Re: [PATCH] Add '--create-index' to git-unpack-objects

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Oct 12, 2005, 14:25 UTC
Message-ID: <Pine.LNX.4.63.0510121612340.6307@wbgn013.biozentrum.uni-wuerzburg.de>
In-Reply-To: <20051012173426.56fd5c1c.vsu@altlinux.ru>

Hi,

On Wed, 12 Oct 2005, Sergey Vlasov wrote:

Show 21 quoted lines

> On Wed, 12 Oct 2005 13:02:36 +0200 (CEST) Johannes Schindelin wrote:
> 
> >  static void write_object(void *buf, unsigned long size, const char *type)
> >  {
> >  	unsigned char sha1[20];
> > -	if (write_sha1_file(buf, size, type, sha1) < 0)
> > +	if (create_index) {
> > +		char header[100];
> > +		SHA_CTX c;
> > +
> > +		SHA1_Init(&c);
> > +		SHA1_Update(&c, header, 1+sprintf(header, "%s %lu", type, size));
> > +		SHA1_Update(&c, buf, size);
> > +		SHA1_Final(current_sha1, &c);
> > +	} else if (write_sha1_file(buf, size, type, sha1) < 0)
> >  		die("failed to write object");
> 
> Sorry, but this cannot work.  git-unpack-objects does a streaming
> unpack, and it needs to be able to read back the objects it has written
> out previously (in case a delta later in the stream references some
> older object).

Even worse, my code did not anticipate that the base objects could have been handled earlier (and thus the deltas would never be resolved).

> Saving unpacked objects in memory would obviously be unacceptable.

Actually, this is what git-unpack-objects does. All unresolved deltas are stored in a linked list, and handled later.

Of course, it would be nicer to use a seekable file if you have one. But then, I am not at all sure that base objects should be allowed to come later in the file: since the delta chains must not be cyclic, the objects can be sorted. Thus, it could be guaranteed that the base objects are already unpacked when unpacking the derived object.

Ciao, Dscho