On Fri, May 17, 2019 at 3:55 PM Jeff King <peff@xxxxxxxx> wrote: > > On Fri, May 17, 2019 at 02:20:42PM +0700, Duy Nguyen wrote: > > > On Fri, May 17, 2019 at 12:35 PM Jeff King <peff@xxxxxxxx> wrote: > > > As it turns out, index-pack does not handle these complicated cases at > > > all! In the final fix_unresolved_deltas(), we are only looking for thin > > > deltas, and anything that was not yet resolved is assumed to be a thin > > > object. In many of these cases we _could_ resolve them if we tried > > > harder. But that is good news for us because it means that these > > > expectations about delta relationships are already there, and the > > > pre-fetch done by your patch should always be 100% correct and > > > efficient. > > > > Is it worth keeping some of these notes in the "third pass" comment > > block in index-pack.c to help future readers? > > Perhaps. I started on the patch below, but I had trouble in the commit > message. I couldn't find the part of the code that explains why we would > never produce this combination, though empirically we do not. That still has some value even if your commit ends up with a question mark. There's not much to dig out of 636171cb80 (make index-pack able to complete thin packs., 2006-10-25). Adding Nico, maybe he still remembers... > -- >8 -- > Subject: [PATCH] index-pack: describe an implication of our thin resolving > > After digging into the delta resolution code, I discovered a surprising > (to me, anyway) implication of our strategy: we could never find a > non-thin delta with a thin delta as its base. This is OK because > pack-objects will never produce such a combination, because....? > > Signed-off-by: Jeff King <peff@xxxxxxxx> > --- > builtin/index-pack.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/builtin/index-pack.c b/builtin/index-pack.c > index ccf4eb7e9b..f40f4560d4 100644 > --- a/builtin/index-pack.c > +++ b/builtin/index-pack.c > @@ -1224,6 +1224,13 @@ static void resolve_deltas(void) > * Third pass: > * - append objects to convert thin pack to full pack if required > * - write the final pack hash > + * > + * Note that we assume all deltas at this phase are thin. We take only a > + * single pass over the unresolved objects, and we look for bases only > + * in our set of already-existing objects, _not_ other objects within this > + * pack. This means that we would never find an object A stored as a delta > + * against another object B in this pack, when B is a thin delta against a base > + * not in the pack. > */ > static void fix_unresolved_deltas(struct hashfile *f); > static void conclude_pack(int fix_thin_pack, const char *curr_pack, unsigned char *pack_hash) > -- > 2.22.0.rc0.544.g1eb4087842 > -- Duy
