==== overview slide Hi. I'm here to plug dgit, which is a system for treating the Debian archive like a git remote. I'm going to try to keep my talk fairly short to leave time for questions. Please save your questions for the end. Firstly, a bit of framing. When we work on Debian we take on different roles. The biggest difference is between the maintainer (or maintainers) of a package, working on their own package, and everyone else. I'm going to start by presenting dgit from the point of view of everyone else: NMUers, sponsorship, bug squashers, downstream distros and users, teams doing cross-archive work like transitions and reproducible builds, and so on. Maintainers, please be patient - I'll get to you a bit later in the talk. ==== manpage slide The point of dgit is that it lets everyone treat the archive as if it were a git repository. You can dgit clone any package, in any suite (so, for example, sid or experimental) and you will get a git tree which is exactly the same as dpkg-source -x. So dgit always works the same, from the non-maintainer's point of view, on any package: the operation is completely uniform. You don't need to know anything about the maintainer's verson control workflow tools or source format preferences. You can then work on the package in git, the way you would work in git with any other project. In particular, you can: * commit locally * cherry pick changes from other branches * git reset, git clean * git rebase -i to polish a more complex set of changes into a patch queue * all the other usual gitish stuff If you have the right authority you can also dgit push, to upload. That is, a DD can dgit push any package; a DM can dgit push the packages that the archive thinks they can upload. Before you push you still have to do a build. dgit does not replace existing ways of building source and binary packages, although it does provide some helpful build rune wrappers (more about that later). If you don't want to, or can't, upload to Debian, but do want so share your work with other people, you can git push your dgit branch anywhere suitable, so other can fetch it. So, for example, you could share your branch with your sponsor, who could then approve it by running dgit push. dgit's branches are ordinary git branches for this purpose. A downstream such as a derivative or partial derivative of Debian can use the dgit branches directly as the upstreams for a git-based setup, and work entirley without source packages. ==== data flow slide Behind the scenes, dgit works by providing a set of git repositories in parallel to the existing archive. Every dgit push is actually two underlying operations: the first is a git tag and push, to the dgit git server. The second is a conventional package upload - with a wrinkle: the .dsc of an upload done with dgit contains an extra field with the git commit hash that was pushed. Likewise, fetch and clone combine information from the archive and the git history. If the most recent upload was done with dgit, the commit hash in the dsc enables dgit fetch to find the right commit and present it to you. If the most recent upload was not done with dgit, dgit imports the source package into git - and stitches it into the existing dgit history, if there is one. You do need to treat dgit git branches a bit specially is if you need to build source pacakges (for upload, for example). In this case dgit needs the .orig tarballs for your source package. If you are not doing a new upstream version, dgit fetch will get the relevant .origs for you. If you /are/ doing a new upstream version, then presumably you have obtained the origs as part of preparing your package, or you can build them easily. I should mention that currently only dgit push updates the history on the dgit git server. So until a package is pushed with dgit push for the first time, doesn't exist on that server; and even after then, non-dgit uploads are not recorded, so the history can be out of date. I'm hoping to improve this in the future. ==== NMU linear history slide As a general rule, the dgit history structure should be up to the maintainer - at least, if they care. If you are doing a straightforward NMU you should produce a well-structured linear sequence of commits, as you would for any other git upstream, based on the dgit suite branch. Not only does this mean that if the maintainer is using dgit, they can hopefully easily include your changes; it also means that if they _aren't_ using dgit, at least you have published a history which is suitable for rebasing onto theirs, or whatever it is they might want to do with it. If the source package is `3.0 (quilt)', you shouldn't touch debian/patches; dgit will take care of that for you. This is the other reason why you should provide a tidy linear patch series: if the maintainer likes quilt and is not using dgit, your changes will be automatically presented to them in a fairly plausible format - a series of extra patches - like they should expect from any other NMU. An ordinary NMUer should not normally update a patch in the quilt stack directly. Ie, an NMUer shouldn't squash their changes into an existing patch. This is because while it's easy for the maintainer to squash it themselves, if they want, it's a little harder for the maintainer to disentangle a squashed patch. Working directly on patches can also result in people having to read interdiffs, which are notoriously confusing. So, in summary, if you are doing cross-archive work, or bug squashing, you can start using dgit right away and I hope it will make your life a lot easier. ==== NMU linear history on top of basic dgit history Sadly, unless the maintainer uses dgit, the history you see in dgit will not be the maintainer's history. This is because maintainers' git branches often differ from the source packages in the archive. If you dgit clone a package and it has an X-Vcs-Git header, dgit will set up a remote for that, so you can fetch the maintainer's history and use it if you like. So in that sense dgit clone encompasses debcheckout. But, in the general case, the X-Vcs-Git tree may not be immediately useable to someone not familiar with the package, so dgit doesn't use it for the dgit suite branch. For example, the maintainer's repo might contain only a debian/ directory, or be a quilty tree without patches applied. And the tag and suite naming conventions can vary too. So while the maintainer's history can be useful if you want to do archeaology, it's not in general suitable for use by dgit. There is also the problem that the maintainer's nominated git server might be anywhere, so it might be down, or gone away, or compromised. So, if the maintainer is not using dgit, dgit has to synthesise a git history. The history you see in dgit will then have a very basic branch and commit structure, rather than representing the package's actual history, something like you see here. Which brings me onto the other side of this talk: dgit for maintainers: ==== history comparison slide For the reasons I've explained, downstream dgit users would like you as a maintainer to use dgit push to do your uploads. They will then be able to see, and directly work with, your own history. More generally, the point of using a dvcs like git is to publish your work. The existing ways of publishing git histories for Debian packagess aren't uniformly useable for users: they require the user to understand the maintainer's git working practices. What dgit does is provide a way for you to publish a history which users can rely on actually corresponding to the archive, and use immediately without special knowledge. But it's in your own selfish interest to upload with dgit, too: If you use dgit, you will be able to directly merge NMUs, branches proposed via pull-request emails, and other similar contributions: Because, in this case, the dgit-using contributor will have based their work on your own history. Whereas, if you don't use dgit, dgit-using contributors will be working on a stub history, and may dgit push commits based on that stub. You can dgit fetch that work even if you're not using dgit for your own uploads, but if you're not using dgit push yourself, at the very least you'll have to rebase the NMUers' and other contributors' work onto your own history. Another advantage of using dgit for your maintainer uploads is that it will put your own history on browse.dgit.debian.org, rather than advertising dgit's stub history (which can also be out of date). If you use dgit push, you get an extra check that the source package you are uploading is exactly the same as your git HEAD. This can save you some dsc-based checks. And, of course, as I say, doing your uploads with dgit will improve downstream dgit users' lives. ==== data flow slide dgit is not a replacement for existing git packaging tools; it's intended to complement them. So (unlike git-dpm) dgit does not define a git history structure. Nor, unlike git-buildpackage, does dgit define or suggest a branch structure for naming upstream or downstream branches, pristine tar branches, etc. dgit doesn't require a particular source format; it couldn't, since it needs to work with any package. ==== data flow slide with EQUAL and FF dgit push imposes only two requirements on your git trees, which stem directly from dgit's objectives. The most important requirement is that your git tree is identical to the unpacked source package. This is the whole point of dgit, because it enables dgit to always present to the user a working tree which is the same as they would get from apt-get source. === Status table slide For all native and source format 1.0 packages (which will include people using raw git to manage their delta from upstream), a git tree identical to the source package is already the interchange git branch format. These maintainers can start using dgit right away for all their maintainer uploads. Please do! The other requirement of dgit is simply that the dgit branches are fast-forwarding. So if your tools have made a rebasing branch, you may need to make a fake merge (with git merge -s ours) before pushing. I'm intending to provide some rather more cooked way to do this but I haven't decided the exact shape yet. For `3.0 (quilt)' packages, things are more complicated. The purpose of dgit is to provide a git view which is identical to the source package, and can be worked on without knowledge of the maintainer's chosen source format, patch system or git workflow. This means that the dgit git tree for a `3.0 (quilt)' package is what is sometimes called a `patches-applied packaging branch without .pc directory'. This means that all the changes made in Debian are represented in the main source tree, as well as being contained within a patch in debian/patches. But: I think the most popular tool for working with `3.0 (quilt)' is git-buildpackage, and git-buildpackage likes to work with patches-unapplied tress. I've spoken to Guido Guenther (the git-buildpackage maintainer) about this. We had a really good conversation. We have a plan for interoperating, which involves dgit synthesising an extra git commit, to apply the patches, and then pushing that to the dgit git server. (The extra git commit wouldn't appear on your own branch; if it did, it would cause trouble.) There is another problem which affects both git-buildpackage and git-dpm: namely, .gitignore files. Neither git-buildpackage nor git-dpm represent the .gitignore, which typically occurs in the git tree, as a patch in debian/patches. Rather, these tools tell dpkg-source to ignore it when buildinng the source package. So the source package does not contain .gitignore, although the git tree does. I have spoken to Bernhard Link (the git-dpm maintainer) about this and he wasn't keen on having git-dpm treat .gitignore normally, and record .gitignores in a patch in the git-dpm branch. So unless we can persuade Bernhard to offer this, at least as an option, dgit is probably going to have to synthesise a commit to add them, in much the same way as it will add a commit to convert a git-buildpackage patches-unapplied branch to a directly-editable patches-applied one. ==== data flow slide There are a few other things I ought to cover, since they often come up. They're are relevant to maintainers and non-maintainers: Firstly, some wrinkles. The first wrinkle is that DMs currently need to email me a signed copy of their ssh key, in order to be able to push. This is because the dgit repo server uses ssh as a transport and the project doesn't, right now, have a record of DMs' ssh keys. The second thing that's less than ideal is that the dgit git history does not generally include the package upload history. git-import-dscs can produce a git branch more or less representing the upload history, but dgit doesn't run that itself. It would be difficult for dgit to do so because deciding which set of versions to include is nontrivial and of course it would involve an awful lot of downloading. One could push such a branch to the archive with dgit push. But, it seems to me that the git history structure ought to up to the maintainer, and if the maintainer chooses to use dgit, the maintainers's existing git history is probably better. So I think the real way to improve this is to persuade more maintainers to use dgit. Perhaps for maintainers who do not, we should at some point consider providing centrally an archive-based package history. ==== data flow slide with EQUAL and FF But the most obvious challenge for a maintainer with an existing git branch, but trying to use dgit, is dgit's insistence that the source package and git tree are the same. Sadly, quite a few source packages contain files not in the maintainers' git branches, but which are needed to build: most commonly, autotools output. Such git branches are not useable with dgit. But nowadays most people recommend that the package build should always rerun autotools. If you do that, then neither your git tree nor your source package need contain the autotools output and all is well. Alternatively, you can commit the autotools output to git. Merge conflicts, which do occur occasionally if you do this, are easily resolved by rerunning autotools. And a second way dgit's rule can bite is .gitignore. Most existing packaging tools remove .gitignore from source packages by default. But, dgit requires that the source package and git tree are the same, so if your git tree has .gitignore in it, your source package should too. ==== manpage slide So it is normally best to use one of dgit's build operations to build for upload; in generally, all that those build operations do differently to the underlying tool, is pass some appropriate -I options, to exclude exactly and only the .git directory. With `1.0' this is sufficient. With `3.0 (quilt)', there is more work to be done in the tools, as I mentioned earlier. ==== manpage clean slide Finally, there is one compelling advantage of dgit's git-based approach. Many packages have strangely-behaved or plain buggy clean targets. Because dgit knows that your git tree is canonical, it can help work around this: you can tell dgit to use git-clean instead, avoiding the package's clean target entirely. If you're not in the habit of forgetting to say git-add, you can set a configuration option to have dgit always use git-clean. Then you will never have to fight a buggy clean target, in a strange package, ever again. ==== Future plans slide I have a number of plans for the future, some of which I may even get around to: I need to implement the automatic adjustment of git-buildpackage and git-dpm's git trees into the dgit branch format, and perhaps provide some additional helper tools. dgit can't currently upload to DELAYED queues, which makes it a bit awkward for a fire-and-forget NMU. One way to implement this would be to push to the suite branch but send the package upload to DELAYED. But such an upload which was deleted by the maintainer would still present be in the suite branch, and the changes might be resurrected by a future dgit user. At the very least there should be a way for the maintainer to revert a push which was accompanied by a DELAYED upload. The other implementation would have the pushed branch stashed away somewhere and "revealed" only when it was ready, if it was still a fast forward. That makes it tricker for someone else to find it to work on top of it, and is more work to implement, but it's probably better. It would be easy for dgit, when pushing an NMU, to construct the NMUdiff and email it to the BTS. Of course a maintainer can fetch your NMU as git commits easily with dgit fetch, but maybe the maintainer isn't using dgit (or, even, isn't using git at all), so you for the benefit of those maintainers we should still be sending these diffs. dgit push is slightly more work than it needs to be when doing a source-only upload. In this case there is not really any need for a separate `dgit build-source' step. I think there should be a way to have dgit push make the source package as well, since it easily can. The dgit git server has most of the moving parts to become a fairly-general-purpose git server with access control equivalent to the Debian archive. In this role it could replace some uses of Alioth, which is good because Alioth is doing too many other things. In particular, with the use of signed pushes, we could have traceability of pushes, which makes a gitish packaging workflow more practical - in particular, maintainers would not need to as thoroughly audit work found on for-upload branches on the git server, before pushing the code to the archive. (I suspect many maintainers already don't do this as thoroughly as they should.) dgit requires the suite branches to be fast forwarding. If you have a workflow involving git rebase, you need to make a fake merge (ie, a merge made with git merge -s ours), at the tip of your branch, to make it be a fast forward from the previous state of the suite branch. And then when you want to work on the package again, you need to strip that same fake merge. This can be done with raw git operations (git merge and git reset) quite easily, but it's ugly. Also doing it by hand like this doesn't provide many safety catches. I intend to provide some tooling which will help with this, but I haven't exactly decided what the UI should be and how it should work. The final thing I can do myself is arrange for non-dgit uploads to be automatically converted, and appended to the dgit suite branches on the dgit git server. That would make the dgit history, as shown on browse.dgit.debian.org for example, for any package be more useful, starting with the first dgit-based upload. In particular it would avoid the browse view ever being out of date. There are also some things that I need help with. The documentation for how dgit works together with existing git management tools is rather sparse. Ideally each git workflow tool would explain how (or whether) it works with dgit. Tasks that should be covered include how to incorporate changes made in an NMU into the tool's favoured history format, and how to upgrade a package to a new upstream version. I have spoken to the git-buildpackage and git-dpm maintainers about this. I'm optimistic particularly about git-buildpackage. There is a difficulty with empty directories in source packages. git refuses to handle empty directories - they tend to just vanish from commits and checkouts. But I'm told there are some source packages whose orig tarballs contain empty directories, and build systems depend on those empty directories. Currently if you dgit clone such a package, it won't build. I don't have a particularly good answer to this problem right now. It's no good to have dgit create these directories in the working tree, when it notices, because the git branches might be transported by plain git and used elsewhere. This can be worked around in the source package by either adding something to debian/rules to create these directories, or by adding some files to the directories to make them nonempty. But at the moment a maintainer is not required to do either of these things. And then there are a couple of exciting possibilities for new uses: Firstly, it would be very nice to have a way for a sponsee to provide a dgitish git branch, which the sponsor could work with directly and ultimately dgit push. I'm really need to talk about this with some people more familiar than me with the sponsorship queue infrastructure. Secondly, dgit is not just for Debian. Other distros currently using only source packages can use it too. Read-only support doesn't necessarily depend on specific support at the distro end. So, you can already dgit fetch from ubuntu; I'd welcome patches for read-only support for Mint, for example. dgit push support requires a suitably set up git server, and maybe some thought, depending on the distro's views on push access control. I hear rumours that Ubuntu might grow a dgit git server and support dgit push. Patches very welcome! ==== Thanks and more info slide Finally, there are some people I need to thank. At least half of the design of dgit is Joey Hess's, at the now-famous session in Vaumarcus. Debian System Administration, and particularly Peter Palfrader, have been absolutely brilliant. The level of service and competence has been outstanding. Ever time I noticed some aspect I had forgotten, and made a blatherous and ill-thought-out request, DSA would propose a much better alternative and implement their end of it it almost immediately. Also, I'd like to thank the ftpmasters for setting up the new ftpmaster data service. This has enabled me to extend dgit's useability to non-DDs. So that's most of what I have prepared. There's, of course, a lot more detailed information in the manpages. I'm going to take questions now.