Memory leak in ApnsChannelPool

[Sycamore-M]

Hi，

We have an offline push service. After running for a few months, an out of memory issue occurred.

By analyzing memory snapshots, it was found that there are hundreds of thousands of DefaultPromise stacked in ApnsChannelPool.pendingAcquisitionPromises

We had to restart the service regularly before this issue was fixed.

pushy:0.15.4
jdk:11.0.25
netty:4.1.101.Final

[Sycamore-M]

I have found that when the SSL certificate expires, ApnsChannelPool.pendingAcquisionPromises will continue to expand.

[jchambers]

We discussed this a bit more in #1095. I think we agree that this isn't a memory "leak" per se (all of the memory is still accounted for and recoverable), and this is a thing that can happen with a combination of expired certificates and loose control. Since everything is working as expected within Pushy and since there are external mitigations for this problem (check certificate expiration, limit pending sends), I think it makes sense to close this for now.

Thank you for reporting the issue and for proposing a solution, though!