Issue #18427: str.replace could crash the interpreter with huge strings.

This fixes two places where 'int'  was used to represent
the size of strings, instead of 'Py_ssize_t'.

(The issue is not present in the corresponding code in the 3.x branches)

Fixes #18427

Misc/NEWS

@@ -24,6 +24,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #18427: str.replace could crash the interpreter with huge strings.
+
 - Issue #18347: ElementTree's html serializer now preserves the case of
   closing tags.
 
@@ -88,7 +90,7 @@ IDLE
 
 - Issue #7136: In the Idle File menu, "New Window" is renamed "New File".
   Patch by Tal Einat, Roget Serwy, and Todd Rovito.
-  
+
 - Issue #8515: Set __file__ when run file in IDLE.
   Initial patch by Bruce Frederiksen.
 

Objects/stringobject.c

@@ -882,9 +882,9 @@ string_print(PyStringObject *op, FILE *fp, int flags)
             size -= chunk_size;
         }
 #ifdef __VMS
-        if (size) fwrite(data, (int)size, 1, fp);
+        if (size) fwrite(data, (size_t)size, 1, fp);
 #else
-        fwrite(data, 1, (int)size, fp);
+        fwrite(data, 1, (size_t)size, fp);
 #endif
         Py_END_ALLOW_THREADS
         return 0;
@@ -2332,7 +2332,7 @@ return_self(PyStringObject *self)
 }
 
 Py_LOCAL_INLINE(Py_ssize_t)
-countchar(const char *target, int target_len, char c, Py_ssize_t maxcount)
+countchar(const char *target, Py_ssize_t target_len, char c, Py_ssize_t maxcount)
 {
     Py_ssize_t count=0;
     const char *start=target;