Call rb_bug when Ruby mutator thread panics

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

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 {

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 {}

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) };

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"))();
         }
     }));
 }