Re: [PATCH] pretty: Fix bug in truncation support for %>, %< and %><

[Jonathan Nieder <jrnieder@gmail.com>]

Hi,

Ramsay Jones wrote:

> Some systems experience failures in t4205-*.sh (tests 18-20, 27)
> which all relate to the use of truncation with the %< padding
> placeholder. This capability was added in the commit a7f01c6b
> ("pretty: support truncating in %>, %< and %><", 19-04-2013).

This is reproducible when running the test suite for 1.8.3-rc0 on some
Debian test machines (ARM, ia64, powerpc) using gcc 4.6:

  https://buildd.debian.org/status/logs.php?pkg=git&ver=1%3A1.8.3~rc0-1

> The truncation support was implemented with the assistance of a
> new strbuf function (strbuf_utf8_replace). This function contains
> the following code:
>
>        strbuf_attach(sb_src, strbuf_detach(&sb_dst, NULL),
>                      sb_dst.len, sb_dst.alloc);
>
> Unfortunately, this code is subject to unspecified behaviour. In
> particular, the order of evaluation of the argument expressions
> (along with the associated side effects) is not specified by the
> C standard. Note that the second argument expression is a call to
> strbuf_detach() which, as a side effect, sets the 'len' and 'alloc'
> fields of the sb_dst argument to zero.

Makes sense.

[...]
> In order to remove the undesired behaviour, we replace the above
> line of code with:
>
>        strbuf_swap(sb_src, &sb_dst);
>        strbuf_release(&sb_dst);
>
> which achieves the desired effect without provoking unspecified
> behaviour.

Nice cleanup.  I haven't tested the patch but it looks obviously
correct and I assume you've tested it, so for what it's worth,

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>