macOS: workaround a dyld/libunwind deadlock issue since 12.1

Apple reintroduced the old bug that we previously worked around in
fad04d3 with a similar patch to this.

This is needed anywhere that we may attempt to stop threads.

Fixes #43578

Author: vtjnash

src/signals-mach.c

@@ -31,6 +31,11 @@ extern void *_keymgr_get_and_lock_processwide_ptr(unsigned int key);
 extern int _keymgr_get_and_lock_processwide_ptr_2(unsigned int key, void **result);
 extern int _keymgr_set_lockmode_processwide_ptr(unsigned int key, unsigned int mode);
 
+// private dyld3/dyld4 stuff
+extern void _dyld_atfork_prepare(void) __attribute__((weak_import));
+extern void _dyld_atfork_parent(void) __attribute__((weak_import));
+//extern void _dyld_fork_child(void) __attribute__((weak_import));
+
 static void attach_exception_port(thread_port_t thread, int segv_only);
 
 // low 16 bits are the thread id, the next 8 bits are the original gc_state
@@ -521,6 +526,28 @@ static kern_return_t profiler_segv_handler
 }
 #endif
 
+static int jl_lock_profile_mach(void)
+{
+    jl_lock_profile();
+    void *unused = NULL;
+    int keymgr_locked = _keymgr_get_and_lock_processwide_ptr_2(KEYMGR_GCC3_DW2_OBJ_LIST, &unused) == 0;
+    if (_dyld_atfork_prepare != NULL && _dyld_atfork_parent != NULL)
+        _dyld_atfork_prepare();
+    return keymgr_locked;
+}
+
+static void jl_unlock_profile_mach(int keymgr_locked)
+{
+    if (_dyld_atfork_prepare != NULL && _dyld_atfork_parent != NULL)
+        _dyld_atfork_parent();
+    if (keymgr_locked)
+        _keymgr_unlock_processwide_ptr(KEYMGR_GCC3_DW2_OBJ_LIST);
+    jl_unlock_profile();
+}
+
+#define jl_lock_profile()       int keymgr_locked = jl_lock_profile_mach()
+#define jl_unlock_profile()     jl_unlock_profile_mach(keymgr_locked)
+
 void *mach_profile_listener(void *arg)
 {
     (void)arg;
@@ -538,8 +565,6 @@ void *mach_profile_listener(void *arg)
         // sample each thread, round-robin style in reverse order
         // (so that thread zero gets notified last)
         jl_lock_profile();
-        void *unused = NULL;
-        int keymgr_locked = _keymgr_get_and_lock_processwide_ptr_2(KEYMGR_GCC3_DW2_OBJ_LIST, &unused) == 0;
         jl_shuffle_int_array_inplace(profile_round_robin_thread_order, jl_n_threads, &profile_cong_rng_seed);
         for (int idx = jl_n_threads; idx-- > 0; ) {
             // Stop the threads in the random round-robin order.
@@ -609,8 +634,6 @@ void *mach_profile_listener(void *arg)
             // We're done! Resume the thread.
             jl_thread_resume(i, 0);
         }
-        if (keymgr_locked)
-            _keymgr_unlock_processwide_ptr(KEYMGR_GCC3_DW2_OBJ_LIST);
         jl_unlock_profile();
         if (running) {
             // Reset the alarm