From 870a79426bd37bb2eaa068cda3d97f05615ca4d5 Mon Sep 17 00:00:00 2001
From: Peter Zhu <peter@peterzhu.ca>
Date: Tue, 29 Jul 2025 16:50:58 -0400
Subject: [PATCH] Skip weak references that are special consts

If a reference marked weak becomes a special const, it will crash because
it is not a GC handled object. We should skip special consts here.
---
 gc/mmtk/mmtk.c           | 9 +++++++++
 gc/mmtk/mmtk.h           | 1 +
 gc/mmtk/src/abi.rs       | 1 +
 gc/mmtk/src/weak_proc.rs | 4 ++++
 4 files changed, 15 insertions(+)

diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index c318c6f..1c7d2d9 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -342,6 +342,14 @@ rb_mmtk_update_global_tables(int table)
     rb_gc_vm_weak_table_foreach(rb_mmtk_update_table_i, NULL, NULL, true, (enum rb_gc_vm_weak_tables)table);
 }
 
+static bool
+rb_mmtk_special_const_p(MMTk_ObjectReference object)
+{
+    VALUE obj = (VALUE)object;
+
+    return RB_SPECIAL_CONST_P(obj);
+}
+
 // Bootup
 MMTk_RubyUpcalls ruby_upcalls = {
     rb_mmtk_init_gc_worker_thread,
@@ -360,6 +368,7 @@ MMTk_RubyUpcalls ruby_upcalls = {
     rb_mmtk_update_global_tables,
     rb_mmtk_global_tables_count,
     rb_mmtk_update_finalizer_table,
+    rb_mmtk_special_const_p,
 };
 
 // Use max 80% of the available memory by default for MMTk
diff --git a/gc/mmtk/mmtk.h b/gc/mmtk/mmtk.h
index 72b4d9d..b00133a 100644
--- a/gc/mmtk/mmtk.h
+++ b/gc/mmtk/mmtk.h
@@ -68,6 +68,7 @@ typedef struct MMTk_RubyUpcalls {
     void (*update_global_tables)(int tbl_idx);
     int (*global_tables_count)(void);
     void (*update_finalizer_table)(void);
+    bool (*special_const_p)(MMTk_ObjectReference object);
 } MMTk_RubyUpcalls;
 
 typedef struct MMTk_RawVecOfObjRef {
diff --git a/gc/mmtk/src/abi.rs b/gc/mmtk/src/abi.rs
index 81e2467..2214441 100644
--- a/gc/mmtk/src/abi.rs
+++ b/gc/mmtk/src/abi.rs
@@ -313,6 +313,7 @@ pub struct RubyUpcalls {
     pub update_global_tables: extern "C" fn(tbl_idx: c_int),
     pub global_tables_count: extern "C" fn() -> c_int,
     pub update_finalizer_table: extern "C" fn(),
+    pub special_const_p: extern "C" fn(object: ObjectReference) -> bool,
 }
 
 unsafe impl Sync for RubyUpcalls {}
diff --git a/gc/mmtk/src/weak_proc.rs b/gc/mmtk/src/weak_proc.rs
index 204dd20..2a7b0b0 100644
--- a/gc/mmtk/src/weak_proc.rs
+++ b/gc/mmtk/src/weak_proc.rs
@@ -138,6 +138,10 @@ impl GCWork<Ruby> for ProcessWeakReferences {
             .expect("Mutators should not be holding the lock.");
 
         for ptr_ptr in weak_references.iter_mut() {
+            if (upcalls().special_const_p)(**ptr_ptr) {
+                continue;
+            }
+
             if !(**ptr_ptr).is_reachable() {
                 **ptr_ptr = crate::binding().weak_reference_dead_value;
             }