pr-1005/dscho/scalar-the-beginning-v6
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27 Oct 08:27
tl;dr: This series contributes the core part of the Scalar command to the Git project. This command provides an opinionated way to create and configure Git repositories with a focus on very large repositories. Changes since v5: * Fixed the commit message talking about make -C contrib/scalar/Makefile. * Fixed the git ls-tree invocation suggested in the manual for scalar clone. * Invoking make -C contrib/scalar, then changing a source file of libgit.a and then immediately invoking make -C contrib/scalar again will now implicitly rebuild libgit.a. Changes since v4: * scalar delete now refuses to delete anything if it was started from within the enlistment. * scalar delete releases any handles to the object store before deleting the enlistment. * The OBJECTS list in the Makefile will now include Scalar. * scalar register now supports secondary worktrees, in addition to the primary worktree. Changes since v3: * Moved the "Changes since" section to the top, to make it easier to see what changed. * Reworded the commit message of the first patch. * Removed the [RFC] prefix because I did not hear any objections against putting this into contrib/. Changes since v2: * Adjusted the description of the list command in the manual page , as suggested by Bagas. * Addressed two style nits in cmd_run(). * The documentation of git reconfigure -a was improved. Changes since v1: * A couple typos were fixed * The code parsing the output of ls-remote was made more readable * The indentation used in scalar.txt now consistently uses tabs * We no longer hard-code core.bare = false when registering with Scalar Background ========== Years ago, Microsoft wanted to move the source code of the Windows operating system to Git. The challenge there was to prove that Git could scale to massive monorepos. The VFS for Git (formerly GVFS) project was born to take up that challenge. The final solution included a virtual filesystem (with both user-mode and kernel components) and a customized fork of Git for Windows. This solution contained several key concepts, such as only populating a portion of the working directory, demand-fetching blobs, and performing periodic repo maintenance in the background. However, the required kernel drivers made it difficult to port the solution to other platforms. But it was realized that many of these key concepts were independent of the actual VFS and its projection of the working directory. The Scalar project was created to make that separation, refine the key concepts, and then extract those features into the new Scalar command. The present =========== The Scalar project provides a completely functional non-virtual experience for monorepos. But why stop there. The Scalar project was designed to be a self-destructing vehicle to allow those key concepts to be moved into core Git itself for the benefit of all. For example, partial clone, sparse-checkout, and background maintenance have already been upstreamed and removed from Scalar proper. This patch series provides a C-based implementation of the final remaining portions of the Scalar command. This will make it easier for users to experiment with the Scalar command. It will also make it substantially easier to experiment with moving functionality from Scalar into core Git, while maintaining backwards-compatibility for existing Scalar users. The C-based Scalar has been shipped to Scalar users, and can be tested by any interested reader: https://github.com/microsoft/git/releases/ (it offers a Git for Windows installer, a macOS package and an Ubuntu package, Scalar has been included since v2.33.0.vfs.0.0). Next steps ========== I have lined up a few more patch series on top of this one: 1. Implement a scalar diagnose command. 2. Use the built-in FSMonitor (that patch series obviously needs to wait for FSMonitor to be integrated). 3. Modify the config machinery to be more generous about concurrent writes, say, to the user-wide config. 4. A few patches to optionally build and install scalar as part of a regular Git install (also teaching git help scalar to find the Scalar documentation These are included in my vfs-with-scalar branch thicket [https://github.com/dscho/git/commits/vfs-with-scalar]. On top of that, this branch thicket also includes patches I do not plan on upstreaming, mainly because they are too specific either to VFS for Git, or they support Azure Repos (which does not offer partial clones but speaks the GVFS protocol, which can be used to emulate partial clones). One other thing is very interesting about that vfs-with-scalar branch thicket: it contains a GitHub workflow which will run Scalar's quite extensive Functional Tests suite. This test suite is quite comprehensive and caught us a lot of bugs in the past, not only in the Scalar code, but also core Git. Epilogue ======== Now, to address some questions that I imagine every reader has who made it this far: * Why not put the Scalar functionality directly into core Git, even a built-in? Creating a Git builtin requires scrutiny over every aspect of the feature, which is difficult to do while also maintaining the command-line interface contract and expected behavior of the Scalar command (there are existing users, after all). By having the Scalar command in contrib/, we present a simple option for users to have these features in the short term while the Git contributor community decides which bits to absorb into Git built-ins. * Why implement the Scalar command in the Git codebase? We ported Scalar to the microsoft/git fork for several reasons. First, we realized it was possible in the first place, now that the core features exist inside Git itself. Second, compiling Scalar directly within a version of Git allows us to remove a version compatibility check from each config option that might or might not apply based on the installed Git version. Finally, this new location has greatly simplified our release process and the installation process for users. We now have ways to install Scalar with microsoft/git via winget, brew, and apt-get. This has been the case since we shipped v2.32.0 to our users, read: this setup has served us well already. * Why contribute Scalar to the Git project? We are biased, of course, yet we do have evidence that the Scalar command is a helpful tool that offers an simple way to handle huge repositories with ease. By contributing it to the core Git project, we are able to share it with more users, especially some users who do not want to install the microsoft/git fork. We intend to include Scalar as a component in git-for-windows/git, but are contributing it here first. Further, we think there is benefit to the Git developer community as this presents an example of how to set certain defaults that work for large repositories. Derrick Stolee (4): scalar: 'register' sets recommended config and starts maintenance scalar: 'unregister' stops background maintenance scalar: implement 'scalar list' scalar: implement the `run` command Johannes Schindelin (10): scalar: create a rudimentary executable scalar: start documenting the command scalar: create test infrastructure scalar: let 'unregister' handle a deleted enlistment directory gracefully scalar: implement the `clone` subcommand scalar: teach 'clone' to support the --single-branch option scalar: allow reconfiguring an existing enlistment scalar: teach 'reconfigure' to optionally handle all registered enlistments scalar: implement the `version` command scalar: accept -C and -c options before the subcommand Matthew John Cheetham (1): scalar: implement the `delete` command Makefile | 9 + contrib/scalar/.gitignore | 5 + contrib/scalar/Makefile | 57 +++ contrib/scalar/scalar.c | 845 +++++++++++++++++++++++++++++++ contrib/scalar/scalar.txt | 155 ++++++ contrib/scalar/t/Makefile | 78 +++ contrib/scalar/t/t9099-scalar.sh | 88 ++++ 7 files changed, 1237 insertions(+) create mode 100644 contrib/scalar/.gitignore create mode 100644 contrib/scalar/Makefile create mode 100644 contrib/scalar/scalar.c create mode 100644 contrib/scalar/scalar.txt create mode 100644 contrib/scalar/t/Makefile create mode 100755 contrib/scalar/t/t9099-scalar.sh base-commit: ebf3c04b262aa27fbb97f8a0156c2347fecafafb Submitted-As: https://lore.kernel.org/git/pull.1005.v6.git.1635323239.gitgitgadget@gmail.com In-Reply-To: https://lore.kernel.org/git/pull.1005.git.1630359290.gitgitgadget@gmail.com In-Reply-To: https://lore.kernel.org/git/pull.1005.v2.git.1630691688.gitgitgadget@gmail.com In-Reply-To: https://lore.kernel.org/git/pull.1005.v3.git.1631129086.gitgitgadget@gmail.com In-Reply-To: https://lore.kernel.org/git/pull.1005.v4.git.1631630356.gitgitgadget@gmail.com In-Reply-To: https://lore.kernel.org/git/pull.1005.v5.git.1633604349.gitgitgadget@gmail.com
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2021-10-27T08:27:19Z -
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