On Tue, Aug 16, 2016 at 12:31:45PM -0400, Jeff King wrote: > On Tue, Aug 16, 2016 at 09:18:39AM -0700, Josh Triplett wrote: > > > Commit 5e7dcad771cb873e278a0571b46910d7c32e2f6c in September 2013 added > > support to upload-pack to show the symbolic target of non-HEAD symbolic > > refs. However, commit d007dbf7d6d647dbcf0f357545f43f36dec46f3b in > > November 2013 reverted that, because it used a capability to transmit > > the information, and capabilities have a limited size (limited by the > > pkt-line format which can't send lines longer than 64k) and can't > > transmit an arbitrary number of symrefs. > > > > (Incidentally, couldn't the same problem occur if the HEAD points to a > > long enough path to exceed 64k? > > Yes. But it's a lot easier to say "your 64k symref is ridiculous; don't > do that" than it is to say "oh, you happened to have a lot of symrefs in > your repository, so we overflowed and failed". > > Besides which, the whole protocol cannot handle refnames larger than > 64k, so it's not a new problem. Absolutely agreed. I mentioned it only because if a server provided any mechanism to send it symbolic ref targets, this could lead to a situation where you could push a repository that you could not subsequently pull. Depending on the protocols involved, that could potentially require manual admin intervention, or could even result in a DoS. Not something that can arise with git itself, as send-pack/receive-pack doesn't support sending symbolic ref targets, but I could imagine a server doing so to allow setting HEAD. > > I'd like to be able to see the targets of non-HEAD symbolic refs for a > > repository (symbolic refs under refs/). I'm interested in extending > > upload-pack to expose those somehow. What seems like a sensible format > > to do so? > > > > Would it make sense to advertise a new capability for symbolic ref > > targets, which would allow the client to send back a dedicated request > > for the targets of all symrefs? > > It will definitely require a new capability. You cannot just send a > "\0symref=..." trailer after each ref, because older clients treat > multiple "\0" trailers as overwriting one another (so it essentially > overwrites the old capabilities). > > Sadly you cannot use a capability to fix that, because all of this > happens before the client agrees to any capabilities (you can find > discussion of a "v2" protocol on the list which solves this, but it's > sort of languishing in the design phase). As a potential 1.1 version, which could work in a backward-compatible way with existing servers and no additional round-trip: what if, in the smart HTTP protocol, the client advertised client capabilities with an additional HTTP header (e.g. "Git-Client-Caps: symrefs othershiny featurenames"? git-http-backend could then pass those capabilities to git-upload-pack (--client-caps='...'), which could take them into account in the initial response? That wouldn't work as a single-pass approach for SSH, since the client can't know if the server's upload-pack supports --client-caps, but it would work for the smart HTTP protocol. > So you are stuck introducing a new phase into the protocol, which is > probably rather tricky (especially with the http protocol, which is very > sensitive to extra round-trips). I guess the least-invasive way would be > to communicate the desires in the "want" phase, and then have the server > dump it out with the packfile. Like: > > - server claims "I support symref-wants" in the capability phase > > - during the negotiation phase, in addition to "want" and "have", the > client may send "symref <ref>" packets. Probably <ref> should be a > wildcard to avoid having to ask about each ref individually. > > - before outputting the packfile in the final phase, if any "symref" > wants were sent by the client, the server dumps a list of "symref > <from> <to>" packets, followed by a flush packet. > > That should Just Work over the existing http protocol without requiring > an extra request. It'd require one extra request for git ls-remote, which normally doesn't need the second round-trip, but that still seems reasonable. -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html