On Fri, Jan 31, 2020 at 3:01 PM Junio C Hamano <gitster@xxxxxxxxx> wrote: > > Masaya Suzuki <masayasuzuki@xxxxxxxxxx> writes: > > >> > +prerequisite = "-" obj-id SP comment LF > >> > +comment = *CHAR > >> > >> Do readers know what CHAR consists of? Anything other than NUL and > >> LF? > > > > RFC 5234 defines core rules > > (https://tools.ietf.org/html/rfc5234#appendix-B.1), and these CHAR etc > > are defined there. It should be OK to use these rules. > > That's not what I asked. Do readers know that? Did you tell them > that we expect they are familiar with the RFC convention? The patch says "We will use ABNF notation to define the Git bundle format. See protocol-common.txt for the details.", and protocol-common.txt says "ABNF notation as described by RFC 5234 is used within the protocol documents, except the following replacement core rules are used:". In order to interpret this ABNF definition, it's not enough to read RFC 5234, but the reader has to read protocol-common.txt. Otherwise, they cannot understand what `obj-id` is and what `refname` is. Those are not defined in RFC 5234. They're defined in protocol-common.txt. Based on the fact that (1) this document instructs the reader to see protocol-common.txt in the beginning and (2) protocol-common.txt is needed to interpret this definition and protocol-common.txt says RFC 5234 describes ABNF format, the readers should know ABNF is defined in RFC 5234 and ABNF includes those LF, CHAR, and SP as a part of the definition after reading the first sentence and referenced documents. > > It might be easier to make the above simple ABNF understandable to > those without knowledge of RFC 5234 by spelling out what CHAR in the > context of the above description means. Or to tell them "go over > there and learn CHAR then come back". We need to do one of them. As I said above, the first sentence says "See protocol-common.txt" which includes the reference to the RFC and other non-terminals. Note that, not only CHAR, but obj-id and refname are not defined here as well. The readers need to reference protocol-common.txt to get the definition of them. > > > I want to make sure the meaning of prerequisites. > > > > 1. Are they meant for a delta base? Or are they meant to represent a > > partial/shallow state? > > They are meant as the "bottom boundary" of the range of the pack > data stored in the bundle. > > Think of "git rev-list --objects $heads --not $prerequisites". If > we limit ourselves to commits, in the simplest case, "git log > maint..master". Imagine your repository has everything up to > 'maint' (and nothing else) and then you are "git fetch"-ing from > another repository that advanced the tip that now points at > 'master'. Imagine the data transferred over the network. Imagine > that data is frozen on disk somehow. That is what a bundle is. > > So, 'maint' is the prerequisite---for the person who builds the > bundle, it can safely be assumed that the bundle will be used only > by those who already has 'maint'. > > There is nothing about 'partial' or 'shallow'. And even though a > bundle typically has deltified objects in the packfile, it does not > have to. Some objects are delitifed against prerequisite, and the > logic to generate thin packs may even prefer to use the > prerequisites as the delta base, but it is merely a side effect that > the prerequisites are at the "bottom boundary" of the range. OK. Then, it's better to make this clear. If you follow the analogy of saved git-fetch response, it's possible that these prerequisites are interpreted same as "shallow" lines of the shallow clone response. It's more like "have" lines of git-fetch request. > > 2. Do they need to be commits? Or can they be any object type? > > > > From what I can see, it seems that they should always be commits. > > > > 3. Does the receiver have to have all reachable objects from prerequisites? > > I would say that the receiver needs to have everything that is > needed to "complete" prereqs. > > Bundle transfer predates shallow or incomplete repositories, but I > think that we can (and we should if needed) update it to adjust to > these situations by using the appropriate definition of what it > means to "complete". In a lazy clone, it may be sufficient to have > promisor remote that has everything reachable from them. In a > shallow clone, the repository may have to be deep enough to have > them and objects immediately reachable from them (e.g. trees and > blobs for a commit at the "bottom boundary"). I think there are two completeness of a packfile: * Delta complete: If an object in a packfile is deltified, the delta base exists in the same packfile. * Object complete: If an object in a packfile contains a reference to another object, that object exists in the same packfile. For example, initial shallow clone response should contain a delta-complete object-incomplete packfile. Incremental fetch response and bundles with prereqs would have a delta-incomplete object-incomplete packfile. Creating delta-incomplete object-complete packfile is possible (e.g. create a parallel history with all blobs slightly modified and deltify against the original branch. I can create a packfile with all objects in one history with all objects deltified with the other history), but it's a rare case. The reader of a bundle SHOULD have all objects reachable from prereqs.
