add indirection on thread start to avoid ObjC exception issues

In Mac OS X 10.11, something about the use of exceptions triggers
a libunwind stack traversal, and that traversal runs into trouble
with Racket's stack mangling for threads. Inserting generated code
in the stack frame sequence causes libunwind to give up and avoids
a crash (e.g., with `-j -l drracket` on startup).

racket/src/racket/src/jit.h

@@ -348,6 +348,7 @@ struct scheme_jit_common_record {
   void *list_ref_code, *list_tail_code;
   void *finish_tail_call_code, *finish_tail_call_fixup_code;
   void *module_run_start_code, *module_exprun_start_code, *module_start_start_code;
+  void *thread_start_child_code;
   void *box_flonum_from_stack_code, *box_flonum_from_reg_code;
   void *fl1_fail_code[JIT_NUM_FL_KINDS], *fl2rr_fail_code[2][JIT_NUM_FL_KINDS];
   void *fl2fr_fail_code[2][JIT_NUM_FL_KINDS], *fl2rf_fail_code[2][JIT_NUM_FL_KINDS];

racket/src/racket/src/jitcommon.c

@@ -3639,6 +3639,33 @@ static int more_common0(mz_jit_state *jitter, void *_data)
     scheme_jit_register_sub_func(jitter, sjc.module_start_start_code, scheme_eof);
   }
 
+  /* *** thread_start_child_code *** */
+  /* A simple indirection to generate code that libunwind can't follow,
+     particularly as used by exceptions in the Objective-C runtime */
+  {
+    int in;
+    
+    sjc.thread_start_child_code = jit_get_ip();
+    jit_prolog(2);
+    in = jit_arg_p();
+    jit_getarg_p(JIT_R0, in); /* child */
+    in = jit_arg_p();
+    jit_getarg_p(JIT_R1, in); /* child_thunk */
+    CHECK_LIMIT();
+    mz_push_locals();
+
+    jit_prepare(2);
+    jit_pusharg_p(JIT_R1);
+    jit_pusharg_p(JIT_R0);
+    (void)mz_finish(scheme_do_thread_start_child);
+    CHECK_LIMIT();
+    mz_pop_locals();
+    jit_ret();
+    CHECK_LIMIT();
+
+    scheme_jit_register_sub_func(jitter, sjc.thread_start_child_code, scheme_eof);
+  }
+
   return 1;
 }
 

racket/src/racket/src/jitstack.c

@@ -689,6 +689,7 @@ void scheme_jit_now(Scheme_Object *f)
 typedef void *(*Module_Run_Proc)(Scheme_Env *menv, Scheme_Env *env, Scheme_Object **name);
 typedef void *(*Module_Exprun_Proc)(Scheme_Env *menv, int set_ns, Scheme_Object **name);
 typedef void *(*Module_Start_Proc)(struct Start_Module_Args *a, Scheme_Object **name);
+typedef void (*Thread_Start_Child_Proc)(Scheme_Thread *child, Scheme_Object *child_thunk);
 
 void *scheme_module_run_start(Scheme_Env *menv, Scheme_Env *env, Scheme_Object *name)
 {
@@ -717,8 +718,23 @@ void *scheme_module_start_start(struct Start_Module_Args *a, Scheme_Object *name
     return scheme_module_start_finish(a);
 }
 
+void scheme_thread_start_child(Scheme_Thread *child, Scheme_Object *child_thunk)
+{
+  Thread_Start_Child_Proc proc = (Thread_Start_Child_Proc)sjc.thread_start_child_code;
+  if (proc)
+    proc(child, child_thunk);
+  else
+    scheme_do_thread_start_child(child, child_thunk);
+}
+
+
 #else
 
 void* scheme_jit_find_code_end(void *p) { return NULL; }
 
+void scheme_thread_start_child(Scheme_Thread *child, Scheme_Object *child_thunk)
+{
+  return scheme_do_thread_start_child(child, child_thunk);
+}
+
 #endif

racket/src/racket/src/schpriv.h

@@ -667,6 +667,9 @@ void scheme_suspend_remembered_threads(void);
 void scheme_resume_remembered_threads(void);
 #endif
 
+void scheme_thread_start_child(Scheme_Thread *child, Scheme_Object *child_thunk);
+void scheme_do_thread_start_child(Scheme_Thread *child, Scheme_Object *child_thunk);
+
 int scheme_wait_until_suspend_ok(void);
 
 #ifdef MZ_USE_MZRT

racket/src/racket/src/thread.c

@@ -1,6 +1,6 @@
 /*
   Racket
-  Copyright (c) 2004-2015 PLT Design Inc.
+  Copyright (c) 2004-2014 PLT Design Inc.
   Copyright (c) 1995-2001 Matthew Flatt
 
     This library is free software; you can redistribute it and/or
@@ -3167,6 +3167,11 @@ static void start_child(Scheme_Thread * volatile child,
   }
 }
 
+void scheme_do_thread_start_child(Scheme_Thread *child, Scheme_Object *child_eval)
+{
+  return start_child(child, child_eval);
+}
+
 static Scheme_Object *make_subprocess(Scheme_Object *child_thunk,
 				      void *child_start, 
 				      Scheme_Config *config,
@@ -3223,7 +3228,7 @@ static Scheme_Object *make_subprocess(Scheme_Object *child_thunk,
   child->stack_start = child_start;
 
   /* Sets the child's jmpbuf for swapping in later: */
-  start_child(child, child_thunk);
+  scheme_thread_start_child(child, child_thunk);
 
   if (scheme_notify_multithread && turn_on_multi) {
     scheme_notify_multithread(1);