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Unbalanced change hooks (part 1)

From: Alan Mackenzie
Subject: Unbalanced change hooks (part 1)
Date: Sun, 31 Jul 2016 10:16:17 +0000
User-agent: Mutt/1.5.24 (2015-08-30)

Hello, Emacs.

In certain buffer modifications, after-change-hooks is being called, yet
before-change-hooks is not being called.  This is a Bad Thing, and is at
the root of bug #24074/#24094.  The documentation (page "Change Hooks"
in the Elisp manual) is quite clear, if a little implicit, that both
hooks, or neither (when inhibit_modification_hooks is non-nil) get
called on a buffer modification.

The first of these problems is in Finsert_file_contents, where
before-change-hooks is invoked by a call to prepare_to_modify_buffer
(which calls signal_before_change), and after-change-hooks is invoked by
a call to signal_after_change.

Both of these invocations are conditional (which is correct), but
different conditions are applied to the before-... and after-...
invocations (which is not correct).  The after-... condition tests both
parameters `visit' and `replace', but the before-... condition tests
only `visit'.  It seems likely that the test on `replace' was added at a
later date, and it was mistakenly missed out of the before-...

I propose to amend Finsert_file_contents so that the same condition is
tested for the invocation of both hooks, and to enforce this by
recording the state in a bool variable.  Comments on this proposed
change are requested:

diff --git a/src/fileio.c b/src/fileio.c
index b1f9d3c..0431cbc 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -3440,6 +3440,7 @@ by calling `format-decode', which see.  */)
   /* SAME_AT_END_CHARPOS counts characters, because
      restore_window_points needs the old character count.  */
   ptrdiff_t same_at_end_charpos = ZV;
+  bool run_change_hooks;
   if (current_buffer->base_buffer && ! NILP (visit))
     error ("Cannot do file visiting in an indirect buffer");
@@ -4077,7 +4078,9 @@ by calling `format-decode', which see.  */)
     /* For a special file, all we can do is guess.  */
     total = READ_BUF_SIZE;
-  if (NILP (visit) && total > 0)
+  run_change_hooks = ((NILP (visit) || !NILP (replace))
+                      && total > 0);
+  if (run_change_hooks)
       if (!NILP (BVAR (current_buffer, file_truename))
          /* Make binding buffer-file-name to nil effective.  */
@@ -4313,8 +4316,7 @@ by calling `format-decode', which see.  */)
   /* Call after-change hooks for the inserted text, aside from the case
      of normal visiting (not with REPLACE), which is done in a new buffer
      "before" the buffer is changed.  */
-  if (inserted > 0 && total > 0
-      && (NILP (visit) || !NILP (replace)))
+  if (run_change_hooks)
       signal_after_change (PT, 0, inserted);
       update_compositions (PT, PT, CHECK_BORDER);

Unfortunately, this amendment, by itself, doesn't fix #240[79]4, since
there are other causes for the change hooks being improperly invoked.

Alan Mackenzie (Nuremberg, Germany).

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