utils: fix process races when killing processes

[hblanks]

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-5109

Description

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

airflow.utils.helpers.reap_process_group() can throw uncaught OSErrors
if processes exit at the wrong time in its execution. Fix this by
catching all OSErrors that can arise due to a process race, and
returning from them when the error is ESRCH (process not found).

Traceback from a production system can be found in the JIRA above.

Tests

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

this functionality is already tested in tests.utils.test_helpers; reproducibly testing for additional cases in a process race can only be done with substantially more complicated tests.

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

No new functionality, so no new docs.

Code Quality

• Passes flake8

[potiuk]

Very nice!

[ashb]

@potiuk When merging PRs can you ensure the merge message contains the Jira ticket reference please.