[AIRFLOW-6043] Update dag reference to all tasks in sub_dag

[Khrol]

Graph UI is broken when tasks are filtered by root if
root is in the middle of chain of tasks.

The origin of the problem is in sub_dag method that keeps references
to the original DAG. deepcopy call is optimized and the optimization is
not 100% correct.

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-6043

Description

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

t1 = DummyOperator(task_id="t1", dag=DAG)
t2 = DummyOperator(task_id="t2", dag=DAG)
t3 = DummyOperator(task_id="t3", dag=DAG)
t1 >> t2
t2 >> t3

Tests

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

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

[codecov-io]

Codecov Report

Merging #6638 into master will decrease coverage by 0.24%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6638      +/-   ##
==========================================
- Coverage   83.71%   83.46%   -0.25%     
==========================================
  Files         672      672              
  Lines       37568    37566       -2     
==========================================
- Hits        31450    31355      -95     
- Misses       6118     6211      +93

Impacted Files	Coverage Δ
airflow/models/dag.py	90.85% <100%> (ø)	⬆️
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	76.38% <0%> (-22.23%)	⬇️
airflow/configuration.py	89.13% <0%> (-3.63%)	⬇️
airflow/models/taskinstance.py	93.46% <0%> (+0.14%)	⬆️
airflow/hooks/postgres_hook.py	94.36% <0%> (+1.4%)	⬆️
airflow/hooks/dbapi_hook.py	91.52% <0%> (+1.69%)	⬆️
... and 2 more

---

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 5e5685a...8256ca4. Read the comment docs.

[ashb]

Does this make sense? This sub_dag is an odd beast that is similarly named but subtly different to an actual subdag via the SubDag operator.

When is this code path exercised?

[Khrol]

This code is not related to SubDag operator at all and is about completely different thing. Naming is very confusing, true.

This sub_dag method is called from
https://github.com/apache/airflow/blob/master/airflow/www/views.py#L1349 when Graph UI is filtered by root. It takes some mental effort to understand what happens in def graph to render corresponding view but the logic there is fine. The problem is in the incorrect deep_copy optimizations in sub_dag.

[ashb]

And when is "filter by root" passed in from the UI?

[Khrol]

https://youtu.be/hNzwsutZX1E

This is very useful for big DAGs.

[ashb]

I NEVER KNEW THAT FEATURE EXISTED!!!