[AIRFLOW-6250] handle_failure needs better default for context and test_mode

[yuqian90]

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-6250

Description

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

on_failure_callback almost always want to know the dag_id and taskinstance that failed. These info are in the context passed to on_failure_callback, which is passed in from handle_failure(). However, in some rare scenarios, if handle_failure is called in scheduler_job.py and backfill_job.py, the only argument passed is the error message. context is left as None.

So in these cases, on_failure_callback will not even know what's the dag_id of the dag that just failed.

This PR fixes this by setting context to get_template_context() if it's not given.

Tests

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

Added test in test_taskinstance.py

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 (not including Jira issue reference)
  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.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[ashb]

Small style changes, otherwise looks good.

[codecov-io]

Codecov Report

Merging #6812 into master will decrease coverage by 0.28%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6812      +/-   ##
==========================================
- Coverage    84.6%   84.31%   -0.29%     
==========================================
  Files         676      676              
  Lines       38331    38335       +4     
==========================================
- Hits        32431    32324     -107     
- Misses       5900     6011     +111

Impacted Files	Coverage Δ
airflow/models/taskinstance.py	93.98% <100%> (+0.18%)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	78.2% <0%> (-20.52%)	⬇️
airflow/utils/dag_processing.py	87.42% <0%> (-0.58%)	⬇️
airflow/models/taskfail.py	100% <0%> (+4.34%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8107651...8e7305e. Read the comment docs.