From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 21 Nov 2007 16:32:55 -0800
Subject: Addendum to "MaintNotes"
Abstract: Imagine that Git development is racing along as usual, when our friendly
neighborhood maintainer is struck down by a wayward bus. Out of the
hordes of suckers (loyal developers), you have been tricked (chosen) to
step up as the new maintainer. This howto will show you "how to" do it.
Content-type: text/asciidoc
How to maintain Git
===================
Activities
----------
The maintainer's Git time is spent on three activities.
- Communication (45%)
Mailing list discussions on general design, fielding user
questions, diagnosing bug reports; reviewing, commenting on,
suggesting alternatives to, and rejecting patches.
- Integration (50%)
Applying new patches from the contributors while spotting and
correcting minor mistakes, shuffling the integration and
testing branches, pushing the results out, cutting the
releases, and making announcements.
- Own development (5%)
Scratching my own itch and sending proposed patch series out.
The Policy
----------
The policy on Integration is informally mentioned in "A Note
from the maintainer" message, which is periodically posted to
this mailing list after each feature release is made.
- Feature releases are numbered as vX.Y.0 and are meant to
contain bugfixes and enhancements in any area, including
functionality, performance and usability, without regression.
- One release cycle for a feature release is expected to last for
eight to ten weeks.
- Maintenance releases are numbered as vX.Y.Z and are meant
to contain only bugfixes for the corresponding vX.Y.0 feature
release and earlier maintenance releases vX.Y.W (W < Z).
- 'master' branch is used to prepare for the next feature
release. In other words, at some point, the tip of 'master'
branch is tagged with vX.Y.0.
- 'maint' branch is used to prepare for the next maintenance
release. After the feature release vX.Y.0 is made, the tip
of 'maint' branch is set to that release, and bugfixes will
accumulate on the branch, and at some point, the tip of the
branch is tagged with vX.Y.1, vX.Y.2, and so on.
- 'next' branch is used to publish changes (both enhancements
and fixes) that (1) have worthwhile goal, (2) are in a fairly
good shape suitable for everyday use, (3) but have not yet
demonstrated to be regression free. New changes are tested
in 'next' before merged to 'master'.
- 'seen' branch is used to publish other proposed changes that do
not yet pass the criteria set for 'next'.
- The tips of 'master' and 'maint' branches will not be rewound to
allow people to build their own customization on top of them.
Early in a new development cycle, 'next' is rewound to the tip of
'master' once, but otherwise it will not be rewound until the end
of the cycle.
- Usually 'master' contains all of 'maint' and 'next' contains all
of 'master'. 'seen' contains all the topics merged to 'next', but
is rebuilt directly on 'master'.
- The tip of 'master' is meant to be more stable than any
tagged releases, and the users are encouraged to follow it.
- The 'next' branch is where new action takes place, and the
users are encouraged to test it so that regressions and bugs
are found before new topics are merged to 'master'.
Note that before v1.9.0 release, the version numbers used to be
structured slightly differently. vX.Y.Z were feature releases while
vX.Y.Z.W were maintenance releases for vX.Y.Z.
A Typical Git Day
-----------------
A typical Git day for the maintainer implements the above policy
by doing the following:
- Scan mailing list. Respond with review comments, suggestions
etc. Kibitz. Collect potentially usable patches from the
mailing list. Patches about a single topic go to one mailbox (I
read my mail in Gnus, and type \C-o to save/append messages in
files in mbox format).
- Write his own patches to address issues raised on the list but
nobody has stepped up solving. Send it out just like other
contributors do, and pick them up just like patches from other
contributors (see above).
- Review the patches in the saved mailboxes. Edit proposed log
message for typofixes and clarifications, and add Acks
collected from the list. Edit patch to incorporate "Oops,
that should have been like this" fixes from the discussion.
- Classify the collected patches and handle 'master' and
'maint' updates:
- Obviously correct fixes that pertain to the tip of 'maint'
are directly applied to 'maint'.
- Obviously correct fixes that pertain to the tip of 'master'
are directly applied to 'master'.
- Other topics are not handled in this step.
This step is done with "git am".
$ git checkout master ;# or "git checkout maint"
$ git am -sc3 mailbox
$ make test
In practice, almost no patch directly goes to 'master' or
'maint'.
- Review the last issue of "What's cooking" message, review the
topics ready for merging (topic->master and topic->maint). Use
"Meta/cook -w" script (where Meta/ contains a checkout of the
'todo' branch) to aid this step.
And perform the merge. Use "Meta/Reintegrate -e" script (see
later) to aid this step.
$ Meta/cook -w last-issue-of-whats-cooking.mbox
$ git checkout master ;# or "git checkout maint"
$ echo ai/topic | Meta/Reintegrate -e ;# "git merge ai/topic"
$ git log -p ORIG_HEAD.. ;# final review
$ git diff ORIG_HEAD.. ;# final review
$ make test ;# final review
- Handle the remaining patches:
- Anything unobvious that is applicable to 'master' (in other
words, does not depend on anything that is still in 'next'
and not in 'master') is applied to a new topic branch that
is forked from the tip of 'master' (or the last feature release,
which is a bit older than 'master'). This includes both
enhancements and unobvious fixes to 'master'. A topic
branch is named as ai/topic where "ai" is two-letter string
named after author's initial and "topic" is a descriptive name
of the topic (in other words, "what's the series is about").
- An unobvious fix meant for 'maint' is applied to a new
topic branch that is forked from the tip of 'maint' (or the
oldest and still relevant maintenance branch). The
topic may be named as ai/maint-topic.
- Changes that pertain to an existing topic are applied to
the branch, but:
- obviously correct ones are applied first;
- questionable ones are discarded or applied to near the tip;
- Replacement patches to an existing topic are accepted only
for commits not in 'next'.
The initial round is done with:
$ git checkout ai/topic ;# or "git checkout -b ai/topic master"
$ git am -sc3 mailbox
and replacing an existing topic with subsequent round is done with:
$ git checkout master...ai/topic ;# try to reapply to the same base
$ git am -sc3 mailbox
to prepare the new round on a detached HEAD, and then
$ git range-diff @{-1}...
$ git diff @{-1}
to double check what changed since the last round, and finally
$ git checkout -B @{-1}
to conclude (the last step is why a topic already in 'next' is
not replaced but updated incrementally).
Whether it is the initial round or a subsequent round, the topic
may not build even in isolation, or may break the build when
merged to integration branches due to bugs. There may already
be obvious and trivial improvements suggested on the list. The
maintainer often adds an extra commit, with "SQUASH???" in its
title, to fix things up, before publishing the integration
branches to make it usable by other developers for testing.
These changes are what the maintainer is not 100% committed to
(trivial typofixes etc. are often squashed directly into the
patches that need fixing, without being applied as a separate
"SQUASH???" commit), so that they can be removed easily as needed.
- Merge maint to master as needed:
$ git checkout master
$ git merge maint
$ make test
- Merge master to next as needed:
$ git checkout next
$ git merge master
$ make test
- Review the last issue of "What's cooking" again and see if topics
that are ready to be merged to 'next' are still in good shape
(e.g. has there any new issue identified on the list with the
series?)
- Prepare 'jch' branch, which is used to represent somewhere
between 'master' and 'seen' and often is slightly ahead of 'next'.
$ Meta/Reintegrate master..seen >Meta/redo-jch.sh
The result is a script that lists topics to be merged in order to
rebuild 'seen' as the input to Meta/Reintegrate script. Remove
later topics that should not be in 'jch' yet. Add a line that
consists of '### match next' before the name of the first topic
in the output that should be in 'jch' but not in 'next' yet.
- Now we are ready to start merging topics to 'next'. For each
branch whose tip is not merged to 'next', one of three things can
happen:
- The commits are all next-worthy; merge the topic to next;
- The new parts are of mixed quality, but earlier ones are
next-worthy; merge the early parts to next;
- Nothing is next-worthy; do not do anything.
This step is aided with Meta/redo-jch.sh script created earlier.
If a topic that was already in 'next' gained a patch, the script
would list it as "ai/topic~1". To include the new patch to the
updated 'next', drop the "~1" part; to keep it excluded, do not
touch the line. If a topic that was not in 'next' should be
merged to 'next', add it at the end of the list. Then:
$ git checkout -B jch master
$ Meta/redo-jch.sh -c1
to rebuild the 'jch' branch from scratch. "-c1" tells the script
to stop merging at the first line that begins with '###'
(i.e. the "### match next" line you added earlier).
At this point, build-test the result. It may reveal semantic
conflicts (e.g. a topic renamed a variable, another added a new
reference to the variable under its old name), in which case
prepare an appropriate merge-fix first (see appendix), and
rebuild the 'jch' branch from scratch, starting at the tip of
'master'.
Then do the same to 'next'
$ git checkout next
$ sh Meta/redo-jch.sh -c1 -e
The "-e" option allows the merge message that comes from the
history of the topic and the comments in the "What's cooking" to
be edited. The resulting tree should match 'jch' as the same set
of topics are merged on 'master'; otherwise there is a mismerge.
Investigate why and do not proceed until the mismerge is found
and rectified.
$ git diff jch next
When all is well, clean up the redo-jch.sh script with
$ sh Meta/redo-jch.sh -u
This removes topics listed in the script that have already been
merged to 'master'. This may lose '### match next' marker;
add it again to the appropriate place when it happens.
- Rebuild 'seen'.
$ Meta/Reintegrate master..seen >Meta/redo-seen.sh
Edit the result by adding new topics that are not still in 'seen'
in the script. Then
$ git checkout -B seen jch
$ sh Meta/redo-seen.sh
When all is well, clean up the redo-seen.sh script with
$ sh Meta/redo-seen.sh -u
Double check by running
$ git branch --no-merged seen
to see there is no unexpected leftover topics.
At this point, build-test the result for semantic conflicts, and
if there are, prepare an appropriate merge-fix first (see
appendix), and rebuild the 'seen' branch from scratch, starting at
the tip of 'jch'.
- Update "What's cooking" message to review the updates to
existing topics, newly added topics and graduated topics.
This step is helped with Meta/cook script.
$ Meta/cook
This script inspects the history between master..seen, finds tips
of topic branches, compares what it found with the current
contents in Meta/whats-cooking.txt, and updates that file.
Topics not listed in the file but are found in master..seen are
added to the "New topics" section, topics listed in the file that
are no longer found in master..seen are moved to the "Graduated to
master" section, and topics whose commits changed their states
(e.g. used to be only in 'seen', now merged to 'next') are updated
with change markers "<<" and ">>".
Look for lines enclosed in "<<" and ">>"; they hold contents from
old file that are replaced by this integration round. After
verifying them, remove the old part. Review the description for
each topic and update its doneness and plan as needed. To review
the updated plan, run
$ Meta/cook -w
which will pick up comments given to the topics, such as "Will
merge to 'next'", etc. (see Meta/cook script to learn what kind
of phrases are supported).
- Compile, test and install all four (five) integration branches;
Meta/Dothem script may aid this step.
- Format documentation if the 'master' branch was updated;
Meta/dodoc.sh script may aid this step.
- Push the integration branches out to public places; Meta/pushall
script may aid this step.
Observations
------------
Some observations to be made.
* Each topic is tested individually, and also together with other
topics cooking first in 'seen', then in 'jch' and then in 'next'.
Until it matures, no part of it is merged to 'master'.
* A topic already in 'next' can get fixes while still in
'next'. Such a topic will have many merges to 'next' (in
other words, "git log --first-parent next" will show many
"Merge branch 'ai/topic' to next" for the same topic.
* An unobvious fix for 'maint' is cooked in 'next' and then
merged to 'master' to make extra sure it is Ok and then
merged to 'maint'.
* Even when 'next' becomes empty (in other words, all topics
prove stable and are merged to 'master' and "git diff master
next" shows empty), it has tons of merge commits that will
never be in 'master'.
* In principle, "git log --first-parent master..next" should
show nothing but merges (in practice, there are fixup commits
and reverts that are not merges).
* Commits near the tip of a topic branch that are not in 'next'
are fair game to be discarded, replaced or rewritten.
Commits already merged to 'next' will not be.
* Being in the 'next' branch is not a guarantee for a topic to
be included in the next feature release. Being in the
'master' branch typically is.
* Due to the nature of "SQUASH???" fix-ups, if the original author
agrees with the suggested changes, it is OK to squash them to
appropriate patches in the next round (when the suggested change
is small enough, the author should not even bother with
"Helped-by"). It is also OK to drop them from the next round
when the original author does not agree with the suggestion, but
the author is expected to say why somewhere in the discussion.
Appendix
--------
Preparing a "merge-fix"
~~~~~~~~~~~~~~~~~~~~~~~
A merge of two topics may not textually conflict but still have
conflict at the semantic level. A classic example is for one topic
to rename an variable and all its uses, while another topic adds a
new use of the variable under its old name. When these two topics
are merged together, the reference to the variable newly added by
the latter topic will still use the old name in the result.
The Meta/Reintegrate script that is used by redo-jch and redo-seen
scripts implements a crude but usable way to work this issue around.
When the script merges branch $X, it checks if "refs/merge-fix/$X"
exists, and if so, the effect of it is squashed into the result of
the mechanical merge. In other words,
$ echo $X | Meta/Reintegrate
is roughly equivalent to this sequence:
$ git merge --rerere-autoupdate $X
$ git commit
$ git cherry-pick -n refs/merge-fix/$X
$ git commit --amend
The goal of this "prepare a merge-fix" step is to come up with a
commit that can be squashed into a result of mechanical merge to
correct semantic conflicts.
After finding that the result of merging branch "ai/topic" to an
integration branch had such a semantic conflict, say seen~4, check the
problematic merge out on a detached HEAD, edit the working tree to
fix the semantic conflict, and make a separate commit to record the
fix-up:
$ git checkout seen~4
$ git show -s --pretty=%s ;# double check
Merge branch 'ai/topic' to seen
$ edit
$ git commit -m 'merge-fix/ai/topic' -a
Then make a reference "refs/merge-fix/ai/topic" to point at this
result:
$ git update-ref refs/merge-fix/ai/topic HEAD
Then double check the result by asking Meta/Reintegrate to redo the
merge:
$ git checkout seen~5 ;# the parent of the problem merge
$ echo ai/topic | Meta/Reintegrate
$ git diff seen~4
This time, because you prepared refs/merge-fix/ai/topic, the
resulting merge should have been tweaked to include the fix for the
semantic conflict.
Note that this assumes that the order in which conflicting branches
are merged does not change. If the reason why merging ai/topic
branch needs this merge-fix is because another branch merged earlier
to the integration branch changed the underlying assumption ai/topic
branch made (e.g. ai/topic branch added a site to refer to a
variable, while the other branch renamed that variable and adjusted
existing use sites), and if you changed redo-jch (or redo-seen) script
to merge ai/topic branch before the other branch, then the above
merge-fix should not be applied while merging ai/topic, but should
instead be applied while merging the other branch. You would need
to move the fix to apply to the other branch, perhaps like this:
$ mf=refs/merge-fix
$ git update-ref $mf/$the_other_branch $mf/ai/topic
$ git update-ref -d $mf/ai/topic
