[fix] Background task update_config launched multiple times #1128

[pandafy]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Fixes #1128

TODOS

• Add tests

[nemesifier]

What do you think of something like the following?

Before sending the config_modified signal for a device we check for a cache key that is unique per device, if the cache key is set, we don't send the signal otherwise we proceed.

After sending the config_modified signal for a device we can set the cache key (unique for each device) with a short duration, eg: 5 seconds (we need some heuristics to find out the right amount of seconds but it cannot be too long otherwise different changes unrelated to one another may not fire config updates).

I outlined the ideal behavior in #1142. For now, let's focus on mitigating this issue.

[nemesifier]

Why check if it's None? Wouldn't just check if it's truthy be enough?

[pandafy]

Yes, you are correct. I just wanted to be 100% sure that the cache does not contain the key.

[coveralls]

coverage: 98.607% (-0.03%) from 98.633%
when pulling f4e3b99 on issues/1128-update_config
into 84fa8fc on master.

[nemesifier]

This reminds me of the django-allauth rate limiting which we had to disable in tests.

The problem with this approach of modifying each test is that all contributors have to be aware of this, which is not scalable for future maintenance.

We need to look for a solution that doesn't require contributors to be aware of this all the time.

What if we make the timeout configurable with a setting? That would allow us to disable it globally during tests and enable it only in the subset of tests where it is needed.

[nemesifier]

Suggested change

	prevents emitting duplicate signals inside the same logical window;
	prevents emitting duplicate signals inside the same change window;

[nemesifier]

Suggested change

	if that key exists the method returns early without emitting.
	if that key exists the method returns without emitting the signal again.