ACCESS_VIOLATION errors due to nullptr writes in wepoll.c

[ratzrattillo]

I experienced ACCESS_VIOLATION errors when using libzmq in a rust project, originating from wepoll.c.

Affected lines: https://github.com/zeromq/libzmq/blob/master/external/wepoll/wepoll.c#L1223 https://github.com/zeromq/libzmq/blob/master/external/wepoll/wepoll.c#L1842

I opened an issue in the wepoll project at: https://github.com/piscisaureus/wepoll/issues/32

[notgull]

This issue probably comes from using the distribution wepoll with the patched version that the polling crate uses. polling patches wepoll so that a null completion packet can be used to wake up an ongoing poll operation, which is necessary for futures to be able to run themselves. If the distribution wepoll is overriding the patched wepoll, this means that a null packet sent by polling will cause a segmentation fault.

See smol-rs/polling#85 and Aclysma/wepoll-ffi#1. I doubt this is an issue with libzmq at this stage.