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Re: [PATCH] diff: ensure correct lifetime of external_diff_cmd




Hi,

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!
Dscho