Index: docs/Proposals/GitHubMove.rst =================================================================== --- /dev/null +++ docs/Proposals/GitHubMove.rst @@ -0,0 +1,685 @@ +============================== +Moving LLVM Projects to GitHub +============================== + +Introduction +============ + +This is a proposal to move our current revision control system from our own +hosted Subversion to GitHub. Below are the financial and technical arguments as +to why we need such a move and how will people (and validation infrastructure) +continue to work with a Git-based LLVM. + +There will be a survey pointing at this document which we'll use to gague the +community's reaction and, if we collectively decide to move, the time-frame. Be +sure to make your view count. + +This proposal is divided into the following parts: + +* Outline of the reasons to move to Git and GitHub +* Description on the options +* What some examples of workflow will look like (compared to currently) +* The proposed migration plan + +What This Proposal is *Not* About +================================= + +Changing the development policy: the development of LLVM will continue as it +exists now. + +This proposal relates only to moving the hosting of our source-code repository +from SVN hosted on our own servers to Git hosted on GitHub. We are not proposing +other workflow changes here. That is, it should not be assumed that moving to +GitHub implies using GitHub's issue tracking, or using the GitHub UI for +pull-requests and/or code-review. + +Every existing contributors will get commit access on demand in the same +condition as currently. Those who don't have an existing GitHub account will +have to create one in order to continue having commit access. + +Why Git, and Why GitHub? +======================== + +Why Move At All? +---------------- + +One of the reasons for the move, and why this discussion started in the first +place, is that we currently host our own Subversion server and Git mirror in a +voluntary basis. The LLVM Foundation sponsors the server and provides limited +support, but there is only so much it can do. + +Volunteers are not sysadmins themselves, but compiler engineers that happen +to know a thing or two about hosting servers. We also don't have 24/7 support, +and we sometimes wake up to see that continuous integration is broken because +the SVN server is either down or unresponsive. + +On the other hand, there are multiple services out there (GitHub, GitLab, +BitBucket among others) that offer that same service (24/7 stability, disk +space, Git server, code browsing, forking facilities, etc) for free. + +Why Git? +-------- + +It seems that Git is new coders first choice nowadays . A lot of them have never +used SVN, CVS, or anything else. Websites like GitHub have changed the landscape +of open source contributions, reducing the cost of first contribution and +fostering collaboration. + +Git is also the version control many (most?) LLVM developers use. Despite the +sources being stored in a SVN server, these developers are already using Git +through the Git-SVN integration. + +Git allows you to: + +* Commit, squash, merge, and fork locally without touching the remote server. +* Maintain as many local branches as you like, letting you maintain multiple + threads of development. +* Collaborate on these branches (e.g. through your own fork of llvm on GitHub). +* Inspect the repository history (blame, log, bisect) without Internet access. + +In addition, because Git seems to be replacing many OSS projects' version +control systems, there are many tools that are built over Git. Future tooling is +much more likely to support Git first (if not only). + +Why GitHub? +----------- + +GitHub, like GitLab and BitBucket, provides free code hosting for open source +projects. Any of these could replace the code-hosting infrastructure that we +have today. + +These services also have a dedicated team to monitor, migrate, improve and +distribute the contents of the repositories depending on region and load. + +All things being equal, GitHub has one important advantage over GitLab and +BitBucket: It offers read-write **SVN** access to the repository +(https://github.com/blog/626-announcing-svn-support). +This would enable people to continue working post-migration as though our code +were still canonically in an SVN repository. + +In addition, there are already multiple LLVM mirrors on GitHub, indicating that +part of our community has already settled there. + +On Managing Revision Numbers with Git +------------------------------------- + +The current SVN repository hosts all the LLVM sub-projects alongside each other. +A single revision number (e.g. r123456) thus identifies a consistent version of +all LLVM sub-projects. + +Git does not use sequential integer revision number but instead uses a hash to +identify each commit. (Linus mentioned that the lack of such revision number +is "the only real design mistake" in Git [TorvaldRevNum]_.) + +The loss of a sequential integer revision number has been a sticking point in +past discussions about Git: + +- "The 'branch' I most care about is mainline, and losing the ability to say + 'fixed in r1234' (with some sort of monotonically increasing number) would + be a tragic loss." [LattnerRevNum]_ +- "I like those results sorted by time and the chronology should be obvious, but + timestamps are incredibly cumbersome and make it difficult to verify that a + given checkout matches a given set of results." [TrickRevNum]_ +- "There is still the major regression with unreadable version numbers. + Given the amount of Bugzilla traffic with 'Fixed in...', that's a + non-trivial issue." [JSonnRevNum]_ +- "Sequential IDs are important for LNT and llvmlab bisection tool." [MatthewsRevNum]_. + +However, Git can emulate this increasing revision number: +`git rev-list --count `. This identifier is unique only within a +single branch, but this means the tuple `(num, branch-name)` uniquely identifies +a commit. + +We can thus use this revision number to ensure that e.g. `clang -v` reports a +user-friendly revision number (e.g. `master-12345` or `4.0-5321`). This should +be enough to address the objections raised above with respect to this aspect of +Git. + +What About Branches and Merges? +------------------------------- + +In contrast to SVN, Git makes branching easy. Git's commit history is represented +as a DAG, a departure from SVN's linear history. + +However, we propose to *enforce linear history* in our canonical Git repository +repository. (This is not uncommon amongst many large users of Git.) + +.. + TODO: Is this going to work when people push via the SVN bridge? + +We'll do this with a combination of client-side and server-side hooks. GitHub +offers a feature called `Status Checks`: a branch protected by `status checks` +requires commits to be whitelisted before the push can happen. A supplied +pre-push hook on the client side will run and check the history, before +whitelisting the commit being pushed [statuschecks]_. + +What About Commit Emails? +------------------------- + +An extra bot will need to be set up to continue to send emails for every commit. +We'll keep the exact same email format as we currently have (a change is possible +later, but beyond the scope of the current discussion), the only difference +being changing the URL from `http://llvm.org/viewvc/...` to +`http://github.org/llvm/...`. + + +One or Multiple Repositories? +============================= + +There are two major proposals for how to structure our Git repository: The +"multirepo" and the "monorepo". + +1. *Multirepo* - Moving each SVN sub-project into its own separate Git repository. +2. *Monorepo* - Moving all the LLVM sub-projects into a single Git repository. + +The first proposal would mimic the existing official separate read-only Git +repositories (e.g. http://llvm.org/git/compiler-rt.git), while the second one +would mimic an export of the SVN repository (i.e. it would look similar to +https://github.com/llvm-project/llvm-project, where each sub-project has its own +top-level directory). + +With the Monorepo, the existing read-only repositories (i.e. for example +http://llvm.org/git/compiler-rt.git) with git-svn read-write access would be +maintained + +There are other impacts that are less immediates and less technicals: the first +proposal of keeping the repository separate implies that the sub-projects are +more independent from each other, while the second proposal +encourage better code sharing and refactoring across projects, for example +reusing a datastructure initially in LLDB by moving it into libSupport. It +would also be easier to decide to extract some pieces of libSupport and/or +ADT to a new top-level *independent* library that can be reused in libcxxabi for +instance. Finally, it also encourages to update all the sub-projects when +changing API or refactoring code ("git grep" works across sub-projects for +instance). + +As another example, some developers think that the division between e.g. clang +and clang-tools-extra is not useful. With the monorepo, we can move code around +as we wish. With the multirepo, moving clang-tools-extra into clang would be +much more complicated, and might end up loosing history. + +Some concerns have been raised that having a single repository would be a burden +for downstream users that have interest in only a single repository, however +this is addressed by keeping a read-only Git repo for each project just as we +do today. Also the GitHub SVN bridge allows to contribute to a single +sub-project the same way it is possible today (see below before/after section +for more details). + +Finally, nobody will be forced to compile projects they don't want to build. +The exact structure is TBD, but even if you use the monorepo directly, we'll +ensure that it's easy to set up your build to compile only a few particular +sub-projects. + +How Do We Handle A Single Revision Number Across Multiple Repositories? +----------------------------------------------------------------------- + +A key need is to be able to check out multiple projects (i.e. lldb+llvm or +clang+llvm+libcxx for example) at a specific revision. + +Under the monorepo, this is a non-issue. That proposal maintains property of +the existing SVN repository that the sub-projects move synchronously, and a +single revision number (or commit hash) identifies the state of the development +across all projects. + +Under the multirepo, things are more involved. We describe here the proposed +solution. + +Fundamentally, separated Git repositories imply that a tuple of revisions +(one entry per repository) is needed to describe the state across +repositories/sub-projects. +For example, a given version of clang would be +**. + +To make this more convenient, a separate *umbrella* repository would be +provided. This repository would be used for the sole purpose of understanding +the sequence (with some granularity) in which commits were added across +repository and to provide a single revision number. + +This umbrella repository will be read-only and periodically updated +to record the above tuple. The proposed form to record this is to use Git +[submodules]_, possibly along with a set of scripts to help check out a +specific revision of the LLVM distribution. + +A regular LLVM developer does not need to interact with the umbrella repository +-- the individual repositories can be checked out independently -- but you would +need to use the umbrella repository to bisect or to check out old revisions of +llvm plus another sub-project at a consistent version. + +One example of such a repository is Takumi's llvm-project-submodule +(https://github.com/chapuni/llvm-project-submodule). You can use +`git submodule init` to check out only the sub-projects you're interested in, and +other submodule commands to e.g. update all submodules to an older revision. + +This umbrella repository will be updated automatically by a bot (running on +notice from a webhook on every push, and periodically). Note that commits in +different repositories pushed within the same time frame may be visible together +or in undefined order in the umbrella repository. + +Workflow Before/After +===================== + +This section goes through a few examples of workflows. + +Checkout/Clone a Single Project, without Commit Access +------------------------------------------------------ + +Except the URL, nothing changes. The possibilities today are:: + + svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm + # or with Git + git clone http://llvm.org/git/llvm.git + +After the move to GitHub, you would do either:: + + git clone https://github.com/llvm-project/llvm.git + # or using the GitHub svn native bridge + svn co https://github.com/llvm-project/llvm/trunk + +The above works for both the monorepo and the multirepo, as we'll maintain the +existing read-only views of the individual sub-projects. + +Checkout/Clone a Single Project, with Commit Access +--------------------------------------------------- + +**Currently** +:: + + # direct SVN checkout + svn co https://user@llvm.org/svn/llvm-project/llvm/trunk llvm + # or using the read-only Git view, with git-svn + git clone http://llvm.org/git/llvm.git + cd llvm + git svn init https://llvm.org/svn/llvm-project/llvm/trunk --username= + git config svn-remote.svn.fetch :refs/remotes/origin/master + git svn rebase -l # -l avoids fetching ahead of the git mirror. + +Commits are performed using `svn commit` or `git commit` and `git svn dcommit`. + +**Multirepo Proposal** + +With the multirepo proposal, nothing changes but the URL, and commits can be +performed using `svn commit` or `git commit` and `git push`:: + + git clone https://github.com/llvm/llvm.git llvm + # or using the GitHub svn native bridge + svn co https://github.com/llvm/llvm/trunk/ llvm + +**Monorepo Proposal** + +With the monorepo, there are multiple possibilities to achieve this. First, +you could just clone the full repository:: + + git clone https://github.com/llvm/llvm-projects.git llvm + # or using the GitHub svn native bridge + svn co https://github.com/llvm/llvm-projects/trunk/ llvm + +At this point you have every sub-project (llvm, clang, lld, lldb, ...), which +**doesn't imply you have to build all of them**. You can still build only +compiler-rt for instance. In this way it's not different from someone who would +check out all the projects with SVN today. + +You can commit as normal using `git commit` and `git push` or `svn commit`, and +read the history for a single project (`git log libcxx` for example). + +If you don't want to have the sources for all the sub-projects checked out for, +there are again a few options. + +First, you could hide the other directories using a Git sparse checkout:: + + git config core.sparseCheckout true + echo /compiler-rt > .git/info/sparse-checkout + git read-tree -mu HEAD + +The data for all sub-projects is still in your `.git` directory, but in your +checkout, you only see `compiler-rt`. Git compresses its history, so +a clone of everything is only about 2x as much data as a clone of llvm only (and +in any case this is dwarfed by the size of e.g. a llvm build dir). + +Before you push, you'll need to fetch and rebase as normal. However when you +fetch you'll likely pull in changes to sub-projects you don't care about. You +may need to rebuild and retest, but only if the fetch included changes to a +sub-project that your change depends on. You can check this by running:: + + git log origin/master@{1}..origin/master libcxx + +This shows you all of the changes to `libcxx` since you last fetched. This +command can be hidden in a script so that `git llvmpush` would perform all these +steps, fail only if such a dependent change exists, and show immediately the +change that prevented the push. An immediate repeat of the command would +(almost) certainly result in a successed push. (This is +an extra step that you don't need in the multirepo, but for those of us who +work on a sub-project that depends on llvm, it has the advantage that we can +check whether we pulled in any changes to say clang *or* llvm.) + +A second option is to use svn via the GitHub svn native bridge:: + + svn co https://github.com/llvm/llvm-projects/trunk/compiler-rt compiler-rt —username=... + +This checks out only compiler-rt and provides commit access using "svn commit", +in the same way as it would do today. + +Finally, you could use *git-svn* and one of the sub-project mirrors:: + + # Clone from the single read-only Git repo + git clone http://llvm.org/git/llvm.git + cd llvm + # Configure the SVN remote and initialize the svn metadata + $ git svn init https://github.com/joker-eph/llvm-project/trunk/llvm —username=... + git config svn-remote.svn.fetch :refs/remotes/origin/master + git svn rebase -l + +In this case the repository contains only a single sub-project, and commits can +be made using `git svn dcommit`, again **exactly as we do today**. + +Checkout/Clone Multiple Projects, with Commit Access +---------------------------------------------------- + +Let's look how to assemble llvm+clang+libcxx at a given revision. + +**Currently** +:: + + svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm -r $REVISION + cd llvm/tools + svn co http://llvm.org/svn/llvm-project/clang/trunk clang -r $REVISION + cd ../projects + svn co http://llvm.org/svn/llvm-project/libcxx/trunk libcxx -r $REVISION + +Or using git-svn:: + + git clone http://llvm.org/git/llvm.git + cd llvm/ + git svn init https://llvm.org/svn/llvm-project/llvm/trunk --username= + git config svn-remote.svn.fetch :refs/remotes/origin/master + git svn rebase -l + git checkout `git svn find-rev -B r258109` + cd tools + git clone http://llvm.org/git/clang.git + cd clang/ + git svn init https://llvm.org/svn/llvm-project/clang/trunk --username= + git config svn-remote.svn.fetch :refs/remotes/origin/master + git svn rebase -l + git checkout `git svn find-rev -B r258109` + cd ../../projects/ + git clone http://llvm.org/git/libcxx.git + cd libcxx + git svn init https://llvm.org/svn/llvm-project/libcxx/trunk --username= + git config svn-remote.svn.fetch :refs/remotes/origin/master + git svn rebase -l + git checkout `git svn find-rev -B r258109` + +Note that the list would be longer with more sub-projects. + +**Multirepo Proposal** + +With the multirepo proposal, the umbrella repository enters the dance. This is +where the mapping from a single revision number to the individual repositories +revisions is stored.:: + + git clone https://github.com/llvm-beanz/llvm-submodules + cd llvm-submodules + git checkout $REVISION + git submodule init + git submodule update clang llvm libcxx + +At this point the clang, llvm, and libcxx individual repositories are cloned +and stored alongside each other. There exist flags you can use to inform CMake +of your directory structure, and alternatively you can just symlink `clang` to +`llvm/tools/clang`, etc. + +**Monorepo Proposal** + +The repository contains natively the source for every sub-projects at the right +revision, which makes this straightforward:: + + git clone https://github.com/llvm/llvm-projects.git llvm + cd llvm + git checkout $REVISION + +As before, at this point clang, llvm, and libcxx are stored in directories +alongside each other. + +Commit an API Change in LLVM and Update the Sub-projects +-------------------------------------------------------- + +Today this is easy for subversion users, and possible but not straighfoward for +git-svn users. Few Git users try to e.g. update LLD or Clang in the same commit +as they change an LLVM API. + +The multirepo proposal does not address this: one would have to commit and push +separately in every individual repository. It might be possible to establish a +protocol whereby users add a special token to their commit messages that causes +the umbrella repo's updater bot to group all of them into a single revision. + +The single repository proposal handles this natively and makes this use case +trivial. + +Branching/Stashing/Updating for Local Development or Experiments +---------------------------------------------------------------- + +**Currently** + +SVN does not allow this use case, but developers that are currently using +git-svn can do it. Let's look in practice what it means when dealing with +multiple sub-projects. + +To update the repository to tip of trunk:: + + git pull + cd tools/clang + git pull + cd ../../projects/libcxx + git pull + +To create a new branch:: + + git checkout -b MyBranch + cd tools/clang + git checkout -b MyBranch + cd ../../projects/libcxx + git checkout -b MyBranch + +To switch branches:: + + git checkout AnotherBranch + cd tools/clang + git checkout -b AnotherBranch + cd ../../projects/libcxx + git checkout -b AnotherBranch + +**Multirepo Proposal** + +The multirepo works the same as the current Git workflow: every command needs +to be applied to each of the individual repositories. + +**Monorepo Proposal** + +Regular Git commands are sufficient, because everything is in a single +repository: + +To update the repository to tip of trunk:: + + git pull + +To create a new branch:: + + git checkout -b MyBranch + +To switch branches:: + + git checkout AnotherBranch + +Bisecting +--------- + +Assuming a developer is looking for a bug in clang (or lld, or lldb, ...). + +**Currently** + +SVN does not have builtin bisection support. Using the existing Git read-only +view of the repositories, it is possible to use the native Git bisection script +over the llvm repository, and use some scripting to synchronize the clang +repository to match the llvm revision. + +**Multirepo Proposal** + +With the multi-repositories proposal, the cross-repository synchronization is +achieved using the umbrella repository. This repository contains only +submodules for the other sub-projects. The native Git bisection can be used on +the umbrella repository directly. A subtlety is that the bisect script itself +needs to make sure the submodules are updated accordingly. + +For example, to find which commit introduces a regression where clang-3.9 +crashes but not clang-3.8 passes, one should be able to simply do:: + + git bisect start release_39 release_38 + git bisect run ./bisect_script.sh + +With the `bisect_script.sh` script being:: + + #!/bin/sh + cd $UMBRELLA_DIRECTORY + git submodule update llvm clang libcxx #.... + cd $BUILD_DIR + + ninja clang || exit 125 # an exit code of 125 asks "git bisect" + # to "skip" the current commit + + ./bin/clang some_crash_test.cpp + +When the `git bisect run` command returns, the umbrella repository is set to +the state where the regression is introduced, one can inspect the history on +every sub-projects compared to the previous revision in the umbrella (it is +possible that one commit in the umbrella repository includes multiple commits +in the sub-projects). + +**Monorepo Proposal** + +Bisecting on the monorepo is straightforward and almost identical to the +multirepo situation explained above. The granularity is finer since each +individual commits in every sub-projects participate in the bisection. The +bisection script does not need to include the `git submodule update` step. + +Living Downstream +----------------- + +Depending on which of the multirepo or the monorepo proposal gets accepted, +and depending on the integration scheme, downstream projects may be differently +impacted and have different options. + +* If you were pulling from the SVN repo before the switch to Git. The monorepo + will allow you to continue to use SVN. The main caveat is that you'll need to + be prepared for a one-time change to the revision numbers. The multirepo + proposal does not provide a great solution for this. + +* If you were pulling from one of the existing read-only Git repos, this also + will continue to work as before as they will continue to exist in any of the + proposal. + +Under the monorepo proposal, you have a third option: migrating your fork to +the monorepo. This can be particularly beneficial if your fork touches +multiple sub-projects (e.g. llvm and clang), because now you can commingle +commits to llvm and clang in a single repository. + +As a demonstration, we've migrated the "Cherry" fork to the monorepo in two ways: + +* Using a script that rewrites history (including merges) so that it looks like + the fork always lived in the monorepo [LebarCherry]_. The upside of this is + when you check out an old revision, you get a copy of all llvm sub-projects at + a consistent revision. (For instance, if it's a clang fork, when you check + out an old revision you'll get a consistent version of llvm proper.) The + downside is that this changes the fork's commit hashes. + +* Merging the fork into the monorepo [AminiCherry]_. This preserves the fork's + commit hashes, but when you check out an old commit you only get the one + sub-project. + +If you keep a split-repository solution downstream, upstreaming patches is +always possible: you can apply the patches in the appropriate subdirectory of +the monorepo. + +Monorepo Variant +================ + +A variant of the monorepo proposal is to group together in a single repository +only the projects that are *rev-locked* to LLVM (clang, lld, lldb, ...) and +leave projects like libcxx and compiler-rt in their own individual and separate +repository. + +Note however that many users of the monorepo would benefit from having all of +the pieces needed for a full toolchain present in one repository. And for +newcomers, getting and building a toolchain is easier. + +Also, developers who hack only on one of these sub-projects can continue to use +the single sub-project Git mirrors, so their workflow is unchanged. (That is, +they aren't forced to download or check out all of llvm, clang, etc. just to +make a change to libcxx.) + +Previews +======== + +FIXME: make something more official/testable and update all the URLs in the +examples above. + +Example of a working version: + +* Repository: https://github.com/llvm-beanz/llvm-submodules +* Update bot: http://beanz-bot.com:8180/jenkins/job/submodule-update/ + + +Remaining Issues +================ + +LNT and llvmlab will need to be updated: they rely on unique monotonically +increasing integer across branch [MatthewsRevNum]_. + +Straw Man Migration Plan +======================== + +STEP #1 : Before The Move + +1. Update docs to mention the move, so people are aware of what is going on. +2. Set up a read-only version of the GitHub project, mirroring our current SVN + repository. +3. Add the required bots to implement the commit emails, as well as the + umbrella repository update (if the multirepo is selected) or the read-only + Git views for the sub-projects (if the monorepo is selected). + +STEP #2 : Git Move + +4. Update the buildbots to pick up updates and commits from the GitHub + repository. Not all bots have to migrate at this point, but it'll help + provide infrastructure testing. +5. Update Phabricator to pick up commits from the GitHub repository. +6. Instruct downstream integrators to pick up commits from the GitHub + repository. +7. Review and prepare an update for the LLVM documentation. + +Until this point nothing has changed for developers, it will just +boil down to a lot of work for buildbot and other infrastructure +owners. + +Once all dependencies are cleared, and all problems have been solved: + +STEP #3: Write Access Move + +8. Collect developers' GitHub account information, and add them to the project. +9. Switch the SVN repository to read-only and allow pushes to the GitHub repository. +10. Update the documentation +11. Mirror Git to SVN. + +STEP #4 : Post Move + +10. Archive the SVN repository. +11. Update links on the LLVM website pointing to viewvc/klaus/phab etc. to + point to GitHub instead. + +.. [LattnerRevNum] Chris Lattner, http://lists.llvm.org/pipermail/llvm-dev/2011-July/041739.html +.. [TrickRevNum] Andrew Trick, http://lists.llvm.org/pipermail/llvm-dev/2011-July/041721.html +.. [JSonnRevNum] Joerg Sonnenberg, http://lists.llvm.org/pipermail/llvm-dev/2011-July/041688.html +.. [TorvaldRevNum] Linus Torvald, http://git.661346.n2.nabble.com/Git-commit-generation-numbers-td6584414.html +.. [MatthewsRevNum] Chris Matthews, http://lists.llvm.org/pipermail/cfe-dev/2016-July/049886.html +.. [submodules] Git submodules, https://git-scm.com/book/en/v2/Git-Tools-Submodules) +.. [statuschecks] GitHub status-checks, https://help.github.com/articles/about-required-status-checks/ +.. [LebarCherry] Port *Cherry* to a single repository rewriting history, http://lists.llvm.org/pipermail/llvm-dev/2016-July/102787.html +.. [AminiCherry] Port *Cherry* to a single repository preserving history, http://lists.llvm.org/pipermail/llvm-dev/2016-July/102804.html Index: docs/index.rst =================================================================== --- docs/index.rst +++ docs/index.rst @@ -507,6 +507,7 @@ CodeOfConduct Proposals/GitHubSubMod + Proposals/GitHubMove :doc:`CodeOfConduct` Proposal to adopt a code of conduct on the LLVM social spaces (lists, events,