On Fri, 20 Oct 2017 10:18:39 -0700 Brandon Williams <bmw...@google.com> wrote:
> Some of the pain points with the current protocol spec are: After some in-office discussion, I think that the most important pain point is that we have to implement each protocol twice: once for HTTP(S), and once for SSH (and friends) that support bidirectional byte streams. If it weren't for this, I think that what is discussed in this document (e.g. ls-refs, fetch-object) can be less invasively accomplished with v1, specifying "extra parameters" (explained in this e-mail [1]) to merely tweak the output of upload-pack instead of replacing it nearly completely, thus acting more as optimizations than changing the mode of operation entirely. [1] https://public-inbox.org/git/20171010193956.168385-1-jonathanta...@google.com/ > * The server's initial response is the ref advertisement. This > advertisement cannot be omitted and can become an issue due to the > sheer number of refs that can be sent with large repositories. For > example, when contacting the internal equivalent of > `https://android.googlesource.com/`, the server will send > approximately 1 million refs totaling 71MB. This is data that is > sent during each and every fetch and is not scalable. For me, this is not a compelling one, because we can provide a ref whitelist as an "extra parameter" in v1. > * Capabilities were implemented as a hack and are hidden behind a NUL > byte after the first ref sent from the server during the ref > advertisement: > > <SHA1> <Ref Name>\0<capabilities space separated> <symref> <agent> > > Since they are sent in the context of a pkt-line they are also subject > to the same length limitations (1k bytes with old clients). While we > may not be close to hitting this limitation with capabilities alone, it > has become a problem when trying to abuse capabilities for other > purposes (e.g. > [symrefs](https://public-inbox.org/git/20160816161838.klvjhhoxsftvkfmd@x/)). > > * Various other technical debt (e.g. abusing capabilities to > communicate agent and symref data, service name set using a query > parameter). I think these 2 are the same - I would emphasize the fact that we cannot add more stuff here, rather than the fact that we're putting this behind NUL. > Special Packets > ----------------- > > In protocol v2 these special packets will have the following semantics: > > * '0000' Flush Packet (flush-pkt) - indicates the end of a message > * '0001' End-of-List delimiter (delim-pkt) - indicates the end of a list To address the pain point of HTTP(S) being different from the others (mentioned above), I think the packet semantics should be further qualified: - Communications must be divided up into packets terminated by a flush-pkt. Also, each side must be implemented without knowing whether packets-in-progress can or cannot be seen by the other side. - Each request packet must have a corresponding, possibly empty, response packet. - A request packet may be sent even if a response packet corresponding to a previously sent request packet is awaited. (This allows us to retain the existing optimization in fetch-pack wherein, during negotiation, the "have" request-response packet pairs are interleaved.) This will allow us to more easily share code between HTTP(S) and the others. In summary, I think that we need a big motivation to make the jump from v1 to v2, instead of merely making small changes to v1 (and I do think that the proposed new commands, such as "ls-refs" and "fetch-object", can be implemented merely by small changes). And I think that the ability to better share code between HTTP(S) and others provides that motivation.
