bug#31888: 27.0.50; Segmentation fault in replace-buffer-contents

[Eli Zaretskii]

> From: Stefan Monnier <monnier@IRO.UMontreal.CA>
> Cc: joaotavora@gmail.com, 31888@debbugs.gnu.org, k.michal@zoho.com, acm@muc.de
> Date: Fri, 29 Jun 2018 16:40:15 -0400
> 
> IIUC replace-buffer-contents is meant to be used in cases where there's
> a good probability that the old and new contents are almost identical,
> save for a few details here and there.
> 
> So, it may very well be the case that out of the 1MB that is covered by
> BEGV..ZV only 10bytes in the middle were deleted/inserted/modified, in
> which case running a-c-f on those 10bytes will likely lead to
> a significantly more efficient recomputation for things like font-lock.
> 
> This refinement is not indispensable, but we make this effort in pretty
> much every other similar circumstance.  It's usually fairly easy/cheap
> to provide those tighter bounds.

Sigh.  Does the below look reasonable?

diff --git a/src/editfns.c b/src/editfns.c
index 4d3c838..9002211 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -3238,9 +3238,21 @@ differences between the two buffers.  */)
      Instead, we announce a single modification for the entire
      modified region.  But don't do that if the caller inhibited
      modification hooks, because then they don't want that.  */
+  ptrdiff_t from, to;
   if (!inhibit_modification_hooks)
     {
-      prepare_to_modify_buffer (BEGV, ZV, NULL);
+      ptrdiff_t k, l;
+
+      /* Find the first character position to be changed.  */
+      for (k = 0; k < size_a && !bit_is_set (ctx.deletions, k); k++)
+       ;
+      from = BEGV + k;
+
+      /* Find the last character position to be changed.  */
+      for (l = size_a; l > 0 && !bit_is_set (ctx.deletions, l - 1); l--)
+       ;
+      to = BEGV + l;
+      prepare_to_modify_buffer (from, to, NULL);
       specbind (Qinhibit_modification_hooks, Qt);
       modification_hooks_inhibited = true;
     }
@@ -3293,8 +3305,9 @@ differences between the two buffers.  */)
 
   if (modification_hooks_inhibited)
     {
-      signal_after_change (BEGV, size_a, ZV - BEGV);
-      update_compositions (BEGV, ZV, CHECK_BORDER);
+      ptrdiff_t updated_to = to + ZV - BEGV - size_a;
+      signal_after_change (from, to - from, updated_to - from);
+      update_compositions (from, updated_to, CHECK_INSIDE);
     }
 
   return Qnil;