Wakeup worker when there are resources freed up (#882)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Alex Grönholm <alex.gronholm@nextday.fi>

docs/versionhistory.rst

@@ -22,6 +22,9 @@ APScheduler, see the :doc:`migration section <migration>`.
 - Fixed dialect name checks in the SQLAlchemy job store
 - Fixed JSON and CBOR serializers unable to serialize enums
 - Fixed infinite loop in CalendarIntervalTrigger with UTC timezone (PR by unights)
+- Fixed scheduler not resuming job processing when ``max_concurrent_jobs`` had been
+  reached and then a job was completed, thus making job processing possible again
+  (PR by MohammadAmin Vahedinia)
 
 **4.0.0a4**
 

src/apscheduler/_schedulers/async_.py

@@ -981,7 +981,7 @@ def _get_task_callable(self, task: Task) -> Callable:
     async def _process_jobs(self, *, task_status: TaskStatus) -> None:
         wakeup_event = anyio.Event()
 
-        async def job_added(event: Event) -> None:
+        async def check_queue_capacity(event: Event) -> None:
             if len(self._running_jobs) < self.max_concurrent_jobs:
                 wakeup_event.set()
 
@@ -993,7 +993,11 @@ async def job_added(event: Event) -> None:
             task_group = await exit_stack.enter_async_context(create_task_group())
 
             # Fetch new jobs every time
-            exit_stack.enter_context(self.event_broker.subscribe(job_added, {JobAdded}))
+            exit_stack.enter_context(
+                self.event_broker.subscribe(
+                    check_queue_capacity, {JobAdded, JobReleased}
+                )
+            )
 
             # Signal that we are ready, and wait for the scheduler start event
             task_status.started()

tests/test_schedulers.py

@@ -15,7 +15,13 @@
 
 import anyio
 import pytest
-from anyio import WouldBlock, create_memory_object_stream, fail_after, sleep
+from anyio import (
+    Lock,
+    WouldBlock,
+    create_memory_object_stream,
+    fail_after,
+    sleep,
+)
 from pytest import MonkeyPatch
 from pytest_mock import MockerFixture, MockFixture
 
@@ -819,6 +825,28 @@ async def test_wait_until_stopped(self) -> None:
         # This should be a no-op
         await scheduler.wait_until_stopped()
 
+    async def test_max_concurrent_jobs(self) -> None:
+        lock = Lock()
+        scheduler = AsyncScheduler(max_concurrent_jobs=1)
+        tasks_done = 0
+
+        async def acquire_release() -> None:
+            nonlocal tasks_done
+            lock.acquire_nowait()
+            await sleep(0.1)
+            tasks_done += 1
+            if tasks_done == 2:
+                await scheduler.stop()
+
+            lock.release()
+
+        with fail_after(3):
+            async with scheduler:
+                await scheduler.configure_task("dummyjob", func=acquire_release)
+                await scheduler.add_job("dummyjob")
+                await scheduler.add_job("dummyjob")
+                await scheduler.run_until_stopped()
+
 
 class TestSyncScheduler:
     def test_configure(self) -> None: