From 3c6c503c5097819db6355fca904ae132e2cdcceb Mon Sep 17 00:00:00 2001
From: Takashi Kokubun <takashikkbn@gmail.com>
Date: Mon, 26 Oct 2026 22:12:50 -0700
Subject: [PATCH] Wait for a pending GC before re-disabling GC after GC.start

GC.start runs even while GC is disabled, so mmtk_handle_user_collection_request
enables GC around the collection and disables it again afterward. While GC is
enabled, another Ractor's GC.start can request a GC that hasn't run yet when
this thread tries to re-disable GC. disable_collection fails in that state,
and we panicked with "failed to re-disable GC after GC".

This showed up as an intermittent crash of test_ractor_nt_switch.rb's
gc_finalize_deferred test on the MMTk CI. Instead of panicking, block for the
pending GC and try again. Since a GC is already requested, the retry joins it
instead of starting a new one.
---
 gc/mmtk/src/api.rs   | 10 +++++++---
 test/mmtk/test_gc.rb | 23 +++++++++++++++++++++++
 2 files changed, 30 insertions(+), 3 deletions(-)
 create mode 100644 test/mmtk/test_gc.rb

diff --git a/gc/mmtk/src/api.rs b/gc/mmtk/src/api.rs
index ef0d7bf..80b0a43 100644
--- a/gc/mmtk/src/api.rs
+++ b/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);
+        }
     }
 }
 
diff --git a/test/mmtk/test_gc.rb b/test/mmtk/test_gc.rb
new file mode 100644
index 0000000..abbe531
--- /dev/null
+++ b/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