Wait for a pending GC before re-disabling GC after GC.start

gc/mmtk/src/api.rs

@@ -305,9 +305,13 @@ pub extern "C" fn mmtk_handle_user_collection_request(
     crate::mmtk().handle_user_collection_request(tls, force, exhaustive);
 
     if gc_was_disabled {
-        crate::mmtk()
-            .disable_collection()
-            .unwrap_or_else(|_| panic!("failed to re-disable GC after GC"));
+        // While GC was enabled, another Ractor may have requested a GC that hasn't run yet.
+        // MMTk can't disable GC until that GC runs, and it needs this thread to stop first.
+        // So block for it and try again. Since a GC is already requested, this waits for that
+        // GC instead of starting a new one.
+        while crate::mmtk().disable_collection().is_err() {
+            crate::mmtk().handle_user_collection_request(tls, true, false);
+        }
     }
 }
 

test/mmtk/test_gc.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require_relative "helper"
+
+module MMTk
+  class TestGC < TestCase
+    def test_start_from_ractors_while_disabled
+      assert_separately([], <<~RUBY)
+        Warning[:experimental] = false
+
+        GC.disable
+        ractors = 4.times.map do
+          Ractor.new do
+            10.times { GC.start }
+          end
+        end
+        ractors.each(&:join)
+
+        assert_equal(true, GC.enable)
+      RUBY
+    end
+  end
+end