Re: [PATCH] technical doc: add a design doc for the evolve command

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

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Hi Stefan,

On Wed, 14 Nov 2018, sxenos@google.com wrote:

> From: Stefan Xenos <sxenos@google.com>
> 
> This document describes what an obsolescence graph for
> git would look like, the behavior of the evolve command,
> and the changes planned for other commands.

Thanks, this is a good discussion starter.

> +Objective
> +---------
> +Track the edits to a commit over time in an obsolescence graph.

I am not sure that we necessarily need this to be a graph. I think part of
the problems with not being able to GC *any* of this is by this
requirement to have it stored in a graph, rather than having mappings from
which you could reconstruct any non-GC'ed parts of that graph, if you
really want.

> +Background
> +----------
> +Imagine you have three dependent changes up for review and you receive feedback
> +that requires editing all three changes. While you're editing one, 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 +that the old commit is obsolete and has been replaced by the new one.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 obsolescence graph. However, the obsolescence
> +graph will also bring other benefits:
> +
> +- Users can view the history of a commit 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 obsolescence graph, users can collaborate more
> +  easily on changes-in-progress. This is better than pushing and pulling the
> +  changes themselves since the obsolescence 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.

Okay.

> +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.
> +
> +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.
> +
> +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.

Why is this missing most notably `hg evolve`? Also, there should be *at
least* a brief introduction how `hg evolve` works. They do have the
benefit of real-world testing, and probably encountered problems and came
up with solutions, and we would be remiss if we did not learn from them.

Also, please do not forget `git imerge`.

Further, I see that this document tries to suggest a proliferation of new
commands (`git change`, `git evolve`, `git obslog` and whatever I glanced
over). This smells a little bit like it wants to be condensed into a
single-purpose command, maybe `evolve`, maybe something better if you can
think of anything.

I guess I will have to stop now and read up on how `hg evolve` works. It
is a it of a pity that that was not described in this document, first
thing, as it forces everybody who is interested in this patch to duplicate
my effort and also go hunt for information about Mercurial.

Ciao,
Johannes

> +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
> +    obsolescence 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. 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 obsolescence graph
> +    without discarding the commits themselves that are already present in
> +    branches and the reflog.
> +
> +
> +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 -l
> +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 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 --continue
> +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@gmail.com> 1540841596 -0700
> +committer Stefan Xenos <sxenos@gmail.com> 1540841596 -0700
> +parent-type content
> +parent-type obsolete
> +parent-type origin
> +
> +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 whose hash matches
> +4b825dc642cb6eb9a060e54bf8d69288fbee4904 exactly, but future versions of git may
> +attach other trees here. For forward-compatibility fsck should ignore such trees
> +if found on future repository versions. Similarly, current versions of git
> +should always fill in an empty commit comment and tools like fsck should ignore
> +the content of the commit comment if present in a future repository version.
> +This will allow future versions of git to add metadata to the meta-commit
> +comments or tree without breaking forwards compatibility.
> +
> +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 is a list of enum values whose order matches the
> +order of the parents. Possible parent types are:
> +
> +- content: the content parent identifies the commit that this meta-commit is
> +  describing.
> +- obsolete: indicates that this parent is made obsolete by the content parent.
> +- origin: indicates that this parent was generated from the given commit.
> +
> +There must be exactly one content parent for each meta-commit and it is always
> +be 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 obsolete parents. An amend operation creates
> +a single obsolete parent. A merge used to resolve divergence (see divergence,
> +below) will create multiple obsolete parents. A meta-commit may have zero
> +obsolete 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.
> +
> +An obsolete 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).
> +
> +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/remotemetas/<remotename> 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.
> +
> +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.
> +
> +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 “obsolete”
> +edges.
> +
> +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 an obsolete 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 change/mychange:refs/for/master
> +
> +# User fetches a collaborator’s modifications to their change
> +> git fetch remotename change/mychange
> +# Which updates the ref remotechange/remotename/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.
> +
> +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 an
> +obsolete 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 an
> +   obsolete 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 remotechange/origin/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.
> +
> +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 obsolete 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 [--abort][--continue][--quit] [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, a merge conflict is discovered, or a divergent parent is
> +discovered.
> +
> +The --abort option returns all changes to the state they were in prior to
> +invoking evolve, and the --quit option terminates the current evolution without
> +changing the current state.
> +
> +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 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:
> +change/foo = A
> +change/bar = B
> +change/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:
> +change/foo = A
> +change/bar = Dmeta
> +change/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:
> +
> +change/changeA=A
> +change/changeB=Cmeta
> +change/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 change/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:
> +
> +change/changeA=A
> +change/changeB=Emeta
> +change/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:
> +
> +change/changeA=A
> +change/changeB=Cmeta
> +change/changeB2=Dmeta
> +
> +$ git merge --noamend change/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:
> +change/changeA=A
> +change/changeB=Cmeta
> +change/changeB2=Dmeta
> +change/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.
> +
> +Change
> +------
> +The “change” command can be used to list, rename, reset or delete change. It
> +takes arguments similar to the “branch” command.
> +
> +The -l argument lists all local changes that aren’t present in the given branch.
> +If the branch name is omitted, all local changes are listed.
> +
> +The -r argument list all remote changes.
> +
> +The -m argument renames a change, given its old and new name.
> +
> +The -d argument deletes a change. This is one way to resolve divergence.
> +
> +The -n argument renames the current change, or creates a change of the given
> +name for the current commit if no such change exists yet. If given an optional
> +commit hash, the change is created for that commit rather than head. If there
> +are multiple local changes for the same commit and they are all aliases for the
> +same metacommit hash, they are all deleted except the newly-created name. If
> +given the name of a metacommit, the new change points to that metacommit.
> +
> +The --purge argument 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 word “obsolete”.
> +
> +Obslog
> +------
> +Obslog command lists the change history for the current commit.
> +
> +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.
> +
> +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, and 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 create and update change branches in refs/metas them at the same time we
> +would 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:
> +
> +change/changeB = D
> +change/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)
> +
> +change/changeB = E
> +change/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.
> -- 
> 2.19.1.930.g4563a0d9d0-goog
> 
>