]
             else:
@@ -272,36 +290,43 @@ class UniverseService:
             )
 
             if force_rebuild:
+                # Delete in chunks to avoid huge IN clauses
+                for i in range(0, len(ticker_list), _CHUNK):
+                    chunk = ticker_list[i:i + _CHUNK]
                     await db.execute(
                         UniverseSnapshot.__table__.delete().where(
-                    UniverseSnapshot.ticker.in_(ticker_list)
+                            UniverseSnapshot.ticker.in_(chunk)
                         )
                     )
-            await db.flush()
+                await db.commit()
 
             # Load registry metadata (sector/industry/exchange/CIK)
+            registry_map: Dict[str, UniverseTickerRegistry] = {}
+            for i in range(0, len(ticker_list), _CHUNK):
+                chunk = ticker_list[i:i + _CHUNK]
                 reg_result = await db.execute(
                     select(UniverseTickerRegistry).where(
-                UniverseTickerRegistry.ticker.in_(ticker_list)
+                        UniverseTickerRegistry.ticker.in_(chunk)
                     )
                 )
-        registry_map: Dict[str, UniverseTickerRegistry] = {
+                for r in reg_result.scalars().all():
-            r.ticker: r for r in reg_result.scalars().all()
+                    registry_map[r.ticker] = r
-        }
 
         total_snapshots = 0
         total_failed = 0
+        total_batches = (len(ticker_list) + _PRICE_BATCH - 1) // _PRICE_BATCH
 
-        # Process in batches of _PRICE_BATCH for yfinance bulk download
+        # ---- Process in batches — each batch uses its own DB session ----
         for batch_start in range(0, len(ticker_list), _PRICE_BATCH):
             batch = ticker_list[batch_start:batch_start + _PRICE_BATCH]
+            batch_num = batch_start // _PRICE_BATCH + 1
 
-            # ---- Fetch monthly prices (synchronous, in thread) ----
+            # Fetch monthly prices (synchronous yfinance, runs in thread)
             price_data = await asyncio.to_thread(
                 self._fetch_bulk_monthly_prices, batch, start_date, end_date
             )
 
-            # ---- Fetch shares_outstanding from SEC EDGAR (concurrent) ----
+            # Fetch shares_outstanding from SEC EDGAR (async, rate-limited)
             sem = asyncio.Semaphore(_SEC_CONCURRENCY)
 
             async def _fetch_one(tkr: str) -> Tuple[str, List]:
@@ -321,7 +346,7 @@ class UniverseService:
                 tkr, history = item
                 shares_map[tkr] = history
 
-            # ---- Build snapshot rows ----
+            # Build snapshot rows
             batch_rows = []
             for ticker in batch:
                 shares_history = shares_map.get(ticker, [])
@@ -335,8 +360,6 @@ class UniverseService:
                 # Use the LATEST available shares_outstanding (most recent SEC filing).
                 # yfinance returns split-adjusted prices retroactively, so using the
                 # post-split shares count gives correct market_cap across all periods.
-                # Small error (~10-20%) may occur for companies with large buyback programs,
-                # but this is acceptable for screening purposes.
                 latest_shares = shares_history[-1][1] if shares_history else None
 
                 for snap_date_key, close in ticker_prices.items():
@@ -351,14 +374,13 @@ class UniverseService:
                     # Sanity checks: skip absurd values (data quality)
                     if close > 1_000_000 or close <= 0:   # max BRK-A ~$600K
                         continue
-                    if shares < 100_000:                  # too few shares for a real public co
+                    if shares < 100_000:                  # too few shares for a public co
                         continue
 
                     market_cap = shares * close
                     if math.isnan(market_cap) or market_cap <= 0 or market_cap > 5e12:
                         continue
 
-                    # Normalize to first of month
                     snapshot_dt = datetime(
                         snap_date.year, snap_date.month, 1, tzinfo=timezone.utc
                     )
@@ -373,8 +395,9 @@ class UniverseService:
                         "exchange": reg.exchange if reg else None,
                     })
 
-            # ---- Batch upsert ----
+            # Upsert with a fresh short-lived DB session (releases connection immediately after)
             if batch_rows:
+                async with _db_ctx() as db:
                     for i in range(0, len(batch_rows), _CHUNK):
                         chunk = batch_rows[i:i + _CHUNK]
                         stmt = pg_insert(UniverseSnapshot).values(chunk)
@@ -393,14 +416,12 @@ class UniverseService:
                         total_snapshots += result.rowcount
                     await db.commit()
 
-            batch_num = batch_start // _PRICE_BATCH + 1
-            total_batches = (len(ticker_list) + _PRICE_BATCH - 1) // _PRICE_BATCH
             logger.info(
                 f"Universe: batch {batch_num}/{total_batches} — "
                 f"{len(batch)} tickers, {len(batch_rows)} snapshot rows"
             )
             # Yield to event loop between batches to keep API responsive
-            await asyncio.sleep(1)
+            await asyncio.sleep(0)
 
         logger.info(
             f"Universe: build complete — {len(ticker_list)} tickers, "