[AIRFLOW-5480] Fix flaky impersonation test

[mik-laj]

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[feluelle]

@mik-laj do you know why this is "flaky"? How does this change fix it?

[codecov-io]

Codecov Report

Merging #6098 into master will increase coverage by 70.54%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #6098       +/-   ##
==========================================
+ Coverage    9.55%   80.1%   +70.54%     
==========================================
  Files         606     606               
  Lines       34893   34893               
==========================================
+ Hits         3334   27951    +24617     
+ Misses      31559    6942    -24617

Impacted Files	Coverage Δ
airflow/plugins_manager.py	86.91% <0%> (+0.93%)	⬆️
airflow/executors/dask_executor.py	2% <0%> (+2%)	⬆️
airflow/config_templates/airflow_local_settings.py	80% <0%> (+2.5%)	⬆️
airflow/kubernetes/pod_launcher.py	91.85% <0%> (+2.96%)	⬆️
airflow/exceptions.py	100% <0%> (+3.03%)	⬆️
airflow/executors/__init__.py	67.34% <0%> (+6.12%)	⬆️
airflow/utils/log/colored_log.py	93.18% <0%> (+13.63%)	⬆️
airflow/settings.py	88.4% <0%> (+15.21%)	⬆️
airflow/utils/decorators.py	90.9% <0%> (+15.9%)	⬆️
airflow/stats.py	69.23% <0%> (+16.92%)	⬆️
... and 497 more

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[mik-laj]

@feluelle II'm not sure why it worked before, but I know why it works now. This test verifies that if we have PYTHONPATH set it will be correctly passed during interpersonation.
We set PYTHONPATH in this place: https://github.com/apache/airflow/blob/master/run-tests#L48
This allows you to search modules from a different location. Please note that airflow modules are available because it is installed on the system - pip install -e .

I would like to add quote from this file:

# AIRFLOW-1893 - Originally, impersonation tests were incomplete missing the use case when
# DAGs access custom packages usually made available through the PYTHONPATH environment
# variable. This file includes a DAG that imports a custom package made available and if
# run via the previous implementation of impersonation, will fail by not being able to
# import the custom package.
# This DAG is used to test that impersonation propagates the PYTHONPATH environment
# variable correctly.

[feluelle]

Got it, thanks. 👍 Nice one! :)

[mik-laj]

It blocks my work a bit. I hope it will be accepted soon.

One more thing that I noticed was that it could be related to what Docker image I use. A colleague did not have this problem locally if he did not re-build the Docker image.