Only report gc_in_progress if the world is stopped

MMTk core's implementation of `gc_in_progress` returns true when a GC is
requested in MMTk, and completes right at the end of the process. this
means it returns true while the mutators are still running before
they've fully stopped, so there's a window there where it's not safe to
do things that require `gc_in_progress` to be false.

We're seeing this on Ractors where Ractor.take is trying to use
rb_objspace_each_objects, which checks that a GC is not running, but
it's reporting true, because a GC has just been requested.

This commit doesn't rely on MMTk to tell us whether a GC is running, but
uses an AtomicBool in the Ruby binding to track whether or not we've
fully stopped the mutators.

Realistically I think that rb_gc_impl_during_gc_p is badly named now
that we have GC running in seperate threads and are trying to move
towards concurrent GC, but we should tackle this later

Author: eightbitraptor

gc/mmtk/src/api.rs

@@ -269,7 +269,7 @@ pub extern "C" fn mmtk_gc_enabled_p() -> bool {
 
 #[no_mangle]
 pub extern "C" fn mmtk_gc_in_progress_p() -> bool {
-    crate::mmtk().gc_in_progress()
+    crate::collection::mutators_stopped()
 }
 
 // =============== Object allocation ===============

gc/mmtk/src/collection.rs

@@ -18,6 +18,11 @@ use std::sync::atomic::Ordering;
 use std::thread;
 
 static CURRENT_GC_MAY_MOVE: AtomicBool = AtomicBool::new(false);
+static MUTATORS_STOPPED: AtomicBool = AtomicBool::new(false);
+
+pub fn mutators_stopped() -> bool {
+    MUTATORS_STOPPED.load(Ordering::SeqCst)
+}
 
 pub struct VMCollection {}
 
@@ -31,6 +36,7 @@ impl Collection<Ruby> for VMCollection {
         F: FnMut(&'static mut mmtk::Mutator<Ruby>),
     {
         (upcalls().stop_the_world)();
+        MUTATORS_STOPPED.store(true, Ordering::SeqCst);
 
         if crate::mmtk().get_plan().current_gc_may_move_object() {
             CURRENT_GC_MAY_MOVE.store(true, Ordering::Relaxed);
@@ -48,6 +54,8 @@ impl Collection<Ruby> for VMCollection {
     }
 
     fn resume_mutators(_tls: VMWorkerThread) {
+        MUTATORS_STOPPED.store(false, Ordering::SeqCst);
+
         if CURRENT_GC_MAY_MOVE.load(Ordering::Relaxed) {
             (upcalls().after_updating_jit_code)();
         }