Issue #18458: Prevent crashes with newer versions of libedit. Its readline

emulation has changed from 0-based indexing to 1-based like gnu readline.
Original patch by Ronald Oussoren.

Misc/NEWS

@@ -32,6 +32,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #18458: Prevent crashes with newer versions of libedit.  Its readline
+  emulation has changed from 0-based indexing to 1-based like gnu readline.
+  Original patch by Ronald Oussoren.
+
 - Issue #18919: If the close() method of a writer in the sunau or wave module
   failed, second invocation of close() and destructor no more raise an
   exception.  Second invocation of close() on sunau writer now has no effects.

Modules/readline.c

@@ -54,14 +54,16 @@ extern char **completion_matches(char *, CPFunction *);
  * with the "real" readline and cannot be detected at compile-time,
  * hence we use a runtime check to detect if we're using libedit
  *
- * Currently there is one know API incompatibility:
+ * Currently there is one known API incompatibility:
  * - 'get_history' has a 1-based index with GNU readline, and a 0-based
- *   index with libedit's emulation.
+ *   index with older versions of libedit's emulation.
  * - Note that replace_history and remove_history use a 0-based index
- *   with both implementation.
+ *   with both implementations.
  */
 static int using_libedit_emulation = 0;
 static const char libedit_version_tag[] = "EditLine wrapper";
+
+static int libedit_history_start = 0;
 #endif /* __APPLE__ */
 
 static void
@@ -555,21 +557,21 @@ get_history_item(PyObject *self, PyObject *args)
         return NULL;
 #ifdef  __APPLE__
     if (using_libedit_emulation) {
-        /* Libedit emulation uses 0-based indexes,
-         * the real one uses 1-based indexes,
-         * adjust the index to ensure that Python
-         * code doesn't have to worry about the
-         * difference.
+        /* Older versions of libedit's readline emulation
+         * use 0-based indexes, while readline and newer
+         * versions of libedit use 1-based indexes.
          */
         int length = _py_get_history_length();
-        idx --;
+
+        idx = idx - 1 + libedit_history_start;
 
         /*
          * Apple's readline emulation crashes when
          * the index is out of range, therefore
          * test for that and fail gracefully.
          */
-        if (idx < 0 || idx >= length) {
+        if (idx < (0 + libedit_history_start)
+                || idx >= (length + libedit_history_start)) {
             Py_RETURN_NONE;
         }
     }
@@ -883,6 +885,17 @@ setup_readline(void)
      */
     if (using_libedit_emulation)
         rl_initialize();
+
+    /* Detect if libedit's readline emulation uses 0-based
+     * indexing or 1-based indexing.
+     */
+    add_history("1");
+    if (history_get(1) == NULL) {
+        libedit_history_start = 0;
+    } else {
+        libedit_history_start = 1;
+    }
+    clear_history();
 #endif /* __APPLE__ */
 
     using_history();
@@ -1090,11 +1103,8 @@ call_readline(FILE *sys_stdin, FILE *sys_stdout, char *prompt)
         if (length > 0)
 #ifdef __APPLE__
             if (using_libedit_emulation) {
-                /*
-                 * Libedit's emulation uses 0-based indexes,
-                 * the real readline uses 1-based indexes.
-                 */
-                line = history_get(length - 1)->line;
+                /* handle older 0-based or newer 1-based indexing */
+                line = history_get(length + libedit_history_start - 1)->line;
             } else
 #endif /* __APPLE__ */
             line = history_get(length)->line;