Fix: Issue where cron would end up in nested transaction blocks and unable to escape. (#586)

* Use sessionmaker instead of context block

Essentially there were two bugs:
1) The logging lines before explicit `Session.begin` caused the root
   "A transaction is already begun on this Session.", which is
   resolved by moving the logging within the explicit session block,
   as intended.
2) We were reusing a Session across cron runs. The architecture was
   intended to not do so, but we didn't have an actual sessionmaker
   available to cron. We probably ought to get rid of scoped_session
   elsewhere, but baby steps to resolve the immediate issue.

In testing, this works correctly with the fix to `db.begin`, and
tests with deliberately reintroducing the logging bug, the changes
to session creation prevent the every-15sec stack trace spam.

---------

Almost-entirely-authored-by: TLSM <duolsm@outlook.com>

files/__main__.py

@@ -194,11 +194,12 @@ limiter = flask_limiter.Limiter(
 # ...and then after that we can load the database.
 
 engine: Engine = create_engine(DATABASE_URL)
-db_session: scoped_session = scoped_session(sessionmaker(
+db_session_factory: sessionmaker = sessionmaker(
 	bind=engine,
 	autoflush=False,
 	future=True,
-))
+)
+db_session: scoped_session = scoped_session(db_session_factory)
 
 # now that we've that, let's add the cache, compression, and mail extensions to our app...
 

files/commands/cron.py

@@ -4,9 +4,9 @@ import time
 from datetime import datetime, timezone
 from typing import Final
 
-from sqlalchemy.orm import scoped_session, Session
+from sqlalchemy.orm import sessionmaker, Session
 
-from files.__main__ import app, db_session
+from files.__main__ import app, db_session_factory
 from files.classes.cron.tasks import (DayOfWeek, RepeatableTask,
                                       RepeatableTaskRun, ScheduledTaskState)
 
@@ -41,7 +41,7 @@ def cron_app_worker():
 	logging.info("Starting scheduler worker process")
 	while True:
 		try:
-			_run_tasks(db_session)
+			_run_tasks(db_session_factory)
 		except Exception as e:
 			logging.exception(
 				"An unhandled exception occurred while running tasks",
@@ -77,7 +77,7 @@ def _acquire_lock_exclusive(db: Session, table: str):
 		raise
 
 
-def _run_tasks(db_session_factory: scoped_session):
+def _run_tasks(db_session_factory: sessionmaker):
 	'''
 	Runs tasks, attempting to guarantee that a task is ran once and only once.
 	This uses postgres to lock the table containing our tasks at key points in
@@ -116,10 +116,11 @@ def _run_tasks(db_session_factory: scoped_session):
 			task.run_time_last = now
 			task.run_state_enum = ScheduledTaskState.RUNNING
 
+		# This *must* happen before we start doing db queries, including sqlalchemy db queries
+		db.begin()
 		task_debug_identifier = f"(ID {task.id}:{task.label})"
 		logging.info(f"Running task {task_debug_identifier}")
 
-		db.begin()
 		run: RepeatableTaskRun = task.run(db, task.run_time_last_or_created_utc)
 
 		if run.exception: