[GitHub] [airflow] cmdoptesc opened a new pull request #5032: [AIRFLOW-4232] Add `none_skipped` trigger rule

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cmdoptesc opened a new pull request #5032: [AIRFLOW-4232] Add `none_skipped` trigger rule
URL: https://github.com/apache/airflow/pull/5032
 
 
   Make sure you have checked _all_ steps below.
   
   ### Jira
   
   - [x] My PR addresses the following [Airflow Jira](https://issues.apache.org/jira/browse/AIRFLOW-4232) issues and references them in the PR title.
   
   ### Description
   
   - [x] Here are some details about my PR, including screenshots of any UI changes:
   
   Add a "none_skipped" trigger rule so that downstream tasks will only run if all their upstream tasks have succeeded or failed.
   
   ##### Example:
   * Task 1 is a transform task that creates a temp table.
   * Task 2 is a cleanup task that drops the temp table.
   
   Task 1 >> Task 2
   
   If Task 1 succeeds or fails, Task 2 should drop the temp table as cleanup. However if the task never runs, we should just skip it.
   
   Or perhaps Task 2 is a logging task that only logs successes and failures, but doesn't log skips to cut down on noise.
   
   ### Tests
   
   - [x] My PR adds the following unit tests __OR__ does not need testing for this extremely good reason:
   
   ### Commits
   
   - [x] 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](http://chris.beams.io/posts/git-commit/)":
     1. Subject is separated from body by a blank line
     1. Subject is limited to 50 characters (not including Jira issue reference)
     1. Subject does not end with a period
     1. Subject uses the imperative mood ("add", not "adding")
     1. Body wraps at 72 characters
     1. Body explains "what" and "why", not "how"
   
   ### Documentation
   
   - [x] 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](https://github.com/apache/airflow/blob/master/UPDATING.md) so we can assign it to a appropriate release
   
   ### Code Quality
   
   - [x] Passes `flake8`
   

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