[AIRFLOW-1077] Warn about subdag deadlock case

[akosel]

Dear Airflow maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-1077] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-1077

Description

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

Addresses a case that may be counter-intuitive to users where a subdag count greater than the concurrency limit can lead to deadlock. It also suggests a workaround for dealing with this issue.

Tests

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

It's a comment so there isn't anything to test.

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
  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"

[mention-bot]

@akosel, thanks for your PR! By analyzing the history of the files in this pull request, we identified @mistercrunch, @jlowin and @AntoineAugusti to be potential reviewers.

[saguziel]

The capitalization is all over the place in this doc, let's stick with "SubDag" since that is what is in code.

[saguziel]

I think you should create a new issue for this rather than use that one

[tispratik]

I think i am running into the same issue. Airflow says the subdags are running, but none of the steps within it are executing. When i restarted airflow, some progress happened, but then it got blocked again.

[codecov-io]

Codecov Report

Merging #2367 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #2367   +/-   ##
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  Coverage      71%     71%           
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  Files         150     150           
  Lines       11740   11740           
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  Hits         8336    8336           
  Misses       3404    3404

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[SamWildmo]

I think i also encountered this but it happened once so i cant be sure.

[yegeniy]

This is a great warning for those who are not on the latest version, but the default executor for SubDags is now the SequentialExecutor.

[SamWildmo]

@yegeiny if someone can overwrite it to local executor then this PR is still needed. If not then maybe commiters should close this.

[yegeniy]

Great point. I think it's still totally over-writeable.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.