Return if a GC ran or not for handle_user_collection_request (#1205)

Closes #1204

---------

Co-authored-by: Yi Lin <[email protected]>

src/memory_manager.rs

@@ -566,13 +566,21 @@ pub fn total_bytes<VM: VMBinding>(mmtk: &MMTK<VM>) -> usize {
     mmtk.get_plan().get_total_pages() << LOG_BYTES_IN_PAGE
 }
 
-/// Trigger a garbage collection as requested by the user.
+/// The application code has requested a collection. This is just a GC hint, and
+/// we may ignore it.
+///
+/// Returns whether a GC was ran or not. If MMTk triggers a GC, this method will block the
+/// calling thread and return true when the GC finishes. Otherwise, this method returns
+/// false immediately.
 ///
 /// Arguments:
 /// * `mmtk`: A reference to an MMTk instance.
 /// * `tls`: The thread that triggers this collection request.
-pub fn handle_user_collection_request<VM: VMBinding>(mmtk: &MMTK<VM>, tls: VMMutatorThread) {
-    mmtk.handle_user_collection_request(tls, false, false);
+pub fn handle_user_collection_request<VM: VMBinding>(
+    mmtk: &MMTK<VM>,
+    tls: VMMutatorThread,
+) -> bool {
+    mmtk.handle_user_collection_request(tls, false, false)
 }
 
 /// Is the object alive?

src/mmtk.rs

@@ -403,6 +403,10 @@ impl<VM: VMBinding> MMTK<VM> {
     /// The application code has requested a collection. This is just a GC hint, and
     /// we may ignore it.
     ///
+    /// Returns whether a GC was ran or not. If MMTk triggers a GC, this method will block the
+    /// calling thread and return true when the GC finishes. Otherwise, this method returns
+    /// false immediately.
+    ///
     /// # Arguments
     /// * `tls`: The mutator thread that requests the GC
     /// * `force`: The request cannot be ignored (except for NoGC)
@@ -412,11 +416,11 @@ impl<VM: VMBinding> MMTK<VM> {
         tls: VMMutatorThread,
         force: bool,
         exhaustive: bool,
-    ) {
+    ) -> bool {
         use crate::vm::Collection;
         if !self.get_plan().constraints().collects_garbage {
             warn!("User attempted a collection request, but the plan can not do GC. The request is ignored.");
-            return;
+            return false;
         }
 
         if force || !*self.options.ignore_system_gc && VM::VMCollection::is_collection_enabled() {
@@ -432,7 +436,10 @@ impl<VM: VMBinding> MMTK<VM> {
                 .store(true, Ordering::Relaxed);
             self.gc_requester.request();
             VM::VMCollection::block_for_gc(tls);
+            return true;
         }
+
+        false
     }
 
     /// MMTK has requested stop-the-world activity (e.g., stw within a concurrent gc).