Clean up Ractor cache after fork

Ractors created in a parent process should be properly shut down in the
child process.  They need their cache cleared and status set to
"terminated"

Co-authored-by: John Hawthorn <john@hawthorn.email>

Author: tenderlove

bootstraptest/test_ractor.rb

@@ -2210,3 +2210,10 @@ def initialize(a)
   end
   'ok'
 }
+
+# fork after creating Ractor
+assert_equal 'ok', %q{
+  Ractor.new { Ractor.receive }
+  _, status = Process.waitpid2 fork { }
+  status.success? ? "ok" : status
+}

ractor.c

@@ -2072,6 +2072,8 @@ rb_ractor_main_alloc(void)
 }
 
 #if defined(HAVE_WORKING_FORK)
+// Set up the main Ractor for the VM after fork.
+// Puts us in "single Ractor mode"
 void
 rb_ractor_atfork(rb_vm_t *vm, rb_thread_t *th)
 {
@@ -2087,6 +2089,14 @@ rb_ractor_atfork(rb_vm_t *vm, rb_thread_t *th)
     VM_ASSERT(vm->ractor.blocking_cnt == 0);
     VM_ASSERT(vm->ractor.cnt == 1);
 }
+
+void
+rb_ractor_terminate_atfork(rb_vm_t *vm, rb_ractor_t *r)
+{
+    rb_gc_ractor_cache_free(r->newobj_cache);
+    r->newobj_cache = NULL;
+    r->status_ = ractor_terminated;
+}
 #endif
 
 void rb_thread_sched_init(struct rb_thread_sched *, bool atfork);

ractor_core.h

@@ -223,6 +223,7 @@ void rb_ractor_terminate_interrupt_main_thread(rb_ractor_t *r);
 void rb_ractor_terminate_all(void);
 bool rb_ractor_main_p_(void);
 void rb_ractor_atfork(rb_vm_t *vm, rb_thread_t *th);
+void rb_ractor_terminate_atfork(rb_vm_t *vm, rb_ractor_t *th);
 VALUE rb_ractor_require(VALUE feature);
 VALUE rb_ractor_autoload_load(VALUE space, ID id);
 

thread.c

@@ -4740,6 +4740,9 @@ rb_thread_atfork_internal(rb_thread_t *th, void (*atfork)(rb_thread_t *, const r
 
     // OK. Only this thread accesses:
     ccan_list_for_each(&vm->ractor.set, r, vmlr_node) {
+        if (r != vm->ractor.main_ractor) {
+            rb_ractor_terminate_atfork(vm, r);
+        }
         ccan_list_for_each(&r->threads.set, i, lt_node) {
             atfork(i, th);
         }