[AIRFLOW-5641] support running git sync container as root

[houqp]

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-5641

Description

Fixed a regression that prevents user from using root as user for git sync container.

Also changed _get_security_context_val so that it behaves as documented.

Tests

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

See changed files in tests folder

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 #6312 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6312      +/-   ##
==========================================
+ Coverage   80.34%   80.35%   +<.01%     
==========================================
  Files         616      616              
  Lines       35733    35734       +1     
==========================================
+ Hits        28711    28713       +2     
+ Misses       7022     7021       -1

Impacted Files	Coverage Δ
airflow/kubernetes/worker_configuration.py	96.4% <ø> (ø)	⬆️
airflow/executors/kubernetes_executor.py	58.99% <100%> (+0.09%)	⬆️
airflow/utils/dag_processing.py	56.55% <0%> (-0.35%)	⬇️
airflow/models/taskinstance.py	93.77% <0%> (+0.5%)	⬆️

---

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 c0d98a7...8bd02cd. Read the comment docs.

[ashb]

Test that if the git_sync_run_as_user is not specified at all that we still set the default of 65533 please

[houqp]

@ashb I tried testing that, Airflow exited on error because that config key exists in default config, so it is required. ('kubernetes', 'git_sync_run_as_user'): '' is the closest thing to that test.

[houqp]

Turns out I was setting gi_sync_run_as_user to None, which was causing the crash. I have added a test case to avoid setting this config key entirely, which results in the default 65533 uid.

[ashb]

This is only a problem against master right? (The code looks quite different on the release branch)

[ashb]

Marking this for inclusion in 1.10.11 -- but that depends on @dimberman's backporting work.