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

From: Junio C Hamano <gitster@pobox.com>
Date: Sep 4, 2025, 16:37 UTC
Message-ID: <xmqqqzwm9nrt.fsf@gitster.g>
In-Reply-To: <aLmJOdxUYiyHpiLA@pks.im>

Patrick Steinhardt <ps@pks.im> 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