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

[Duy Nguyen <pclouds@gmail.com>]

On Thu, Nov 15, 2018 at 2:00 AM <sxenos@google.com> wrote:
> +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.

I sometimes "modify" a commit by "git reset @^", pick up the changes
then "git commit -c @{1}". I don't think this counts as a typical
modification and is probably hard to detect automatically. But I hope
there's some way for me to tell git "yes this is a modified commit of
that one, record that!".

> +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

I guess as an example, how the name metas/this_is_a_test is
constructed does not matter much. But it's probably better to stick
with some sort of id because subject line will change over time and
the original one may become irrelevant. Perhaps we could use the
original commit id as name.

> +$ 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

How do we group changes of a topic together? I think branch-diff could
take advantage of that.

> +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”.

This feels a bit forced. Could we just organize it like a normal
history? Something like

*
|\
| * last version of the commit
*
|\
| * second last version of the commit
*
|\

Basically all commits will be linked in a new merge history. Real
commits are on the second parent, first parent is to link changes
together. This makes it possible to just use "git log --first-parent
--patch" (or "git log --oneline --graph") to examine the change. More
details (e.g. parent-type) could be stored as normal trailers in the
commit message of these merges.
-- 
Duy