Re: [PATCH] diff: ensure correct lifetime of external_diff_cmd
- Date: Thu, 10 Jan 2019 16:47:02 +0100 (STD)
- From: Johannes Schindelin <Johannes.Schindelin@xxxxxx>
- Subject: Re: [PATCH] diff: ensure correct lifetime of external_diff_cmd
On Wed, 9 Jan 2019, Eric Sunshine wrote:
> On Wed, Jan 9, 2019 at 5:19 PM Kim Gybels <kgybels@xxxxxxxxxxxx> wrote:
> > According to getenv(3)'s notes:
> > [...]
> > Since strings returned by getenv() are allowed to change on subsequent
> > calls to getenv(), make sure to duplicate when caching external_diff_cmd
> > from environment.
> > [...]
> > Signed-off-by: Kim Gybels <kgybels@xxxxxxxxxxxx>
> > ---
> > diff --git a/diff.c b/diff.c
> > @@ -492,6 +492,9 @@ static const char *external_diff(void)
> > external_diff_cmd = getenv("GIT_EXTERNAL_DIFF");
> > if (!external_diff_cmd)
> > external_diff_cmd = external_diff_cmd_cfg;
> > + else
> > + external_diff_cmd = xstrdup(external_diff_cmd);
> Make sense.
> Not shown in the context is that 'external_diff_cmd' is static, so
> this is not (in the traditional sense) leaking the dup'd string.
Ah! And that also explains why we do not need to take care of releasing
the memory via `free()` (which is what I was wondering about).
> I do find that the logic is obscured by doing the xstrdup() in the
> 'else' arm; it would be easier to grok if the condition was reversed and
> xstrdup() done in the 'then' arm.
> However, you might also consider using xstrdup_or_null(), like this:
> external_diff_cmd = xstrdup_or_null(getenv(...));
> if (!external_diff_cmd)
> ...as before...
> > done_preparing = 1;
> > return external_diff_cmd;
> > }
I like this version slightly better, too.
Thanks for diagnosing and fixing this annoying bug!