[GitHub] [airflow] drewsonne opened a new pull request #5085: [WIP][AIRFLOW-594] Add dag source plugin hooks

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drewsonne opened a new pull request #5085: [WIP][AIRFLOW-594] Add dag source plugin hooks
URL: https://github.com/apache/airflow/pull/5085
 
 
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   ### 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, "\[AIRFLOW-594\] My Airflow PR"
     - https://issues.apache.org/jira/browse/AIRFLOW-594
     - In case you are fixing a typo in the documentation you can prepend your commit with \[AIRFLOW-594\], code changes always need a Jira issue.
   
   ### Description
   
   - [x] Here are some details about my PR, including screenshots of any UI changes:
   
     This PR is a precursor to loading DAGs from pip packages. It provides a new plugin structure, `AirflowPluginDagSource` with two methods `collect_dags_from_plugins` and `put_dag_plugins_on_disk` where a Plugin author can:
    -  implement the former and provide logic to inject the `airflow.model.DAG` objects into a provided `airflow.modal.DagBag` object using their own custom logic,
    - or implement the latter and provide logic to write the dag python files to disk in the dag folder where they will be collected by the normal DAG loading setup.
   
   From discussion below, potential use cases could be:
    - to load dags from pip packages
    - to load dags from (for example) S3, GCP Cloud Storage, Azure Blob Storage (using the built in hooks/connections);
    - integration with say, https://github.com/wooga/airconditioner without mixing yaml and python for dag definitions;
    - tighter integration with https://github.com/python-bonobo/bonobo
    - as an intermediary step to handle some automatic lineage injection on DAG load
    - perform some validation or linting of DAGs before loading.
   
   ### Tests
   
   - [ ] 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.
     - When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
     - All the public functions and the classes in the PR contain docstrings that explain what it does
   
   ### Code Quality
   
   - [x] Passes `flake8`
   

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