How to maintain Git

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REQUEST EXIT

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.Z 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.W and are meant to contain only bugfixes for the corresponding vX.Y.Z feature release and earlier maintenance releases vX.Y.Z.V (V < W).
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.Z.
maint branch is used to prepare for the next maintenance release. After the feature release vX.Y.Z 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.Z.1, vX.Y.Z.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.
pu 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. pu 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.

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. 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. The topic is 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 above except the "replacement" are all done with:

$ git checkout ai/topic ;# or "git checkout -b ai/topic master"
$ git am -sc3 mailbox

while patch replacement is often done by:

$ git format-patch ai/topic~$n..ai/topic ;# export existing

then replace some parts with the new patch, and reapplying:

$ git checkout ai/topic
$ git reset --hard ai/topic~$n
$ git am -sc3 -s 000*.txt

The full test suite is always run for 'maint' and 'master'
after patch application; for topic branches the tests are run
as time permits.

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 pu and often is slightly ahead of next.

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

The result is a script that lists topics to be merged in order to
rebuild 'pu' 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 pu.

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

Edit the result by adding new topics that are not still in 'pu'
in the script.  Then

$ git checkout -B pu jch
$ sh Meta/redo-pu.sh

When all is well, clean up the redo-pu.sh script with

$ sh Meta/redo-pu.sh -u

Double check by running

$ git branch --no-merged pu

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 'pu' 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..pu, 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..pu are
added to the "New topics" section, topics listed in the file that
are no longer found in master..pu are moved to the "Graduated to
master" section, and topics whose commits changed their states
(e.g. used to be only in 'pu', 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 pu, 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.

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-pu 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 pu~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 pu~4
$ git show -s --pretty=%s ;# double check
Merge branch 'ai/topic' to pu
$ 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 pu~5 ;# the parent of the problem merge
$ echo ai/topic | Meta/Reintegrate
$ git diff pu~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-pu) 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