PyUnicode_Join(): Two primary aims:

1. u1.join([u2]) is u2
2. Be more careful about C-level int overflow.

Since PySequence_Fast() isn't needed to achieve #1, it's not used -- but
the code could sure be simpler if it were.

Objects/unicodeobject.c

@@ -3975,49 +3975,110 @@ int fixtitle(PyUnicodeObject *self)
     return 1;
 }
 
-PyObject *PyUnicode_Join(PyObject *separator,
-			 PyObject *seq)
+PyObject *
+PyUnicode_Join(PyObject *separator, PyObject *seq)
 {
+    PyObject *internal_separator = NULL;
     Py_UNICODE *sep;
-    int seplen;
+    size_t seplen;
     PyUnicodeObject *res = NULL;
-    int reslen = 0;
-    Py_UNICODE *p;
-    int sz = 100;
+    size_t sz;      /* # allocated bytes for string in res */
+    size_t reslen;  /* # used bytes */
+    Py_UNICODE *p;  /* pointer to free byte in res's string area */
+    PyObject *it;   /* iterator */
+    PyObject *item;
     int i;
-    PyObject *it;
+    PyObject *temp;
 
     it = PyObject_GetIter(seq);
     if (it == NULL)
         return NULL;
 
-    if (separator == NULL) {
-	Py_UNICODE blank = ' ';
-	sep = ␣
-	seplen = 1;
+    item = PyIter_Next(it);
+    if (item == NULL) {
+        if (PyErr_Occurred())
+            goto onError;
+        /* empty sequence; return u"" */
+        res = _PyUnicode_New(0);
+        goto Done;
     }
-    else {
-	separator = PyUnicode_FromObject(separator);
-	if (separator == NULL)
+
+    /* If this is the only item, maybe we can get out cheap. */
+    res = (PyUnicodeObject *)item;
+    item = PyIter_Next(it);
+    if (item == NULL) {
+        if (PyErr_Occurred())
             goto onError;
-	sep = PyUnicode_AS_UNICODE(separator);
-	seplen = PyUnicode_GET_SIZE(separator);
+        /* There's only one item in the sequence. */
+        if (PyUnicode_CheckExact(res)) /* whatever.join([u]) -> u */
+            goto Done;
     }
 
+    /* There are at least two to join (item != NULL), or there's only
+     * one but it's not an exact Unicode (item == NULL).  res needs
+     * conversion to Unicode in either case.
+     * Caution:  we may need to ensure a copy is made, and that's trickier
+     * than it sounds because, e.g., PyUnicode_FromObject() may return
+     * a shared object (which must not be mutated).
+     */
+    if (! PyUnicode_Check(res) && ! PyString_Check(res)) {
+        PyErr_Format(PyExc_TypeError,
+                "sequence item 0: expected string or Unicode,"
+    	        " %.80s found",
+    	       res->ob_type->tp_name);
+    	Py_XDECREF(item);
+        goto onError;
+    }
+    temp = PyUnicode_FromObject((PyObject *)res);
+    if (temp == NULL) {
+        Py_XDECREF(item);
+        goto onError;
+    }
+    Py_DECREF(res);
+    if (item == NULL) {
+    	/* res was the only item */
+        res = (PyUnicodeObject *)temp;
+        goto Done;
+    }
+    /* There are at least two items.  As above, temp may be a shared object,
+     * so we need to copy it.
+     */
+    reslen = PyUnicode_GET_SIZE(temp);
+    sz = reslen + 100;  /* breathing room */
+    if (sz < reslen || sz > INT_MAX) /* overflow -- no breathing room */
+    	sz = reslen;
     res = _PyUnicode_New(sz);
-    if (res == NULL)
+    if (res == NULL) {
+        Py_DECREF(item);
         goto onError;
+    }
     p = PyUnicode_AS_UNICODE(res);
-    reslen = 0;
+    Py_UNICODE_COPY(p, PyUnicode_AS_UNICODE(temp), (int)reslen);
+    p += reslen;
+    Py_DECREF(temp);
 
-    for (i = 0; ; ++i) {
-	int itemlen;
-	PyObject *item = PyIter_Next(it);
-	if (item == NULL) {
-	    if (PyErr_Occurred())
+    if (separator == NULL) {
+	Py_UNICODE blank = ' ';
+	sep = &blank;
+	seplen = 1;
+    }
+    else {
+	internal_separator = PyUnicode_FromObject(separator);
+	if (internal_separator == NULL) {
+	    Py_DECREF(item);
 	    goto onError;
-	    break;
 	}
+	sep = PyUnicode_AS_UNICODE(internal_separator);
+	seplen = PyUnicode_GET_SIZE(internal_separator);
+    }
+
+    i = 1;
+    do {
+	size_t itemlen;
+	size_t newreslen;
+
+	/* Catenate the separator, then item. */
+	/* First convert item to Unicode. */
 	if (!PyUnicode_Check(item)) {
 	    PyObject *v;
 	    if (!PyString_Check(item)) {
@@ -4034,36 +4095,55 @@ PyObject *PyUnicode_Join(PyObject *separator,
 	    if (item == NULL)
 		goto onError;
 	}
+        /* Make sure we have enough space for the separator and the item. */
 	itemlen = PyUnicode_GET_SIZE(item);
-	while (reslen + itemlen + seplen >= sz) {
-	    if (_PyUnicode_Resize(&res, sz*2) < 0) {
+	newreslen = reslen + seplen + itemlen;
+	if (newreslen < reslen ||  newreslen > INT_MAX)
+	    goto Overflow;
+	if (newreslen > sz) {
+	    do {
+	        size_t oldsize = sz;
+	        sz += sz;
+	        if (sz < oldsize || sz > INT_MAX)
+	            goto Overflow;
+	    } while (newreslen > sz);
+	    if (_PyUnicode_Resize(&res, (int)sz) < 0) {
 		Py_DECREF(item);
 		goto onError;
 	    }
-	    sz *= 2;
             p = PyUnicode_AS_UNICODE(res) + reslen;
 	}
-	if (i > 0) {
-	    Py_UNICODE_COPY(p, sep, seplen);
+	Py_UNICODE_COPY(p, sep, (int)seplen);
 	p += seplen;
-	    reslen += seplen;
-	}
-	Py_UNICODE_COPY(p, PyUnicode_AS_UNICODE(item), itemlen);
+	Py_UNICODE_COPY(p, PyUnicode_AS_UNICODE(item), (int)itemlen);
 	p += itemlen;
-	reslen += itemlen;
 	Py_DECREF(item);
-    }
-    if (_PyUnicode_Resize(&res, reslen) < 0)
+	reslen = newreslen;
+
+        ++i;
+	item = PyIter_Next(it);
+    } while (item != NULL);
+    if (PyErr_Occurred())
+	goto onError;
+
+    if (_PyUnicode_Resize(&res, (int)reslen) < 0)
 	goto onError;
 
-    Py_XDECREF(separator);
+ Done:
+    Py_XDECREF(internal_separator);
     Py_DECREF(it);
     return (PyObject *)res;
 
+ Overflow:
+    PyErr_SetString(PyExc_OverflowError,
+                    "join() is too long for a Python string");
+    Py_DECREF(item);
+    /* fall through */
+
  onError:
-    Py_XDECREF(separator);
-    Py_XDECREF(res);
+    Py_XDECREF(internal_separator);
     Py_DECREF(it);
+    Py_XDECREF(res);
     return NULL;
 }