One thing that is not clear to me is whether this is the desired direction. I took at look at the git review notes but it was hard to get a sense of where people are at. Would love input on the design. On Wed, Oct 5, 2022 at 4:59 PM Stefan Xenos via GitGitGadget <gitgitgadget@xxxxxxxxx> wrote: > > From: Stefan Xenos <sxenos@xxxxxxxxxx> > > This document describes what a change graph for > git would look like, the behavior of the evolve command, > and the changes planned for other commands. > > It was originally proposed in 2018, see > https://public-inbox.org/git/20181115005546.212538-1-sxenos@xxxxxxxxxx/ > > Signed-off-by: Stefan Xenos <sxenos@xxxxxxxxxx> > Signed-off-by: Chris Poucet <poucet@xxxxxxxxxx> > --- > Documentation/technical/evolve.txt | 1070 ++++++++++++++++++++++++++++ > 1 file changed, 1070 insertions(+) > create mode 100644 Documentation/technical/evolve.txt > > diff --git a/Documentation/technical/evolve.txt b/Documentation/technical/evolve.txt > new file mode 100644 > index 00000000000..2051ea77b8a > --- /dev/null > +++ b/Documentation/technical/evolve.txt > @@ -0,0 +1,1070 @@ > +Evolve > +====== > + > +Objective > +========= > +Create an "evolve" command to help users craft a high quality commit history. > +Users can improve commits one at a time and in any order, then run git evolve to > +rewrite their recent history to ensure everything is up-to-date. We track > +amendments to a commit over time in a change graph. Users can share their > +progress with others by exchanging their change graphs using the standard push, > +fetch, and format-patch commands. > + > +Status > +====== > +This proposal has not been implemented yet. > + > +Background > +========== > +Imagine you have three sequential changes up for review and you receive feedback > +that requires editing all three changes. We'll define the word "change" > +formally later, but for the moment let's say that a change is a work-in-progress > +whose final version will be submitted as a commit in the future. > + > +While you're editing one change, more feedback arrives on one of the others. > +What do you do? > + > +The evolve command is a convenient way to work with chains of commits that are > +under review. Whenever you rebase or amend a commit, the repository remembers > +that the old commit is obsolete and has been replaced by the new one. Then, at > +some point in the future, you can run "git evolve" and the correct sequence of > +rebases will occur in the correct order such that no commit has an obsolete > +parent. > + > +Part of making the "evolve" command work involves tracking the edits to a commit > +over time, which is why we need an change graph. However, the change > +graph will also bring other benefits: > + > +- Users can view the history of a change directly (the sequence of amends and > + rebases it has undergone, orthogonal to the history of the branch it is on). > +- It will be possible to quickly locate and list all the changes the user > + currently has in progress. > +- It can be used as part of other high-level commands that combine or split > + changes. > +- It can be used to decorate commits (in git log, gitk, etc) that are either > + obsolete or are the tip of a work in progress. > +- By pushing and pulling the change graph, users can collaborate more > + easily on changes-in-progress. This is better than pushing and pulling the > + commits themselves since the change graph can be used to locate a more > + specific merge base, allowing for better merges between different versions of > + the same change. > +- It could be used to correctly rebase local changes and other local branches > + after running git-filter-branch. > +- It can replace the change-id footer used by gerrit. > + > +Goals > +----- > +Legend: Goals marked with P0 are required. Goals marked with Pn should be > +attempted unless they interfere with goals marked with Pn-1. > + > +P0. All commands that modify commits (such as the normal commit --amend or > + rebase command) should mark the old commit as being obsolete and replaced by > + the new one. No additional commands should be required to keep the > + change graph up-to-date. > +P0. Any commit that may be involved in a future evolve command should not be > + garbage collected. Specifically: > + - Commits that obsolete another should not be garbage collected until > + user-specified conditions have occurred and the change has expired from > + the reflog. User specified conditions for removing changes include: > + - The user explicitly deleted the change. > + - The change was merged into a specific branch. > + - Commits that have been obsoleted by another should not be garbage > + collected if any of their replacements are still being retained. > +P0. A commit can be obsoleted by more than one replacement (called divergence). > +P0. Users must be able to resolve divergence (convergence). > +P1. Users should be able to share chains of obsolete changes in order to > + collaborate on WIP changes. > +P2. Such sharing should be at the user’s option. That is, it should be possible > + to directly share a change without also sharing the file states or commit > + comments from the obsolete changes that led up to it, and the choice not to > + share those commits should not require changing any commit hashes. > +P2. It should be possible to discard part or all of the change graph > + without discarding the commits themselves that are already present in > + branches and the reflog. > +P2. Provide sufficient information to replace gerrit's Change-Id footers. > + > +Similar technologies > +-------------------- > +There are some other technologies that address the same end-user problem. > + > +Rebase -i can be used to solve the same problem, but users can't easily switch > +tasks midway through an interactive rebase or have more than one interactive > +rebase going on at the same time. It can't handle the case where you have > +multiple changes sharing the same parent when that parent needs to be rebased > +and won't let you collaborate with others on resolving a complicated interactive > +rebase. You can think of rebase -i as a top-down approach and the evolve command > +as the bottom-up approach to the same problem. > + > +Revup amend (https://github.com/Skydio/revup/blob/main/docs/amend.md) > +allows insertion of cached changes into any commit in > +the current history, and then reapplies the rest of history on top of > +those changes. It uses a "git apply --cached" engine under the hood so > +doesn't touch the working directory (although it will soon use the new > +git merge-tree). When paired with "revup upload" which creates and > +pushes multiple branches in the background for you, its possible to > +work on a "graph" of changes on a single branch linearly, then have > +the true graph structure created at upload time. > + > +git-revise (https://github.com/mystor/git-revise) does some very > +similar things except it uses "git merge-file" combined with manually > +merging the resulting trees. git branchstack > +(https://github.com/krobelus/git-branchstack) can also create branches > +in the background with the same mechanism. > + > +These tools don't store any external state, but as such also don't > +provide any specific collaboration mechanism for individual changes. > + > +Several patch queue managers have been built on top of git (such as topgit, > +stgit, and quilt). They address the same user need. However they also rely on > +state managed outside git that needs to be kept in sync. Such state can be > +easily damaged when running a git native command that is unaware of the patch > +queue. They also typically require an explicit initialization step to be done by > +the user which creates workflow problems. > + > +Mercurial implements a very similar feature in its EvolveExtension. The behavior > +of the evolve command itself is very similar, but the storage format for the > +change graph differs. In the case of mercurial, each change set can have one or > +more obsolescence markers that point to other changesets that they replace. This > +is similar to the "Commit Headers" approach considered in the other options > +appendix. The approach proposed here stores obsolescence information in a > +separate metacommit graph, which makes exchanging of obsolescence information > +optional. > + > +Mercurial's default behavior makes it easy to find and switch between > +non-obsolete changesets that aren't currently on any branch. We introduce the > +notion of a new ref namespace that enables a similar workflow via a different > +mechanism. Mercurial has the notion of changeset phases which isn't present > +in git and creates new ways for a changeset to diverge. Git doesn't need > +to deal with these issues, but it has to deal with the problems of picking an > +upstream branch as a target for rebases and protecting obsolescence information > +from GC. We also introduce some additional transformations (see > +obsolescence-over-cherry-pick, below) that aren't present in the mercurial > +implementation. > + > +Semi-related work > +----------------- > +There are other technologies that address different problems but have some > +similarities with this proposal. > + > +Replacements (refs/replace) are superficially similar to obsolescences in that > +they describe that one commit should be replaced by another. However, they > +differ in both how they are created and how they are intended to be used. > +Obsolescences are created automatically by the commands a user runs, and they > +describe the user’s intent to perform a future rebase. Obsolete commits still > +appear in branches, logs, etc like normal commits (possibly with an extra > +decoration that marks them as obsolete). Replacements are typically created > +explicitly by the user, they are meant to be kept around for a long time, and > +they describe a replacement to be applied at read-time rather than as the input > +to a future operation. When a replaced commit is queried, it is typically hidden > +and swapped out with its replacement as though the replacement has already > +occurred. > + > +Git-imerge is a project to help make complicated merges easier, particularly > +when merging or rebasing long chains of patches. It is not an alternative to > +the change graph, but its algorithm of applying smaller incremental merges > +could be used as part of the evolve algorithm in the future. > + > +Overview > +======== > +We introduce the notion of “meta-commits” which describe how one commit was > +created from other commits. A branch of meta-commits is known as a change. > +Changes are created and updated automatically whenever a user runs a command > +that creates a commit. They are used for locating obsolete commits, providing a > +list of a user’s unsubmitted work in progress, and providing a stable name for > +each unsubmitted change. > + > +Users can exchange edit histories by pushing and fetching changes. > + > +New commands will be introduced for manipulating changes and resolving > +divergence between them. Existing commands that create commits will be updated > +to modify the meta-commit graph and create changes where necessary. > + > +Example usage > +------------- > +# First create three dependent changes > +$ echo foo>bar.txt && git add . > +$ git commit -m "This is a test" > +created change metas/this_is_a_test > +$ echo foo2>bar2.txt && git add . > +$ git commit -m "This is also a test" > +created change metas/this_is_also_a_test > +$ echo foo3>bar3.txt && git add . > +$ git commit -m "More testing" > +created change metas/more_testing > + > +# List all our changes in progress > +$ git change list > +metas/this_is_a_test > +metas/this_is_also_a_test > +* metas/more_testing > +metas/some_change_already_merged_upstream > + > +# Now modify the earliest change, using its stable name > +$ git reset --hard metas/this_is_a_test > +$ echo morefoo>>bar.txt && git add . && git commit --amend --no-edit > + > +# Use git-evolve to fix up any dependent changes > +$ git evolve > +rebasing metas/this_is_also_a_test onto metas/this_is_a_test > +rebasing metas/more_testing onto metas/this_is_also_a_test > +Done > + > +# Use git-obslog to view the history of the this_is_a_test change > +$ git log --obslog > +93f110 metas/this_is_a_test@{0} commit (amend): This is a test > +930219 metas/this_is_a_test@{1} commit: This is a test > + > +# Now create an unrelated change > +$ git reset --hard origin/master > +$ echo newchange>unrelated.txt && git add . > +$ git commit -m "Unrelated change" > +created change metas/unrelated_change > + > +# Fetch the latest code from origin/master and use git-evolve > +# to rebase all dependent changes. > +$ git fetch origin master > +$ git evolve origin/master > +deleting metas/some_change_already_merged_upstream > +rebasing metas/this_is_a_test onto origin/master > +rebasing metas/this_is_also_a_test onto metas/this_is_a_test > +rebasing metas/more_testing onto metas/this_is_also_a_test > +rebasing metas/unrelated_change onto origin/master > +Conflict detected! Resolve it and then use git evolve --continue to resume. > + > +# Sort out the conflict > +$ git mergetool > +$ git evolve origin/master > +Done > + > +# Share the full history of edits for the this_is_a_test change > +# with a review server > +$ git push origin metas/this_is_a_test:refs/for/master > +# Share the lastest commit for “Unrelated change”, without history > +$ git push origin HEAD:refs/for/master > + > +Detailed design > +=============== > +Obsolescence information is stored as a graph of meta-commits. A meta-commit is > +a specially-formatted merge commit that describes how one commit was created > +from others. > + > +Meta-commits look like this: > + > +$ git cat-file -p <example_meta_commit> > +tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904 > +parent aa7ce55545bf2c14bef48db91af1a74e2347539a > +parent d64309ee51d0af12723b6cb027fc9f195b15a5e9 > +parent 7e1bbcd3a0fa854a7a9eac9bf1eea6465de98136 > +author Stefan Xenos <sxenos@xxxxxxxxx> 1540841596 -0700 > +committer Stefan Xenos <sxenos@xxxxxxxxx> 1540841596 -0700 > +parent-type c r o > + > +This says “commit aa7ce555 makes commit d64309ee obsolete. It was created by > +cherry-picking commit 7e1bbcd3”. > + > +The tree for meta-commits is always the empty tree, but future versions of git > +may attach other trees here. For forward-compatibility fsck should ignore such > +trees if found on future repository versions. This will allow future versions of > +git to add metadata to the meta-commit tree without breaking forwards > +compatibility. > + > +The commit comment for a meta-commit is an auto-generated user-readable string > +describing the command that produced the meta commit. These strings are shown > +to the user when they view the obslog. > + > +Parent-type > +----------- > +The “parent-type” field in the commit header identifies a commit as a > +meta-commit and indicates the meaning for each of its parents. It is never > +present for normal commits. It contains a space-deliminated list of enum values > +whose order matches the order of the parents. Possible parent types are: > + > +- c: (content) the content parent identifies the commit that this meta-commit is > + describing. > +- r: (replaced) indicates that this parent is made obsolete by the content > + parent. > +- o: (origin) indicates that the content parent was generated by cherry-picking > + this parent. > +- a: (abandoned) used in place of a content parent for abandoned changes. Points > + to the final content commit for the change at the time it was abandoned. > + > +There must be exactly one content or abandoned parent for each meta-commit and > +it is always the first parent. The content commit will always be a normal commit > +and not a meta-commit. However, future versions of git may create meta-commits > +for other meta-commits and the fsck tool must be aware of this for forwards > +compatibility. > + > +A meta-commit can have zero or more replaced parents. An amend operation creates > +a single replaced parent. A merge used to resolve divergence (see divergence, > +below) will create multiple replaced parents. A meta-commit may have no > +replaced parents if it describes a cherry-pick or squash merge that copies one > +or more commits but does not replace them. > + > +A meta-commit can have zero or more origin parents. A cherry-pick creates a > +single origin parent. Certain types of squash merge will create multiple origin > +parents. Origin parents don't directly cause their origin to become obsolete, > +but are used when computing blame or locating a merge base. The section > +on obsolescence over cherry-picks describes how the evolve command uses > +origin parents. > + > +A replaced parent or origin parent may be either a normal commit (indicating > +the oldest-known version of a change) or another meta-commit (for a change that > +has already been modified one or more times). > + > +The parent-type field needs to go after the committer field since git's rules > +for forwards-compatibility require that new fields to be at the end of the > +header. Putting a new field in the middle of the header would break fsck. > + > +The presence of an abandoned parent indicates that the change should be pruned > +by the evolve command, and removed from the repository's history. Any follow-up > +changes should rebased onto the parent of the pruned commit. The abandoned > +parent points to the version of the change that should be restored if the user > +attempts to restore the change. > + > +Changes > +------- > +A branch of meta-commits describes how a commit was produced and what previous > +commits it is based on. It is also an identifier for a thing the user is > +currently working on. We refer to such a meta-branch as a change. > + > +Local changes are stored in the new refs/metas namespace. Remote changes are > +stored in the refs/remote/<remotename>/metas namespace. > + > +The list of changes in refs/metas is more than just a mechanism for the evolve > +command to locate obsolete commits. It is also a convenient list of all of a > +user’s work in progress and their current state - a list of things they’re > +likely to want to come back to. > + > +Strictly speaking, it is the presence of the branch in the refs/metas namespace > +that marks a branch as being a change, not the fact that it points to a > +metacommit. Metacommits are only created when a commit is amended or rebased, so > +in the case where a change points to a commit that has never been modified, the > +change points to that initial commit rather than a metacommit. > + > +Changes are also stored in the refs/hiddenmetas namespace. Hiddenmetas holds > +metadata for historical changes that are not currently in progress by the user. > +Commands like filter-branch and other bulk import commands create metadata in > +this namespace. > + > +Note that the changes in hiddenmetas get special treatment in several ways: > + > +- They are not cleaned up automatically once merged, since it is expected that > + they refer to historical changes. > +- User commands that modify changes don't append to these changes as they would > + to a change in refs/metas. > +- They are not displayed when the user lists their local changes. > + > +Obsolescence > +------------ > +A commit is considered obsolete if it is reachable from the “replaces” edges > +anywhere in the history of a change and it isn’t the head of that change. > +Commits may be the content for 0 or more meta-commits. If the same commit > +appears in multiple changes, it is not obsolete if it is the head of any of > +those changes. > + > +Note that there is an exception to this rule. The metas namespace takes > +precedence over the hiddenmetas namespace for the purpose of obsolescence. That > +is, if a change appears in a replaces edge of a change in the metas namespace, > +it is obsolete even if it also appears as the head of a change in the > +hiddenmetas namespace. > + > +This special case prevents the hiddenmetas namespace from creating divergence > +with the user's work in progress, and allows the user to resolve historical > +divergence by creating new changes in the metas namespace. > + > +Divergence > +---------- > +From the user’s perspective, two changes are divergent if they both ask for > +different replacements to the same commit. More precisely, a target commit is > +considered divergent if there is more than one commit at the head of a change in > +refs/metas that leads to the target commit via an unbroken chain of “replaces” > +parents. > + > +Much like a merge conflict, divergence is a situation that requires user > +intervention to resolve. The evolve command will stop when it encounters > +divergence and prompt the user to resolve the problem. Users can solve the > +problem in several ways: > + > +- Discard one of the changes (by deleting its change branch). > +- Merge the two changes (producing a single change branch). > +- Copy one of the changes (keep both commits, but one of them gets a new > + metacommit appended to its history that is connected to its predecessor via an > + origin edge rather than a replaces edge. That new change no longer obsoletes > + the original.) > + > +Obsolescence across cherry-picks > +-------------------------------- > +By default the evolve command will treat cherry-picks and squash merges as being > +completely separate from the original. Further amendments to the original commit > +will have no effect on the cherry-picked copy. However, this behavior may not be > +desirable in all circumstances. > + > +The evolve command may at some point support an option to look for cases where > +the source of a cherry-pick or squash merge has itself been amended, and > +automatically apply that same change to the cherry-picked copy. In such cases, > +it would traverse origin edges rather than ignoring them, and would treat a > +commit with origin edges as being obsolete if any of its origins were obsolete. > + > +Garbage collection > +------------------ > +For GC purposes, meta-commits are normal commits. Just as a commit causes its > +parents and tree to be retained, a meta-commit also causes its parents to be > +retained. > + > +Change creation > +--------------- > +Changes are created automatically whenever the user runs a command like “commit” > +that has the semantics of creating a new change. They also move forward > +automatically even if they’re not checked out. For example, whenever the user > +runs a command like “commit --amend” that modifies a commit, all branches in > +refs/metas that pointed to the old commit move forward to point to its > +replacement instead. This also happens when the user is working from a detached > +head. > + > +This does not mean that every commit has a corresponding change. By default, > +changes only exist for recent locally-created commits. Users may explicitly pull > +changes from other users or keep their changes around for a long time, but > +either behavior requires a user to opt-in. Code review systems like gerrit may > +also choose to keep changes around forever. > + > +Note that the changes in refs/metas serve a dual function as both a way to > +identify obsolete changes and as a way for the user to keep track of their work > +in progress. If we were only concerned with identifying obsolete changes, it > +would be sufficient to create the change branch lazily the first time a commit > +is obsoleted. Addressing the second use - of refs/metas as a mechanism for > +keeping track of work in progress - is the reason for eagerly creating the > +change on first commit. > + > +Change naming > +------------- > +When a change is first created, the only requirement for its name is that it > +must be unique. Good names would also serve as useful mnemonics and be easy to > +type. For example, a short word from the commit message containing no numbers or > +special characters and that shows up with low frequency in other commit messages > +would make a good choice. > + > +Different users may prefer different heuristics for their change names. For this > +reason a new hook will be introduced to compute change names. Git will invoke > +the hook for all newly-created changes and will append a numeric suffix if the > +name isn’t unique. The default heuristics are not specified by this proposal and > +may change during implementation. > + > +Change deletion > +--------------- > +Changes are normally only interesting to a user while a commit is still in > +development and under review. Once the commit has submitted wherever it is > +going, its change can be discarded. > + > +The normal way of deleting changes makes this easy to do - changes are deleted > +by the evolve command when it detects that the change is present in an upstream > +branch. It does this in two ways: if the latest commit in a change either shows > +up in the branch history or the change becomes empty after a rebase, it is > +considered merged and the change is discarded. In this context, an “upstream > +branch” is any branch passed in as the upstream argument of the evolve command. > + > +In case this sometimes deletes a useful change, such automatic deletions are > +recorded in the reflog allowing them to be easily recovered. > + > +Sharing changes > +--------------- > +Change histories are shared by pushing or fetching meta-commits and change > +branches. This provides users with a lot of control of what to share and > +repository implementations with control over what to retain. > + > +Users that only want to share the content of a commit can do so by pushing the > +commit itself as they currently would. Users that want to share an edit history > +for the commit can push its change, which would point to a meta-commit rather > +than the commit itself if there is any history to share. Note that multiple > +changes can refer to the same commits, so it’s possible to construct and push a > +different history for the same commit in order to remove sensitive or irrelevant > +intermediate states. > + > +Imagine the user is working on a change “mychange” that is currently the latest > +commit on master. They have two ways to share it: > + > +# User shares just a commit without its history > +> git push origin master > + > +# User shares the full history of the commit to a review system > +> git push origin metas/mychange:refs/for/master > + > +# User fetches a collaborator’s modifications to their change > +> git fetch remotename metas/mychange > +# Which updates the ref remote/remotename/metas/mychange > + > +This will cause more intermediate states to be shared with the server than would > +have been shared previously. A review system like gerrit would need to keep > +track of which states had been explicitly pushed versus other intermediate > +states in order to de-emphasize (or hide) the extra intermediate states from the > +user interface. > + > +Merge-base > +---------- > +Merge-base will be changed to search the meta-commit graph for common ancestors > +as well as the commit graph, and will generally prefer results from the > +meta-commit graph over the commit graph. Merge-base will consider meta-commits > +from all changes, and will traverse both origin and obsolete edges. > + > +The reason for this is that - when merging two versions of the same commit > +together - an earlier version of that same commit will usually be much more > +similar than their common parent. This should make the workflow of collaborating > +on unsubmitted patches as convenient as the workflow for collaborating in a > +topic branch by eliminating repeated merges. > + > +Configuration > +------------- > +The core.enableChanges configuration variable enables the creation and update > +of change branches. This is enabled by default. > + > +User interface > +-------------- > +All git porcelain commands that create commits are classified as having one of > +four behaviors: modify, create, copy, or import. These behaviors are discussed > +in more detail below. > + > +Modify commands > +--------------- > +Modification commands (commit --amend, rebase) will mark the old commit as > +obsolete by creating a new meta-commit that references the old one as a > +replaced parent. In the event that multiple changes point to the same commit, > +this is done independently for every such change. > + > +More specifically, modifications work like this: > + > +1. Locate all existing changes for which the old commit is the content for the > + head of the change branch. If no such branch exists, create one that points > + to the old commit. Changes that include this commit in their history but not > + at their head are explicitly not included. > +2. For every such change, create a new meta-commit that references the new > + commit as its content and references the old head of the change as a > + replaced parent. > +3. Move the change branch forward to point to the new meta-commit. > + > +Copy commands > +------------- > +Copy commands (cherry-pick, merge --squash) create a new meta-commit that > +references the old commits as origin parents. Besides the fact that the new > +parents are tagged differently, copy commands work the same way as modify > +commands. > + > +Create commands > +--------------- > +Creation commands (commit, merge) create a new commit and a new change that > +points to that commit. The do not create any meta-commits. > + > +Import commands > +--------------- > +Import commands (fetch, pull) do not create any new meta-commits or changes > +unless that is specifically what they are importing. For example, the fetch > +command would update remote/origin/metas/change35 and fetch all referenced > +meta-commits if asked to do so directly, but it wouldn’t create any changes or > +meta-commits for commits discovered on the master branch when running “git fetch > +origin master”. > + > +Other commands > +-------------- > +Some commands don’t fit cleanly into one of the above categories. > + > +Semantically, filter-branch should be treated as a modify command, but doing so > +is likely to create a lot of irrelevant clutter in the changes namespace and the > +large number of extra change refs may introduce performance problems. We > +recommend treating filter-branch as an import command initially, but making it > +behave more like a modify command in future follow-up work. One possible > +solution may be to treat commits that are part of existing changes as being > +modified but to avoid creating changes for other rewritten changes. Another > +solution may be to record the modifications as changes in the hiddenmetas > +namespace. > + > +Once the evolve command can handle obsolescence across cherry-picks, such > +cherry-picks will result in a hybrid move-and-copy operation. It will create > +cherry-picks that replace other cherry-picks, which will have both origin edges > +(pointing to the new source commit being picked) and replacement edges (pointing > +to the previous cherry-pick being replaced). > + > +Evolve > +------ > +The evolve command performs the correct sequence of rebases such that no change > +has an obsolete parent. The syntax looks like this: > + > +git evolve [upstream…] > + > +It takes an optional list of upstream branches. All changes whose parent shows > +up in the history of one of the upstream branches will be rebased onto the > +upstream branch before resolving obsolete parents. > + > +Any change whose latest state is found in an upstream branch (or that ends up > +empty after rebase) will be deleted. This is the normal mechanism for deleting > +changes. Changes are created automatically on the first commit, and are deleted > +automatically when evolve determines that they’ve been merged upstream. > + > +Orphan commits are commits with obsolete parents. The evolve command then > +repeatedly rebases orphan commits with non-orphan parents until there are either > +no orphan commits left, or a merge conflict is discovered. It will also > +terminate if it detects a divergent parent or a cycle that can't be resolved > +using any of the enabled transformations. > + > +When evolve discovers divergence, it will first check if it can resolve the > +divergence automatically using one of its enabled transformations. Supported > +transformations are: > + > +- Check if the user has already merged the divergent changes in a follow-up > + change. That is, look for an existing merge in a follow-up change where all > + the parents are divergent versions of the same change. Squash that merge with > + its parents and use the result as the resolution for the divergence. > + > +- Attempt to auto-merge all the divergent changes (disabled by default). > + > +Each of the transformations can be enabled or disabled by command line options. > + > +Cycles can occur when two changes reference one another as parents. This can > +happen when both changes use an obsolete version of the other change as their > +parent. Although there are never cycles in the commit graph, users can create > +cycles in the change graph by rebasing changes onto obsolete commits. The evolve > +command has a transformation that will detect and break cycles by arbitrarily > +picking one of the changes to go first. If this generates a merge conflict, > +it tries each of the other changes in sequence to see if any ordering merges > +cleanly. If no possible ordering merges cleanly, it picks one and terminates > +to let the user resolve the merge conflict. > + > +If the working tree is dirty, evolve will attempt to stash the user's changes > +before applying the evolve and then reapply those changes afterward, in much > +the same way as rebase --autostash does. > + > +Checkout > +-------- > +Running checkout on a change by name has the same effect as checking out a > +detached head pointing to the latest commit on that change-branch. There is no > +need to ever have HEAD point to a change since changes always move forward when > +necessary, no matter what branch the user has checked out > + > +Meta-commits themselves cannot be checked out by their hash. > + > +Reset > +----- > +Resetting a branch to a change by name is the same as resetting to the content > +(or abandoned) commit at that change’s head. > + > +Commit > +------ > +Commit --amend gets modify semantics and will move existing changes forward. The > +normal form of commit gets create semantics and will create a new change. > + > +$ touch foo && git add . && git commit -m "foo" && git tag A > +$ touch bar && git add . && git commit -m "bar" && git tag B > +$ touch baz && git add . && git commit -m "baz" && git tag C > + > +This produces the following commits: > +A(tree=[foo]) > +B(tree=[foo, bar], parent=A) > +C(tree=[foo, bar, baz], parent=B) > + > +...along with three changes: > +metas/foo = A > +metas/bar = B > +metas/baz = C > + > +Running commit --amend does the following: > +$ git checkout B > +$ touch zoom && git add . && git commit --amend -m "baz and zoom" > +$ git tag D > + > +Commits: > +A(tree=[foo]) > +B(tree=[foo, bar], parent=A) > +C(tree=[foo, bar, baz], parent=B) > +D(tree=[foo, bar, zoom], parent=A) > +Dmeta(content=D, obsolete=B) > + > +Changes: > +metas/foo = A > +metas/bar = Dmeta > +metas/baz = C > + > +Merge > +----- > +Merge gets create, modify, or copy semantics based on what is being merged and > +the options being used. > + > +The --squash version of merge gets copy semantics (it produces a new change that > +is marked as a copy of all the original changes that were squashed into it). > + > +The “modify” version of merge replaces both of the original commits with the > +resulting merge commit. This is one of the standard mechanisms for resolving > +divergence. The parents of the merge commit are the parents of the two commits > +being merged. The resulting commit will not be a merge commit if both of the > +original commits had the same parent or if one was the parent of the other. > + > +The “create” version of merge creates a new change pointing to a merge commit > +that has both original commits as parents. The result is what merge produces now > +- a new merge commit. However, this version of merge doesn’t directly resolve > +divergence. > + > +To select between these two behaviors, merge gets new “--amend” and “--noamend” > +options which select between the “create” and “modify” behaviors respectively, > +with noamend being the default. > + > +For example, imagine we created two divergent changes like this: > + > +$ touch foo && git add . && git commit -m "foo" && git tag A > +$ touch bar && git add . && git commit -m "bar" && git tag B > +$ touch baz && git add . && git commit --amend -m "bar and baz" > +$ git tag C > +$ git checkout B > +$ touch bam && git add . && git commit --amend -m "bar and bam" > +$ git tag D > + > +At this point the commit graph looks like this: > + > +A(tree=[foo]) > +B(tree=[bar], parent=A) > +C(tree=[bar, baz], parent=A) > +D(tree=[bar, bam], parent=A) > +Cmeta(content=C, obsoletes=B) > +Dmeta(content=D, obsoletes=B) > + > +There would be three active changes with heads pointing as follows: > + > +metas/changeA=A > +metas/changeB=Cmeta > +metas/changeB2=Dmeta > + > +ChangeB and changeB2 are divergent at this point. Lets consider what happens if > +perform each type of merge between changeB and changeB2. > + > +Merge example: Amend merge > +One way to resolve divergent changes is to use an amend merge. Recall that HEAD > +is currently pointing to D at this point. > + > +$ git merge --amend metas/changeB > + > +Here we’ve asked for an amend merge since we’re trying to resolve divergence > +between two versions of the same change. There are no conflicts so we end up > +with this: > + > +E(tree=[bar, baz, bam], parent=A) > +Emeta(content=E, obsoletes=[Cmeta, Dmeta]) > + > +With the following branches: > + > +metas/changeA=A > +metas/changeB=Emeta > +metas/changeB2=Emeta > + > +Notice that the result of the “amend merge” is a replacement for C and D rather > +than a new commit with C and D as parents (as a normal merge would have > +produced). The parents of the amend merge are the parents of C and D which - in > +this case - is just A, so the result is not a merge commit. Also notice that > +changeB and changeB2 are now aliases for the same change. > + > +Merge example: Noamend merge > +Consider what would have happened if we’d used a noamend merge instead. Recall > +that HEAD was at D and our branches looked like this: > + > +metas/changeA=A > +metas/changeB=Cmeta > +metas/changeB2=Dmeta > + > +$ git merge --noamend metas/changeB > + > +That would produce the sort of merge we’d normally expect today: > + > +F(tree=[bar, baz, bam], parent=[C, D]) > + > +And our changes would look like this: > +metas/changeA=A > +metas/changeB=Cmeta > +metas/changeB2=Dmeta > +metas/changeF=F > + > +In this case, changeB and changeB2 are still divergent and we’ve created a new > +change for our merge commit. However, this is just a temporary state. The next > +time we run the “evolve” command, it will discover the divergence but also > +discover the merge commit F that resolves it. Evolve will suggest converting F > +into an amend merge in order to resolve the divergence and will display the > +command for doing so. > + > +Rebase > +------ > +In general the rebase command is treated as a modify command. When a change is > +rebased, the new commit replaces the original. > + > +Rebase --abort is special. Its intent is to restore git to the state it had > +prior to running rebase. It should move back any changes to point to the refs > +they had prior to running rebase and delete any new changes that were created as > +part of the rebase. To achieve this, rebase will save the state of all changes > +in refs/metas prior to running rebase and will restore the entire namespace > +after rebase completes (deleting any newly-created changes). Newly-created > +metacommits are left in place, but will have no effect until garbage collected > +since metacommits are only used if they are reachable from refs/metas. > + > +Change > +------ > +The “change” command can be used to list, rename, reset or delete change. It has > +a number of subcommands. > + > +The "list" subcommand lists local changes. If given the -r argument, it lists > +remote changes. > + > +The "rename" subcommand renames a change, given its old and new name. If the old > +name is omitted and there is exactly one change pointing to the current HEAD, > +that change is renamed. If there are no changes pointing to the current HEAD, > +one is created with the given name. > + > +The "forget" subcommand deletes a change by deleting its ref from the metas/ > +namespace. This is the normal way to delete extra aliases for a change if the > +change has more than one name. By default, this will refuse to delete the last > +alias for a change if there are any other changes that reference this change as > +a parent. > + > +The "update" subcommand adds a new state to a change. It uses the default > +algorithm for assigning change names. If the content commit is omitted, HEAD is > +used. If given the optional --force argument, it will overwrite any existing > +change of the same name. This latter form of "update" can be used to effectively > +reset changes. > + > +The "update" command can accept any number of --origin and --replace arguments. > +If any are present, the resulting change branch will point to a metacommit > +containing the given origin and replacement edges. > + > +The "abandon" command deletes a change using obsolescence markers. It marks the > +change as being obsolete and having been replaced by its parent. If given no > +arguments, it applies to the current commit. Running evolve will cause any > +abandoned changes to be removed from the branch. Any child changes will be > +reparented on top of the parent of the abandoned change. If the current change > +is abandoned, HEAD will move to point to its parent. > + > +The "restore" command restores a previously-abandoned change. > + > +The "prune" command deletes all obsolete changes and all changes that are > +present in the given branch. Note that such changes can be recovered from the > +reflog. > + > +Combined with the GC protection that is offered, this is intended to facilitate > +a workflow that relies on changes instead of branches. Users could choose to > +work with no local branches and use changes instead - both for mailing list and > +gerrit workflows. > + > +Log > +--- > +When a commit is shown in git log that is part of a change, it is decorated with > +extra change information. If it is the head of a change, the name of the change > +is shown next to the list of branches. If it is obsolete, it is decorated with > +the text “obsolete, <n> commits behind <changename>”. > + > +Log gets a new --obslog argument indicating that the obsolescence graph should > +be followed instead of the commit graph. This also changes the default > +formatting options to make them more appropriate for viewing different > +iterations of the same commit. > + > +Pull > +---- > + > +Pull gets an --evolve argument that will automatically attempt to run "evolve" > +on any affected branches after pulling. > + > +We also introduce an "evolve" enum value for the branch.<name>.rebase config > +value. When set, the evolve behavior will happen automatically for that branch > +after every pull even if the --evolve argument is not used. > + > +Next > +---- > + > +The "next" command will reset HEAD to a non-obsolete commit that refers to this > +change as its parent. If there is more than one such change, the user will be > +prompted. If given the --evolve argument, the next commit will be evolved if > +necessary first. > + > +The "next" command can be thought of as the opposite of > +"git reset --hard HEAD^" in that it navigates to a child commit rather than a > +parent. > + > +Prev > +---- > + > +The "prev" command will reset HEAD to the latest version of the parent change. > +If the parent change isn't obsolete, this is equivalent to > +"git reset --hard HEAD^". If the parent commit is obsolete, it resets to the > +latest replacement for the parent commit. > + > +Other options considered > +======================== > +We considered several other options for storing the obsolescence graph. This > +section describes the other options and why they were rejected. > + > +Commit header > +------------- > +Add an “obsoletes” field to the commit header that points backwards from a > +commit to the previous commits it obsoletes. > + > +Pros: > +- Very simple > +- Easy to traverse from a commit to the previous commits it obsoletes. > +Cons: > +- Adds a cost to the storage format, even for commits where the change history > + is uninteresting. > +- Unconditionally prevents the change history from being garbage collected. > +- Always causes the change history to be shared when pushing or pulling changes. > + > +Git notes > +--------- > +Instead of storing obsolescence information in metacommits, the metacommit > +content could go in a new notes namespace - say refs/notes/metacommit. Each note > +would contain the list of obsolete and origin parents. An automerger could > +be supplied to make it easy to merge the metacommit notes from different remotes. > + > +Pros: > +- Easy to locate all commits obsoleted by a given commit (since there would only > + be one metacommit for any given commit). > +Cons: > +- Wrong GC behavior (obsolete commits wouldn’t automatically be retained by GC) > + unless we introduced a special case for these kinds of notes. > +- No way to selectively share or pull the metacommits for one specific change. > + It would be all-or-nothing, which would be expensive. This could be addressed > + by changes to the protocol, but this would be invasive. > +- Requires custom auto-merging behavior on fetch. > + > +Tags > +---- > +Put the content of the metacommit in a message attached to tag on the > +replacement commit. This is very similar to the git notes approach and has the > +same pros and cons. > + > +Simple forward references > +------------------------- > +Record an edge from an obsolete commit to its replacement in this form: > + > +refs/obsoletes/<A> > + > +pointing to commit <B> as an indication that B is the replacement for the > +obsolete commit A. > + > +Pros: > +- Protects <B> from being garbage collected. > +- Fast lookup for the evolve operation, without additional search structures > + (“what is the replacement for <A>?” is very fast). > + > +Cons: > +- Can’t represent divergence (which is a P0 requirement). > +- Creates lots of refs (which can be inefficient) > +- Doesn’t provide a way to fetch only refs for a specific change. > +- The obslog command requires a search of all refs. > + > +Complex forward references > +-------------------------- > +Record an edge from an obsolete commit to its replacement in this form: > + > +refs/obsoletes/<change_id>/obs<A>_<B> > + > +Pointing to commit <B> as an indication that B is the replacement for obsolete > +commit A. > + > +Pros: > +- Permits sharing and fetching refs for only a specific change. > +- Supports divergence > +- Protects <B> from being garbage collected. > + > +Cons: > +- Creates lots of refs, which is inefficient. > +- Doesn’t provide a good lookup structure for lookups in either direction. > + > +Backward references > +------------------- > +Record an edge from a replacement commit to the obsolete one in this form: > + > +refs/obsolescences/<B> > + > +Cons: > +- Doesn’t provide a way to resolve divergence (which is a P0 requirement). > +- Doesn’t protect <B> from being garbage collected (which could be fixed by > + combining this with a refs/metas namespace, as in the metacommit variant). > + > +Obsolescences file > +------------------ > +Create a custom file (or files) in .git recording obsolescences. > + > +Pros: > +- Can store exactly the information we want with exactly the performance we want > + for all operations. For example, there could be a disk-based hashtable > + permitting constant time lookups in either direction. > + > +Cons: > +- Handling GC, pushing, and pulling would all require custom solutions. GC > + issues could be addressed with a repository format extension. > + > +Squash points > +------------- > +We treat changes like topic branches, and use special squash points to mark > +places in the commit graph that separate changes. > + > +We create and update change branches in refs/metas at the same time we > +would have in the metacommit proposal. However, rather than pointing to a > +metacommit branch they point to normal commits and are treated as “squash > +points” - markers for sequences of commits intended to be squashed together on > +submission. > + > +Amends and rebases work differently than they do now. Rather than actually > +containing the desired state of a commit, they contain a delta from the previous > +version along with a squash point indicating that the preceding changes are > +intended to be squashed on submission. Specifically, amends would become new > +changes and rebases would become merge commits with the old commit and new > +parent as parents. > + > +When the changes are finally submitted, the squashes are executed, producing the > +final version of the commit. > + > +In addition to the squash points, git would maintain a set of “nosquash” tags > +for commits that were used as ancestors of a change that are not meant to be > +included in the squash. > + > +For example, if we have this commit graph: > + > +A(...) > +B(parent=A) > +C(parent=B) > + > +...and we amend B to produce D, we’d get: > + > +A(...) > +B(parent=A) > +C(parent=B) > +D(parent=B) > + > +...along with a new change branch indicating D should be squashed with its > +parents when submitted: > + > +metas/changeB = D > +metas/changeC = C > + > +We’d also create a nosquash tag for A indicating that A shouldn’t be included > +when changeB is squashed. > + > +If a user amends the change again, they’d get: > + > +A(...) > +B(parent=A) > +C(parent=B) > +D(parent=B) > +E(parent=D) > + > +metas/changeB = E > +metas/changeC = C > + > +Pros: > +- Good GC behavior. > +- Provides a natural way to share changes (they’re just normal branches). > +- Merge-base works automatically without special cases. > +- Rewriting the obslog would be easy using existing git commands. > +- No new data types needed. > +Cons: > +- No way to connect the squashed version of a change to the original, so no way > + to automatically clean up old changes. This also means users lose all benefits > + of the evolve command if they prematurely squash their commits. This may occur > + if a user thinks a change is ready for submission, squashes it, and then later > + discovers an additional change to make. > +- Histories would look very cluttered (users would see all previous edits to > + their commit in the commit log, and all previous rebases would show up as > + merges). Could be quite hard for users to tell what is going on. (Possible > + fix: also implement a new smart log feature that displays the log as though > + the squashes had occurred). > +- Need to change the current behavior of current commands (like amend and > + rebase) in ways that will be unexpected to many users. > -- > gitgitgadget >
