[AIRFLOW-4881] Persist the max_tries field of task instances during zombie collection

[tamasflamich]

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Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-4881

Description

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In case a task instance

• has more attempts than retries (it can happen when the state of the task instance is explicitly cleared) and
• task instance is prematurely terminated (without graceful shutdown)
  then zombie collection process of the scheduler can mark the task instance failed instead of up_for_retry.

This happens because the max_retries field is not persisted but set to its initial value (retries).

Tests

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

Commits

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  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

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  • All the public functions and the classes in the PR contain docstrings that explain what it does
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Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #5514 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5514      +/-   ##
==========================================
- Coverage   79.07%   79.07%   -0.01%     
==========================================
  Files         489      489              
  Lines       30744    30748       +4     
==========================================
+ Hits        24312    24315       +3     
- Misses       6432     6433       +1

Impacted Files	Coverage Δ
airflow/models/dagbag.py	92.16% <100%> (+0.03%)	⬆️
airflow/utils/dag_processing.py	60.47% <100%> (+0.2%)	⬆️
airflow/models/taskinstance.py	93.02% <0%> (-0.17%)	⬇️

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[seelmann]

Just FYI, this will conflict with #5511 and when applied this change should no longer be required because the full TaskInstance including max_tries is loaded from DB when calling handle_failure().