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Make supervision tree fetching more error proof #57

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gomoripeti
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If a child is wrongly specified as supervisor, but it does not
implement this behaviour, the supervisor:which_children/0 call can
return unpredictable values. (Especially if the worker is a gen_server
which can return any sort of funny value in its handle_call).

This issue has been seen in more than one open-source library.

Hopefully closes #37

If a child is wrongly specified as `supervisor`, but it does not
implement this behaviour, the `supervisor:which_children/0` call can
return unpredictable values. (Especially if the worker is a `gen_server`
which can return any sort of funny value in its `handle_call`).

This issue has been seen in more than one open-source library.

Hopefully closes erlanglab#37
@baransu
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baransu commented Jun 1, 2017

So if some branch of sup-tree is not OTP correct for example it would skip whole branch?

@gomoripeti
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I think in most cases a worker child is incorrectly declared as a supervisor child so there are no grandchildren and it is safe to handle this process as a leaf.
If this process is a custom parent which has children but does not implement the which_children api then, yes, the subtree under it would be ignored. But in this case epl would have a hard time to figure out grandchildren anyway.
(I only know of rabbitmq which has its own supervisor implementation, but it also supports the which_children call)

@baransu
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baransu commented Jun 1, 2017

We want to enforce correct OTP structure so rather throw error than ignore something incorrect.

/ cc @michalslaski

@arkgil
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arkgil commented Jun 4, 2017

Or instead of throwing an error we could show an information in the UI that this branch of the tree cannot be resolved.

@gomoripeti
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It makes total sense not to hide the discrepancy. The easiest way would be to just crash with a very descriptive error message. But it would be more user-friendly to either print an error message (only once and not every second when the sup-tree is updated) or indicate this in the UI somehow. If it is not just a crash I would like to leave the implementation to you guys.

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Crashes wobserver
3 participants