[GitHub] XD-DENG opened a new pull request #3688: [AIRFLOW-2843] ExternalTaskSensor-check if external task exists

[GitBox <gi...@apache.org>]

XD-DENG opened a new pull request #3688: [AIRFLOW-2843] ExternalTaskSensor-check if external task exists
URL: https://github.com/apache/incubator-airflow/pull/3688
 
 
   ### Jira
   
   - [x] My PR addresses the following [Airflow Jira](https://issues.apache.org/jira/browse/AIRFLOW/) issues and references them in the PR title. For example, "\[AIRFLOW-XXX\] My Airflow PR"
     - https://issues.apache.org/jira/browse/AIRFLOW-2843
     - In case you are fixing a typo in the documentation you can prepend your commit with \[AIRFLOW-XXX\], code changes always need a Jira issue.
   
   ### Description
   
   - [x] Here are some details about my PR, including screenshots of any UI changes:
   
   #### Background
   `ExternalTaskSensor` will keep waiting (given restrictions of retries, poke_interval, etc), even if the external task specified doesn't exist at all. In some cases, this waiting may still make sense as new DAG may backfill.
   
   But it may be good to provide an option to cease waiting immediately if the external task specified doesn't exist.
   
   #### Proposal
   Provide an argument `check_existence`. Set to `True` to check if the external task exists, and immediately cease waiting if the external task does not exist.
   
   **The default value is set to `False` (no check or ceasing will happen), so it will not affect any existing DAGs or user expectation.**
   
   ### Tests
   
   - [ ] My PR adds the following unit tests __OR__ does not need testing for this extremely good reason:
   
   ### 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](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
   
   - [ ] In case of new functionality, my PR adds documentation that describes how to use it.
     - When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
   
   ### Code Quality
   
   - [ ] Passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
   

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