CORS exception cause gunicorn to not find create_app method

[ghost]

Hello,

I observed this issue when using the flask-CORS plugin. The error that shows up is:

...
[2019-04-30 22:42:16 +0000] [9] [INFO] Worker exiting (pid: 9)
Failed to find application object 'create_app()' in 'bug'
[2019-04-30 22:42:16 +0000] [10] [INFO] Worker exiting (pid: 10)
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/gunicorn/arbiter.py", line 203, in run
    self.manage_workers()
  File "/usr/local/lib/python3.7/site-packages/gunicorn/arbiter.py", line 545, in manage_workers
...

Note the Failed to find application object 'create_app()' in 'bug' error. create_app() clearly exists.

To reproduce the error do the following:

With the directory structure:

.
├── Dockerfile
├── __init__.py
└── requirements.txt

Dockerfile

FROM python:3.7.3-alpine3.9
WORKDIR /usr/src/app
ADD requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
ADD __init__.py  /usr/src/app/bug/__init__.py
CMD ["gunicorn", "bug:create_app()", "-b", "0.0.0.0:5000", "--workers=4"]

init.py

from flask import Flask
app = Flask(__name__)

@app.route('/')
def index():
    return "Hello world!"

def create_app():
    CORS(app, resources={r"/*": {"origins": "*"}})
    return app

requirements.txt

flask
flask-cors
gunicorn

Run:

docker build -t gunicornbug ./
docker run -d --name gunibug -p 5000:5000 gunicornbug
docker logs gunibug

and you will see the errors I described.

If you remove the CORS line so that __init__.py reads:

from flask import Flask
app = Flask(__name__)

@app.route('/')
def index():
    return "Hello world!"

def create_app():
    return app

and then do another docker build -t gunicornbug ./

The application runs fine when doing curl localhost:5000/

I deliberately didn't include an from flask_cors import CORS at the top to reproduce the error. Not sure what's going on here. This is a dummy example, the project where this error occurred is more complicated so I made a simpler example for reference. Let me know if you need any more info.

[jamadden]

Are you saying that you'd prefer a different error message? gunicorn currently catches NameError deliberately in order to print a friendly error message --- the idea is that if module:var fails with a NameError on var, it's probably because module doesn't define var. If var() is actually a callable that itself raises NameError, gunicorn has no way to distinguish that.

TIL, actually. I didn't realize gunicorn actually evals the var part of module:var. I assumed it just did getattr(mod, var). But doing that doesn't let someone write var() --- gunicorn can't distinguish a callable that's meant to return the wsgi app from the wsgi app itself...unless it were to try to do some checking of the callable's signature, which seems like a hack.

[ghost]

At least with how I interpreted the error message was that var() doesn't exist in module. This (for me) was misleading and I was thinking at first it was a PYTHON_PATH problem or something else. If the eval is failing that would be useful info to know in the error message. For example had it said something similar to:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'CORS' is not defined

that would make more sense. If CORS is in module this error above would (ideally) still show. To illustrate:

module/
├── __init__.py

where __init__.py is :

def create_app():
    CORS

I do:

Patricks-MacBook-Pro:gunicorn-bug pburton$ python3
Python 3.7.3 (default, Mar 27 2019, 09:23:15)
[Clang 10.0.1 (clang-1001.0.46.3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import module
>>> module.create_app()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/pburton/gunicorn-bug/module/__init__.py", line 2, in create_app
    CORS
NameError: name 'CORS' is not defined
>>>

I see that this name doesn't exist within module and it's also a NameError.

[benoitc]

hrm you run like looks weird. gunicorn should use a wsgi instance and will not use the callback normally. ie it should use the the app from flask

[jamadden]

@benoitc I'm not sure what you mean. Are you saying the module:var() syntax isn't really meant to be used that way? In which case instead of writing bug:create_app(), one should do:

def create_app():
    CORS(app, resources={r"/*": {"origins": "*"}})
    return app
app = create_app()

with bug:app? That pushes the NameError back to import time.

[ghost]

Found what appears to be a related issue: #1333

[jamadden]

Yup, the code from that PR is still in place. Enabling debugging for gunicorn will print the original traceback as you requested.

[benoitc]

@jamadden i am reffering to this line in the docker file : CMD ["gunicorn", "bug:create_app()", "-b", "0.0.0.0:5000", "--workers=4"]

[benoitc]

fixed in latest master