Spotty push notifications delivery on iOS

[eoger]

I've noticed some Push notifications sent from the FxA servers never seem to arrive on iOS on a regular basis. We've investigated on the FxA side and it looks like Autopush is returning an error (we think a 502) calling the Push endpoint, but we didn't get more information.

From the slack conversion on #push, some errors were logged but couldn't get serialized to a string:
"message": "Uncaught exception in write_error: "

[jrconlin]

Will contact you offline, but I'm going to need a few things to diagnose what's going on here.

If possible, could you provide:
Sample message content for a notification.
if this is prod or stage
potentially some APNs credentials so I can test this outside of some sandbox type application, etc.

[eoger]

This is happening on prod. I've sent you my test app by email. Thanks for looking into this!

[scheinercc]

Could this be related? mozilla-mobile/fenix#8582

[jrconlin]

@scheinercc I don't believe so, since the bridge system for FCM is different than for iOS, we generally detect unregistered users better. Still #1415 may help address any similar problem if it's present.