The Universe of Disco

My Git Habits
Miles Gould asked his Twitter followers whether they used git-add -p or git-commit -a and how often. My reply was too long for Twitter, so here it is.

First the short version: I use git-add -p frequently, and git-commit -a almost never. The exception is when I'm working on the repo that holds my blog, where I rarely commit changes to more than one or two files at a time. Then I'll usually just git-commit -a -m ....

But I use git-add -p all the time. Typically what will happen is that I will be developing some fairly complicated feature. It will necessitate a bunch of changes and reshuffling elsewhere in the system. I'll make commits on the topic branch as I go along without worrying too much about whether the commits are neatly packaged.

Often I'll be in the middle of something, with a dirty work tree, when it's time to leave for the day. Then I'll just commit everything with the subject WIP ("work-in-progress"). First thing the next morning I'll git-reset HEAD^ and continue where I left off.

So the model is that the current head is usually a terrible mess, accumulating changes as it moves forward in time. When I'm done, I will merge the topic into master and run the tests.

If they pass, I am not finished. The merge I just created is only a draft merge. The topic branch is often full of all sorts of garbage, commits where I tried one approach, found it didn't work later on, and then tried a different approach, places where I committed debugging code, and so on. So it is now time to clean up the topic branch. Only the cleaned-up topic branch gets published.

Cleaning up messy topic branches

1. Compose the commits: Repeat until the working tree is clean:
   
   1. Eyeball the output of git-diff
   2. Think of an idea for an intelligible commit
   3. Use git-add -p to stage the planned commit
   4. Use git diff --cached to make sure it makes sense
   5. Commit it
2. Order the commits: Use git-rebase --interactive

By separating these tasks, I can proceed something like this: I eyeball the diff, and the first thing I see is something about the penguin feature. I can immediately say "Great, I'll make up a commit of all the stuff related to the penguin feature", and proceed to the git-add -p step without worrying that there might be other stuff that should precede the penguin feature in the commit sequence. I can focus on just getting the penguin commit right without needing to think about any of the other changes.

When the time comes to put the commits in order, I can do it well because by then I have abstracted away all the details, and reduced each group of changes to a single atomic unit with a one-line description.

For the most complicated cases, I will print out the diffs, read them over, and mark them up in six colors of highlighter: code to throw away gets marked in orange; code that I suspect is erroneous is pink. I make many notes in pen to remind me how I want to divide up the changes into commits. When a commit occurs to me I'll jot a numbered commit message, and then mark all the related parts of the diff with that number. Once I have the commits planned, I'll reset the topic ref and then run through the procedure above, using git-add -p repeatedly to construct the commits I planned on paper. Since I know ahead of time what they are I might do them in the right order, but more likely I'll just do them in the order I thought of them and then reorder them at the end, as usual.

For simple cases I'll just do a series of git-rebase --interactive passes, pausing at any leftover WIP commits to run the loop above, reordering the commits to squash related commits together, and so on.

The very simplest cases of all require no cleanup, of course.

For example, here's my current topic branch, called c-domain, with the oldest commits at the top:

        055a2f7 correction to bulk consumer template
        d9630bd DomainActivator half of Pobox Domain consumer
        ebebb4a Add HasDomain role to provide ->domain reader for domain consumers
        ade6ac6 stubbed domain test
        e170e77 start templates for Pobox domain consumers
        067ca81 stubbed Domain::ThumbTwiddler
        685a3ee cost calculations for DomainActivator
        ec8b1cc test fixes; trivial domain test passes now
        845b1f2 rename InvoiceCharge::CreateDomain to ..::RegisterDomain
(e)     6083a97 add durations to Domain consumers and charges
        c64fda0 tests for Domain::Activator consumer
        41e4292 repeat activator tests for 1-year and 3-year durations
        7d68065 tests for activator's replacement
(d)     87f3b09 move days_in_year to Moonpig::Util
        3cd9f3b WIP
        e5063d4 add test for sent invoice in domain.t
        c8dbf41 WIP
        9e6ffa4 add missing MakesReplacement stuff
        fc13059 bring in Net::OpenSRS module
(c)     52c18fb OpenSRS interface
        893f16f notes about why domain queries might fail
(b)     f64361f rename "croak" method to "fail" to avoid conflicts
        4e500ec Domain::Activator initial_invoice_charge_pairs
(a)     3c5cdd4 WIP

f64361f (b) is a prime candidate for later squashing. 5c218fb (c) introduced a module with a "croak" method. This turned out to be a stupid idea, because this conflicted with the croak function from Perl's Carp module, which we use everywhere. I needed to rename it. By then, the intervening commit already existed. I probably should have squashed these right away, but I didn't think of it at the time. No problem! Git means never having to say "If only I'd realized sooner."

Similarly, 6083a97 (e) added a days_in_year function that I later decided at 87f3b09 (d) should be in a utility module in a different repository. 87f3b09 will eventually be squashed into 6083a97 so that days_in_year never appears in this code at all.

I don't know what is in the WIP commits c8dbf41 or 3cd9f3b, for which I didn't invent commit messages. I don't know why those are left in the tree, but I can figure it out later.

An example cleanup

The first order of business is to get rid of those WIP commits. I'll git-reset HEAD^ to bring 3c5cdd4 into the working directory, then use git-status to see how many changes there are:

         M lib/Pobox/Moonpig/Consumer/Domain/Activator.pm
         M lib/Pobox/Moonpig/Role/HasDomain.pm
         M lib/Pobox/Moonpig/TemplateSet.pm
        ?? bin/register_domains
         M t/consumer/domain.t
        ?? t/lib/MockOpenSRS.pm

Not too bad, probably no need for a printout. The new bin/register-domains program can go in right away by itself:

        % git add bin
        % git commit -m 'new register_domains utility program'

        % git add t/lib
        % git add -p
        ...
        % git s
        MM lib/Pobox/Moonpig/Consumer/Domain/Activator.pm
         M lib/Pobox/Moonpig/Role/HasDomain.pm
         M lib/Pobox/Moonpig/TemplateSet.pm
        A  t/lib/MockOpenSRS.pm
        MM t/consumer/domain.t
        % git ix
        ...

        % git commit -m 'mock OpenSRS object; add tests'

        % git stash
        % make test
        ...
        OK
        % git stash pop

Continuing:

        % git diff
        ...
        % git add -p lib/Pobox/Moonpig/Role/HasDomain.pm
        ...
        % git commit -m 'Domains do not have explicit start dates'
        % git diff
        ...
        % git add -p
        ...
        % git commit --fixup :/mock

It goes on like that for a while:

        % git diff
        ...
        % git add -p ...
        ...
        % git commit -m 'Activator consumer can generate special charges'
        % git diff
        ...
        % git checkout lib/Pobox/Moonpig/Role/HasDomain.pm

        % git diff
        ...
        % git add -u
        % git commit -m 'separate templates for domain-registering and domain-renewing consumers'

        a3c0b92 new register_domains utility program
        53d704d mock OpenSRS object; add tests
        a24acd8 Domains do not have explicit start dates
        17a915d fixup! mock OpenSRS object; add tests
        86e472b Activator consumer can generate special charges
        5b2ad2b separate templates for domain-registering and domain-renewing consumers

        % git rebase -i --autosquash HEAD~6

        a3c0b92 new register_domains utility program
        ea8dacd Domains do not have explicit start dates
        297366a separate templates for domain-registering and domain-renewing consumers
        4ef0e28 mock OpenSRS object; add tests
        c3ab1eb Activator consumer can generate special charges

Here's a comparison the original c-domain branch and my new cleanup version:

        % git vee c-domain
        * c3ab1eb (HEAD, cleanup) Activator consumer can generate special charges
        * 4ef0e28 mock OpenSRS object; add tests
        * 297366a separate templates for domain-registering and domain-renewing consumer
        * ea8dacd Domains do not have explicit start dates
        * a3c0b92 new register_domains utility program
        | * 3c5cdd4 (origin/c-domain, c-domain) WIP
        |/  
        o 4e500ec Domain::Activator initial_invoice_charge_pairs

A cleaned-up branch should usually have the same final tree as the tree at the end of the original branch. I have another tool, git-treehash, which compares trees. By default it compares HEAD with ORIG_HEAD, so after I use git-rebase to squash or to split commits, I sometimes run "git treehash" to make sure that the tree hasn't changed. In this example, I do:

        % git treehash c-domain HEAD
        d360408d1afa90e0176aaa73bf8d3cae641a0850 HEAD
        f0fd6ea0de7dbe60520e2a69fbec210260370d78 c-domain

        % git diff c-domain
        diff --git a/lib/Pobox/Moonpig/Role/HasDomain.pm b/lib/Pobox/Moonpig/Role/HasDomain.pm
        index 3d8bb8c..21cb752 100644
        --- a/lib/Pobox/Moonpig/Role/HasDomain.pm
        +++ b/lib/Pobox/Moonpig/Role/HasDomain.pm
        @@ -5,7 +5,6 @@ use Carp qw(croak confess);
         use ICG::Handy qw(is_domain);
         use Moonpig::Types qw(Factory Time);
         use Moose::Util::TypeConstraints qw(duck_type enum subtype);
        -use MooseX::SetOnce;

         with (
           'Moonpig::Role::StubBuild',

The next task is probably to deal with the older WIP commits. This time I'll omit all the details. But the enclosing procedure looks like this:

        % git checkout -b wip-cleanup c8dbf41
        % git reset HEAD^
        % ... (a lot of git-add -p as above) ...
        ...

        % git vee c8dbf41
        * 4c6ff45 (wip-cleanup) get rid of unused twiddler test
        * b328de5 test full payment cycle
        * 201a4f2 abstract out pay_invoice operation
        * 55ae45e add upper limit (default 30d) to wait_until utility
        | * c8dbf41 WIP
        |/  
        o e5063d4 add test for sent invoice in domain.t

        % git treehash c8dbf41 HEAD
        7f52ba68923e2ede8fda407ffa9c06c5c48338ae
        % git checkout cleanup
        % git rebase wip-cleanup

        % git vee c-domain
        * a425aa1 (HEAD, cleanup) Activator consumer can generate special charges
        * 2bb0932 mock OpenSRS object; add tests
        * a77bfcb separate templates for domain-registering and domain-renewing consumer
        * 4c44db2 Domains do not have explicit start dates
        * fab500f new register_domains utility program
        = 38018b6 Domain::Activator initial_invoice_charge_pairs
        = aebbae6 rename "croak" method to "fail" to avoid conflicts
        = 45a224d notes about why domain queries might fail
        = 80e4a90 OpenSRS interface
        = 27f4562 bring in Net::OpenSRS module
        = f5cb624 add missing MakesReplacement stuff
        * 4c6ff45 (wip-cleanup) get rid of unused twiddler test
        * b328de5 test full payment cycle
        * 201a4f2 abstract out pay_invoice operation
        * 55ae45e add upper limit (default 30d) to wait_until utility
        | * 3c5cdd4 (origin/c-domain, c-domain) WIP
        | = 4e500ec Domain::Activator initial_invoice_charge_pairs
        | = f64361f rename "croak" method to "fail" to avoid conflicts
        | = 893f16f notes about why domain queries might fail
        | = 52c18fb OpenSRS interface
        | = fc13059 bring in Net::OpenSRS module
        | = 9e6ffa4 add missing MakesReplacement stuff
        | * c8dbf41 WIP
        |/  
        o e5063d4 add test for sent invoice in domain.t

I've been planning for a long time to write a tool to help me with breaking up WIP commits like this, and with branch cleanup in general: It will write each changed hunk into a file, and then let me separate the hunk files into several subdirectories, each of which represents one commit, and then it will create the commits automatically from the directory contents. This is still only partly finished, but I think when it's done it will eliminate the six-color diff printouts.

[ Addendum 20120404: Further observation has revealed that I almost never use git-commit -a, even when it would be quicker to do so. Instead, I almost always use git-add -u and then git-commit the resulting index. This is just an observation, and not a claim that my practice is either better or worse than using git-commit -a. ]

[ Addendum 20120825: There is now a followup article about how to manage rewriting of published history. ]

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Why can't Git resolve all conflicted merges?
I like to be prepared ahead of time for questions, and one such question is why Git can't resolve all merge conflicts automatically. People do show up on IRC asking this from time to time. If you're a sophisticated user the answer is obvious, but I've made a pretty good living teaching classes to people who don't find such things obvious.

What we need is a nice example. In the past my example was sort of silly. You have a file that contains the instruction:

           Pay potato tax every April 15

           Pay potato tax every April 15
             (Except in years of potato blight.)

           Pay all tax due every April 15

           Pay all tax due every April 15
             (Except in years of potato blight.)

But a few months ago I ran into a somewhat less silly example. At work we had two seprate projects, "Moonpig" and "Stick", each in its own repository. Moonpig contained a subsystem, "Collections", which we decided would make more sense as part of Stick. I did this work, removing the Collections code from the Moonpig project and integrating it into the Stick project. From the point of view of the Moonpig repository, the Collections system was deleted entirely.

Meanwhile, on a parallel branch of Moonpig, R.J.B. Signes made some changes that included bug fixes to the Collections. After I removed the collections, he tried to merge his changes into the master branch, and got a merge conflict, because some of the files to which he was making bug fixes were no longer there.

The correct resolution was to perform the rest of the merge without the bug fixes, which Git could conceivably have done. But then the unapplied bug fixes needed to be applied to the Collections module that was now in the completely separate Stick project, and there is no way Git could have done this, or even to have known it should be done. Human intervention was the only answer.

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