Apparently, `sigprocmask()` on macOS affects all threads. You must use `pthread_sigmask()` to affect only the current thread. (To be fair, POSIX says that `sigprocmask()` has unspecified behavior in the presence of threads. However, having it affect all threads is bizarre. On Linux, it affects only the calling thread.)

Moreover, `siglongjmp()` on macOS is implemented in terms of `sigprocmask()`. Hence, `siglongjmp()` (with `true` for the second parameter) is not safe to use in a multithreaded program.

Instead, we must decompose `siglongjmp()` into its component parts, so that we can correctly use `pthread_sigmask()`.

This fixes the deadlocks on macOS.