Subject: Addendum to "MaintNotes"
From: Junio C Hamano <[email-masked]>
Date: Wed, 21 Nov 2007 16:32:55 -0800

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
the 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.

 - Maintenance releases are numbered as vX.Y.Z (0 < 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).

 - The 'master' branch is used to prepare for the next feature
   release. In other words, at some point, the tip of 'master'
   branch is tagged as vX.(Y+1).0, when vX.Y.0 is the latest
   feature release.

 - '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.  Reviews from contributors on
   the mailing list help to make the determination.  After a topic
   is merged to 'next', it is tested for at least 7 calendar days
   before getting merged to 'master'.

 - 'seen' branch is used to publish other proposed changes that do
   not yet pass the criteria set for 'next' (see above), but there
   is no promise that 'seen' will contain everything.  A topic that
   had no reviewer reaction may not be picked up.

   - A new topic will first get merged to 'seen', unless it is
     trivially correct and clearly urgent, in which case it may be
     directly merged to 'next' or even to 'master'.

   - If a topic that was picked up to 'seen' becomes and stays
     inactive for 3 calendar weeks without having seen a clear
     consensus that it is good enough to be moved to 'next', the
     topic may be discarded from 'seen'.  Interested parties are
     still free to revive the topic.  For the purpose of this
     guideline, the definition of being "inactive" is that nobody
     has discussed the topic, no new iteration of the topic was
     posted, and no responses to the review comments were given.

 - 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'.

 - When a problem is found in a topic in 'next', the topic is marked
   not to be merged to 'master'.  Follow-up patches are discussed on
   the mailing list and applied to the topic after being reviewed and
   then the topic is merged (again) to 'next'.  After going through
   the usual testing in 'next', the entire (fixed) topic is merged
   to 'master'.

 - One release cycle for a feature release is expected to last for
   eight to ten weeks.  A few "release candidate" releases are
   expected to be tagged about a week apart before the final
   release, and a "preview" release is tagged about a week before
   the first release candidate gets tagged.

 - After the preview release is tagged, topics that were well
   reviewed may be merged to 'master' before spending the usual 7
   calendar days in 'next', with the expectation that any bugs in
   them can be caught and fixed in the release candidates before
   the final release.

 - After the first release candidate is tagged, the contributors are
   strongly encouraged to focus on finding and fixing new regressions
   introduced during the cycle, over addressing old bugs and any new
   features.  Topics stop getting merged down from 'next' to 'master',
   and new topics stop getting merged to 'next'. Unless they are fixes
   to new regressions in the cycle, that is.

 - Soon after a feature release is made, the tip of 'maint' gets
   fast-forwarded to point at the release.  Topics that have been
   kept in 'next' are merged down to 'master' and a new development
   cycle starts.

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.

Because most of the lines of code in Git are written by individual
contributors, and contributions come in the form of e-mailed patches
published on the mailing list, the project maintains a mapping from
individual commits to the Message-Id of the e-mail that resulted in
the commit, to help tracking the origin of the changes. The notes
in "refs/notes/amlog" are used for this purpose, and are published
along with the broken-out branches to the maintainer's repository.

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 to solve.  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'.

   Applying the e-mailed patches using "git am" automatically records
   the mappings from 'Message-Id' to the applied commit in the "amlog"
   notes. Periodically check that this is working with "git show -s
   --notes=amlog $commit".

   This mapping is maintained with the aid of the "post-applypatch"
   hook found in the 'todo' branch. That hook should be installed
   before applying patches. It is also helpful to carry forward any
   relevant amlog entries when rebasing, so the following config may
   be useful:

      [notes]
	rewriteRef = refs/notes/amlog

   Avoid "cherry-pick", as it does not propagate notes by design. Use
   either "git commit --amend" or "git rebase" to make corrections to
   an existing commit, even for a single-patch topic.

   Make sure that a push refspec for 'refs/notes/amlog' is in the
   remote configuration for publishing repositories. A few sample
   configurations look like the following:

      [remote "github"]
	url = https://github.com/gitster/git
	pushurl = github.com:gitster/git.git
	mirror

      [remote "github2"]
	url = https://github.com/git/git
	fetch = +refs/heads/*:refs/remotes/github2/*
	pushurl = github.com:git/git.git
	push = refs/heads/maint:refs/heads/maint
	push = refs/heads/master:refs/heads/master
	push = refs/heads/next:refs/heads/next
	push = +refs/heads/seen:refs/heads/seen
	push = +refs/notes/amlog

 - 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

   If the tip of 'master' is updated, also generate the preformatted
   documentation and push the out result to git-htmldocs and
   git-manpages repositories.

 - 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 --whitespace=warn 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 --whitespace=warn 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.
   The expectation is that the original author will make corrections
   in a reroll.

 - By now, new topic branches are created and existing topic
   branches are updated.  The integration branches 'next', 'jch',
   and 'seen' need to be updated to contain them.

 - If there are topics that have been merged to 'master' and should
   be merged to 'maint', merge them to 'maint', and update the
   release notes to the next maintenance release.

 - Review the latest issue of "What's cooking" again.  Are topics
   that have been sufficiently long in 'next' ready to be merged to
   'master'?  Are topics we saw earlier and are in 'seen' now got
   positive reviews and are ready to be merged to 'next'?

 - If there are topics that have been cooking in 'next' long enough
   and should be merged to 'master', merge them to 'master', and
   update the release notes to the next feature release.

 - If there were patches directly made on 'maint', merge 'maint' to
   'master'; make sure that the result is what you want.

     $ git checkout master
     $ git merge -m "Sync with 'maint'" --no-log maint
     $ git log -p --first-parent ORIG_HEAD..
     $ make test

 - Prepare to update the 'jch' branch, which is used to represent
   somewhere between 'master' and 'seen' and often is slightly ahead
   of 'next', and the 'seen' branch, which is used to hold the rest.

     $ Meta/Reintegrate master..jch >Meta/redo-jch.sh

   The result is a script that lists topics to be merged in order to
   rebuild the current 'jch'.  Do the same for 'seen'.

 - Review the Meta/redo-jch.sh and Meta/redo-seen.sh scripts.  The
   former should have a line '### match next'---the idea is that
   merging the topics listed before the line on top of 'master'
   should result in a tree identical to that of 'next'.

 - As newly created topics are usually merged near the tip of
   'seen', add them to the end of the Meta/redo-seen.sh script.
   Among the topics that were in 'seen', there may be ones that
   are not quite ready for 'next' but are getting there.  Move
   them from Meta/redo-seen.sh to the end of Meta/redo-jch.sh.
   The expectation is that you'd use 'jch' as your daily driver
   as the first guinea pig, so you should choose carefully.

 - Now we are ready to start rebuilding 'jch' and 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 before the '### match next' line.  Then:

     $ git checkout --detach master
     $ sh 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', this time without using "-c1" to merge all topics.

   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^{/^### match next'}'
   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.

   If 'master' was updated before you started redoing 'next', then

     $ git diff 'jch^{/^### match next}' next

   would show differences that went into 'master' (which 'jch' has,
   but 'next' does not yet---often it is updates to the release
   notes).  Merge 'master' back to 'next' if that is the case.

     $ git merge -m "Sync with 'master'" --no-log master

   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' on top of 'jch'.

     $ 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 a 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 around this issue.
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