upcxx_memberof_general failures with smp-conduit and non-fork smp spawners introduced by recent GASNet-EX

[Dan Bonachea]

Recent development versions of GASNet-EX (newer than release 2024.5.0) introduce non-fork spawning options for smp-conduit executables.

The implementation of upcxx_memberof_general currently assumes that smp-conduit implies fork-based spawning, and deploys an optimization that is only guaranteed to be valid with fork-based spawning (see memberof.hpp:109). smp-conduit's new non-fork spawning options invalidate that equivalence and render the optimization unsafe on systems with ASLR loading.

This issue is responsible for recent nightly failures of the memberof-{seq,par} test on an smp macOS tester since it began using --with-smp-spawner=pmi.