Handle vm_bug in MMTk GC worker threads

Do not attempt to get the execution context because it does not exist
for GC worker threads which are not Ruby threads.

error.c

@@ -50,6 +50,10 @@
 #include "ruby_assert.h"
 #include "vm_core.h"
 
+#if USE_MMTK
+#include "internal/mmtk_support.h"
+#endif
+
 #include "builtin.h"
 
 /*!
@@ -1036,6 +1040,11 @@ rb_bug_without_die(const char *fmt, va_list args)
     const char *file = NULL;
     int line = 0;
 
+#if USE_MMTK
+    if (rb_mmtk_enabled_p() && rb_mmtk_is_mmtk_worker()) {
+        file = NULL;
+    } else
+#endif
     if (GET_EC()) {
         file = rb_source_location_cstr(&line);
     }

internal/mmtk_support.h

@@ -103,6 +103,8 @@ VALUE rb_mmtk_harness_begin(VALUE _);
 VALUE rb_mmtk_harness_end(VALUE _);
 
 // Debugging
+bool rb_mmtk_is_mmtk_worker(void);
+bool rb_mmtk_is_mutator(void);
 void rb_mmtk_assert_mmtk_worker(void);
 void rb_mmtk_assert_mutator(void);
 

mmtk_support.c

@@ -1223,13 +1223,13 @@ rb_mmtk_harness_end(VALUE _)
 // Debugging
 ////////////////////////////////////////////////////////////////////////////////
 
-static inline bool
+bool
 rb_mmtk_is_mmtk_worker(void)
 {
     return rb_mmtk_gc_thread_tls != NULL;
 }
 
-static inline bool
+bool
 rb_mmtk_is_mutator(void)
 {
     return ruby_native_thread_p();