Re: Please add a lossage-limit option for `view-lossage'

["Bruno Félix Rezende Ribeiro" <oitofelix@gnu.org>]

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Hello Drew,

Drew Adams <drew.adams@oracle.com> writes:

> The request (and thus the change that RMS asked to be
> made) was this:
>   Any chance that the number of keystrokes displayed
>   by `view-lossage', currently hard-coded at 100,
>   could be made a user option?
>                   ^^^^^^^^^^^
> Since then, the hard-coded limit was upped to 300.
> But there still is no user option, is there?  Could
> we please add an option for this now?  The code is
> in C - it's not simple for a user to change the
> value without a Lisp variable for it.

I took this task as a goal in my first attempt to write code to Emacs C
core.  What I could get for now is in the patch attached.  It seems to
work fine for ‘num-recent-keys’[1] strictly positive, but it segfaults
promptly for other values (understandably).

What I did was to replace the C macro constant ‘NUM_RECENT_KEYS’ , with
the Lisp-visible variable ‘num-recent-keys’, defined using ‘DEFVAR_INT’,
and then define a C function called ‘ensure_recent_keys_size’ to keep
the size of the vector ‘recent-keys’ truthful to that; shrinking or
enlarging the vector.[2]

The problem is, I had to place calls to ‘ensure_recent_keys_size’
scattered throughout the source --- strategically before references to
‘num_recent_keys’.  This is clearly not the right approach.  What is
needed is a way to intercept changes to that variable and do the job
once, before anything else happens.  I think this would fix my fragile
hack.  Furthermore, that interception routine could prevent values
smaller than 1 --- set from the Lisp world --- to be rejected (not only
non-integers), hopefully preventing the crashes I’m experiencing now.

In the Lisp reference documentation, I was unable to find a way to do
that.  I thought that if that mechanism existed it’d be important enough
to be documented.  I couldn’t find it in a quick look around the code
neither.  How is the interception of change of variables from the Lisp
world canonically handled by Emacs C source code?


Footnotes: 

[1]  This is the requested user option.

[2] Actually creating another vector of the right size and copying the
    data over as needed.



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From f87911cf8f6d12046f25e2964bbbdbcd3ca0581d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bruno=20F=C3=A9lix=20Rezende=20Ribeiro?= <oitofelix@gnu.org>
Date: Mon, 6 Apr 2020 08:30:16 -0300
Subject: [PATCH] =?UTF-8?q?Transform=20C=20constant=20=E2=80=98NUM=5FRECEN?=
 =?UTF-8?q?T=5FKEYS=E2=80=99=20into=20=E2=80=98num-recent-keys=E2=80=99=20?=
 =?UTF-8?q?Lisp=20variable?=
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Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

* src/keyboard.c: remove NUM_RECENT_KEYS constant.
  (ensure_recent_keys_size): new function.
  (command_loop_1, record_char, recent-keys): add calls to
  ‘ensure_recent_key_size’.
  (syms_of_keyboard): add ‘num-recent-keys’ Lisp integer variable.
* lisp/cus-start.el: add ‘num-recent-keys’.
---
 lisp/cus-start.el |  1 +
 src/keyboard.c    | 62 ++++++++++++++++++++++++++++++++++++++-----------------
 2 files changed, 44 insertions(+), 19 deletions(-)

diff --git a/lisp/cus-start.el b/lisp/cus-start.el
index dff4d9a..001f79b 100644
--- a/lisp/cus-start.el
+++ b/lisp/cus-start.el
@@ -353,6 +353,7 @@ minibuffer-prompt-properties--setter
 	     (indent-tabs-mode indent boolean)
 	     ;; keyboard.c
 	     (meta-prefix-char keyboard character)
+             (num-recent-keys keyboard integer)
 	     (auto-save-interval auto-save integer)
              (auto-save-no-message auto-save boolean "27.1")
 	     (auto-save-timeout auto-save (choice (const :tag "off" nil)
diff --git a/src/keyboard.c b/src/keyboard.c
index 9ce168c..cbbb572 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -103,15 +103,13 @@ Copyright (C) 1985-1989, 1993-1997, 1999-2020 Free Software Foundation,
 /* True in the single-kboard state, false in the any-kboard state.  */
 static bool single_kboard;
 
-#define NUM_RECENT_KEYS (300)
-
 /* Index for storing next element into recent_keys.  */
 static int recent_keys_index;
 
 /* Total number of elements stored into recent_keys.  */
 static int total_keys;
 
-/* This vector holds the last NUM_RECENT_KEYS keystrokes.  */
+/* This vector holds the last num_recent_keys keystrokes.  */
 static Lisp_Object recent_keys;
 
 /* Vector holding the key sequence that invoked the current command.
@@ -371,6 +369,7 @@ #define READABLE_EVENTS_IGNORE_SQUEEZABLES	(1 << 2)
 static void deliver_user_signal (int);
 static char *find_user_signal_name (int);
 static void store_user_signal_events (void);
+static void ensure_recent_keys_size (void);
 
 /* Advance or retreat a buffered input event pointer.  */
 
@@ -448,6 +447,20 @@ kset_system_key_syms (struct kboard *kb, Lisp_Object val)
 }
 
 \f
+void
+ensure_recent_keys_size (void)
+{
+  if (ASIZE (recent_keys) != num_recent_keys)
+    {
+      Lisp_Object new_recent_keys = make_nil_vector (num_recent_keys);
+      memcpy (XVECTOR (new_recent_keys)->contents, XVECTOR (recent_keys)->contents,
+              min (ASIZE (recent_keys), num_recent_keys)
+              * sizeof *XVECTOR (recent_keys)->contents);
+      recent_keys = new_recent_keys;
+    }
+}
+
+\f
 static bool
 echo_keystrokes_p (void)
 {
@@ -1421,10 +1434,11 @@ command_loop_1 (void)
       /* Execute the command.  */
 
       {
-	total_keys += total_keys < NUM_RECENT_KEYS;
+        ensure_recent_keys_size ();
+	total_keys += total_keys < num_recent_keys;
 	ASET (recent_keys, recent_keys_index,
 	      Fcons (Qnil, cmd));
-	if (++recent_keys_index >= NUM_RECENT_KEYS)
+	if (++recent_keys_index >= num_recent_keys)
 	  recent_keys_index = 0;
       }
       Vthis_command = cmd;
@@ -3249,16 +3263,18 @@ record_char (Lisp_Object c)
       Lisp_Object ev1, ev2, ev3;
       int ix1, ix2, ix3;
 
+      ensure_recent_keys_size ();
+
       if ((ix1 = recent_keys_index - 1) < 0)
-	ix1 = NUM_RECENT_KEYS - 1;
+	ix1 = num_recent_keys - 1;
       ev1 = AREF (recent_keys, ix1);
 
       if ((ix2 = ix1 - 1) < 0)
-	ix2 = NUM_RECENT_KEYS - 1;
+	ix2 = num_recent_keys - 1;
       ev2 = AREF (recent_keys, ix2);
 
       if ((ix3 = ix2 - 1) < 0)
-	ix3 = NUM_RECENT_KEYS - 1;
+	ix3 = num_recent_keys - 1;
       ev3 = AREF (recent_keys, ix3);
 
       if (EQ (XCAR (c), Qhelp_echo))
@@ -3309,12 +3325,13 @@ record_char (Lisp_Object c)
     {
       if (!recorded)
 	{
-	  total_keys += total_keys < NUM_RECENT_KEYS;
+          ensure_recent_keys_size ();
+	  total_keys += total_keys < num_recent_keys;
 	  ASET (recent_keys, recent_keys_index,
                 /* Copy the event, in case it gets modified by side-effect
                    by some remapping function (bug#30955).  */
                 CONSP (c) ? Fcopy_sequence (c) : c);
-	  if (++recent_keys_index >= NUM_RECENT_KEYS)
+	  if (++recent_keys_index >= num_recent_keys)
 	    recent_keys_index = 0;
 	}
       else if (recorded < 0)
@@ -3328,10 +3345,11 @@ record_char (Lisp_Object c)
 
 	  while (recorded++ < 0 && total_keys > 0)
 	    {
-	      if (total_keys < NUM_RECENT_KEYS)
+              ensure_recent_keys_size ();
+	      if (total_keys < num_recent_keys)
 		total_keys--;
 	      if (--recent_keys_index < 0)
-		recent_keys_index = NUM_RECENT_KEYS - 1;
+		recent_keys_index = num_recent_keys - 1;
 	      ASET (recent_keys, recent_keys_index, Qnil);
 	    }
 	}
@@ -10420,22 +10438,24 @@ DEFUN ("recent-keys", Frecent_keys, Srecent_keys, 0, 1, 0,
 {
   bool cmds = !NILP (include_cmds);
 
+  ensure_recent_keys_size ();
+
   if (!total_keys
-      || (cmds && total_keys < NUM_RECENT_KEYS))
+      || (cmds && total_keys < num_recent_keys))
     return Fvector (total_keys,
 		    XVECTOR (recent_keys)->contents);
   else
     {
       Lisp_Object es = Qnil;
-      int i = (total_keys < NUM_RECENT_KEYS
+      int i = (total_keys < num_recent_keys
 	       ? 0 : recent_keys_index);
-      eassert (recent_keys_index < NUM_RECENT_KEYS);
+      eassert (recent_keys_index < num_recent_keys);
       do
 	{
 	  Lisp_Object e = AREF (recent_keys, i);
 	  if (cmds || !CONSP (e) || !NILP (XCAR (e)))
 	    es = Fcons (e, es);
-	  if (++i >= NUM_RECENT_KEYS)
+	  if (++i >= num_recent_keys)
 	    i = 0;
 	} while (i != recent_keys_index);
       es = Fnreverse (es);
@@ -11690,9 +11710,6 @@ syms_of_keyboard (void)
     staticpro (&modifier_symbols);
   }
 
-  recent_keys = make_nil_vector (NUM_RECENT_KEYS);
-  staticpro (&recent_keys);
-
   this_command_keys = make_nil_vector (40);
   staticpro (&this_command_keys);
 
@@ -11858,6 +11875,13 @@ syms_of_keyboard (void)
 result of looking up the original command in the active keymaps.  */);
   Vthis_original_command = Qnil;
 
+  DEFVAR_INT ("num-recent-keys", num_recent_keys,
+	      doc: /* Number of input events to consider. */);
+  num_recent_keys = 300;
+
+  recent_keys = make_nil_vector (num_recent_keys);
+  staticpro (&recent_keys);
+
   DEFVAR_INT ("auto-save-interval", auto_save_interval,
 	      doc: /* Number of input events between auto-saves.
 Zero means disable autosaving due to number of characters typed.  */);
-- 
2.7.4


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-- 
Bruno Félix Rezende Ribeiro (oitofelix) [0x28D618AF]
<http://oitofelix.freeshell.org/>

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