FREE_AT_EXIT: Don't free main stack post-fork

When a forked process was started in a thread, this would result in a
double-free during the child process exit.

    RUBY_FREE_AT_EXIT=1 ./miniruby -e 'Thread.new { fork { } }.join; Process.waitpid'

This is because the main thread in the forked process was not the
initial VM thread, and the new thread's stack was freed as part of
objectspace iteration.

This change also allows rb_threadptr_root_fiber_release to run without
EC being available.

2 files changed, 4 insertions, 3 deletions

diff --git a/cont.c b/cont.c
index e345ac42c1..2b60860edd 100644
--- a/cont.c
+++ b/cont.c
@@ -2587,12 +2587,12 @@ rb_threadptr_root_fiber_release(rb_thread_t *th)
         /* ignore. A root fiber object will free th->ec */
     }
     else {
-        rb_execution_context_t *ec = GET_EC();
+        rb_execution_context_t *ec = rb_current_execution_context(false);
 
         VM_ASSERT(th->ec->fiber_ptr->cont.type == FIBER_CONTEXT);
         VM_ASSERT(th->ec->fiber_ptr->cont.self == 0);
 
-        if (th->ec == ec) {
+        if (ec && th->ec == ec) {
             rb_ractor_set_current_ec(th->ractor, NULL);
         }
         fiber_free(th->ec->fiber_ptr);
diff --git a/vm.c b/vm.c
index 6334459579..96f4189b14 100644
--- a/vm.c
+++ b/vm.c
@@ -3056,7 +3056,8 @@ ruby_vm_destruct(rb_vm_t *vm)
                 rb_objspace_free_objects(objspace);
                 rb_free_generic_iv_tbl_();
                 rb_free_default_rand_key();
-                if (th) {
+                if (th && vm->fork_gen == 0) {
+                    /* If we have forked, main_thread may not be the initial thread */
                     xfree(stack);
                     ruby_mimfree(th);
                 }