Support standard Python3 environment

[geaden]

While bazel itself works perfectly with Flask, it seems currently it's impossible to deploy app like Python3 quickstart.

The first issue is requirements.txt is not symlinked into bazel-build/path/to/app/app.deploy.runfiles/project (where app.yaml is located). Probably, the issue could be overcome if flask is bundled into external dependency, but gunicorn (or other wsgi server) should be declared somehow.

I manually added symlink to /path/to/app/requirements.txt into bazel-build/path/to/app/app.deploy.runfiles/project and after bazel run //path/to/app:app.deploy it worked, gunicorn dependency was resolved.

The second issue is related to loading main module.

Tried several ways to specify entrypoint in app.yaml:

entrypoint: gunicorn -b :$PORT main:app

This caused the following stacktrace:

Traceback (most recent call last):
  File "/layers/google.python.pip/pip/lib/python3.8/site-packages/gunicorn/arbiter.py", line 589, in spawn_worker
    worker.init_process()
  File "/layers/google.python.pip/pip/lib/python3.8/site-packages/gunicorn/workers/base.py", line 134, in init_process
    self.load_wsgi()
  File "/layers/google.python.pip/pip/lib/python3.8/site-packages/gunicorn/workers/base.py", line 146, in load_wsgi
    self.wsgi = self.app.wsgi()
  File "/layers/google.python.pip/pip/lib/python3.8/site-packages/gunicorn/app/base.py", line 67, in wsgi
    self.callable = self.load()
  File "/layers/google.python.pip/pip/lib/python3.8/site-packages/gunicorn/app/wsgiapp.py", line 58, in load
    return self.load_wsgiapp()
  File "/layers/google.python.pip/pip/lib/python3.8/site-packages/gunicorn/app/wsgiapp.py", line 48, in load_wsgiapp
    return util.import_app(self.app_uri)
  File "/layers/google.python.pip/pip/lib/python3.8/site-packages/gunicorn/util.py", line 359, in import_app
    mod = importlib.import_module(module)
  File "/opt/python3.8/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 973, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'main'

And

entrypoint: gunicorn -b :$PORT --chdir /path/to/app main:app

entrypoint: gunicorn -b :$PORT path.to.app.main:app

produced:

Traceback (most recent call last):
  File "/opt/python3.8/lib/python3.8/pkgutil.py", line 493, in find_loader
    spec = importlib.util.find_spec(fullname)
  File "/opt/python3.8/lib/python3.8/importlib/util.py", line 94, in find_spec
    parent = __import__(parent_name, fromlist=['__path__'])
ModuleNotFoundError: No module named '/workspace/path/to/app/main'

Any plans on supporting flask in python appengine standard environment?

Thanks.