matt-land opened a new pull request #4136: Fix for scheduler infinite loop when evaluating non-UTC DAGs after DST URL: https://github.com/apache/incubator-airflow/pull/4136 …ing than their start state Make sure you have checked _all_ steps below. ### Jira - [ ] My PR addresses the following [Airflow-1710](https://issues.apache.org/jira/browse/AIRFLOW/) issues and references them in the PR title. For example, "\[AIRFLOW-1710\] My Airflow PR" - https://issues.apache.org/jira/browse/AIRFLOW-1710 - 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: This fixes the infinite loop in the AF jobs scheduler scanning a DAG with a start date in Daylight Savings Time while evaluating a potential run date in Standard Time. The infinite loop Airflow gets trapped in is below: `while next_run_date <= last_run.execution_date: next_run_date = dag.following_schedule(next_run_date)` dag.following_schedule is not incrementing as expected. It instead starts to repeating values that dag.following_schedule has already returned. This is caused by an interaction between a pendulum instance timezone having a DST offset this is 'illegal' for the evaluated next run date. IE, If I call following_schedule with a dag.timezone that has a DST offset of -5, but on the date being evaluated the value should be -6, the function stops incrementing and repeats ranges of values. The fix is to simply drop the dst offset from dag.timezone. dag.following_schedule will then increment normally, it fixes the infinite loop. ### Tests - [X] My PR adds the following unit tests __OR__ does not need testing for this extremely good reason: I need help with my PR ### 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 `flake8`
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