Improve locked narrowing around low-level hooks.

* src/buffer.c (syms_of_buffer): Two new variables,
'long-line-locked-narrowing-region-size' and
'long-line-locked-narrowing-bol-search-limit', to make the locked
narrowing around low-level hooks configurable.

Increase the default value of 'long-line-threshold'.  After
carefully considering the (few) bug reports about long line
optimizations, I concluded that the previous default value was too
low.

* src/xdisp.c (get_locked_narrowing_begv)
(get_locked_narrowing_zv): Two new functions.
(handle_fontified_prop, reseat): Use them.

* src/keyboard.c (safe_run_hooks_maybe_narrowed): Use them.

* src/dispextern.h (struct it): Add two new fields to store the
values returned by these functions.
Make them externally visible.

* src/editfns.c: (Fsave_restriction): Update docstring.

src/buffer.c

@@ -5882,7 +5882,40 @@ this threshold.
 If nil, these display shortcuts will always remain disabled.
 
 There is no reason to change that value except for debugging purposes.  */);
-  XSETFASTINT (Vlong_line_threshold, 10000);
+  XSETFASTINT (Vlong_line_threshold, 50000);
+
+  DEFVAR_INT ("long-line-locked-narrowing-region-size",
+	      long_line_locked_narrowing_region_size,
+	      doc: /* Region size for locked narrowing in buffers with long lines.
+
+This variable has effect only in buffers which contain one or more
+lines whose length is above `long-line-threshold', which see.  For
+performance reasons, in such buffers, low-level hooks such as
+`fontification-functions' or `post-command-hook' are executed on a
+narrowed buffer, with a narrowing locked with `narrowing-lock'.  This
+variable specifies the size of the narrowed region around point.
+
+To disable that narrowing, set this variable to 0.
+
+There is no reason to change that value except for debugging purposes.  */);
+  long_line_locked_narrowing_region_size = 500000;
+
+  DEFVAR_INT ("long-line-locked-narrowing-bol-search-limit",
+	      long_line_locked_narrowing_bol_search_limit,
+	      doc: /* Limit for beginning of line search in buffers with long lines.
+
+This variable has effect only in buffers which contain one or more
+lines whose length is above `long-line-threshold', which see.  For
+performance reasons, in such buffers, low-level hooks such as
+`fontification-functions' or `post-command-hook' are executed on a
+narrowed buffer, with a narrowing locked with `narrowing-lock'.  The
+variable `long-line-locked-narrowing-region-size' specifies the size
+of the narrowed region around point.  This variable, which should be a
+small integer, specifies the number of characters by which that region
+can be extended backwards to start it at the beginning of a line.
+
+There is no reason to change that value except for debugging purposes.  */);
+  long_line_locked_narrowing_bol_search_limit = 128;
 
   DEFVAR_INT ("large-hscroll-threshold", large_hscroll_threshold,
     doc: /* Horizontal scroll of truncated lines above which to use redisplay shortcuts.

src/dispextern.h

@@ -2342,6 +2342,14 @@ struct it
      optimize display.  */
   ptrdiff_t narrowed_zv;
 
+  /* Begin position of the buffer for the locked narrowing around
+     low-level hooks.  */
+  ptrdiff_t locked_narrowing_begv;
+
+  /* End position of the buffer for the locked narrowing around
+     low-level hooks.  */
+  ptrdiff_t locked_narrowing_zv;
+
   /* C string to iterate over.  Non-null means get characters from
      this string, otherwise characters are read from current_buffer
      or it->string.  */
@@ -3405,6 +3413,8 @@ void init_iterator (struct it *, struct window *, ptrdiff_t,
 ptrdiff_t get_narrowed_begv (struct window *, ptrdiff_t);
 ptrdiff_t get_narrowed_zv (struct window *, ptrdiff_t);
 ptrdiff_t get_closer_narrowed_begv (struct window *, ptrdiff_t);
+ptrdiff_t get_locked_narrowing_begv (ptrdiff_t);
+ptrdiff_t get_locked_narrowing_zv (ptrdiff_t);
 void init_iterator_to_row_start (struct it *, struct window *,
                                  struct glyph_row *);
 void start_display (struct it *, struct window *, struct text_pos);

src/editfns.c

@@ -2659,7 +2659,7 @@ DEFUN ("delete-and-extract-region", Fdelete_and_extract_region,
    the (uninterned) Qoutermost_narrowing tag and records the narrowing
    bounds that were set by the user and that are visible on display.
    This alist is used internally by narrow-to-region, widen,
-   narrowing-lock and narrowing-unlock.  */
+   narrowing-lock, narrowing-unlock and save-restriction.  */
 static Lisp_Object narrowing_locks;
 
 /* Retrieve one of the BEGV/ZV bounds of a narrowing in BUF from the
@@ -3061,11 +3061,12 @@ DEFUN ("save-restriction", Fsave_restriction, Ssave_restriction, 0, UNEVALLED, 0
        doc: /* Execute BODY, saving and restoring current buffer's restrictions.
 The buffer's restrictions make parts of the beginning and end invisible.
 \(They are set up with `narrow-to-region' and eliminated with `widen'.)
-This special form, `save-restriction', saves the current buffer's restrictions
-when it is entered, and restores them when it is exited.
+This special form, `save-restriction', saves the current buffer's
+restrictions, as well as their locks if they have been locked with
+`narrowing-lock', when it is entered, and restores them when it is exited.
 So any `narrow-to-region' within BODY lasts only until the end of the form.
-The old restrictions settings are restored
-even in case of abnormal exit (throw or error).
+The old restrictions settings are restored even in case of abnormal exit
+\(throw or error).
 
 The value returned is the value of the last form in BODY.
 

src/keyboard.c

@@ -1911,8 +1911,8 @@ safe_run_hooks_maybe_narrowed (Lisp_Object hook, struct window *w)
   specbind (Qinhibit_quit, Qt);
 
   if (current_buffer->long_line_optimizations_p)
-    narrow_to_region_locked (make_fixnum (get_narrowed_begv (w, PT)),
-			     make_fixnum (get_narrowed_zv (w, PT)),
+    narrow_to_region_locked (make_fixnum (get_locked_narrowing_begv (PT)),
+			     make_fixnum (get_locked_narrowing_zv (PT)),
 			     hook);
 
   run_hook_with_args (2, ((Lisp_Object []) {hook, hook}),

src/xdisp.c

@@ -3533,6 +3533,34 @@ get_closer_narrowed_begv (struct window *w, ptrdiff_t pos)
   return max ((pos / len - 1) * len, BEGV);
 }
 
+ptrdiff_t
+get_locked_narrowing_begv (ptrdiff_t pos)
+{
+  if (long_line_locked_narrowing_region_size == 0)
+    return BEGV;
+  int begv;
+  int len = long_line_locked_narrowing_region_size / 2;
+  begv = max (pos - len, BEGV);
+  int limit = long_line_locked_narrowing_bol_search_limit;
+  while (limit)
+    {
+      if (begv == BEGV || FETCH_BYTE (CHAR_TO_BYTE (begv) - 1) == '\n')
+	return begv;
+      begv--;
+      limit--;
+    }
+  return begv;
+}
+
+ptrdiff_t
+get_locked_narrowing_zv (ptrdiff_t pos)
+{
+  if (long_line_locked_narrowing_region_size == 0)
+    return ZV;
+  int len = long_line_locked_narrowing_region_size / 2;
+  return min (pos + len, ZV);
+}
+
 static void
 unwind_narrowed_begv (Lisp_Object point_min)
 {
@@ -4368,13 +4396,13 @@ handle_fontified_prop (struct it *it)
 
       if (current_buffer->long_line_optimizations_p)
 	{
-	  ptrdiff_t begv = it->narrowed_begv;
-	  ptrdiff_t zv = it->narrowed_zv;
+	  ptrdiff_t begv = it->locked_narrowing_begv;
+	  ptrdiff_t zv = it->locked_narrowing_zv;
 	  ptrdiff_t charpos = IT_CHARPOS (*it);
 	  if (charpos < begv || charpos > zv)
 	    {
-	      begv = get_narrowed_begv (it->w, charpos);
-	      zv = get_narrowed_zv (it->w, charpos);
+	      begv = get_locked_narrowing_begv (charpos);
+	      zv = get_locked_narrowing_zv (charpos);
 	    }
 	  narrow_to_region_locked (make_fixnum (begv), make_fixnum (zv),
 				   Qfontification_functions);
@@ -7435,12 +7463,20 @@ reseat (struct it *it, struct text_pos pos, bool force_p)
 	{
 	  it->narrowed_begv = get_narrowed_begv (it->w, window_point (it->w));
 	  it->narrowed_zv = get_narrowed_zv (it->w, window_point (it->w));
+	  it->locked_narrowing_begv
+	    = get_locked_narrowing_begv (window_point (it->w));
+	  it->locked_narrowing_zv
+	    = get_locked_narrowing_zv (window_point (it->w));
 	}
       else if ((pos.charpos < it->narrowed_begv || pos.charpos > it->narrowed_zv)
 		&& (!redisplaying_p || it->line_wrap == TRUNCATE))
 	{
 	  it->narrowed_begv = get_narrowed_begv (it->w, pos.charpos);
 	  it->narrowed_zv = get_narrowed_zv (it->w, pos.charpos);
+	  it->locked_narrowing_begv
+	    = get_locked_narrowing_begv (window_point (it->w));
+	  it->locked_narrowing_zv
+	    = get_locked_narrowing_zv (window_point (it->w));
 	}
     }