feat(scheduler): implement thread-safe singleton get_scheduler/reset_scheduler

scripts/task_scheduler.py

@@ -196,12 +196,25 @@ _scheduler_lock = threading.Lock()
 def get_scheduler(db_path: Path, run_task_fn: Callable = None) -> TaskScheduler:
     """Return the process-level TaskScheduler singleton, constructing it if needed.
 
-    run_task_fn is required on the first call (when the singleton is constructed);
-    ignored on subsequent calls. Pass scripts.task_runner._run_task.
+    run_task_fn is required on the first call; ignored on subsequent calls.
+    Safety: inner lock + double-check prevents double-construction under races.
+    The outer None check is a fast-path performance optimisation only.
     """
-    raise NotImplementedError
+    global _scheduler
+    if _scheduler is None:                      # fast path — avoids lock on steady state
+        with _scheduler_lock:
+            if _scheduler is None:              # re-check under lock (double-checked locking)
+                if run_task_fn is None:
+                    raise ValueError("run_task_fn required on first get_scheduler() call")
+                _scheduler = TaskScheduler(db_path, run_task_fn)
+                _scheduler.start()
+    return _scheduler
 
 
 def reset_scheduler() -> None:
-    """Shut down and clear the singleton. TEST TEARDOWN ONLY — not for production use."""
-    raise NotImplementedError
+    """Shut down and clear the singleton. TEST TEARDOWN ONLY."""
+    global _scheduler
+    with _scheduler_lock:
+        if _scheduler is not None:
+            _scheduler.shutdown()
+            _scheduler = None

tests/test_task_scheduler.py

@@ -72,10 +72,7 @@ def _noop_run_task(*args, **kwargs):
 def clean_scheduler():
     """Reset singleton between every test."""
     yield
-    try:
     reset_scheduler()
-    except NotImplementedError:
-        pass
 
 
 def test_default_budgets_used_when_no_config(tmp_db):
@@ -330,3 +327,47 @@ def test_worker_crash_releases_vram(tmp_db):
     # Second task still ran, VRAM was released
     assert 2 in log
     assert s._reserved_vram == 0.0
+
+
+def test_get_scheduler_returns_singleton(tmp_db):
+    """Multiple calls to get_scheduler() return the same instance."""
+    s1 = get_scheduler(tmp_db, _noop_run_task)
+    s2 = get_scheduler(tmp_db, _noop_run_task)
+    assert s1 is s2
+
+
+def test_singleton_thread_safe(tmp_db):
+    """Concurrent get_scheduler() calls produce exactly one instance."""
+    instances = []
+    errors = []
+
+    def _get():
+        try:
+            instances.append(get_scheduler(tmp_db, _noop_run_task))
+        except Exception as e:
+            errors.append(e)
+
+    threads = [threading.Thread(target=_get) for _ in range(20)]
+    for t in threads:
+        t.start()
+    for t in threads:
+        t.join()
+
+    assert not errors
+    assert len(set(id(s) for s in instances)) == 1  # all the same object
+
+
+def test_reset_scheduler_cleans_up(tmp_db):
+    """reset_scheduler() shuts down the scheduler; no threads linger."""
+    s = get_scheduler(tmp_db, _noop_run_task)
+    thread = s._thread
+    assert thread.is_alive()
+
+    reset_scheduler()
+
+    thread.join(timeout=2.0)
+    assert not thread.is_alive()
+
+    # After reset, get_scheduler creates a fresh instance
+    s2 = get_scheduler(tmp_db, _noop_run_task)
+    assert s2 is not s