PHPC-1158 describes a situation where multiple PHP extensions interacting with OpenSSL resulted in a dangling function pointer to libmongoc's thread id callback being left in place. This ultimately produced a segfault during cURL's shutdown routines, which also interacted with OpenSSL and attempted to invoke that callback after libmongoc had already been unloaded (i.e. dlclose()).

This previously came up in CDRIVER-555. The fix (746d250, released in 1.1.2) introduced the logic we see today that checks the locking callback before assigning or unsetting libmongoc's callbacks.

I propose that _mongoc_openssl_thread_cleanup() be improved to clear the locking and id callbacks independently if either is still assigned to the libmongoc function. _mongoc_openssl_thread_startup() can be left as-is and only assign locking and id callbacks if the locking callback is currently unset.