[AIRFLOW-2204] Fix webserver debug mode

[bbonagura9]

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title.

Description

• Here are some details about my PR, including screenshots of any UI changes:

The command airflow webserver -d crashes because it tries to call the method run from the cached_app (from airflow.www.app) returned value, i.e. a DispatcherMiddleware instance. To run the flask app in debug mode (without gunicorn) it has to be created directly via create_app (also from airflow.www.app), that returns the Flask instance.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Very small changes, just a variable assign pushed a few lines down the usual flow and a line added for the debug flow.
(Some are to fix PEP8 compliance and pass git diff upstream/master -u -- "*.py" | flake8 --diff)

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":

  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[codecov-io]

Codecov Report

Merging #3118 into master will decrease coverage by 0.02%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master    #3118      +/-   ##
==========================================
- Coverage   73.06%   73.03%   -0.03%     
==========================================
  Files         180      180              
  Lines       12595    12596       +1     
==========================================
- Hits         9202     9200       -2     
- Misses       3393     3396       +3

Impacted Files	Coverage Δ
airflow/bin/cli.py	55.02% <25%> (-0.09%)	⬇️
airflow/jobs.py	79.91% <0%> (-0.18%)	⬇️

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[jgao54]

good catch, and code lgtm! Would prefer to add a simple test to prevent regression in the future. Perhaps something like:

    def test_cli_webserver_debug(self):
        import subprocess
        p = subprocess.Popen(["airflow", "webserver", "-d"])
        sleep(3) # wait for webserver to start
        return_code = p.poll()
        self.assertEqual(None, return_code, "webserver terminated with return code {} in debug mode.".format(return_code))
        p.terminate()
        p.wait()

[bbonagura9]

@jgao54 thanks for the feedback.
I've just added the test as you suggested.