[PyQt] sip: Accessing members results in incorrect/missing data

[MatthijsBurgh]

Phil Thompson-5 wrote
> On 22/01/2020 13:57, MatthijsBurgh wrote:
>> The first changeset already fixed the issue.
>> 
>> The newer changeset creates the situation as desribed in
>> http://python.6.x6.nabble.com/sip-Accessing-members-results-in-incorrect-missing-data-tp5266521p5266740.html,
>> it looks like the release function isn't called anymore. I thought 
>> Python GC
>> was able to break up circular references, but it looks like it isn't
>> happening.
>> 
>> By running the following code:
>> ```
>> v = Vector(1,2,3)
>> R = Rotation()
>> F = Frame(R,v)
>> v2 = F.p
>> v3 = Frame(F).p
>> v3 = Vector(2,3,4)
>> ```
>> The `release_Frame` is called(print shows up) after the second `v3`
>> assignment when using your first changeset, but the print doesn't show 
>> up
>> with your second changeset, today's one.
>> 
>> So to me it looks like this will cause a memory leak.
> 
> In my own test cases the cycle is broken. Have you run gc.collect()? If 
> you print out the reference counts to they look correct?
> 
>> Side note: I would prefer the usage of a specific key for the member
>> reference to the containing class.
>> I don't think using the same key for both references will cause a 
>> problem
>> with overwriting. As a class can't contain a member of the same type, 
>> only a
>> member as a pointer to the same type. In which case, the reference 
>> isn't
>> kept, if I understand the code correct.
> 
> There can be a problem (now fixed) if the type of the variable is 
> defined in a different module from the type of the container.
> 
> Phil
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In your last changeset (but also the second one), the new frame keeps alive.
Even after `v3` is assigned a new value. Therefore the unassigned Frame,
from which only `p` was accessed, stays alive till the end of the Python
session.

This happens because `Frame` has a ref to `p`, because of the caching of the
python object. And `p` has a reference to `Frame`, to keep the class alive,
to be able to access the member `p`.

I don't see an easy solution for this at the moment. Do you?

Matthijs




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