Initialize ractor thgroup in thread_do_start_proc

Followup: ruby#13013

Author: byroot

thread.c

@@ -576,6 +576,8 @@ thread_do_start_proc(rb_thread_t *th)
 
     if (th->invoke_type == thread_invoke_type_ractor_proc) {
         VALUE self = rb_ractor_self(th->ractor);
+        th->thgroup = th->ractor->thgroup_default = rb_obj_alloc(cThGroup);
+
         VM_ASSERT(FIXNUM_P(args));
         args_len = FIX2INT(args);
         args_ptr = ALLOCA_N(VALUE, args_len);
@@ -796,7 +798,6 @@ struct thread_create_params {
     // for normal proc thread
     VALUE args;
     VALUE proc;
-    VALUE group;
 
     // for ractor
     rb_ractor_t *g;
@@ -855,13 +856,7 @@ thread_create_core(VALUE thval, struct thread_create_params *params)
     }
 
     th->priority = current_th->priority;
-
-    if (params->group) {
-        th->thgroup = params->group;
-    }
-    else {
-        th->thgroup = current_th->thgroup;
-    }
+    th->thgroup = current_th->thgroup;
 
     th->pending_interrupt_queue = rb_ary_hidden_new(0);
     th->pending_interrupt_queue_checked = 0;
@@ -1001,19 +996,12 @@ rb_thread_create(VALUE (*fn)(void *), void *arg)
 VALUE
 rb_thread_create_ractor(rb_ractor_t *r, VALUE args, VALUE proc)
 {
-    VALUE thgroup = r->thgroup_default = rb_obj_alloc(cThGroup);
-#if RACTOR_CHECK_MODE > 0
-    rb_ractor_setup_belonging_to(thgroup, r->pub.id);
-#endif
-
     struct thread_create_params params = {
         .type = thread_invoke_type_ractor_proc,
         .g = r,
-        .group = thgroup,
         .args = args,
         .proc = proc,
     };
-    RB_GC_GUARD(thgroup);
     return thread_create_core(rb_thread_alloc(rb_cThread), &params);;
 }