fix: release admission transaction before dispatch

src/wf_scheduling/poll.py

@@ -16,6 +16,7 @@ contains no fixture input, fixture environments, or canned results.
 
 from __future__ import annotations
 
+from dataclasses import dataclass
 from datetime import datetime, timezone
 from typing import Any, cast
 
@@ -49,6 +50,14 @@ class BlockedSchedule(ValueError):
     """A schedule is blocked by a corrupt record and fails closed."""
 
 
+@dataclass(frozen=True, slots=True)
+class _AdmissionOutcome:
+    """Result of one atomic admission and any dispatch required afterward."""
+
+    result: str | None
+    dispatch_run_id: str | None = None
+
+
 def source_for_trigger(trigger: Any) -> OccurrenceSource:
     """Build a calendar source from a schedule trigger model."""
     kind = trigger.kind if hasattr(trigger, "kind") else trigger.get("kind")
@@ -316,27 +325,37 @@ class Scheduler:
         return None
 
     def _admit(self, sched: Any, intended: datetime, now: datetime) -> str | None:
-        """Admit one occurrence under the schedule store's local transaction."""
+        """Admit atomically, then dispatch a new run after the transaction.
+
+        The durable admission and all schedule-side markers must commit as one
+        compound transition. Registration can wait on the event loop, though,
+        so it deliberately happens after that transaction releases; otherwise
+        an event-loop admin mutation can wait on a worker that is waiting for
+        the event loop to register its task.
+        """
         with schedule_store_transaction(self.schedule_store):
-            return self._admit_locked(sched, intended, now)
+            outcome = self._admit_locked(sched, intended, now)
+        if outcome.dispatch_run_id is not None:
+            self._execute_guarded(outcome.dispatch_run_id, now)
+        return outcome.result
 
     def _admit_locked(
         self, sched: Any, intended: datetime, now: datetime
-    ) -> str | None:
+    ) -> _AdmissionOutcome:
         # The poller's listing is only a fairness snapshot. Re-read under the
         # admission transaction so an edit committed before this point wins;
         # an edit after this point waits and affects a later occurrence.
         try:
             current = self.schedule_store.get_schedule(sched.id)
         except KeyError:
-            return None
+            return _AdmissionOutcome(None)
         if current.revision != sched.revision:
-            return "schedule-changed"
+            return _AdmissionOutcome("schedule-changed")
         sched = current
         if getattr(sched, "blocked_reason", None):
             raise BlockedSchedule(getattr(sched, "blocked_reason"))
         if not sched.enabled or sched.deleted or sched.paused:
-            return None
+            return _AdmissionOutcome(None)
         # Fail closed before any new admission when a corrupt active view
         # exists for this schedule.
         for run in self._active(sched.id):
@@ -370,7 +389,7 @@ class Scheduler:
                     now=now,
                     admitted_at=now,
                 )
-            return existing
+            return _AdmissionOutcome(existing)
         prepared = self.preparer.prepare(sched=sched, intended=intended, now=now)
         if isinstance(prepared, PreparationRejected):
             self._record(
@@ -385,7 +404,7 @@ class Scheduler:
             if cand is not None and cand.intended_at == intended:
                 self.schedule_store.save_candidate(None, schedule_id=sched.id)
             _save_consumed_max(self.schedule_store, sched.id, intended)
-            return None
+            return _AdmissionOutcome(None)
         active = self._active(sched.id)
         if sched.overlap == "skip" and active:
             self._record(
@@ -399,7 +418,7 @@ class Scheduler:
             if cand is not None and cand.intended_at == intended:
                 self.schedule_store.save_candidate(None, schedule_id=sched.id)
             _save_consumed_max(self.schedule_store, sched.id, intended)
-            return None
+            return _AdmissionOutcome(None)
         if sched.overlap == "parallel" and len(active) >= sched.max_active_runs:
             self._record(
                 kind="skipped-overlap",
@@ -412,7 +431,7 @@ class Scheduler:
             if cand is not None and cand.intended_at == intended:
                 self.schedule_store.save_candidate(None, schedule_id=sched.id)
             _save_consumed_max(self.schedule_store, sched.id, intended)
-            return None
+            return _AdmissionOutcome(None)
         if self._task_load() >= self.capacity:
             if sched.misfire == "latest":
                 self.schedule_store.save_candidate(
@@ -424,7 +443,7 @@ class Scheduler:
                     schedule_id=sched.id,
                 )
                 _save_consumed_max(self.schedule_store, sched.id, intended)
-                return "held"
+                return _AdmissionOutcome("held")
             if (now - intended).total_seconds() > sched.lateness_allowance_s:
                 self._record(
                     kind="skipped-misfire",
@@ -434,8 +453,8 @@ class Scheduler:
                     revision=sched.revision,
                 )
                 _save_consumed_max(self.schedule_store, sched.id, intended)
-                return None
+                return _AdmissionOutcome(None)
-            return "held-undecided"
+            return _AdmissionOutcome("held-undecided")
         run_id = self.run_store.allocate_run_id()
         from wf_api.run_lifecycle import persist_admission
 
@@ -471,8 +490,7 @@ class Scheduler:
 
         self.run_store.mark_pending_dispatch(run_id)
         materialize_admitted_view(store=self.run_store, admission=admission)
-        self._execute_guarded(run_id, now)
+        return _AdmissionOutcome(run_id, dispatch_run_id=run_id)
-        return run_id
 
     def _execute_guarded(self, run_id: str, now: datetime) -> None:
         """Dispatch one provably undispatched run behind a durable transition.