fix: publish schedules after creation watermark

src/wf_api/schedules.py

@@ -21,7 +21,7 @@ from wf_scheduling.history import FileScheduleHistoryRecorder, HistoryEntry
 from wf_scheduling.models import OccurrenceRecord, Schedule
 from wf_scheduling.occurrences import occurrence_id
 from wf_scheduling.prepare import PreparationRejected, SchedulePreparer
-from wf_scheduling.store import StaleScheduleRevisionError
+from wf_scheduling.store import ScheduleExistsError, StaleScheduleRevisionError
 
 from .models import (
     JsonProjector,
@@ -207,11 +207,20 @@ class WorkflowScheduleApi:
             }
         )
         self._validate_definition(schedule)
-        stored = store.create_schedule(schedule)
         # Creation never backfills time before the revision: the consumed
-        # watermark starts at creation so catch-up only ever covers
+        # watermark starts at creation so catch-up only ever covers downtime
-        # downtime after this revision, never pre-creation instants.
+        # after this revision, never pre-creation instants. Check the id
+        # before this write so a duplicate create cannot overwrite the
+        # existing schedule's watermark; publishing the schedule last keeps
+        # a failed watermark write invisible to the poller.
+        try:
+            store.get_schedule(schedule_id)
+        except KeyError:
+            pass
+        else:
+            raise ScheduleExistsError(f"schedule id already exists: {schedule_id!r}")
         store.save_consumed(schedule_id, now)
+        stored = store.create_schedule(schedule)
         return _PROJECT_SCHEDULE(stored.model_dump(mode="json"))
 
     async def get_schedule(self, *, schedule_id: str) -> ScheduleResult:

tests/wf_api/test_schedules.py

@@ -528,6 +528,26 @@ async def test_create_initializes_consumed_no_backfill(tmp_path: Path) -> None:
     assert consumed is not None and consumed >= created_at
 
 
+async def test_create_watermark_failure_does_not_publish_schedule(
+    tmp_path: Path,
+) -> None:
+    """A failed creation watermark cannot expose a backfillable schedule."""
+    api, sched_store, _ = _fault_harness(tmp_path / "torn_create")
+    sched_store.armed = True
+
+    with pytest.raises(OSError, match="injected consumed failure"):
+        await api.create_schedule(
+            schedule_id="s",
+            deployment_id="dep.personal",
+            trigger=_cron(),
+            misfire="latest",
+        )
+
+    with pytest.raises(KeyError):
+        sched_store.get_schedule("s")
+    assert sched_store.get_consumed("s") is None
+
+
 async def test_update_watermark_failure_leaves_revision(tmp_path: Path) -> None:
     api, sched_store, _ = _fault_harness(tmp_path / "torn_update")