# Intro `git replay` has initially been developed entirely by Elijah Newren mostly between June and July 2022 at: https://github.com/newren/git/commits/replay I took over a few months ago to polish and upstream it as GitLab is interested in replacing libgit2, and for that purpose needs a command to do server side (so without using a worktree) rebases, cherry-picks and reverts. I reduced the number of commits and features in this first patch series, compared to what Elijah already developed. Especially I stopped short of replaying merge commits and replaying interactively. These and other features might be upstreamed in the future after this patch series has graduated. Thanks to Elijah, Patrick Steinhardt and Dscho for early reviews and discussions. Based on ae73b2c8f1 (The seventh batch, 2023-04-04) # Quick Overview (from Elijah) `git replay`, at a basic level, can perhaps be thought of as a "default-to-dry-run rebase" -- meaning no updates to the working tree, or to the index, or to any references. However, it differs from rebase in that it: * Works for branches that aren't checked out * Works in a bare repository * Can replay multiple branches simultaneously (with or without common history in the range being replayed) * Preserves relative topology by default (merges are replayed too) * Focuses on performance * Has several altered defaults as a result of the above I sometimes think of `git replay` as "fast-replay", a patch-based analogue to the snapshot-based fast-export & fast-import tools. # Reasons for diverging from cherry-pick & rebase (from Elijah) There are multiple reasons to diverge from the defaults in cherry-pick and rebase. * Server side needs * Both cherry-pick and rebase, via the sequencer, are heavily tied to updating the working tree, index, some refs, and a lot of control files with every commit replayed, and invoke a mess of hooks[1] that might be hard to avoid for backward compatibility reasons (at least, that's been brought up a few times on the list). * cherry-pick and rebase both fork various subprocesses unnecessarily, but somewhat intrinsically in part to ensure the same hooks are called that old scripted implementations would have called. * "Dry run" behavior, where there are no updates to worktree, index, or even refs might be important. * Should not assume users only want to operate on HEAD (see next section) * Decapitate HEAD-centric assumptions * cherry-pick forces commits to be played on top of HEAD; inflexible. * rebase assumes the range of commits to be replayed is upstream..HEAD by default, though it allows one to replay upstream..otherbranch -- but it still forcibly and needlessly checks out otherbranch before starting to replay things. * Assuming HEAD is involved severely limits replaying multiple (possibly divergent) branches. * Once you stop assuming HEAD has a certain meaning, there's not much reason to have two separate commands anymore (except for the funny extra not-necessarily-compatible options both have gained over time). * (Micro issue: Assuming HEAD is involved also makes it harder for new users to learn what rebase means and does; it makes command lines hard to parse. Not sure I want to harp on this too much, as I have a suspicion I might be creating a tool for experts with complicated use cases, but it's a minor quibble.) * Performance * jj is slaughtering us on rebase speed[2]. I would like us to become competitive. (I dropped a few comments in the link at [2] about why git is currently so bad.) * From [3], there was a simple 4-patch series in linux.git that took 53 seconds to rebase. Switching to ort dropped it to 16 seconds. While that sounds great, only 11 *milliseconds* were needed to do the actual merges. That means almost *all* the time (>99%) was overhead! Big offenders: * --reapply-cherry-picks should be the default * can_fast_forward() should be ripped out, and perhaps other extraneous revision walks * avoid updating working tree, index, refs, reflogs, and control structures except when needed (e.g. hitting a conflict, or operation finished) * Other performance ideas: * single-file control structures instead of directory of files * avoid forking subprocesses unless explicitly requested (e.g. --exec, --strategy, --run-hooks). For example, definitely do not invoke `git commit` or `git merge`. * Sanitize hooks: * dispense with all per-commit hooks for sure (pre-commit, post-commit, post-checkout). * pre-rebase also seems to assume exactly 1 ref is written, and invoking it repeatedly would be stupid. Plus, it's specific to "rebase". So...ignore? (Stolee's --ref-update option for rebase probably broke the pre-rebase assumptions already...) * post-rewrite hook might make sense, but fast-import got exempted, and I think of replay like a patch-based analogue to the snapshot-based fast-import. * When not running server side, resolve conflicts in a sparse-cone sparse-index worktree to reduce number of files written to a working tree. (See below as well) * [High risk of possible premature optimization] Avoid large numbers of newly created loose objects, when replaying large numbers of commits. Two possibilities: (1) Consider using tmp-objdir and pack objects from the tmp-objdir at end of exercise, (2) Lift code from git-fast-import to immediately stuff new objects into a pack? * Multiple branches and non-checked out branches * The ability to operate on non-checked out branches also implies that we should generally be able to replay when in a dirty working tree (exception being when we expect to update HEAD and any of the dirty files is one that needs to be updated by the replay). * Also, if we are operating locally on a non-checked out branch and hit a conflict, we should have a way to resolve the conflict without messing with the user's work on their current branch. * Idea: new worktree with sparse cone + sparse index checkout, containing only files in the root directory, and whatever is necessary to get the conflicts * Companion to above idea: control structures should be written to $GIT_COMMON_DIR/replay-${worktree}, so users can have multiple replay sessions, and so we know which worktrees are associated with which replay operations. - [1] https://lore.kernel.org/git/pull.749.v3.git.git.1586044818132.gitgitgadget@xxxxxxxxx/ - [2] https://github.com/martinvonz/jj/discussions/49 - [3] https://lore.kernel.org/git/CABPp-BE48=97k_3tnNqXPjSEfA163F8hoE+HY0Zvz1SWB2B8EA@xxxxxxxxxxxxxx/ # Important limitations * The code die()s if there are any conflict. No resumability. No nice output. No interactivity. * No replaying merges, nor root commits. Only regular commits. * Signed commits are not properly handled. It's not clear what to do to such commits when replaying on the server side. # Commit overview * 1/14 replay: introduce new builtin This creates a minimal `git replay` command by moving the code from the `fast-rebase` test helper from `t/helper/` into `builtin/` and doing some renames and a few other needed changes. * - 2/14 replay: start using parse_options API - 3/14 replay: die() instead of failing assert() - 4/14 replay: introduce pick_regular_commit() - 5/14 replay: don't simplify history - 6/14 replay: add an important FIXME comment about gpg signing - 7/14 replay: remove progress and info output - 8/14 replay: remove HEAD related sanity check These slowly change the command to make it behave more like a regular commands and to start cleaning up its output. * 9/14 replay: very coarse worktree updating Make it handle conflicts in a very coarse way. This might not work on bare repos, but it allows existing tests to pass and it's nice to help cli users a bit when they get conflicts. * 10/14 replay: make it a minimal server side command After the cleaning up in previous ommits, it's now time to radically change the way it works by stopping it to do ref updates, to update the index and worktree, to consider HEAD as special. Instead just make it output commands that should be passed to `git update-ref --stdin`. * - 11/14 replay: use standard revision ranges - 12/14 replay: introduce guess_new_base() - 13/14 replay: add different modes - 14/14 replay: stop assuming replayed branches do not diverge These finish the clean up and add new interesting features at the same time, as well as related documentation and tests. # Note about tests and documentation Note that the `fast-rebase` test helper was used before this series in t6429-merge-sequence-rename-caching.sh So when `git replay` is created from `fast-rebase` in patch 1/14, this test script is also converted to use `git replay`. This ensures that `git replay` doesn't break too badly during the first 10 patches in this patch series. Tests and documentation are introduced specifically for `git replay` only in 11/14 and later patches as it doesn't make much sense to document and test behavior that we know is going to change soon. So it's only when the command is crystalizing towards its final form that we start documenting and testing it. # Possibly controversial issues * bare or not bare: this series works towards a command with the end goal of it being usable and used on bare repos, contrary to existing commands like `git rebase` and `git cherry-pick`, but the tests currently don't check that, and in case of conflicts it won't currently work on bare repos. One reason for that is that existing tests in t6429 should continue to work, and one of these tests requires some output in case of conflict. And it's nice for users to get some help in case of conflict. It's also nice for users if commands that should work on both bare and non bare repos work well on non bare repos first as they are less likely to use them on bare repos. So let's have a command that works well on non-bare repos first, even if its end goal is to work fine on bare repos too. We plan to improve things for bare repos soon after this first patch series graduates. * exit status: a successful, non-conflicted replay exits with code 0. When the replay has conflicts, the exit status is 1. If the replay is not able to complete (or start) due to some kind of error, the exit status is something other than 0 or 1. It has been suggested in an internal review that conflicts might want to get a more specific error code as an error code of 1 might be quite easy to return by accident. It doesn't seem to me from their docs (which might want to be improved, I didn't look at the code) that other commands like `git merge` and `git rebase` exit with a special error code in case of conflict. * to guess or not to guess: commit 12/14 introduces the guess_new_base() function which tries to find a base to rebase onto when the --onto option is not provided, making this option actually optional instead of mandatory. Given that it's an heuristic and the command end goal is to be used on server side, we might want to introduce this as an iterative improvement later. I still think it's interesting to have it in for now though, as it shows that --onto and --advance (which is introduced in the following commit) should indeed be options. If --onto was always mandatory in the series, people could argue that it shouldn't be an option and its argument should always be the first (unconditional) argument of the command. * make worktree and index changes optional: commit 10/14 stops updating the index and worktree, but it might be better especially for cli users to make that optional. The issue is that this would make the command more complex while we are developing a number of important features. It seems to me that this should rather be done in an iterative improvement after the important features have landed. * when and where to add tests and docs: although t6429 has tests that are changed to use the new command instead of the fast-rebase test-tool command as soon as the former is introduced, there is no specific test script and no doc for the new command until commit 11/14 when standard revision ranges are used. This is done to avoid churn in tests and docs while the final form of the command hasn't crystalized enough. Adding tests and doc at this point makes this commit quite big and possibly more difficult to review than if they were in separate commits though. On the other hand when tests and docs are added in specific commits some reviewers say it would be better to introduce them when the related changes are made. * --advance and --contained: these two advanced options might not belong to this first series and could perhaps be added in a followup series in separate commits. On the other hand the code for --contained seems involved with the code of --advance and it's nice to see soon that git replay can indeed do cherry-picking and rebase many refs at once, and this way fullfil these parts of its promise. * replaying diverging branches: 14/14 the last patch in the series, which allow replaying diverging branches, can be seen as a fundamental fix or alternatively as adding an interesting feature. So it's debatable if it should be in its own patch along with its own tests as in this series, or if it should be merged into a previous patch and which one. * only 2 patches: this patch series can be seen from a high level point of view as 1) introducing the new `git replay` command, and 2) using `git replay` to replace, and get rid of, the fast-rebase test-tool command. The fact that not much of the original fast-rebase code and interface is left would agree with that point of view. On the other hand, fast-rebase can also be seen as a first iteration towards `git replay`. So it can also make sense to see how `git replay` evolved from it. Elijah Newren (14): replay: introduce new builtin replay: start using parse_options API replay: die() instead of failing assert() replay: introduce pick_regular_commit() replay: don't simplify history replay: add an important FIXME comment about gpg signing replay: remove progress and info output replay: remove HEAD related sanity check replay: very coarse worktree updating replay: make it a minimal server side command replay: use standard revision ranges replay: introduce guess_new_base() replay: add different modes replay: stop assuming replayed branches do not diverge .gitignore | 1 + Documentation/git-replay.txt | 130 +++++++ Makefile | 2 +- builtin.h | 1 + builtin/replay.c | 419 +++++++++++++++++++++++ command-list.txt | 1 + git.c | 1 + t/helper/test-fast-rebase.c | 233 ------------- t/helper/test-tool.c | 1 - t/helper/test-tool.h | 1 - t/t3650-replay-basics.sh | 160 +++++++++ t/t6429-merge-sequence-rename-caching.sh | 43 ++- 12 files changed, 739 insertions(+), 254 deletions(-) create mode 100644 Documentation/git-replay.txt create mode 100644 builtin/replay.c delete mode 100644 t/helper/test-fast-rebase.c create mode 100755 t/t3650-replay-basics.sh -- 2.40.0.228.gb2eb5bb98e
