M3 plugin platform: SDK + loader, RLS-isolated tables, migrations, jobs

Backend foundation for the plugin platform (SPEC §11), Phases 1-3.

Phase 1 — SDK + loader:
- app/plugins/types.py: frozen Plugin/UiManifest/Slot/Page/JobSpec contract.
- app/plugins/sdk.py: sdk(name) facade — the only app.* import allowed for plugins
  (scoped session, plugin.{name}.* publish, blob, outbox, settings, SSRF http_get, KV, enqueue).
- app/plugins/loader.py: entry-point discovery (webmail.plugins), hook forwarding
  (API + worker), router mounting at /api/plugins/{name}, ui-manifest aggregation.
- plugin_kv core table (migration 0013) + GET /api/plugins/ui-manifest.

Phase 2 — non-removable core plugins + DB-enforced privacy:
- builtin flag (calendar/contacts will be builtin=True; uninstall refuses them).
- RLS per-user isolation on plg_* tables: least-privilege webmail_plugin role
  (migration 0014), plugin_session(user_id) sets LOCAL ROLE + app.current_user_id,
  rls.enable_rls() (ENABLE+FORCE RLS + USING/WITH CHECK policy). Mail untouched.
- Per-plugin Alembic runner (shared env, dedicated version table) + install/uninstall
  --drop-data CLI; skeleton fixture proves install -> run -> uninstall --drop-data
  leaves zero residue (§12 M3 acceptance).

Phase 3 — plugin jobs (§11.2):
- JobSpec registered as arq function plg.{plugin}.{func} + cron (5-field parser);
  singleton=True guarded by a per-(plugin,user) Redis lock; wired onto the shared
  interactive worker.

Gates: ruff, mypy --strict (94 files + tests), full suite 326 passed. RLS isolation,
migration round-trip, and singleton lock covered by tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rcj3TpydMTzTSmU7kunzf8

backend/app/plugins/jobs.py

@@ -0,0 +1,130 @@
+"""Plugin background jobs on the shared arq worker (SPEC §11.2).
+
+Each plugin ``JobSpec`` is registered as an arq function under a namespaced name
+``plg.{plugin}.{func}`` (enqueue on demand from hooks/routes via ``sdk.enqueue``),
+and, when it has a ``schedule``, also as an arq cron job. ``singleton=True`` wraps
+the job in a Redis NX lock keyed by (plugin, func, user) so at most one instance
+runs per user (or globally for scheduleds with no user). Jobs share the worker
+pool + retry semantics with core jobs and log under the ``plg`` logger.
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import Any
+
+from arq import cron
+from arq.cron import CronJob
+from arq.worker import Function, func
+
+from app.plugins import loader
+from app.plugins.types import JobSpec, Plugin
+from app.redis_client import get_redis
+
+logger = logging.getLogger("plg")
+
+# Safety expiry on the singleton lock so a crashed job can't wedge a user forever.
+# A job that runs longer than this could overlap; acceptable for M3.
+_LOCK_TTL = 3600
+
+# Standard 5-field cron order and each field's inclusive value bounds.
+_FIELD_ORDER = ("minute", "hour", "day", "month", "weekday")
+_FIELD_BOUNDS = {
+    "minute": (0, 59),
+    "hour": (0, 23),
+    "day": (1, 31),
+    "month": (1, 12),
+    "weekday": (0, 6),
+}
+
+
+def job_name(plugin_name: str, spec: JobSpec) -> str:
+    """The arq function name a plugin job is registered under."""
+    return f"plg.{plugin_name}.{spec.func.__name__}"
+
+
+def _lock_key(plugin_name: str, func_name: str, user: object) -> str:
+    return f"plglock:{plugin_name}:{func_name}:{user}"
+
+
+def _make_runner(plugin: Plugin, spec: JobSpec) -> tuple[Any, str]:
+    """Wrap a job func with the singleton lock (when requested) + namespaced name."""
+    func_name = spec.func.__name__
+    name = job_name(plugin.name, spec)
+
+    async def runner(ctx: dict[str, Any], *args: Any, **kwargs: Any) -> Any:
+        if not spec.singleton:
+            return await spec.func(ctx, *args, **kwargs)
+        user = kwargs.get("user_id", "global")
+        key = _lock_key(plugin.name, func_name, user)
+        acquired = await get_redis().set(key, b"1", nx=True, ex=_LOCK_TTL)
+        if not acquired:
+            logger.info("%s skipped: singleton already running (user=%s)", name, user)
+            return None
+        try:
+            return await spec.func(ctx, *args, **kwargs)
+        finally:
+            await get_redis().delete(key)
+
+    return runner, name
+
+
+def plugin_arq_functions() -> list[Function]:
+    """Every plugin job as an arq function (enqueue-on-demand), namespaced."""
+    funcs: list[Function] = []
+    for plugin in loader.get_plugins():
+        for spec in plugin.jobs:
+            runner, name = _make_runner(plugin, spec)
+            funcs.append(func(runner, name=name))
+    return funcs
+
+
+def _parse_field(token: str, lo: int, hi: int) -> set[int] | None:
+    """Parse one cron field to a value set (``*`` -> None = every)."""
+    if token == "*":
+        return None
+    values: set[int] = set()
+    for part in token.split(","):
+        if part.startswith("*/"):
+            values.update(range(lo, hi + 1, int(part[2:])))
+        elif "-" in part:
+            start, end = part.split("-", 1)
+            values.update(range(int(start), int(end) + 1))
+        else:
+            values.add(int(part))
+    return values
+
+
+def parse_cron(expr: str) -> dict[str, set[int] | None]:
+    """Parse a standard 5-field cron string into arq ``cron`` field kwargs."""
+    parts = expr.split()
+    if len(parts) != 5:
+        raise ValueError(f"cron expression must have 5 fields (min hour dom mon dow): {expr!r}")
+    return {
+        name: _parse_field(token, *_FIELD_BOUNDS[name])
+        for name, token in zip(_FIELD_ORDER, parts, strict=True)
+    }
+
+
+def plugin_cron_jobs() -> list[CronJob]:
+    """Cron-scheduled plugin jobs as arq ``CronJob``s (run on the shared worker)."""
+    jobs: list[CronJob] = []
+    for plugin in loader.get_plugins():
+        for spec in plugin.jobs:
+            if spec.schedule is None:
+                continue
+            runner, name = _make_runner(plugin, spec)
+            f = parse_cron(spec.schedule)
+            jobs.append(
+                cron(
+                    runner,
+                    name=f"{name}.cron",
+                    second=0,
+                    minute=f["minute"],
+                    hour=f["hour"],
+                    day=f["day"],
+                    month=f["month"],
+                    weekday=f["weekday"],
+                )
+            )
+    return jobs