bug#3552: 23.0.94; backward-prefix-chars: Point before start of properties

[Noam Postavsky <npostavs@users.sourceforge.net>]

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On Sat, Jun 4, 2016 at 1:55 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> I would use
>
>   if (pos <= beg)
>     break;
>   DEC_BOTH (pos, pos_byte);

Oh yeah, that makes sense; parallels with the same check at the
beginning of the function. Full patch attached.

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From fb739ee2f83df58266c8bfc6a0e4426fed5b5890 Mon Sep 17 00:00:00 2001
From: Noam Postavsky <npostavs@gmail.com>
Date: Sat, 4 Jun 2016 09:02:20 -0400
Subject: [PATCH] Fbackward_prefix_chars: stay within buffer bounds

The commit 1fd3172d "(Fbackward_prefix_chars): Set point properly while
scanning" (1998-03-18), moved the check against of the position against the
buffer beginning out the loop condition so that we might end up checking
the syntax of characters before the beginning of the buffer.  This can
cause segfaults or trigger a "Point before start of properties" error in
`update_interval' (called indirectly from `char_quoted').

* src/syntax.c (Fbackward_prefix_chars): Stop the loop when beginning of
buffer is reached (Bug #3552).
---
 src/syntax.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/syntax.c b/src/syntax.c
index 8e14bf3..b1ba5c6 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -3109,8 +3109,9 @@ DEFUN ("backward-prefix-chars", Fbackward_prefix_chars, Sbackward_prefix_chars,
       opoint = pos;
       opoint_byte = pos_byte;
 
-      if (pos + 1 > beg)
-	DEC_BOTH (pos, pos_byte);
+      if (pos <= beg)
+        break;
+      DEC_BOTH (pos, pos_byte);
     }
 
   SET_PT_BOTH (opoint, opoint_byte);
-- 
2.8.0