SA_IMMUTABLE and the hazards of messing with signals

I don't agree with this at all. Signal-safety and thread-safety are actually very different concepts with very different constraints. For example, you can make code thread-safe by using mutexes, but that actually doesn't work at all for signal handlers because you will instantly deadlock if a signal handler tries to acquire a mutex held by the thread it interrupted (assuming non-reentrant mutexes; if your mutexes are reentrant the situation is even worse since you won't get mutual exclusion).