From d10fd325ddb32fc92f20a5ad75367dbbed6c26cf Mon Sep 17 00:00:00 2001
From: Peter Zhu <peter@peterzhu.ca>
Date: Fri, 19 Dec 2025 17:16:01 -0500
Subject: [PATCH] Call rb_bug when Ruby mutator thread panics

This will allow the Ruby backtrace, memory mapping, etc. to be outputted
when a Ruby mutator thread panics.
---
 gc/mmtk/mmtk.c     | 7 +++++++
 gc/mmtk/mmtk.h     | 1 +
 gc/mmtk/src/abi.rs | 1 +
 gc/mmtk/src/api.rs | 4 ++++
 gc/mmtk/src/lib.rs | 8 ++++++++
 5 files changed, 21 insertions(+)

diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index 131aaf3..52f653a 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -355,6 +355,12 @@ rb_mmtk_special_const_p(MMTk_ObjectReference object)
     return RB_SPECIAL_CONST_P(obj);
 }
 
+static void
+rb_mmtk_mutator_thread_panic_handler(void)
+{
+    rb_bug("Ruby mutator thread panicked");
+}
+
 // Bootup
 MMTk_RubyUpcalls ruby_upcalls = {
     rb_mmtk_init_gc_worker_thread,
@@ -374,6 +380,7 @@ MMTk_RubyUpcalls ruby_upcalls = {
     rb_mmtk_global_tables_count,
     rb_mmtk_update_finalizer_table,
     rb_mmtk_special_const_p,
+    rb_mmtk_mutator_thread_panic_handler,
 };
 
 // Use max 80% of the available memory by default for MMTk
diff --git a/gc/mmtk/mmtk.h b/gc/mmtk/mmtk.h
index b00133a..18466c6 100644
--- a/gc/mmtk/mmtk.h
+++ b/gc/mmtk/mmtk.h
@@ -69,6 +69,7 @@ typedef struct MMTk_RubyUpcalls {
     int (*global_tables_count)(void);
     void (*update_finalizer_table)(void);
     bool (*special_const_p)(MMTk_ObjectReference object);
+    void (*mutator_thread_panic_handler)(void);
 } MMTk_RubyUpcalls;
 
 typedef struct MMTk_RawVecOfObjRef {
diff --git a/gc/mmtk/src/abi.rs b/gc/mmtk/src/abi.rs
index 2214441..1bd19fb 100644
--- a/gc/mmtk/src/abi.rs
+++ b/gc/mmtk/src/abi.rs
@@ -314,6 +314,7 @@ pub struct RubyUpcalls {
     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,
+    pub mutator_thread_panic_handler: extern "C" fn(),
 }
 
 unsafe impl Sync for RubyUpcalls {}
diff --git a/gc/mmtk/src/api.rs b/gc/mmtk/src/api.rs
index 5217eb4..006e987 100644
--- a/gc/mmtk/src/api.rs
+++ b/gc/mmtk/src/api.rs
@@ -138,6 +138,10 @@ pub unsafe extern "C" fn mmtk_init_binding(
     upcalls: *const RubyUpcalls,
     weak_reference_dead_value: ObjectReference,
 ) {
+    crate::MUTATOR_THREAD_PANIC_HANDLER
+        .set((unsafe { (*upcalls).clone() }).mutator_thread_panic_handler)
+        .unwrap_or_else(|_| panic!("MUTATOR_THREAD_PANIC_HANDLER is already initialized"));
+
     crate::set_panic_hook();
 
     let builder = unsafe { Box::from_raw(builder) };
diff --git a/gc/mmtk/src/lib.rs b/gc/mmtk/src/lib.rs
index d16a5bf..4bcafb5 100644
--- a/gc/mmtk/src/lib.rs
+++ b/gc/mmtk/src/lib.rs
@@ -55,6 +55,11 @@ impl VMBinding for Ruby {
     type VMMemorySlice = RubyMemorySlice;
 }
 
+/// The callback for mutator thread panic handler (which calls rb_bug to output
+/// debugging information such as the Ruby backtrace and memory maps).
+/// This is set before BINDING is set because mmtk_init could panic.
+pub static MUTATOR_THREAD_PANIC_HANDLER: OnceCell<extern "C" fn()> = OnceCell::new();
+
 /// The singleton object for the Ruby binding itself.
 pub static BINDING: OnceCell<RubyBinding> = OnceCell::new();
 
@@ -132,6 +137,9 @@ pub(crate) fn set_panic_hook() {
             handle_gc_thread_panic(panic_info);
         } else {
             old_hook(panic_info);
+            (crate::MUTATOR_THREAD_PANIC_HANDLER
+                .get()
+                .expect("MUTATOR_THREAD_PANIC_HANDLER is not set"))();
         }
     }));
 }