Disabling a task through 'Disable selected tasks' dropdown option does not clear 'last_run_at' field

[ellastewart]

Summary:

Disabling a task through 'Disable selected tasks' dropdown option does not clear 'last_run_at' field whereas selecting an individual task and toggling the 'Enabled' button does. When 'last_run_at' is not cleared, the task can be run on the wrong schedule when the it's re-enabled.

• Celery Version: 4.4.0
• Celery-Beat Version: django-celery-beat 2.1.0

Exact steps to reproduce the issue:

1. Disable a task through selecting the issue and using 'Disable selected tasks' from the menu dropdown
2. Wait a longer time period than the time period between scheduled tasks, and then re-enable the task
3. The task will run immediately instead of at the right time

Detailed information

Please include more detailed information here, such as relevant information about your system setup, things you did to try and debug the problem, log messages, etc.

[Kirade]

Maybe related to this PR
#501

next minor updates will fix this. (2.3.0)