Fix FreeBSD postfork child handler never being called: FreeBSD's libthr
calls _malloc_postfork in both parent and child (see freebsd-src
lib/libthr/thread/thr_fork.c), but jemalloc mapped it to the parent
handler only. Detect the child via getpid() and route to
jemalloc_postfork_child, which resets nthreads and rebuilds the
descriptor queue.
Remove the child_survivor_bytes vs pre_survivor_bytes comparison: on
macOS where jemalloc registers as the default zone, internal allocations
during the postfork handler (pthread_mutex_init) can inflate the
surviving thread's tcache.
Add double-fork test to verify prefork pid is refreshed correctly when a
child process forks again.
This adds a fast-path for threads freeing a small number of allocations to
bins which are not their "home-base" and which encounter lock contention in
attempting to do so. In producer-consumer workflows, such small lock hold times
can cause lock convoying that greatly increases overall bin mutex contention.
Forking a multithreaded process is dangerous but allowed, so long as the child
only executes async-signal-safe functions (e.g. exec). Add a test to ensure
that we don't break this behavior.
When witness is enabled, lock rank order needs to be preserved during
prefork, not only for each arena, but also across arenas. This change
breaks arena_prefork into further stages to ensure valid rank order
across arenas. Also changed test/unit/fork to use a manual arena to
catch this case.
