[Python] gunicorn.errors.HaltServer: <HaltServer 'App failed to load.' 4> in Google Cloud Run...

I have deployed a Cloud Run Function and am frequently seeing this error:

2025-08-26 13:08:18.052 PDT DEBUG: False
2025-08-26 13:08:18.052 PDT Type of DEBUG: <class 'bool'>
2025-08-26 13:08:18.052 PDT Connecting to Cloud SQL with UNIX socket
2025-08-26 13:08:18.161 PDT Failed to find attribute 'app' in 'main'.
2025-08-26 13:08:18.162 PDT [2025-08-26 20:08:18 +0000] [6] [INFO] Worker exiting (pid: 6)
2025-08-26 13:08:18.546 PDT [2025-08-26 20:08:18 +0000] [1] [ERROR] Worker (pid:6) exited with code 4
2025-08-26 13:08:18.550 PDT Traceback (most recent call last): File "/layers/google.python.pip/pip/lib/python3.13/site-packages/gunicorn/arbiter.py", line 223, in run handler()
2025-08-26 13:08:18.550 PDT ~~~~~~~^^
2025-08-26 13:08:18.550 PDT File "/layers/google.python.pip/pip/lib/python3.13/site-packages/gunicorn/arbiter.py", line 256, in handle_term
2025-08-26 13:08:18.550 PDT raise StopIteration
2025-08-26 13:08:18.550 PDT StopIteration
2025-08-26 13:08:18.550 PDT During handling of the above exception, another exception occurred:
2025-08-26 13:08:18.551 PDT Traceback (most recent call last): File "/layers/google.python.pip/pip/bin/gunicorn", line 7, in <module> sys.exit(run())


The code runs fine locally, although I am connecting to CloudSQL in a different way when it's deployed vs locally (CloudSQL Python Connector vs UNIX Socket), but it's worked before without modification to that method.

The first 3 lines are print statements from outside of my main function.

How the beginning of my function looks:

import base64
import json
import os
from importlib.metadata import version
from datetime import date, datetime, timedelta

import pandas as pd
from flask import Response
import pg8000
import sqlalchemy
from dateutil import tz, parser
from dateutil.relativedelta import relativedelta
from google.cloud import bigquery
from google.oauth2 import service_account
from google.cloud.sql.connector import Connector, IPTypes
from sqlalchemy.exc import SQLAlchemyError
from cloudevents.http import CloudEvent
import functions_framework

DEBUG = os.environ.get('DEBUG', 'True').capitalize() == 'True'

print("SQLAlchemy version: ", version('SQLAlchemy'))

print("DEBUG: ", DEBUG)
print("Type of DEBUG: ", type(DEBUG))

if not DEBUG:
# Connect using UNIX socket (preferred for Cloud Run)
print("Connecting to Cloud SQL with UNIX socket")
db_socket_dir = os.environ.get("DB_SOCKET_DIR", "/cloudsql")
# Connect using Unix Socket
pg_engine = sqlalchemy.create_engine(
# Equivalent URL:
# postgresql+pg8000://<db_user>:<db_pass>@/<db_name>
# ?unix_sock=<socket_path>/<cloud_sql_instance_name>/.s.PGSQL.5432
sqlalchemy.engine.url.URL.create(
drivername="postgresql+pg8000",
username=db_user,
password=db_pass,
database=db_name,
query={
"unix_sock": "{}/{}/.s.PGSQL.5432".format(
db_socket_dir, # e.g. "/cloudsql"
instance_connection_name) # i.e "<PROJECT-NAME>:<INSTANCE-REGION>:<INSTANCE-NAME>"
}
),
# Pool size is the maximum number of permanent connections to keep.
pool_size=0,
# Temporarily exceeds the set pool_size if no connections are available.
max_overflow=1,
echo=True
)
else:
db_hostname = DB_HOSTNAME
db_port = 5432
print("Connecting to Cloud SQL using Cloud SQL Python Connector")
# # For local testing only
db_hostname = "123.45.678.90"
db_port = 5432
pg_engine = sqlalchemy.create_engine(
# Equivalent URL:
# postgresql+pg8000://<db_user>:<db_pass>@<db_host>:<db_port>/<db_name>
sqlalchemy.engine.url.URL.create(
drivername="postgresql+pg8000",
username=db_user,
password=db_pass,
host=db_hostname,
port=db_port,
database=db_name
),
# Pool size is the maximum number of permanent connections to keep.
pool_size=1,
# Temporarily exceeds the set pool_size if no connections are available.
max_overflow=0,
echo=True
)

@functions_framework.cloud_event
def main(cloud_event: CloudEvent) -> None:
"""Triggered from a message on a Cloud Pub/Sub topic.
Args:
event (dict): Event payload.
context (google.cloud.functions.Context): Metadata for the event.
"""
start_time = datetime.now()
print("event: ", cloud_event)
if not DEBUG:
pubsub_message = base64.b64decode(cloud_event.data["message"]["data"]).decode('utf-8')
else:
# For local testing only
pubsub_message = base64.b64decode(cloud_event.data["message"]["data"]).decode('utf-8')
print("pubsub_message:", pubsub_message)
settings = json.loads(pubsub_message)
print("settings:", settings)
...


It is deployed as a cloud run function, so I don't have access to the container itself.

Any direction on how to debug this or prevent this is appreciated

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