[jira] [Work started] (AIRFLOW-931) LocalExecutor fails to run queued task with race condition

["Bolke de Bruin (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/AIRFLOW-931?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Work on AIRFLOW-931 started by Bolke de Bruin.
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> LocalExecutor fails to run queued task with race condition
> ----------------------------------------------------------
>
>                 Key: AIRFLOW-931
>                 URL: https://issues.apache.org/jira/browse/AIRFLOW-931
>             Project: Apache Airflow
>          Issue Type: Sub-task
>    Affects Versions: Airflow 1.8, 1.8.0rc4
>            Reporter: Vijay Krishna Ramesh
>            Assignee: Bolke de Bruin
>
> https://gist.github.com/vijaykramesh/707262c83429ab2a3f5ee701879813e3 provides a small example that consistently hits this problem with LocalExecutor.
> Basically when the dag run kicks off (with concurrency > 1) and a LocalExecutor with parallelism > 2 the scheduler marks more than concurrency tasks as queued (https://github.com/apache/incubator-airflow/blob/master/airflow/jobs.py#L1095)
> There is a second check before actually running the task (https://github.com/apache/incubator-airflow/blob/master/airflow/models.py#L1291) that leaves the task in the QUEUED state but then the scheduler never picks it back up.  This causes the DAG to get stuck (as the queued tasks never run) until the scheduler is restarted (at which point the enqueued tasks are considered orphaned, the status is set to NONE, and then they are picked up by the scheduler again and run.



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