Index: ChangeLog
===================================================================
--- ChangeLog	(revision 17550)
+++ ChangeLog	(working copy)
@@ -1,3 +1,7 @@
+Sun Jun 22 07:16:42 2008  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (str_buf_cat): check for self concatenation.
+
 Fri Jun 20 18:24:18 2008  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* string.c (rb_str_buf_append): should infect.
Index: string.c
===================================================================
--- string.c	(revision 17550)
+++ string.c	(working copy)
@@ -693,9 +693,13 @@
     const char *ptr;
     long len;
 {
-    long capa, total;
+    long capa, total, off = -1;;
 
     rb_str_modify(str);
+    if (ptr >= RSTRING(str)->ptr && ptr <= RSTRING(str)->ptr + RSTRING(str)->len) {
+        off = ptr - RSTRING(str)->ptr;
+    }
+    if (len == 0) return 0;
     if (FL_TEST(str, STR_ASSOC)) {
 	FL_UNSET(str, STR_ASSOC);
 	capa = RSTRING(str)->aux.capa = RSTRING(str)->len;
@@ -717,6 +721,9 @@
 	}
 	RESIZE_CAPA(str, capa);
     }
+    if (off != -1) {
+        ptr = RSTRING(str)->ptr + off;
+    }
     memcpy(RSTRING(str)->ptr + RSTRING(str)->len, ptr, len);
     RSTRING(str)->len = total;
     RSTRING(str)->ptr[total] = '\0'; /* sentinel */