1519 callbacks waiting fix

[eff-kay]

Fixes #1519

the description of what this PR does is found in this comment

#1627 (comment)

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[eff-kay]

@alexcjohnson rebasing #1627, took way longer than expected, so I simply created a new branch. This is way cleaner and easier to test. Marc-Andres comment is pretty much what I am doing to fix the issue, in that I traverse the entire tree and filter out duplicate calls.

[alexcjohnson]

💃 Thanks @eff-kay, looks good to me! I refactored your test a little, to sequence based on a Lock rather than delays and to look at number of callback invocations to ensure we aren't missing an intermediate state - but I'm pretty sure it still tests the same behavior.

[eff-kay]

💃 Thanks @eff-kay, looks good to me! I refactored your test a little, to sequence based on a Lock rather than delays and to look at number of callback invocations to ensure we aren't missing an intermediate state - but I'm pretty sure it still tests the same behaviour.

@alexcjohnson I verified the test, it validates the correct thing. I have one small comment on understanding how locks work, other than that feel free to merge the PR.