--- python-cjson-1.0.5.orig/jsontest.py
+++ python-cjson-1.0.5/jsontest.py
@@ -316,6 +316,18 @@
 
     def testWriteLong(self):
         self.assertEqual("12345678901234567890", cjson.encode(12345678901234567890))
+
+    def testWriteLongUnicode(self):
+        # This test causes a buffer overrun in cjson 1.0.5, on UCS4 builds.
+        # The string length is only resized for wide unicode characters if
+        # there is less than 12 bytes of space left. Padding with
+        # narrow-but-escaped characters prevents string resizing.
+        # Note that u'\U0001D11E\u1234' also breaks, but sometimes goes
+        # undetected.
+        s = cjson.encode(u'\U0001D11E\U0001D11E\U0001D11E\U0001D11E'
+                         u'\u1234\u1234\u1234\u1234\u1234\u1234')
+        self.assertEqual(r'"\U0001d11e\U0001d11e\U0001d11e\U0001d11e'
+                         r'\u1234\u1234\u1234\u1234\u1234\u1234"', s)
         
 def main():
     unittest.main()
--- python-cjson-1.0.5.orig/cjson.c
+++ python-cjson-1.0.5/cjson.c
@@ -613,6 +613,25 @@
     char *p;
 
     static const char *hexdigit = "0123456789abcdef";
+#ifdef Py_UNICODE_WIDE
+    const Py_ssize_t expandsize = 10;
+#else
+    const Py_ssize_t expandsize = 6;
+#endif
+
+    /* Initial allocation is based on the longest-possible unichr
+       escape.
+
+       In wide (UTF-32) builds '\U00xxxxxx' is 10 chars per source
+       unichr, so in this case it's the longest unichr escape. In
+       narrow (UTF-16) builds this is five chars per source unichr
+       since there are two unichrs in the surrogate pair, so in narrow
+       (UTF-16) builds it's not the longest unichr escape.
+
+       In wide or narrow builds '\uxxxx' is 6 chars per source unichr,
+       so in the narrow (UTF-16) build case it's the longest unichr
+       escape.
+    */
 
     s = PyUnicode_AS_UNICODE(unicode);
     size = PyUnicode_GET_SIZE(unicode);
@@ -623,7 +642,7 @@
         return NULL;
     }
 
-    repr = PyString_FromStringAndSize(NULL, 2 + 6*size + 1);
+    repr = PyString_FromStringAndSize(NULL, 2 + expandsize*size + 1);
     if (repr == NULL)
         return NULL;
 
@@ -644,15 +663,6 @@
 #ifdef Py_UNICODE_WIDE
         /* Map 21-bit characters to '\U00xxxxxx' */
         else if (ch >= 0x10000) {
-            int offset = p - PyString_AS_STRING(repr);
-
-            /* Resize the string if necessary */
-            if (offset + 12 > PyString_GET_SIZE(repr)) {
-                if (_PyString_Resize(&repr, PyString_GET_SIZE(repr) + 100))
-                    return NULL;
-                p = PyString_AS_STRING(repr) + offset;
-            }
-
             *p++ = '\\';
             *p++ = 'U';
             *p++ = hexdigit[(ch >> 28) & 0x0000000F];