app/session_store.py

"""
app/session_store.py — Redis abstraction with in-memory fallback.

V2 Fix: V1 used a plain dict — sessions lost on restart.
V2 uses Upstash Redis (free tier). If Redis is unavailable, falls back to
an in-memory dict so the episode never crashes. Worst case: sessions are
process-local again, same as V1.

The rest of the codebase never touches Redis directly — only load/save/delete.
"""
import os
import pickle
from typing import Optional

# ── Lazy Redis client ────────────────────────────────────────────────────────
_redis_client = None
_local_cache: dict = {}   # In-memory fallback — activated when Redis is down

REDIS_URL = os.getenv("REDIS_URL", "")
SESSION_TTL = 3600   # 1 hour — episodes expire after inactivity


def _get_redis():
    """Lazy singleton. Returns Redis client or None if unavailable."""
    global _redis_client
    if _redis_client is not None:
        return _redis_client
    if not REDIS_URL:
        return None
    try:
        import redis as redis_lib
        _redis_client = redis_lib.from_url(REDIS_URL, decode_responses=False, socket_timeout=2)
        _redis_client.ping()   # Fail fast if connection is broken
        return _redis_client
    except Exception:
        return None


def load(session_id: str):
    """Fetch EpisodeState from Redis, fall back to local cache."""
    key = f"session:{session_id}"
    r = _get_redis()
    if r:
        try:
            data = r.get(key)
            return pickle.loads(data) if data else None
        except Exception:
            pass
    # Fallback: local memory
    return _local_cache.get(session_id)


def save(session_id: str, state) -> None:
    """Persist EpisodeState to Redis + local cache (dual write for resilience)."""
    key = f"session:{session_id}"
    _local_cache[session_id] = state   # Always write locally
    r = _get_redis()
    if r:
        try:
            r.setex(key, SESSION_TTL, pickle.dumps(state))
        except Exception:
            pass   # Redis outage — local cache is the fallback


def delete(session_id: str) -> None:
    """Remove session after episode completes."""
    _local_cache.pop(session_id, None)
    r = _get_redis()
    if r:
        try:
            r.delete(f"session:{session_id}")
        except Exception:
            pass