bug#33749: [PATCH] Preserve echo area contents when decoding an empty key sequence (Bug#33749)

[Yuri Khan]

* src/keyboard.h (struct kboard): New terminal-local variable
  `input-decode-preserve-echo'.

* src/keyboard.c (read_key_sequence): If the current sequence is
  mapped via `input-decode-map' to an empty sequence and
  `input-decode-preserve-echo' is non-nil, return -1.

* src/keyboard.c (command_loop_1): Save echo area contents before
  calling `read-key-sequence'. If it returns -1 and
  `input-decode-preserve-echo' is non-nil, restore echo area contents.
---
 src/keyboard.c | 27 ++++++++++++++++++++++++++-
 src/keyboard.h |  9 +++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/src/keyboard.c b/src/keyboard.c
index baf2f514409..70390ccf88c 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -1239,6 +1239,7 @@ command_loop_1 (void)
   EMACS_INT prev_modiff = 0;
   struct buffer *prev_buffer = NULL;
   bool already_adjusted = 0;
+  volatile Lisp_Object previous_echo_area_message;
 
   kset_prefix_arg (current_kboard, Qnil);
   kset_last_prefix_arg (current_kboard, Qnil);
@@ -1341,6 +1342,7 @@ command_loop_1 (void)
       Vthis_original_command = Qnil;
       Vthis_command_keys_shift_translated = Qnil;
 
+      previous_echo_area_message = Fcurrent_message ();
       /* Read next key sequence; i gets its length.  */
       raw_keybuf_count = 0;
       Lisp_Object keybuf[READ_KEY_ELTS];
@@ -1362,6 +1364,9 @@ command_loop_1 (void)
         Just loop around and read another command.  */
       if (i == -1)
        {
+         if (! NILP (KVAR (current_kboard, Vinput_decode_preserve_echo))
+             && ! NILP (previous_echo_area_message))
+           message_with_string ("%s", previous_echo_area_message, 0);
          cancel_echoing ();
          this_command_key_count = 0;
          this_single_command_key_start = 0;
@@ -8787,7 +8792,9 @@ void init_raw_keybuf_count (void)
    storing it in KEYBUF, a buffer of size READ_KEY_ELTS.
    Prompt with PROMPT.
    Return the length of the key sequence stored.
-   Return -1 if the user rejected a command menu.
+   Return -1 if the user rejected a command menu, or
+   `input-decode-preserve-echo' is non-nil and the user entered
+   a sequence that is mapped via `input-decode-map' to an empty vector.
 
    Echo starting immediately unless `prompt' is 0.
 
@@ -9540,6 +9547,9 @@ read_key_sequence (Lisp_Object *keybuf, Lisp_Object 
prompt,
          if (done)
            {
              mock_input = diff + max (t, mock_input);
+             if (! NILP (KVAR (current_kboard, Vinput_decode_preserve_echo))
+                 && mock_input == 0)
+               return -1;
              goto replay_sequence;
            }
        }
@@ -10817,6 +10827,7 @@ init_kboard (KBOARD *kb, Lisp_Object type)
   kset_system_key_syms (kb, Qnil);
   kset_window_system (kb, type);
   kset_input_decode_map (kb, Fmake_sparse_keymap (Qnil));
+  kset_input_decode_preserve_echo (kb, Qnil);
   kset_local_function_key_map (kb, Fmake_sparse_keymap (Qnil));
   Fset_keymap_parent (KVAR (kb, Vlocal_function_key_map), Vfunction_key_map);
   kset_default_minibuffer_frame (kb, Qnil);
@@ -11644,6 +11655,19 @@ and its return value (a key sequence) is used.
 The events that come from bindings in `input-decode-map' are not
 themselves looked up in `input-decode-map'.  */);
 
+  DEFVAR_KBOARD ("input-decode-preserve-echo", Vinput_decode_preserve_echo,
+                 doc: /* Preserve the echo area contents while decoding input.
+
+It is possible for `input-decode-map' to map a sequence to an empty vector,
+for example, to ignore irrelevant sequences sent by the terminal.
+
+If this variable is `nil', `read-key-sequence' will loop waiting for the
+sequence to continue.  The echo area may show the original sequence.  This
+is the historic Emacs behavior.
+
+If this variable is set to `t', `read-key-sequence' will return -1, and
+the command loop will restore the echo area contents.  */);
+
   DEFVAR_LISP ("function-key-map", Vfunction_key_map,
                doc: /* The parent keymap of all `local-function-key-map' 
instances.
 Function key definitions that apply to all terminal devices should go
@@ -11956,6 +11980,7 @@ mark_kboards (void)
       mark_object (KVAR (kb, system_key_syms));
       mark_object (KVAR (kb, Vwindow_system));
       mark_object (KVAR (kb, Vinput_decode_map));
+      mark_object (KVAR (kb, Vinput_decode_preserve_echo));
       mark_object (KVAR (kb, Vlocal_function_key_map));
       mark_object (KVAR (kb, Vdefault_minibuffer_frame));
       mark_object (KVAR (kb, echo_string));
diff --git a/src/keyboard.h b/src/keyboard.h
index ce4630b8a37..7c778292f50 100644
--- a/src/keyboard.h
+++ b/src/keyboard.h
@@ -151,6 +151,10 @@ struct kboard
        DEFVAR for more documentation.  */
     Lisp_Object Vinput_decode_map_;
 
+    /* Save and restore the echo area across sequences that are mapped
+       to the empty vector by `input-decode-map'.  */
+    Lisp_Object Vinput_decode_preserve_echo_;
+
     /* Minibufferless frames on this display use this frame's minibuffer.  */
     Lisp_Object Vdefault_minibuffer_frame_;
 
@@ -197,6 +201,11 @@ kset_input_decode_map (struct kboard *kb, Lisp_Object val)
   kb->Vinput_decode_map_ = val;
 }
 INLINE void
+kset_input_decode_preserve_echo (struct kboard *kb, Lisp_Object val)
+{
+  kb->Vinput_decode_preserve_echo_ = val;
+}
+INLINE void
 kset_last_command (struct kboard *kb, Lisp_Object val)
 {
   kb->Vlast_command_ = val;
-- 
2.20.1