Backtrace for SystemStackError

* eval.c (setup_exception): set backtrace in system stack error
  other than the pre-allocated sysstack_error.  [Feature #6216]
* proc.c (Init_Proc): freeze the pre-allocated sysstack_error.
* vm_insnhelper.c (vm_stackoverflow): raise new instance for each
  times without calling any methods to keep the backtrace with no
  further stack overflow.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46502 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 8 insertions, 3 deletions

diff --git a/eval.c b/eval.c
index afad73c735..4dada48882 100644
--- a/eval.c
+++ b/eval.c
@@ -460,6 +460,12 @@ exc_setup_cause(VALUE exc, VALUE cause)
     return exc;
 }
 
+static inline int
+sysstack_error_p(VALUE exc)
+{
+    return exc == sysstack_error || (!SPECIAL_CONST_P(exc) && RBASIC_CLASS(exc) == rb_eSysStackError);
+}
+
 static void
 setup_exception(rb_thread_t *th, int tag, volatile VALUE mesg, VALUE cause)
 {
@@ -497,8 +503,7 @@ setup_exception(rb_thread_t *th, int tag, volatile VALUE mesg, VALUE cause)
 	    rb_iv_set(mesg, "bt", at);
 	}
 	else {
-	    at = get_backtrace(mesg);
-	    if (NIL_P(at)) {
+	    if (sysstack_error_p(mesg) || NIL_P(at = get_backtrace(mesg))) {
 		at = rb_vm_backtrace_object();
 		if (OBJ_FROZEN(mesg)) {
 		    mesg = rb_obj_dup(mesg);
@@ -697,7 +702,7 @@ make_exception(int argc, const VALUE *argv, int isstr)
 	exc = argv[0];
 	n = 1;
       exception_call:
-	if (exc == sysstack_error) return exc;
+	if (sysstack_error_p(exc)) return exc;
 	CONST_ID(exception, "exception");
 	mesg = rb_check_funcall(exc, exception, n, argv+1);
 	if (mesg == Qundef) {