AIRFLOW-2787 Allow is_backfill to handle NULL DagRun.run_id

[gwax]

Make sure you have checked all steps below.

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-2787

Description

• Here are some details about my PR, including screenshots of any UI changes:
  • is_backfill will fail with a AttributeError: 'NoneType' object has no attribute 'startswith' error when called on a DagRun with NULL run_id (allowed by model definition). This PR introduces a None check to prevent the crash.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  • extension to test_is_backfill to check the NULL run_id case

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"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.

Code Quality

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

[gwax]

Test failure is unrelated.

[feng-tao]

LGTM

[feng-tao]

Do you have a pass travis CI link for your pr in your local repo?

[gwax]

I have not seen a full set of passing tests yet but it is different CI targets that fail each time. The tests seem to be failing due to some sort of problem with after kerberos setup before anything even runs.

[feng-tao]

thanks @gwax . Could you rebase the pr with master? There was an issue with the master which fails the CI. It just get reverted.

[feng-tao]

feel free to ping me once you rebase with master and we are good to go.

[gwax]

@feng-tao rebased

[feng-tao]

thanks @gwax . LGTM.