.. Copyright © 2014 Greg Ward <greg@gerg.ca>

------------------------------

Evolve: Shared Mutable History

------------------------------

Once you have mastered the art of mutable history in a single

computers. Say you're working on code that must be tested on a remote

test server, probably in a rack somewhere, only accessible by SSH, and

running an “enterprise-grade” (out-of-date) OS. But you probably

prefer to write code locally: everything is setup the way you like it,

and you can use your preferred editor, IDE, merge/diff tools, etc.

Traditionally, your options are limited: either

  * (ab)use your source control system by committing half-working code

    in order to get it onto the remote test server, or

  * go behind source control's back by using ``rsync`` (or similar) to

    transfer your code back-and-forth until it is ready to commit

The former is less bad with distributed version control systems like

Mercurial, but it's still far from ideal. (One important version

control “best practice” is that every commit should make things just a

little bit better, i.e. you should never commit code that is worse

than what came before.) The latter, avoiding version control entirely,

means that you're walking a tightrope without a safety net. One

accidental ``rsync`` in the wrong direction could destroy hours of

work.

Using Mercurial with ``evolve`` to share mutable history solves all of

these problems. As with single-repository ``evolve``, you can commit

whenever the code is demonstrably better, even if all the tests aren't

passing yet—just ``hg amend`` when they are. And you can transfer

those half-baked changesets between repositories to try things out on

your test server before anything is carved in stone.

Publishing and non-publishing repositories

------------------------------------------

The key to shared mutable history is to keep your changesets in

*draft* phase as you pass them around. Recall that by default, ``hg

push`` promotes changesets from *draft* to *public*, and public

changesets are immutable. You can change this behaviour by

reconfiguring the *target* repository so that it is non-publishing.

(Short version: set ``phases.publish`` to ``false``. Long version

follows.)

Setting things up

-----------------

We'll work an example with three local repositories, although in the

real world they'd most likely be on three different computers. First,

  $ hg init public

We'll need two clones where work gets done::

``dev-repo`` is your local machine, with GUI merge tools and IDEs and

everything configured just the way you like it. ``test-repo`` is the

test server in a rack somewhere behind SSH. So for the most part,

we'll develop in ``dev-repo``, push to ``test-repo``, test and polish

there, and push to ``public``.

  [phases]

  publish = false

  [extensions]

  evolve = /path/to/mutable-history/hgext/evolve.py

  [extensions]

  evolve = /path/to/mutable-history/hgext/evolve.py

Keep in mind that in real life, these repositories would probably be

on separate computers, so you'd have to login to each one to configure

each repository.

To start things off, let's make one public, immutable changeset::

  $ cd test-repo

  $ echo 'my new project' > file1

  $ hg add file1

  $ hg commit -m 'create new project'

and pull that into the development repository::

  $ cd ../dev-repo

  $ hg pull -u

Everything you learned in the `user guide`_ applies to work done in

``dev-repo``. You can commit, amend, uncommit, evolve, and so forth

just as before.

.. _`user guide`: user-guide.html

Things get different when you push changesets to ``test-repo``. Or

rather, things stay the same, which *is* different: because we

configured ``test-repo`` to be non-publishing, draft changesets stay

draft when we push them to ``test-repo``. Importantly, they're also

draft (mutable) in ``test-repo``.

Let's commit a preliminary change and push it to ``test-repo`` for

testing. ::

  $ echo 'fix fix fix' > file1

  $ hg commit -m 'prelim change'

  $ hg push ../test-repo

At this point, ``dev-repo`` and ``test-repo`` have the same changesets

in the same phases:

  [figure SG01: rev 0:0dc9 public, rev 1:f649 draft, same on both repos]

(You may notice a change in notation from the user guide: now

changesets are labelled with their revision number and the first four

digits of the 40-digit hexadecimal changeset ID. Mercurial revision

numbers are never stable when working across repositories, especially

when obsolescence is involved. We'll see why shortly.)

Now let's switch to ``test-repo`` to test our change::

  $ cd ../test-repo

  $ hg update

Don't forget to ``hg update``! Pushing only adds changesets to a

remote repository; it does not update the working directory (unless

you have a hook that updates for you).

Now let's imagine the tests failed because we didn't use proper

punctuation and capitalization (oops). Let's amend our preliminary fix

(and fix the lame commit message while we're at it)::

  $ echo 'Fix fix fix.' > file1

  $ hg amend -m 'fix bug 37'

Now we're in a funny intermediate state (figure 2): revision 1:f649 is

obsolete in ``test-repo``, having been replaced by revision 3:60ff

(revision 2:2a03 is another one of those temporary amend commits that

we saw in the user guide)—but ``dev-repo`` knows nothing of these

recent developments.

  [figure SG02: rev 0:0dc9 public, rev 1:f649, 2:2a03 obsolete, rev 3:60ff draft -- but dev-repo same as in SG01]

Let's resynchronize::

  $ cd ../dev-repo

  $ hg pull -u

As seen in figure 3, this transfers the new changeset *and* the

obsolescence marker for revision 1. However, it does *not* transfer

the temporary amend commit, because it is obsolete. Push and pull

transfer obsolesence markers between repositories, but they do not

normally transfer obsolete changesets.

  [figure SG03: dev-repo grows new rev 2:60ff, marks 1:f649 obsolete]

Because of this deliberately incomplete synchronization, revision

numbers in ``test-repo`` and ``dev-repo`` are no longer consistent. We

*must* use changeset IDs.

This process can repeat. Perhaps you figure out a more elegant fix to

the bug, and want to mutate history so nobody ever knows you had a

less-than-perfect idea. We'll implement it locally in ``dev-repo`` and

push to ``test-repo``::

  $ echo 'Fix, fix, and fix.' > file1

  $ hg amend

  $ hg push

This time around, the temporary amend commit is in ``dev-repo``, and

it is not transferred to ``test-repo``—the same as before, just in the

opposite direction. Figure 4 shows the two repositories after amending

in ``dev-repo`` and pushing to ``test-repo``.

  [figure SG04: each repo has one temporary amend commit, but they're different in each one]

Let's hop over to ``test-repo`` to test the more elegant fix::

  $ cd ../test-repo

This time, all the tests pass, so no further amendment is required.

This bug fix is finished, so we push it to the public repository::

  $ hg push

  [...]

  added 1 changesets with 1 changes to 1 files

Note that only one changeset—the final version, after two

amendments—was actually pushed. Again, Mercurial normally doesn't

transfer obsolete changesets on push and pull. (Specifically, it

doesn't transfer *hidden* changesets: roughly speaking, obsolete

changesets with no non-obsolete descendants. If you're curious, see

the `concept guide`_ for the precise definition of hidden.)

.. _`concept guide`: concepts.html

So the picture in ``public`` is much simpler than in either

``dev-repo`` or ``test-repo``. None of our missteps or amendments are

visible publicly, just the final, beautifully polished changeset:

  [figure SG05: public repo with rev 0:0dc9, 1:de61, both public]

There is one important step left to do. Because we pushed from

``test-repo`` to ``public``, the pushed changeset is in *public* phase

in those two repositories. But ``dev-repo`` knows nothing of this:

that changeset is still *draft* there. If we're not careful, we might

mutate history in ``dev-repo``, obsoleting a changeset that is already

public. Let's avoid that situation for now by pulling from

``test-repo`` down to ``dev-repo``::

  $ cd ../dev-repo

  $ hg pull -u

Getting into trouble

--------------------

Mercurial with ``evolve`` is a powerful tool, and using powerful tools

can have consequences. (You can cut yourself badly with a sharp knife,

but every competent chef keeps several around. Ever try to chop onions

with a spoon?)

In the user guide, we saw examples of *unstable* changesets, which are

the most common type of troubled changeset. (Recall that a

non-obsolete changeset with obsolete ancestors is unstable.)

Two other types of trouble can crop up: *bumped* and *divergent*

changesets. Both are more likely with shared mutable history,

especially mutable history shared by multiple developers.

To demonstrate, let's start with the ``public`` repository as we left

it in the last example, with two immutable changesets (figure 5

above). Two developers, Alice and Bob, start working from this point::

We need to configure Alice's and Bob's working repositories similar to

``test-repo``, i.e. make them non-publishing and enable ``evolve``::

  $ cat >> alice/.hg/hgrc <<EOF

  [phases]

  publish = false

  [extensions]

  evolve = /path/to/mutable-history/hgext/evolve.py

  EOF

  $ cp alice/.hg/hgrc bob/.hg/hgrc

Bumped changesets: only one gets on the plane

---------------------------------------------

If two people show up at the airport with tickets for the same seat on

the same plane, only one of them gets on the plane. The would-be

traveller left behind in the airport terminal is said to have been

*bumped*.

Similarly, if Alice and Bob are collaborating on some mutable

changesets, it's possible to get into a situation where an otherwise

worthwhile changeset cannot be pushed to the public repository; it is

bumped by an alternative changeset that happened to get there first.

Let's demonstrate one way this could happen.

It starts with Alice committing a bug fix. Right now, we don't yet

know if this bug fix is good enough to push to the public repository,

but it's good enough for Alice to commit. ::

  $ cd alice

  $ echo 'fix' > file2

Now Bob has a bad idea: he decides to pull whatever Alice is working

on and tweak her bug fix to his taste::

  $ cd ../bob

  $ echo 'Fix.' > file2

(Note the lack of communication between Alice and Bob. Failing to

communicate with your colleagues is a good way to get into trouble.

Nevertheless, ``evolve`` can usually sort things out, as we will see.)

  [figure SG06: Bob's repo with one amendment]

After some testing, Alice realizes her bug fix is just fine as it is:

no need for further polishing and amending, this changeset is ready to

publish. ::

  $ cd ../alice

This introduces a contradiction: in Bob's repository, changeset 2:e011

(his copy of Alice's fix) is obsolete, since Bob amended it. But in

Alice's repository (and ``public``), that changeset is public: it is

immutable, carved in stone for all eternity. No changeset can be both

obsolete and public, so Bob is in for a surprise the next time he

pulls from ``public``::

  $ cd ../bob

  $ hg pull -q -u

  1 new bumped changesets

Figure 7 shows what just happened to Bob's repository: changeset

2:e011 is now public, so it can't be obsolete. When that changeset was

obsolete, it made perfect sense for it to have a successor, namely

Bob's amendment of Alice's fix (changeset 4:fe88). But it's illogical

for a public changeset to have a successor, so 4:fe88 is in trouble:

it has been *bumped*.

  [figure SG07: 2:e011 now public not obsolete, 4:fe88 now bumped]

As usual when there's trouble in your repository, the solution is to

evolve it::

  $ hg evolve --all

Figure 8 illustrate's Bob's repository after evolving away the bumped

changeset. Ignoring the obsolete changesets, Bob now has a nice,

clean, simple history. His amendment of Alice's bug fix lives on, as

changeset 5:227d—albeit with a software-generated commit message. (Bob

should probably amend that changeset to improve the commit message.)

But the important thing is that his repository no longer has any

troubled changesets, thanks to ``evolve``.

  [figure SG08: 5:227d is new, formerly bumped changeset 4:fe88 now hidden]

Divergent changesets

--------------------

In addition to *unstable* and *bumped*, there is a third kind of

troubled changeset: *divergent*. When an obsolete changeset has two

successors, those successors are divergent.

To illustrate, let's start Alice and Bob at the same

point—specifically, the point where Alice's repository currently

stands. Bob's repository is a bit of a mess, so we'll throw it away

and start him off with a copy of Alice's repository::

  $ cd ..

  $ rm -rf bob

  $ cp -rp alice bob

Now we'll have Bob commit a bug fix that could still be improved::

  $ cd bob

  $ echo 'pretty good fix' >> file1

  $ hg commit -u bob -m 'fix bug 24 (v1)'

This time, Alice meddles with her colleague's work (still a bad

idea)::

  $ cd ../alice

  $ echo 'better (alice)' >> file1

  $ hg amend -u alice -m 'fix bug 24 (v2 by alice)'

Here's where things change from the "bumped" scenario above: this

time, the original author (Bob) decides to amend his changeset too. ::

  $ cd ../bob

  $ echo 'better (bob)' >> file1

  $ hg amend -u bob -m 'fix bug 24 (v2 by bob)'

At this point, the divergence exists, but only in theory: Bob's

original changeset, 3:fe81, is obsolete and has two successors. But

those successors are in different repositories, so the trouble is not

visible to anyone yet. It will be as soon as one of our players pulls

from the other's repository. Let's make Bob the victim again::

  $ hg pull -q -u ../alice

  not updating: not a linear update

  (merge or update --check to force update)

  2 new divergent changesets

The “not a linear update” is our first hint that something is wrong,

but of course “2 new divergent changesets” is the real problem. Figure

9 shows both problems.

  [figure SG09: bob's repo with 2 heads for the 2 divergent changesets, 5:fc16 and 6:694f; wc is at 5:fc16, hence update refused; both are successors of obsolete 3:fe81, hence divergence]

Now we need to get out of trouble. Unfortunately, a `bug`_ in

``evolve`` means that the usual answer (run ``hg evolve --all``) does

not work. Bob has to figure out the solution on his own: in this case,

merge. To avoid distractions, we'll set ``HGMERGE`` to make Mercurial

resolve any conflicts in favour of Bob. ::

  $ HGMERGE=internal:local hg merge

  $ hg commit -m merge

.. _`bug`: https://bitbucket.org/marmoute/mutable-history/issue/48/

This is approximately what ``hg evolve`` would do in this

circumstance, if not for that bug. One annoying difference is that

Mercurial thinks the two divergent changesets are still divergent,

which you can see with a simple revset query::

  $ hg log -q -r 'divergent()'

  5:fc16901f4d7a

  6:694fd0f6b503

(That annoyance should go away when the bug is fixed.)

Conclusion

----------

Mutable history is a powerful tool. Like a sharp knife, an experienced

user can do wonderful things with it, much more wonderful than with a

dull knife (never mind a rusty spoon). At the same time, an

inattentive or careless user can do harm to himself or others.

Mercurial with ``evolve`` goes to great lengths to limit the harm you

can do by trying to handle all possible types of “troubled”

changesets. But having a first-aid kit nearby does not excuse you from

being careful with sharp knives.

Mutable history shared across multiple repositories by a single

developer is a natural extension of this model. Once you are used to

using a single sharp knife on its own, it's pretty straightforward to

chop onions and mushrooms using the same knife, or to alternate

between two chopping boards with different knives.

Mutable history shared by multiple developers is a scary place to go.

Imagine a professional kitchen full of expert chefs tossing their

favourite knives back and forth, with the occasional axe or chainsaw

thrown in to spice things up. If you're confident that you *and your

colleagues* can do it without losing a limb, go for it. But be sure to

practice a lot first before you rely on it!