bug#24751: 26.0.50; Regex stack overflow not detected properly (gets "Variable binding depth exceeds max-specpdl-size")

[Eli Zaretskii <eliz@gnu.org>]

> From: npostavs@users.sourceforge.net
> Cc: 24751@debbugs.gnu.org
> Date: Sun, 01 Jan 2017 23:49:46 -0500
> 
> Everything you've said makes sense after your last message, but I'm
> still unable to put it together and come up with a revised comment.
> Could you make a suggestion?

How about the below?

--- src/regex.c~0	2016-12-11 06:39:19.000000000 +0200
+++ src/regex.c	2017-01-02 12:40:44.266517100 +0200
@@ -1195,24 +1195,28 @@
     gettext_noop ("Range striding over charsets") /* REG_ERANGEX  */
   };
 \f
-/* Avoiding alloca during matching, to placate r_alloc.  */
+/* Whether to allocate memory during matching.  */
 
-/* Define MATCH_MAY_ALLOCATE unless we need to make sure that the
-   searching and matching functions should not call alloca.  On some
-   systems, alloca is implemented in terms of malloc, and if we're
-   using the relocating allocator routines, then malloc could cause a
-   relocation, which might (if the strings being searched are in the
-   ralloc heap) shift the data out from underneath the regexp
-   routines.
-
-   Here's another reason to avoid allocation: Emacs
-   processes input from X in a signal handler; processing X input may
-   call malloc; if input arrives while a matching routine is calling
-   malloc, then we're scrod.  But Emacs can't just block input while
-   calling matching routines; then we don't notice interrupts when
-   they come in.  So, Emacs blocks input around all regexp calls
-   except the matching calls, which it leaves unprotected, in the
-   faith that they will not malloc.  */
+/* Define MATCH_MAY_ALLOCATE to allow the searching and matching
+   functions allocate memory for the failure stack and registers.
+   Normally should be defined, because otherwise searching and
+   matching routines will have much smaller memory resources at their
+   disposal, and therefore might fail to handle complex regexps.
+   Therefore undefine MATCH_MAY_ALLOCATE only in the following
+   exceptional situations:
+
+   . When running on a system where memory is at premium.
+   . When alloca cannot be used at all, perhaps due to bugs in
+     its implementation, or its being unavailable, or due to a
+     very small stack size.  This requires to define REGEX_MALLOC
+     to use malloc instead, which in turn could lead to memory
+     leaks if search is interrupted by a signal.  (For these
+     reasons, defining REGEX_MALLOC when building Emacs
+     automatically undefines MATCH_MAY_ALLOCATE, but outside
+     Emacs you may not care about memory leaks.)  If you want to
+     prevent the memory leaks, undefine MATCH_MAY_ALLOCATE.
+   . When code that calls the searching and matching functions
+     cannot allow memory allocation, for whatever reasons.  */
 
 /* Normally, this is fine.  */
 #define MATCH_MAY_ALLOCATE