MJIT: Do not hang after forking with threads

First, rb_mjit_fork should call rb_thread_atfork to stop threads after
fork in the child process. Unfortunately, we cannot use rb_fork_ruby to
prevent this kind of mistakes because MJIT needs special handling of
waiting_pid and mjit_pause/resume.

Second, mjit_waitpid_finished should be checked regardless of
trap_interrupt. It doesn't seem like the flag is not set when SIGCHLD is
handled for an MJIT child process.

Author: k0kubun

process.c

@@ -4222,6 +4222,7 @@ rb_mjit_fork(void)
 
     after_fork_ruby();
     disable_child_handler_fork_parent(&old);
+    if (pid == 0) rb_thread_atfork();
 
     return pid;
 }

thread.c

@@ -2321,16 +2321,16 @@ rb_threadptr_execute_interrupts(rb_thread_t *th, int blocking_timing)
                 ret |= rb_signal_exec(th, sig);
             }
             th->status = prev_status;
+        }
 
 #if USE_MJIT
-            // Handle waitpid_signal for MJIT issued by ruby_sigchld_handler. This needs to be done
-            // outside ruby_sigchld_handler to avoid recursively relying on the SIGCHLD handler.
-            if (mjit_waitpid_finished) {
-                mjit_waitpid_finished = false;
-                mjit_notify_waitpid(WIFEXITED(mjit_waitpid_status) ? WEXITSTATUS(mjit_waitpid_status) : -1);
-            }
-#endif
+        // Handle waitpid_signal for MJIT issued by ruby_sigchld_handler. This needs to be done
+        // outside ruby_sigchld_handler to avoid recursively relying on the SIGCHLD handler.
+        if (mjit_waitpid_finished && th == th->vm->ractor.main_thread) {
+            mjit_waitpid_finished = false;
+            mjit_notify_waitpid(WIFEXITED(mjit_waitpid_status) ? WEXITSTATUS(mjit_waitpid_status) : -1);
         }
+#endif
 
         /* exception from another thread */
         if (pending_interrupt && threadptr_pending_interrupt_active_p(th)) {