Currently task instances get created and scheduled based on the DAG's start date rather than their own. This PR adds a check before creating a task instance to see that the start date is not after the execution date.
### 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-1837 ### Description - [x] Here are some details about my PR, including screenshots of any UI changes: Currently task instances get created and scheduled based on the DAG's start date rather than their own. This PR adds a check before creating a task instance to see that the start date is not after the execution date. I tried this first in https://github.com/apache/incubator-airflow/pull/4000 but didn't realize that it's intended behavior (at least in many tests) to allow BackfillJob to run before dags'/tasks' start_date. ### Tests - [x] My PR adds the following unit tests __OR__ does not need testing for this extremely good reason: SchedulerJobTest.test_scheduler_task_start_date ### 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 - [N/A] 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 - [x] Passes `git diff upstream/master -u -- "*.py" | flake8 --diff` [ Full content available at: https://github.com/apache/incubator-airflow/pull/4010 ] This message was relayed via gitbox.apache.org for devnull@infra.apache.org
