Add local backtest snapshot runner

app/services/backtest_service.py

@@ -791,6 +791,23 @@ def _backtest_worker_count() -> int:
     return max(1, min(configured, cpu - 1))
 
 
+def _offline_snapshot_mode() -> bool:
+    return os.getenv("BACKTEST_SNAPSHOT_OFFLINE", "").strip().lower() in {"1", "true", "yes", "on"}
+
+
+async def _load_benchmark_closes_for_backtest(
+    db: AsyncSession, *, days: int | None = None, refresh: bool = True
+) -> dict[date, float]:
+    from app.services.benchmark_service import load_benchmark_closes, refresh_benchmark_prices
+
+    if refresh and not _offline_snapshot_mode():
+        if days is None:
+            await refresh_benchmark_prices(db)
+        else:
+            await refresh_benchmark_prices(db, days=days)
+    return await load_benchmark_closes(db)
+
+
 def _mp_context():
     """A start method safe to use from the threaded asyncio server: ``forkserver``
     (workers forked from a clean, single-threaded server — avoids the
@@ -800,6 +817,12 @@ def _mp_context():
     for method in ("forkserver", "fork"):
         if method in methods:
             return multiprocessing.get_context(method)
+    if (
+        _offline_snapshot_mode()
+        and os.getenv("BACKTEST_ALLOW_SPAWN", "").strip().lower() in {"1", "true", "yes", "on"}
+        and "spawn" in methods
+    ):
+        return multiprocessing.get_context("spawn")
     return None
 
 
@@ -1568,10 +1591,9 @@ async def run_backtest(
     # emitted and the rest of the report remains valid.
     benchmark_closes: dict[date, float] = {}
     try:
-        from app.services.benchmark_service import load_benchmark_closes, refresh_benchmark_prices
-
-        await refresh_benchmark_prices(db, days=settings.ohlcv_history_days + 365)
-        benchmark_closes = await load_benchmark_closes(db)
+        benchmark_closes = await _load_benchmark_closes_for_backtest(
+            db, days=settings.ohlcv_history_days + 365
+        )
     except Exception:
         logger.exception("Benchmark load for residual momentum failed")
 
@@ -1693,16 +1715,13 @@ async def run_backtest(
 
         spy_closes: dict | None = None
         try:
-            from app.services.benchmark_service import (
-                load_benchmark_closes,
-                refresh_benchmark_prices,
-            )
-
             oldest = min((cols[0][0] for cols in price_columns.values()), default=None)
-            if oldest is not None:
+            days_needed = None
+            if oldest is not None and not _offline_snapshot_mode():
                 days_needed = (date.today() - date.fromordinal(oldest)).days + 30
-                await refresh_benchmark_prices(db, days=days_needed)
-            spy_closes = await load_benchmark_closes(db)
+            spy_closes = await _load_benchmark_closes_for_backtest(
+                db, days=days_needed, refresh=oldest is not None
+            )
         except Exception:
             logger.exception("Benchmark load for the portfolio sim failed")