Re: [PATCH 09/23] pack v4: commit object encoding

> Nicolas Pitre <nico@fluxnic.net> writes:
> 
> > This goes as follows:
> >
> > - Tree reference: either variable length encoding of the index
> >   into the SHA1 table or the literal SHA1 prefixed by 0 (see
> >   add_sha1_ref()).
> >
> > - Parent count: variable length encoding of the number of parents.
> >   This is normally going to occupy a single byte but doesn't have to.
> >
> > - List of parent references: a list of add_sha1_ref() encoded references,
> >   or nothing if the parent count was zero.
> >
> > - Author reference: variable length encoding of an index into the author
> >   string dictionary table which also covers the time zone.  To make the
> >   overall encoding efficient, the author table is already sorted by usage
> >   frequency so the most used names are first and require the shortest
> >   index encoding.
> >
> > - Author time stamp: variable length encoded.  Year 2038 ready!
> >
> > - Committer reference: same as author reference.
> >
> > - Committer time stamp: same as author time stamp.
> >
> > The remainder of the canonical commit object content is then zlib
> > compressed and appended to the above.
> >
> > Rationale: The most important commit object data is densely encoded while
> > requiring no zlib inflate processing, and all SHA1 references are most
> > likely to be direct indices into the pack index file requiring no SHA1
> > search into the pack index file.
> >
> > Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
> > ---
> >  packv4-create.c | 119 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 119 insertions(+)
> >
> > diff --git a/packv4-create.c b/packv4-create.c
> > index bf33d15..cedbbd9 100644
> > --- a/packv4-create.c
> > +++ b/packv4-create.c
> > @@ -13,6 +13,9 @@
> >  #include "tree-walk.h"
> >  #include "pack.h"
> >  
> > +
> > +static int pack_compression_level = Z_DEFAULT_COMPRESSION;
> > +
> >  struct data_entry {
> >  	unsigned offset;
> >  	unsigned size;
> > @@ -289,6 +292,122 @@ static unsigned char *add_sha1_ref(unsigned char *dst, const unsigned char *sha1
> >  	return dst + 20;
> >  }
> >  
> > +/*
> > + * This converts a canonical commit object buffer into its
> > + * tightly packed representation using the already populated
> > + * and sorted commit_name_table dictionary.  The parsing is
> > + * strict so to ensure the canonical version may always be
> > + * regenerated and produce the same hash.
> > + */
> > +void * conv_to_dict_commit(void *buffer, unsigned long *psize)
> 
> Drop SP between asterisk and "conv_"?
> 
> > +{
> > +	unsigned long size = *psize;
> > +	char *in, *tail, *end;
> > +	unsigned char *out;
> > +	unsigned char sha1[20];
> > +	int nb_parents, index, tz_val;
> > +	unsigned long time;
> > +	z_stream stream;
> > +	int status;
> > +
> > +	/*
> > +	 * It is guaranteed that the output is always going to be smaller
> > +	 * than the input.  We could even do this conversion in place.
> > +	 */
> > +	in = buffer;
> > +	tail = in + size;
> > +	buffer = xmalloc(size);
> > +	out = buffer;
> > +
> > +	/* parse the "tree" line */
> > +	if (in + 46 >= tail || memcmp(in, "tree ", 5) || in[45] != '\n')
> > +		goto bad_data;
> > +	if (get_sha1_hex(in + 5, sha1) < 0)
> > +		goto bad_data;
> 
> Is this strict enough to guarantee roundtrip hash identity?  Because
> get_sha1_hex() accepts hexadecimal represented with uppercase A-F,
> you need to reject such a "broken" commit object, no?