We set in_blocking_func around some blocking C functions so that we don't
delay signal infinitely (if in_blocking_func is set we deliver signals
synchronously).
However, pthread_join is blocking but also call munmap/free to free thread
resources. If we are inside the munmap/free interceptors called from
pthread_join and deliver a signal synchronously, it can lead to deadlocks
and crashes since we re-enter runtime and try to lock the same mutexes
or use the same per-thread data structures.
If we re-enter runtime via an interceptor when in_blocking_func is set,
temporary reset in_blocking_func around the interceptor and restore it back
when we return from the recursive interceptor.
Also move in_blocking_func from ThreadSignalContext to ThreadContext
so that we can CHECK that it's not set in SlotLocker ctor.
Fixes https://github.com/google/sanitizers/issues/1540
Reviewed By: melver
Differential Revision: https://reviews.llvm.org/D127845
