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Re: [RFC PATCH v2] technical doc: add a design doc for the evolve command

Sorry, that should have been uploaded with a subject starting with
[RFC PATCH v2]. I'm not sure why git send-email ignored my argument to
--subject-prefix. I'm going to try to change the subject one more
time. Please excuse me in advance if I accidentally spam the list by
sending the same email again.
On Sun, Nov 18, 2018 at 6:02 PM <sxenos@xxxxxxxxxx> 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.
> Signed-off-by: Stefan Xenos <sxenos@xxxxxxxxxx>
> ---
>  Documentation/technical/change-graph.txt | 928 +++++++++++++++++++++++
>  1 file changed, 928 insertions(+)
>  create mode 100644 Documentation/technical/change-graph.txt
> diff --git a/Documentation/technical/change-graph.txt b/Documentation/technical/change-graph.txt
> new file mode 100644
> index 0000000000..2f4051f65f
> --- /dev/null
> +++ b/Documentation/technical/change-graph.txt
> @@ -0,0 +1,928 @@
> +Change Graph
> +============
> +
> +Objective
> +=========
> +Track the amendments to a commit over time in a change graph. Allow users to
> +exchange such change graphs, and introduce a new "evolve" command that
> +uses the graph to rebase commits based on obsolete parents.
> +
> +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 +that the old commit is obsolete and has been replaced by the new one.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
> +  changes 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. 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.
> +
> +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.
> +
> +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 -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@xxxxxxxxx> 1540841596 -0700
> +committer Stefan Xenos <sxenos@xxxxxxxxx> 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
> +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 no
> +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. Origin parents don't directly cause their origin to become obsolete,
> +but may cause
> +
> +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).
> +
> +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.
> +
> +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 metas/mychange:refs/for/master
> +
> +# User fetches a collaborator’s modifications to their change
> +> git fetch remotename metas/mychange
> +# Which updates the ref remotemetas/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.
> +
> +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 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 remotemetas/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:
> +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.
> +
> +Change
> +------
> +The “change” command can be used to list, rename, reset or delete change. It has
> +a number of subcommands.
> +
> +The "list" subcommand lists all local changes that aren’t present in the given
> +branch. If the branch name is omitted, all local changes are listed. 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 "remove" subcommand deletes a change. This is one way to resolve divergence.
> +
> +The "new" subcommand creates a new change with the given name and start commit.
> +If the name is omitted, it uses the default algorithm for assigning change
> +names. If the start commit is omitted, HEAD is used. The start commit may be
> +a metacommit, in which case the new change points to that position in the commit
> +history. If given the optional --force argument, it will overwrite any existing
> +change of the same name. This latter form of "new" can be used to effectively
> +reset changes.
> +
> +If the "new" command points to a normal commit (and not a metacommit), it 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 "replace" command records a replacement in the obsolescence graph, given a
> +list of obsolete commits or metacommits followed by their replacement. This
> +behaves like a normal "modify" command, except that the replacement is an
> +existing commit. If an obsolete commit points to a metacommit, only a change
> +branch pointing to exactly that metacommit moves forward. If an obsolete commit
> +points to a normal commit, all change branches pointing to that commit move
> +forward. If no change branches moved forward, a new change branch is created
> +using the default name.
> +
> +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>”.
> +
> +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:
> +
> +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.
> --