From f70f4a63b67805c77834e198644b1b5a958fb4f5 Mon Sep 17 00:00:00 2001
From: Ned Deily <nad@acm.org>
Date: Fri, 6 Sep 2013 15:16:19 -0700
Subject: [PATCH] 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          |  3 +++
 Modules/readline.c | 34 ++++++++++++++++++++++------------
 2 files changed, 25 insertions(+), 12 deletions(-)

diff --git a/Misc/NEWS b/Misc/NEWS
index b5cf807e5f..757ac89562 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -56,6 +56,9 @@ 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.
+
 - Issue #18852: Handle case of ``readline.__doc__`` being ``None`` in the new
   readline activation code in ``site.py``.
 
diff --git a/Modules/readline.c b/Modules/readline.c
index ab24636e2f..c154e1ddbd 100644
--- a/Modules/readline.c
+++ b/Modules/readline.c
@@ -63,6 +63,8 @@ extern char **completion_matches(char *, CPFunction *);
  */
 static int using_libedit_emulation = 0;
 static const char libedit_version_tag[] = "EditLine wrapper";
+
+static int libedit_history_start = 0;
 #endif /* __APPLE__ */
 
 #ifdef HAVE_RL_COMPLETION_DISPLAY_MATCHES_HOOK
@@ -627,21 +629,21 @@ 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;
         }
     }
@@ -974,6 +976,17 @@ setup_readline(readlinestate *mod_state)
      */
     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();
@@ -1178,11 +1191,8 @@ 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 = (const char *)history_get(length - 1)->line;
+                /* handle older 0-based or newer 1-based indexing */
+                line = (const char *)history_get(length + libedit_history_start - 1)->line;
             } else
 #endif /* __APPLE__ */
             line = (const char *)history_get(length)->line;
-- 
2.40.0