[AIRFLOW-6956] Extract kill_child_processes_by_pids from DagFileProcessorManager

[mik-laj]

---

Issue link: AIRFLOW-6956

Make sure to mark the boxes below before creating PR: [x]

• Description above provides context of the change
• Commit message/PR title starts with [AIRFLOW-NNNN]. AIRFLOW-NNNN = JIRA ID^*
• Unit tests coverage for changes (not needed for documentation changes)
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as explained in Contribution Workflow Example.

^* For document-only changes commit message can start with [AIRFLOW-XXXX].

---

In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.
Read the Pull Request Guidelines for more information.

[codecov-io]

Codecov Report

Merging #7584 into master will decrease coverage by 0.23%.
The diff coverage is 85%.

@@            Coverage Diff             @@
##           master    #7584      +/-   ##
==========================================
- Coverage   86.81%   86.58%   -0.24%     
==========================================
  Files         896      897       +1     
  Lines       42704    42747      +43     
==========================================
- Hits        37074    37011      -63     
- Misses       5630     5736     +106

Impacted Files	Coverage Δ
airflow/utils/dag_processing.py	89.57% <50%> (+2.92%)	⬆️
airflow/utils/process_utils.py	73.25% <88.88%> (+4.13%)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	47.18% <0%> (-45.08%)	⬇️
...viders/cncf/kubernetes/operators/kubernetes_pod.py	69.69% <0%> (-25.26%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
airflow/config_templates/airflow_local_settings.py	64.28% <0%> (-1.76%)	⬇️
airflow/models/dag.py	91.55% <0%> (-0.03%)	⬇️
airflow/utils/log/cloudwatch_task_handler.py	100% <0%> (ø)

---

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 1d16de7...7d83211. Read the comment docs.

[msb217]

I think creating process_utils file makes sense. Do you think it would make sense to move more functionality to this process_utils file as well? For example, creating more generic util functions and then calling upon them from _kill_process, kill, terminate, start of the class DagFileProcessorProcess? And other files throughout dag_processing.py?

[mik-laj]

Do you have any specific example? Many operations are available via a nice API in the psutil library. Some operations have system APIs, e.g. os.kill, so they don't have to be in utils. I found one function - reap_process_group that could be moved to this module and I will try to do it in a separate PR.