gh-46927: Prevent readline from overriding environment

Lib/test/test_readline.py

@@ -413,6 +413,16 @@ def test_write_read_limited_history(self):
         # So, we've only tested that the read did not fail.
         # See TestHistoryManipulation for the full test.
 
+    def test_environment_is_not_modified(self):
+        # os.environ contains environment at the time "os" module was loaded, so
+        # before the "readline" module is loaded.
+        original_env = dict(os.environ)
+
+        # Force refresh of os.environ and make sure it is the same as before the
+        # refresh.
+        os.reload_environ()
+        self.assertEqual(dict(os.environ), original_env)
+
     @unittest.skipUnless(hasattr(readline, "get_pre_input_hook"),
                          "get_pre_input_hook not available")
     def test_get_pre_input_hook(self):

Misc/NEWS.d/next/Library/2025-05-07-18-31-31.gh-issue-46927.sF02gj.rst

@@ -0,0 +1,2 @@
+Prevent :mod:`readline` from overriding the ``COLUMNS`` and ``LINES``
+environment variables, as values are not updated on terminal resize.

Modules/readline.c

@@ -1351,6 +1351,13 @@ setup_readline(readlinestate *mod_state)
     /* The name must be defined before initialization */
     rl_readline_name = "python";
 
+#ifdef HAVE_RL_CHANGE_ENVIRONMENT
+    /* Prevent readline from setting the LINES and COLUMNS environment
+     * variables: ncurses prefers them over an ioctl() query, so a stale value
+     * left after a resize breaks SIGWINCH / KEY_RESIZE handling (gh-46927). */
+    rl_change_environment = 0;
+#endif
+
     /* the libedit readline emulation resets key bindings etc
      * when calling rl_initialize.  So call it upfront
      */

configure.ac

@@ -6830,6 +6830,17 @@ AS_VAR_IF([with_readline], [no], [
       AC_DEFINE([HAVE_RL_RESIZE_TERMINAL], [1], [Define if you have readline 4.0])
     ])
 
+    # rl_change_environment is in readline 6.3, but not in editline
+    AC_CACHE_CHECK([for rl_change_environment in -l$LIBREADLINE], [ac_cv_readline_rl_change_environment], [
+      AC_LINK_IFELSE(
+        [AC_LANG_PROGRAM([readline_includes], [int x = rl_change_environment])],
+        [ac_cv_readline_rl_change_environment=yes], [ac_cv_readline_rl_change_environment=no]
+      )
+    ])
+    AS_VAR_IF([ac_cv_readline_rl_change_environment], [yes], [
+      AC_DEFINE([HAVE_RL_CHANGE_ENVIRONMENT], [1], [Define if you have readline 6.3])
+    ])
+
     # check for readline 4.2
     AC_CACHE_CHECK([for rl_completion_matches in -l$LIBREADLINE], [ac_cv_readline_rl_completion_matches], [
       AC_LINK_IFELSE(

pyconfig.h.in

@@ -1153,6 +1153,9 @@
 /* Define if you can turn off readline's signal handling. */
 #undef HAVE_RL_CATCH_SIGNAL
 
+/* Define if you have readline 6.3 */
+#undef HAVE_RL_CHANGE_ENVIRONMENT
+
 /* Define to 1 if the system has the type 'rl_compdisp_func_t'. */
 #undef HAVE_RL_COMPDISP_FUNC_T