Let a quarantine expire, and stop two tasks spending one budget

A house's inverter broker dropped at 04:27 and the power flow was quarantined
20 seconds later. Quarantine was terminal — the supervised task returned and
only a publish or an engine restart could bring it back — so five hours of
power and battery readings are missing, and what ended it was an unrelated
`git pull` restarting uvicorn.

Two changes, both in that path:

- the failure budget is per task, not per flow. `power` runs an MQTT subscriber
  and a Victron keepalive publisher against the same broker; they died together
  and spent one shared budget in 41s, giving up before the 60s backoff step was
  ever reached.
- quarantine is now a rest. The task sits out 5min, then 15, then an hour, and
  each time gets its budget back and tries again, so a broker that comes back
  is picked up without anyone watching. `quarantined` reads from whichever
  tasks are currently resting.

The alert for it says when it will try again.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C5H4uLCCpsbipL1R7WKCee

backend/fluksio/flow/alerts.py

@@ -114,10 +114,12 @@ def describe(event: dict[str, Any]) -> Alert | None:
             node=node,
         )
     if kind == "flow_quarantined":
+        retry = event.get("retry_in_s")
+        again = f" Trying again in {round(float(retry) / 60)} min." if retry else ""
         return Alert(
             title=f"Flow '{flow}' was quarantined",
             body=(
-                f"It kept crashing, so the engine stopped restarting it. {error}"
+                f"It kept crashing, so the engine stood it down. {error}{again}"
             ).strip(),
             flow=flow,
         )

backend/fluksio/flow/supervision.py

@@ -3,9 +3,15 @@
 A node's subscription, schedule or poll loop is an `asyncio.Task`, and a task
 that raises is simply gone — the node stays listed as running while nothing
 listens any more. The supervisor restarts those tasks with a growing delay and
-gives up on a flow that keeps failing, because a flow crash-looping every
+stands a flow down when one keeps failing, because a flow crash-looping every
 second is worse than a flow that is visibly stopped.
 
+Standing down is a rest, not a verdict. It used to be the end of the task: the
+flow stayed dead until somebody published it or the engine restarted. On a
+house that runs unattended that turns a broker rebooting at four in the morning
+into a hole in the record until someone notices — so the rest expires, the
+budget comes back, and the task tries again on a much longer clock.
+
 Deliberately a plain task registry rather than a `TaskGroup`: a group cancels
 its siblings when one member fails, which is the opposite of what supervision
 means here.
@@ -25,10 +31,20 @@ from fluksio.flow.events import EventBus
 logger = logging.getLogger(__name__)
 
 BACKOFF = (1.0, 5.0, 30.0, 60.0)
-# A flow that burns through this many restarts in the window is not going to
+# A task that burns through this many restarts in the window is not going to
-# recover by being restarted again.
+# recover by being restarted again *now*.
+#
+# Per task rather than per flow: a flow whose subscriber and its keepalive
+# publisher both go down with the same broker spends one shared budget twice as
+# fast as it was sized for, and gives up before the backoff above has reached
+# its longest step. That is what took the house's inverter readings out for
+# nearly five hours — six crashes across two tasks inside 41 seconds.
 FAILURE_BUDGET = 5
 FAILURE_WINDOW = 300.0
+# How long a task sits out before its budget is handed back. Minutes rather
+# than seconds because a device that is really gone should not be hammered, and
+# capped in the hour because nothing here is worth losing a day of readings to.
+QUARANTINE_BACKOFF = (300.0, 900.0, 3600.0)
 # How long a cancelled task gets to notice. A loop that is still waiting after
 # this is not going to stop on its own — a client closing a socket the broker
 # no longer answers on is the case seen in the wild — and the rebuild asking
@@ -48,8 +64,15 @@ class Supervisor:
         #: task's name, because names are a flow and a node joined by a dot and
         #: matching on that prefix would let 'hea' cancel 'heating'.
         self._flows: dict[str, str] = {}
+        #: Crash times per *task*, keyed by task name.
         self._failures: dict[str, deque[float]] = {}
-        self.quarantined: set[str] = set()
+        #: The tasks sitting out a quarantine, and the flow each belongs to.
+        self._resting: dict[str, str] = {}
+
+    @property
+    def quarantined(self) -> set[str]:
+        """The flows with at least one task currently standing down."""
+        return set(self._resting.values())
 
     def spawn(self, name: str, flow: str, factory: TaskFactory) -> None:
         """Run `factory()` and keep running it until told to stop."""
@@ -62,6 +85,7 @@ class Supervisor:
 
     async def _supervise(self, name: str, flow: str, factory: TaskFactory) -> None:
         attempt = 0
+        rests = 0
         while True:
             try:
                 await factory()
@@ -69,7 +93,10 @@ class Supervisor:
                 raise
             except Exception as exc:
                 if not self._record_failure(name, flow, exc):
-                    return
+                    await self._rest(name, flow, exc, rests)
+                    rests += 1
+                    attempt = 0
+                    continue
             else:
                 # A clean return means the loop decided it was done.
                 return
@@ -78,8 +105,37 @@ class Supervisor:
             attempt += 1
             await asyncio.sleep(delay)
 
+    async def _rest(self, name: str, flow: str, exc: Exception, rests: int) -> None:
+        """Stand the task down, then hand its budget back and let it retry."""
+        delay = QUARANTINE_BACKOFF[min(rests, len(QUARANTINE_BACKOFF) - 1)]
+        logger.error(
+            "Flow '%s' crashed %d times in %.0fs — quarantined, retrying in %.0fs",
+            flow,
+            FAILURE_BUDGET,
+            FAILURE_WINDOW,
+            delay,
+        )
+        self._resting[name] = flow
+        self._publish(
+            {
+                "type": "flow_quarantined",
+                "flow": flow,
+                "task": name,
+                "error": f"{type(exc).__name__}: {exc}",
+                "retry_in_s": delay,
+                "ts": time.time(),
+            }
+        )
+        try:
+            await asyncio.sleep(delay)
+        finally:
+            # Both, and in this order: a task woken by a cancellation is being
+            # torn down, and must not leave the flow looking quarantined.
+            self._resting.pop(name, None)
+            self._failures.pop(name, None)
+
     def _record_failure(self, name: str, flow: str, exc: Exception) -> bool:
-        """Note the crash; False when the flow has spent its budget."""
+        """Note the crash; False when this task has spent its budget."""
         logger.warning("Supervised task '%s' crashed: %s", name, exc, exc_info=True)
         self._publish(
             {
@@ -92,36 +148,20 @@ class Supervisor:
         )
 
         now = time.monotonic()
-        window = self._failures.setdefault(flow, deque())
+        window = self._failures.setdefault(name, deque())
         window.append(now)
         while window and window[0] < now - FAILURE_WINDOW:
             window.popleft()
 
-        if len(window) < FAILURE_BUDGET:
+        return len(window) < FAILURE_BUDGET
-            return True
-
-        logger.error(
-            "Flow '%s' crashed %d times in %.0fs — quarantined",
-            flow,
-            len(window),
-            FAILURE_WINDOW,
-        )
-        self.quarantined.add(flow)
-        self._publish(
-            {
-                "type": "flow_quarantined",
-                "flow": flow,
-                "error": f"{type(exc).__name__}: {exc}",
-                "ts": time.time(),
-            }
-        )
-        return False
 
     async def cancel_all(self) -> None:
         """Stop supervising. Idempotent, and safe to call mid-restart."""
         tasks = list(self._tasks.values())
         self._tasks.clear()
         self._flows.clear()
+        self._resting.clear()
+        self._failures.clear()
         await self._cancel(tasks)
 
     async def cancel_flow(self, flow: str) -> None:
@@ -136,11 +176,12 @@ class Supervisor:
         tasks = [self._tasks.pop(name) for name in names if name in self._tasks]
         for name in names:
             del self._flows[name]
+            # The clean slate: whatever these tasks spent before, the build
+            # that follows starts their budgets again — and a rebuild is not
+            # made to wait out a rest somebody has just fixed the cause of.
+            self._failures.pop(name, None)
+            self._resting.pop(name, None)
         await self._cancel(tasks)
-        # The clean slate: whatever this flow spent before, the build that
-        # follows starts its budget again.
-        self.quarantined.discard(flow)
-        self._failures.pop(flow, None)
 
     async def _cancel(self, tasks: list[asyncio.Task[None]]) -> None:
         """Ask these tasks to stop, and wait no longer than the grace period."""

backend/tests/flow/test_supervision.py

@@ -13,6 +13,9 @@ from fluksio.flow.supervision import FAILURE_BUDGET, Supervisor
 def no_backoff(monkeypatch: pytest.MonkeyPatch):
     """Real backoff would make these tests a minute long."""
     monkeypatch.setattr(supervision, "BACKOFF", (0.0,))
+    # Long enough that a quarantine holds for the length of an assertion, short
+    # enough that the test asking it to expire does not wait five minutes.
+    monkeypatch.setattr(supervision, "QUARANTINE_BACKOFF", (0.05,))
 
 
 async def _settle() -> None:
@@ -61,7 +64,7 @@ def test_a_loop_that_returns_is_left_alone():
     asyncio.run(scenario())
 
 
-def test_a_flow_that_keeps_crashing_is_quarantined():
+def test_a_flow_that_keeps_crashing_is_stood_down():
     attempts = 0
     events: list[dict] = []
     bus = EventBus()
@@ -81,13 +84,14 @@ def test_a_flow_that_keeps_crashing_is_quarantined():
         assert attempts == FAILURE_BUDGET
         assert supervisor.quarantined == {"heating"}
         assert [e["type"] for e in events].count("task_crashed") == FAILURE_BUDGET
-        assert [e["type"] for e in events][-1] == "flow_quarantined"
+        quarantine = [e for e in events if e["type"] == "flow_quarantined"][-1]
+        assert quarantine["retry_in_s"] == 0.05
         await supervisor.cancel_all()
 
     asyncio.run(scenario())
 
 
-def test_the_budget_is_per_flow():
+def test_one_flows_failure_does_not_stand_another_down():
     ran = 0
 
     async def failing() -> None:
@@ -111,6 +115,63 @@ def test_the_budget_is_per_flow():
     asyncio.run(scenario())
 
 
+def test_two_tasks_on_one_flow_do_not_spend_each_others_budget():
+    """The house's power flow: an MQTT subscriber and its keepalive publisher.
+
+    Both die with the same broker. Sharing one budget meant the flow was stood
+    down after half the restarts either task was allowed on its own.
+    """
+    tries = {"sub": 0, "pub": 0}
+
+    def crasher(which: str):
+        async def loop() -> None:
+            tries[which] += 1
+            raise ConnectionError("broker went away")
+
+        return loop
+
+    async def scenario() -> None:
+        supervisor = Supervisor()
+        supervisor.spawn("power.cerbo:mqtt", "power", crasher("sub"))
+        supervisor.spawn("power.keepalive:mqtt-out", "power", crasher("pub"))
+        await _settle()
+
+        assert tries == {"sub": FAILURE_BUDGET, "pub": FAILURE_BUDGET}
+        await supervisor.cancel_all()
+
+    asyncio.run(scenario())
+
+
+def test_a_quarantine_expires_and_the_task_tries_again():
+    """The whole point: a broker that comes back is picked up without a human."""
+    attempts = 0
+
+    async def loop() -> None:
+        nonlocal attempts
+        attempts += 1
+        if attempts <= FAILURE_BUDGET:
+            raise ConnectionError("broker went away")
+        await asyncio.sleep(3600)
+
+    async def scenario() -> None:
+        supervisor = Supervisor()
+        supervisor.spawn("mqtt", "power", loop)
+        await _settle()
+
+        assert attempts == FAILURE_BUDGET
+        assert supervisor.quarantined == {"power"}
+
+        # Sit out the rest, which the fixture has shortened to 50ms.
+        await asyncio.sleep(0.1)
+        await _settle()
+
+        assert attempts == FAILURE_BUDGET + 1
+        assert supervisor.quarantined == set()
+        await supervisor.cancel_all()
+
+    asyncio.run(scenario())
+
+
 def test_cancel_all_is_idempotent():
     async def loop() -> None:
         await asyncio.sleep(3600)
@@ -165,8 +226,8 @@ def test_cancelling_one_flow_lifts_only_its_quarantine():
         assert supervisor.quarantined == {"also-broken"}
         # And its budget, or the flow would be quarantined again on the first
         # crash after the rebuild that was meant to fix it.
-        assert "broken" not in supervisor._failures
+        assert "a" not in supervisor._failures
-        assert "also-broken" in supervisor._failures
+        assert "b" in supervisor._failures
         await supervisor.cancel_all()
 
     asyncio.run(scenario())