# Intro `git replay` has initially been developed entirely by Elijah Newren mostly last year (2022) at: https://github.com/newren/git/commits/replay I took over this year 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. The focus in this series is to make it a good plumbing command that can already be used server side and that replaces the "fast-rebase" test-tool command. So things to make it easier to use on the command line, and more advanced features (like replaying merges) are left out. It looks like GitHub has actually already been using version 3 of this patch series in production with good results. See: https://github.blog/2023-07-27-scaling-merge-ort-across-github/ https://lore.kernel.org/git/304f2a49-5e05-7655-9f87-2011606df5db@xxxxxx/ # Content of this cover letter The "Quick Overview" and "Reasons for diverging from cherry-pick & rebase" sections just below are describing the purpose of the new command in the big scheme of things. They are taken from Elijah's design notes (https://github.com/newren/git/blob/replay/replay-design-notes.txt) and describe what we want this command to become and the reasons for that, not what the command is after only this patch series. Also these design notes were written at least one year ago, so parts of those 2 sections are not true anymore. I have added Phillip Wood's or Felipe Contreras' notes (thanks to them) where that's the case, but some now flawed parts may have missed. After these two sections, starting with the "Important limitations" section, you will find sections describing what is actually in this patch series. More interesting material is available in Elijah's design notes like an "Intro via examples" (https://github.com/newren/git/blob/replay/replay-design-notes.txt#L37-L132), a discussion about "Preserving topology, replaying merges" (https://github.com/newren/git/blob/replay/replay-design-notes.txt#L264-L341) and a "Current status" section describing Elijah's work (https://github.com/newren/git/blob/replay/replay-design-notes.txt#L344-L392) before I started working on upstreaming it. I have not included this material here though, as the documentation added by this patch series for the `git replay` command already includes an "EXAMPLES" section, and other sections of Elijah's design notes might not be interesting for now. Also this cover letter is already pretty long. But reviewers can refer to the links above if they think it can help. # Quick Overview (from Elijah's design notes) `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 in Elijah's original work, not in this series) * 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's design notes) 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. Note: since 356ee4659bb (sequencer: try to commit without forking 'git commit', 2017-11-24) cherry-pick and rebase do not fork subprocesses other than hooks for the cases covered by this patch series (i.e. they do not fork "git commit" for simple picks). * "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. Note: since 767a9c417eb (rebase -i: stop checking out the tip of the branch to rebase, 2020-01-24) it's not true that rebase forcibly and needlessly checks out 'otherbranch'. * Assuming HEAD is involved severely limits replaying multiple (possibly divergent) branches. Note: since 89fc0b53fdb (rebase: update refs from 'update-ref' commands, 2022-07-19) the sequencer can update multiple branches. The issue with divergent branch is with command line arguments and the todo list generation rather than the capabilities of the sequencer. * 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 Note: d42c9ffa0f (rebase: factor out branch_base calculation, 2022-10-17) might already deal with that (according to Felipe Contreras). * avoid updating working tree, index, refs, reflogs, and control structures except when needed (e.g. hitting a conflict, or operation finished) * Other performance ideas (mostly for future work, not in this series) * single-file control structures instead of directory of files (when doing interactive things which is in Elijah's original work, but not in this series) * 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. (This is not is this patch series though.) * 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 exits with code 1 if there are any conflict. No resumability. No nice output. No interactivity. No special exit code depending on the reason. * When a commit becomes empty as it is replayed, it is still replayed as an empty commit, instead of being dropped. * 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. * Notes associated with replayed commits are not updated nor duplicated. (Thanks to Phillip Wood for noticing.) # Commit overview * 1/15 t6429: remove switching aspects of fast-rebase New preparatory commit to make it easier to later replace the fast-rebase test-tool by `git replay` without breaking existing tests. * 2/15 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. (In v3 some `#include ...` were changed to deal with upstream changes in this area as suggested by Junio.) * - 3/15 replay: start using parse_options API - 4/15 replay: die() instead of failing assert() - 5/15 replay: introduce pick_regular_commit() - 6/15 replay: don't simplify history - 7/15 replay: add an important FIXME comment about gpg signing - 8/15 replay: remove progress and info output - 9/15 replay: remove HEAD related sanity check These slowly change the command to make it behave more like regular commands and to start cleaning up its output. * 10/15 replay: make it a minimal server side command After the cleaning up in previous commits, 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`. * 11/15 replay: use standard revision ranges Start addind new interesting features and also documentation and tests, as the interface of the command is cristalizing into its final form. * 12/15 replay: disallow revision specific options and pathspecs For now disallow revision specific options and pathspecs that are allowed and eaten by setup_revisions(), as it's not clear if all of these extra features are really needed, and anyway they would require tests and doc. So we leave them for future improvements. (In v3 a typo was fixed in the commit message and a code comment has been improved as suggested by Elijah.) * - 13/15 replay: add --advance or 'cherry-pick' mode - 14/15 replay: add --contained to rebase contained branches Add new option and features to the command. (In v3, in commit 14/15, the synopsys of the command was improved and a sentence about git rebase in its doc was removed as suggested by Elijah.) * 15/15 replay: stop assuming replayed branches do not diverge This adds another interesting feature, as well as related documentation and tests. (In v3 a typo in the commit message was fixed as suggested by Elijah.) # Notes about `fast-rebase`, tests and documentation The `fast-rebase` test-tool helper was developed by Elijah to experiment with a rebasing tool that would be developed from scratch based on his merge-ort work, could be used to test that merge-ort work, and would not have the speed and interface limitations of `git rebase` or `git cherry-pick`. This `fast-rebase` helper was used before this series in: t6429-merge-sequence-rename-caching.sh So when `git replay` is created from `fast-rebase` in patch 2/15, the t6429 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 11/15 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 plumbing command with the end goal of it being usable and used first on bare repos, contrary to existing commands like `git rebase` and `git cherry-pick`. The tests check that the command works on both bare and non-bare repo though. * 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. There are a few tests checking that. 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. * make worktree and index changes optional: commit 10/15 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 so that the command can be used on bare repos. It seems 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 11/15 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: 15/15 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. # Changes between v3 and v4 Thanks to Toon, Junio and Dscho for their suggestions on the previous version! The very few and minor changes compared to v3 are: * The patch series was rebased onto master at d814540bb7 (The fifth batch, 2023-09-01). This is to fix a few header related conflicts as can be seen in the range-diff. * In patch 10/15 (replay: make it a minimal server side command) a /* Cleanup */ code comment has been removed as suggested by Toon. * In patch 11/15 (replay: use standard revision ranges) the git-replay documentation related to --onto has been improved to better explain which branches will be updated by the update-ref command(s) in the output as suggested by Junio. * In patch 12/15 (replay: disallow revision specific options and pathspecs) an error message has been improved as suggested by Junio. * In patch 13/15 (replay: add --advance or 'cherry-pick' mode) the commit message and the git-replay documentation have been improved to better explain that --advance only works when the revision range passed has a single tip as suggested by Junio. * Also in patch 13/15 (replay: add --advance or 'cherry-pick' mode) an error message has been improved, and a few tests have been added to check that `git replay` fails when it's passed both --advance and --onto and when it's passed none of these options, as suggested by Toon. # Range-diff between v3 and v4 1: 51fa1c7aea = 1: 1eaca9b788 t6429: remove switching aspects of fast-rebase 2: 19f8cf1b2e ! 2: 5ac4beb1ae replay: introduce new builtin @@ t/helper/test-fast-rebase.c => builtin/replay.c #define USE_THE_INDEX_VARIABLE -#include "test-tool.h" --#include "cache.h" +#include "git-compat-util.h" + +#include "builtin.h" 3: 295e876db6 ! 3: 299381aa9b replay: start using parse_options API @@ builtin/replay.c #include "lockfile.h" #include "merge-ort.h" #include "object-name.h" +-#include "read-cache-ll.h" +#include "parse-options.h" #include "refs.h" #include "revision.h" 4: 6ece7d3751 = 4: 3b825c9be0 replay: die() instead of failing assert() 5: 9b4bc659fb = 5: b1e890745d replay: introduce pick_regular_commit() 6: 9ab68d50ab = 6: ec51351889 replay: don't simplify history 7: 37e93faafa = 7: cd4ed07d2d replay: add an important FIXME comment about gpg signing 8: 03036781ed = 8: e45a55917c replay: remove progress and info output 9: 4ea289952e = 9: 0587a76cbb replay: remove HEAD related sanity check 10: fba98eda07 ! 10: d10368e87a replay: make it a minimal server side command @@ builtin/replay.c -#include "commit.h" #include "environment.h" -#include "gettext.h" +-#include "hash.h" #include "hex.h" #include "lockfile.h" #include "merge-ort.h" @@ builtin/replay.c: int cmd_replay(int argc, const char **argv, const char *prefix + } } -+ /* Cleanup */ merge_finalize(&merge_opt, &result); + ret = result.clean; 11: 03f9f20f6e ! 11: 4e09572c43 replay: use standard revision ranges @@ Documentation/git-replay.txt (new) + Starting point at which to create the new commits. May be any + valid commit, and not just an existing branch name. ++ -+The update-ref commands in the output will update the branch(es) -+in the revision range to point at the new commits (in other -+words, this mimics a rebase operation). ++The update-ref command(s) in the output will update the branch(es) in ++the revision range to point at the new commits, similar to the way how ++`git rebase --update-refs` updates multiple branches in the affected ++range. + +<revision-range>:: + Range of commits to replay; see "Specifying Ranges" in @@ Documentation/git-replay.txt (new) + update refs/heads/branch2 ${NEW_branch2_HASH} ${OLD_branch2_HASH} + update refs/heads/branch3 ${NEW_branch3_HASH} ${OLD_branch3_HASH} + -+where the number of refs updated depend on the arguments passed. ++where the number of refs updated depends on the arguments passed and ++the shape of the history being replayed. + +EXIT STATUS +----------- 12: e651250ac7 ! 12: 64b803f1cf replay: disallow revision specific options and pathspecs @@ builtin/replay.c: int cmd_replay(int argc, const char **argv, const char *prefix + * them, after adding related tests and doc though. + */ + if (revs.prune_data.nr) { -+ error(_("invalid pathspec: %s"), revs.prune_data.items[0].match); ++ error(_("no pathspec is allowed: '%s'"), revs.prune_data.items[0].match); + usage_with_options(replay_usage, replay_options); + } + 13: 56e5416dad ! 13: 04f27d81ab replay: add --advance or 'cherry-pick' mode @@ Commit message 'cherry-pick' mode with `--advance`. This new mode will make the target branch advance as we replay commits onto it. + The replayed commits should have a single tip, so that it's clear where + the target branch should be advanced. If they have more than one tip, + this new mode will error out. + Co-authored-by: Christian Couder <chriscool@xxxxxxxxxxxxx> Signed-off-by: Elijah Newren <newren@xxxxxxxxx> Signed-off-by: Christian Couder <chriscool@xxxxxxxxxxxxx> @@ Documentation/git-replay.txt: OPTIONS Starting point at which to create the new commits. May be any valid commit, and not just an existing branch name. + --The update-ref commands in the output will update the branch(es) --in the revision range to point at the new commits (in other --words, this mimics a rebase operation). +-The update-ref command(s) in the output will update the branch(es) in +-the revision range to point at the new commits, similar to the way how +-`git rebase --update-refs` updates multiple branches in the affected +-range. +When `--onto` is specified, the update-ref command(s) in the output will +update the branch(es) in the revision range to point at the new -+commits (in other words, this mimics a rebase operation). ++commits, similar to the way how `git rebase --update-refs` updates ++multiple branches in the affected range. + +--advance <branch>:: + Starting point at which to create the new commits; must be a @@ Documentation/git-replay.txt: OPTIONS +the new commits (in other words, this mimics a cherry-pick operation). <revision-range>:: - Range of commits to replay; see "Specifying Ranges" in +- Range of commits to replay; see "Specifying Ranges" in +- linkgit:git-rev-parse. ++ Range of commits to replay. More than one <revision-range> can ++ be passed, but in `--advance <branch>` mode, they should have ++ a single tip, so that it's clear where <branch> should point ++ to. See "Specifying Ranges" in linkgit:git-rev-parse. + + OUTPUT + ------ @@ Documentation/git-replay.txt: input to `git update-ref --stdin`. It is basically of the form: - update refs/heads/branch2 ${NEW_branch2_HASH} ${OLD_branch2_HASH} update refs/heads/branch3 ${NEW_branch3_HASH} ${OLD_branch3_HASH} --where the number of refs updated depend on the arguments passed. -+where the number of refs updated depend on the arguments passed. When -+using `--advance`, the number of refs updated is always one, but for -+`--onto`, it can be one or more (rebasing multiple branches -+simultaneously is supported). + where the number of refs updated depends on the arguments passed and +-the shape of the history being replayed. ++the shape of the history being replayed. When using `--advance`, the ++number of refs updated is always one, but for `--onto`, it can be one ++or more (rebasing multiple branches simultaneously is supported). EXIT STATUS ----------- @@ builtin/replay.c: static struct commit *create_commit(struct tree *tree, + die(_("argument to --advance must be a reference")); + } + if (rinfo.positive_refexprs > 1) -+ die(_("cannot advance target with multiple source branches because ordering would be ill-defined")); ++ die(_("cannot advance target with multiple sources because ordering would be ill-defined")); + } else { + int positive_refs_complete = ( + rinfo.positive_refexprs == @@ builtin/replay.c: int cmd_replay(int argc, const char **argv, const char *prefix + oid_to_hex(&onto->object.oid)); + } + - /* Cleanup */ merge_finalize(&merge_opt, &result); ret = result.clean; @@ t/t3650-replay-basics.sh: test_expect_success 'using replay on bare repo with di + git -C bare replay --advance main topic1..topic2 >result-bare && + test_cmp expect result-bare +' ++ ++test_expect_success 'replay on bare repo fails with both --advance and --onto' ' ++ test_must_fail git -C bare replay --advance main --onto main topic1..topic2 >result-bare ++' ++ ++test_expect_success 'replay fails when both --advance and --onto are omitted' ' ++ test_must_fail git replay topic1..topic2 >result ++' + test_done 14: 2cc17dfdc7 ! 14: 9ed0919ad5 replay: add --contained to rebase contained branches @@ builtin/replay.c: int cmd_replay(int argc, const char **argv, const char *prefix oid_to_hex(&last_commit->object.oid), ## t/t3650-replay-basics.sh ## -@@ t/t3650-replay-basics.sh: test_expect_success 'using replay on bare repo to perform basic cherry-pick' ' - test_cmp expect result-bare +@@ t/t3650-replay-basics.sh: test_expect_success 'replay fails when both --advance and --onto are omitted' ' + test_must_fail git replay topic1..topic2 >result ' +test_expect_success 'using replay to also rebase a contained branch' ' 15: a6d88fc8f0 ! 15: cf8c984877 replay: stop assuming replayed branches do not diverge @@ builtin/replay.c: int cmd_replay(int argc, const char **argv, const char *prefix if (advance_name) continue; @@ builtin/replay.c: int cmd_replay(int argc, const char **argv, const char *prefix) + } - /* Cleanup */ merge_finalize(&merge_opt, &result); - ret = result.clean; - Christian Couder (1): replay: disallow revision specific options and pathspecs Elijah Newren (14): t6429: remove switching aspects of fast-rebase 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: make it a minimal server side command replay: use standard revision ranges replay: add --advance or 'cherry-pick' mode replay: add --contained to rebase contained branches replay: stop assuming replayed branches do not diverge .gitignore | 1 + Documentation/git-replay.txt | 124 +++++++ Makefile | 2 +- builtin.h | 1 + builtin/replay.c | 431 +++++++++++++++++++++++ command-list.txt | 1 + git.c | 1 + t/helper/test-fast-rebase.c | 241 ------------- t/helper/test-tool.c | 1 - t/helper/test-tool.h | 1 - t/t3650-replay-basics.sh | 214 +++++++++++ t/t6429-merge-sequence-rename-caching.sh | 45 +-- 12 files changed, 799 insertions(+), 264 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 -- 2.42.0.126.gcf8c984877
