Fix PR gdb/15294: list with unlimited listsize broken

Fix PR gdb/15294: list with unlimited listsize broken

Currently, "set listsize -1" is supposed to mean "unlimited" source
lines, but, alas, it doesn't actually work:

 (gdb) set listsize -1
 (gdb) show listsize
 Number of source lines gdb will list by default is unlimited.
 (gdb) list 1
 (gdb) list 1
 (gdb) list 1
 (gdb) set listsize 10
 (gdb) list 1
 1       /* Main function for CLI gdb.
 2          Copyright (C) 2002-2013 Free Software Foundation, Inc.
 3
 4          This file is part of GDB.
 5
 6          This program is free software; you can redistribute it and/or modify
 7          it under the terms of the GNU General Public License as published by
 8          the Free Software Foundation; either version 3 of the License, or
 9          (at your option) any later version.
 10

Before this patch:

     http://sourceware.org/ml/gdb-patches/2012-08/msg00367.html

was applied, the "set listsize" command was a var_integer command, and
"unlimited" was set with 0.  Internally, var_integer maps 0 to INT_MAX

   case var_integer:
      {
      ...
	if (val == 0 && c->var_type == var_integer)
	  val = INT_MAX;

The change in that patch to zuinteger_unlimited command, meant that -1
is left as -1 in the command's control variable (lines_to_list), and
the code in source.c isn't expecting that -- it only expects positive
numbers.

I previously suggested fixing the code and keeping the new behavior,
but I found that "set listsize 0" is currently used in the wild, and
we do have a bunch of other commands where "0" means unlimited, so I'm
thinking that changing this command alone in isolation is not a good
idea.

So I now strongly prefer reverting back the behavior in 7.6 to the
same behavior the command has had since 2006 (0==unlimited, -1=error).
Before that, set listsize -1 would be accepted as unlimited as well.

After 7.6 is out, in mainline, we can get back to reconsidering
changing this command's behavior, if there's a real need for being
able to suppress output.  For now, let's play it safe.

The "list line 1 with unlimited listsize" test in list.exp was
originally written years and years ago expecting 0 to mean "no
output", but GDB never actually worked that way, even when the tests
were written, so the tests had been xfailed then.  This patch now
adjusts the test to the new behavior, so that the test actually
passes, and the xfail is removed.

gdb/
2013-03-28  Pedro Alves  <palves@redhat.com>

	PR gdb/15294

	* source.c (_initialize_source): Change back "set listsize" to an
	integer command.

gdb/testsuite/
2013-03-28  Pedro Alves  <palves@redhat.com>

	PR gdb/15294

	* gdb.base/list.exp (set_listsize): Adjust to accept $arg == 0 to
	mean unlimited instead of $arg < 0.
	(test_listsize): Remove "listsize of 0 suppresses output" test.
	Test that "set listsize 0" ends up with an unlimited listsize.

gdb/doc/
2013-03-28  Pedro Alves  <palves@redhat.com>

	PR gdb/15294

	* gdb.texinfo (List) <set listsize>: Adjust to document that
	listsize 0 means no limit, and remove mention of -1.