Add DAG Docs to Trigger UI

[jyotidhiman0610]

closes #12550

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst)
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[ryanahamilton]

This looks good! Thanks @jyotidhiman0610!

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest master or amend the last commit of the PR, and push it with --force-with-lease.

[jyotidhiman0610]

@ryanahamilton how can I merge this PR?
Also, the 4 failing checks seem unrelated to the PR. Can you please help?

[ryanahamilton]

Appears to be a related error in there:

=================================== FAILURES ===================================
  ________________ TestTriggerDag.test_trigger_dag_params_conf_1 _________________
  
  a = (<tests.www.test_views.TestTriggerDag testMethod=test_trigger_dag_params_conf_1>,)
  
      @wraps(func)
      def standalone_func(*a):
  >       return func(*(a + p.args), **p.kwargs)
  
  /usr/local/lib/python3.6/site-packages/parameterized/parameterized.py:530: 
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
  tests/www/test_views.py:2791: in test_trigger_dag_params_conf
      resp = self.client.get(f'trigger?dag_id={test_dag_id}&conf={test_request_conf}')
  /usr/local/lib/python3.6/site-packages/werkzeug/test.py:1006: in get
      return self.open(*args, **kw)
  /usr/local/lib/python3.6/site-packages/flask/testing.py:227: in open
      follow_redirects=follow_redirects,
  /usr/local/lib/python3.6/site-packages/werkzeug/test.py:970: in open
      response = self.run_wsgi_app(environ.copy(), buffered=buffered)
  /usr/local/lib/python3.6/site-packages/werkzeug/test.py:861: in run_wsgi_app
      rv = run_wsgi_app(self.application, environ, buffered=buffered)
  /usr/local/lib/python3.6/site-packages/werkzeug/test.py:1096: in run_wsgi_app
      app_rv = app(environ, start_response)
  /usr/local/lib/python3.6/site-packages/flask/app.py:2464: in __call__
      return self.wsgi_app(environ, start_response)
  /usr/local/lib/python3.6/site-packages/flask/app.py:2450: in wsgi_app
      response = self.handle_exception(e)
  /usr/local/lib/python3.6/site-packages/flask/app.py:1867: in handle_exception
      reraise(exc_type, exc_value, tb)
  /usr/local/lib/python3.6/site-packages/flask/_compat.py:39: in reraise
      raise value
  /usr/local/lib/python3.6/site-packages/flask/app.py:2447: in wsgi_app
      response = self.full_dispatch_request()
  /usr/local/lib/python3.6/site-packages/flask/app.py:1952: in full_dispatch_request
      rv = self.handle_user_exception(e)
  /usr/local/lib/python3.6/site-packages/flask/app.py:1821: in handle_user_exception
      reraise(exc_type, exc_value, tb)
  /usr/local/lib/python3.6/site-packages/flask/_compat.py:39: in reraise
      raise value
  /usr/local/lib/python3.6/site-packages/flask/app.py:1950: in full_dispatch_request
      rv = self.dispatch_request()
  /usr/local/lib/python3.6/site-packages/flask/app.py:1936: in dispatch_request
      return self.view_functions[rule.endpoint](**req.view_args)
  airflow/www/auth.py:34: in decorated
      return func(*args, **kwargs)
  airflow/www/decorators.py:60: in wrapper
      return f(*args, **kwargs)
  airflow/utils/session.py:65: in wrapper
      return func(*args, session=session, **kwargs)
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
  
  self = <airflow.www.views.Airflow object at 0x7fb436cab6d8>
  session = <sqlalchemy.orm.session.Session object at 0x7fb43604c2b0>
  
      @expose('/trigger', methods=['POST', 'GET'])
      @auth.has_access(
          [
              (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
              (permissions.ACTION_CAN_CREATE, permissions.RESOURCE_DAG_RUN),
          ]
      )
      @action_logging
      @provide_session
      def trigger(self, session=None):
          """Triggers DAG Run."""
          dag_id = request.values.get('dag_id')
          origin = get_safe_url(request.values.get('origin'))
          request_conf = request.values.get('conf')
      
          if request.method == 'GET':
              # Populate conf textarea with conf requests parameter, or dag.params
              default_conf = ''
              if request_conf:
                  default_conf = request_conf
              else:
                  try:
                      dag = current_app.dag_bag.get_dag(dag_id)
      
                      doc_md = wwwutils.wrapped_markdown(getattr(dag, 'doc_md', None))
                      default_conf = json.dumps(dag.params, indent=4)
                  except TypeError:
                      flash("Could not pre-populate conf field due to non-JSON-serializable data-types")
              return self.render_template(
  >               'airflow/trigger.html', dag_id=dag_id, origin=origin, conf=default_conf, doc_md=doc_md
              )
  E           UnboundLocalError: local variable 'doc_md' referenced before assignment
  
  airflow/www/views.py:1423: UnboundLocalError

[jyotidhiman0610]

@ryanahamilton Thank you so much for pointing out the test case, I have done the necessary changes & all the tests have passed.
Request you to kindly take a look.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!