On Tue, 27 Aug 2013, Junio C Hamano wrote:
> Nicolas Pitre <nico@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > ---
> > 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?
Indeed, yes. Same concern as with split_ident_line() I mentioned
before.
> Same for parent commit object names below that are parsed with the
> same helper.
Exact.
Nicolas
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