Re: [PATCH 1/4] Improve message-id generation flow control for format-patch

From: Pierre Habouzit <madcoder@debian.org>
Date: Feb 6, 2008, 22:56 UTC
Message-ID: <20080206225648.GB23510@artemis.madism.org>
In-Reply-To: <20080206225335.GA23510@artemis.madism.org>

On Wed, Feb 06, 2008 at 10:53:35PM +0000, Pierre Habouzit wrote:

Show 36 quoted lines

> On Wed, Feb 06, 2008 at 10:10:30PM +0000, Daniel Barkalow wrote:
> > On Wed, 6 Feb 2008, Pierre Habouzit wrote:
> > 
> > > On Wed, Feb 06, 2008 at 08:31:08PM +0000, Junio C Hamano wrote:
> > > > I wonder how the rule established by b315c5c (strbuf change: be
> > > > sure ->buf is never ever NULL) and at the beginning of strbuf.h
> > > > applies here.  I think the current implementation of strbuf
> > > > happens to allow this, and it is very handy.  Perhaps the rule
> > > > stated there should be loosened and allow copying the buf away
> > > > when you know you have stuff in there (i.e. ->buf != slopbuf).
> > > > Pierre, what do you think?
> > > > 
> > > > What the patch does itself is much nicer than the original.
> > > 
> > >   Why wouldn't you just use strbuf_detach ? I mean replacing:
> > > 
> > > +	info->message_id = buf.buf;
> > > 
> > > with:
> > > 
> > > +	info->message_id = strbuf_detach(&buf, NULL);
> > > 
> > >   isn't really hard to read, and has the nice side effect to prevent
> > > errors that could happen in the future (like reusing buf and screwing
> > > with info->message_id without noticing it). I'd rather stand on the safe
> > > side here, it's more forward-compatible and idiot-proof[0].
> > 
> > Is it actually right to have buf go out of scope right after 
> > strbuf_detach()? It sort of looks like it would leak memory from buf.buf. 
> > I'm happy to do whatever the API wants there, and I didn't see anything to 
> > leave the struct as if strbuf_release were called, but with the string 
> > extracted for the caller.
> 
>   err no, strbuf_detach gives you a pointer you are supposed to free()
> later, and inits the strbuf passed as its argument to be used again,
> though if you don't, you leak nothing.

  In fact, strbuf_detach is the rough equivalent of doing that:

  info->message_id = buf.buf;
  buf.buf = NULL;

  Except that it sets buf.buf to a magic place so that it's never NULL,
and that it also keeps the internal invariants in place. But after a
strbuf_detach, a strbuf doesn't holds any allocated memory anymore.
You'll see in many places in the code that
`return strbuf_detach(&sb, NULL)` is quite idiomatic, and the function
does exactly what it means "Please detach the memory allocated in that
buffer and give it to me".

-- 
·O·  Pierre Habouzit
··O                                                madcoder@debian.org
OOO                                                http://www.madism.org