Subject: Re: [PATCH 2/2] upload-pack: don't ACK non-commits repeatedly in protocol v2

From: Junio C Hamano <[email-masked]>
Date: Thu, 04 Sep 2025 09:37:58 -0700

Patrick Steinhardt <[email-masked]> writes:

> ... when the
> server ACKs an object there is no reason for the client to go deeper, so
> they stop advertising any of its parents.

Very true.  I did point out the difference in behaviour, but I
couldn't actually figure out what goodness we are deriving in this
code by marking grandparents (and possibly their ancestors) as
something they have.  Your change in behaviour could be seen as
stopping us from making unnecessary operations ;-)

If the other side is mischievous, they are not limited to feed us a
commit and then its parent in the naturally expected order.  They
could feed us A and then skip B and give us C that is a child of B.
Pre-painting B when we got A from them, in order to prepare for
seeing B (which we can return without doing anything) in the next
round, would not help us all that much, if they give us C after
giving us A, as we haven't even heard of C yet at that point.

But in the normal case against sane clients that do not skip the
probes, marking immediate parents (like B) when processing A might
be helping?  I dunno.  I also somehow thought that even normal case
we have an option to skip the probes in order to converge faster,
but I am misremembering.

cf. https://lore.kernel.org/git/?q=upload-pack%20fibonacci