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

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Evolve: User Guide

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

.. contents::

Life without ``evolve``

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

Before we dive into learning about ``evolve``, let's look into some

features of core Mercurial that interact with ``evolve``. ``commit``

affects ``evolve``, and ``evolve`` modifies how ``commit --amend``

works.

Example 1: Commit a new changeset

=================================

To create a new changeset, simply run ``hg commit`` as usual.

``evolve`` does not change the behaviour of ``commit`` at all.

However, it's important to understand that new changesets are in the

*draft* phase by default: they are mutable. This means that they can

be modified by Mercurial's existing history-editing commands

(``rebase``, ``histedit``, etc.), and also by the ``evolve``

extension. Specifically, ``evolve`` adds a number of commands that can

be used to modify history: ``amend``, ``uncommit``, ``prune``,

``fold``, and ``evolve``. Generally speaking, changesets remain in

*draft* phase until they are pushed to another repository, at which

point they enter *public* phase. ::

  $ hg commit -m 'implement feature X'

  $ hg phase -r .

  1: draft

(Strictly speaking, changesets only become public when they are pushed

to a *publishing* repository. But all repositories are publishing by

default; you have to explicitly configure repositories to be

*non-publishing*. Non-publishing repositories are an advanced topic

which we'll see when we get to `sharing mutable history`_.)

.. _`sharing mutable history`: sharing.html

Example 2: Amend a changeset (traditional)

==========================================

Imagine you've just committed a new changeset, and then you discover a

mistake. Maybe you forgot to run the tests and a failure slipped in.

You want to modify history so that you push one perfect changeset,

rather than one flawed changeset followed by an "oops" commit. (Or

perhaps you made a typo in the commit message—this is really feature

*Y*, not feature X. You can't fix that with a followup commit.)

This is actually trivial with plain vanilla Mercurial since 2.2: fix

your mistake and run ::

  $ hg commit --amend -m 'implement feature Y'

to create a new, amended changeset. The drawback of doing this with

vanilla Mercurial is that your original, flawed, changeset is removed

from the repository. This is *unsafe* history editing. It's probably

not too serious if all you did was fix a syntax error, but still.

.. figure:: figures/figure-ug01.svg

   Figure 1: unsafe history modification with core Mercurial (not

   using ``evolve``): the original revision 1 is destroyed.

(Incidentally, Mercurial's traditional history modification mechanism

isn't *really* unsafe: any changeset(s) removed from the repository

are kept in a backup directory, so you can manually restore them later

if you change your mind. But it's awkward and inconvenient compared to

the features provided by ``evolve`` and changeset obsolescence.)

Life with ``evolve`` (basic usage)

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

Once you enable the ``evolve`` extension, a number of features are

available to you. First, we're going to explore several examples of

painless, trouble-free history modification.

Example 3: Amend a changeset (with ``evolve``)

==============================================

Outwardly, amending a changeset with ``evolve`` can look exactly the

same as it does with core Mercurial (example 2)::

  $ hg commit --amend -m 'implement feature Y'

Alternately, you can use the new ``amend`` command added by

``evolve``::

  $ hg amend -m 'implement feature Y'

(``hg amend`` is nearly synonymous with ``hg commit --amend``. The

difference is that ``hg amend`` reuses the existing commit message by

default, whereas ``hg commit --amend`` runs your editor if you don't

pass ``-m`` or ``-l``.)

Under the hood, though, things are quite different. Mercurial has

simply marked the old changeset *obsolete*, replacing it with a new

one. We'll explore what this means in detail later, after working

through a few more examples.

Example 4: Prune an unwanted changeset

======================================

Sometimes you make a change, and then decide it was such a bad idea

that you don't want anyone to know about it. Or maybe it was a

debugging hack that you needed to keep around for a while, but do not

intend to ever push publicly. ::

  $ echo 'debug hack' >> file1.c

  $ hg commit -m 'debug hack'

In either case, ``hg prune`` is the answer. ``prune`` simply marks

changesets obsolete without creating any new changesets to replace

them::

  $ hg prune .

  1 changesets pruned

  1 files updated, 0 files merged, 0 files removed, 0 files unresolved

  working directory now at 934359450037

Outwardly, it appears that your “debug hack” commit never happened;

we're right back where we started::

  $ hg parents --template '{rev}:{node|short}  {desc|firstline}\n'

  3:934359450037  implement feature Y

In reality, though, the “debug hack” is still there, obsolete and hidden.

Example 5: Uncommit changes to certain files

============================================

Occasionally you commit more than you intended: perhaps you made

unrelated changes to different files, and thus intend to commit

different files separately. ::

  $ echo 'relevant' >> file1.c

  $ echo 'irrelevant' >> file2.c

If you forget to specify filenames on the ``commit`` command line,

Mercurial commits all those changes together::

  $ hg commit -m 'fix bug 234'          # oops: too many files

Luckily, this mistake is easy to fix with ``uncommit``::

  $ hg uncommit file2.c

  $ hg status

  M file2.c

Let's verify that the replacement changeset looks right (i.e.,

modifies only ``file1.c``)::

  $ hg parents --template '{rev}:{node|short}  {desc|firstline}\n{files}\n'

  6:c8defeecf7a4  fix bug 234

  file1.c

As before, the original flawed changeset is still there, but obsolete

and hidden. It won't be exchanged with other repositories by ``push``,

``pull``, or ``clone``.

Example 6: Fold multiple changesets together into one

=====================================================

If you're making extensive changes to fragile source code, you might

commit more frequently than normal so that you can fallback on a

known good state if one step goes badly. ::

  $ echo step1 >> file1.c

  $ hg commit -m 'step 1'               # revision 7

  $ echo step2 >> file1.c

  $ hg commit -m 'step 2'               # revision 8

  $ echo step3 >> file2.c

  $ hg commit -m 'step 3'               # revision 9

At the end of such a sequence, you often end up with a series of small

changesets that are tedious to review individually. It might make more

sense to combine them into a single changeset using the ``fold``

command.

To make sure we pass the right revisions to ``fold``, let's review the

changesets we just created, from revision 7::

  $ hg log --template '{rev}:{node|short}  {desc|firstline}\n' -r 7::

  7:05e61aab8294  step 1

  8:be6d5bc8e4cc  step 2

  9:35f432d9f7c1  step 3

and fold them::

  $ hg fold -m 'fix bug 64' -r 7::

  3 changesets folded

  1 files updated, 0 files merged, 0 files removed, 0 files unresolved

This time, Mercurial marks three changesets obsolete, replacing them

all with a single *successor*.

(You might be familiar with this operation under other names, like

*squash* or *collapse*.)

Changeset obsolescence under the hood

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

So far, everything has gone just fine. We haven't run into merge

conflicts or other trouble. Before we start exploring advanced usage

that can run into trouble, let's step back and see what happens when

Mercurial marks changesets obsolete. That will make it much easier to

understand the more advanced use cases we'll see later.

When you have the ``evolve`` extension enabled, all history

modification uses the same underlying mechanism: the original

changesets are marked *obsolete* and replaced by zero or more

*successors*. The obsolete changesets are the *precursors* of their

successors. This applies equally to built-in commands (``commit

--amend``), commands added by ``evolve`` (``amend``, ``prune``,

``uncommit``, ``fold``), and even commands provided by other

extensions (``rebase``, ``histedit``).

Another way of looking at it is that obsolescence is second-order

version control, i.e. the history of your history. We'll cover this in

more detail (and mathematical precision) in the `concepts`_ guide.

.. _`concepts`: concepts.html

Under the hood: Amend a changeset

=================================

Consider Example 2, amending a changeset with ``evolve``. We saw above

that you can do this using the exact same command-line syntax as core

Mercurial, namely ``hg commit --amend``. But the implementation is

quite different, and Figure 2 shows how.

.. figure:: figures/figure-ug02.svg

   Figure 2: safe history modification using ``evolve``: the original

   revision 1 is preserved as an obsolete changeset. (The "temporary

   amend commit", marked with T, is an implementation detail stemming

   from limitations in Mercurial's current merge machinery. Future

   versions of Mercurial will not create them.)

In this case, the obsolete changesets are also *hidden*. That is the

usual end state for obsolete changesets. But many scenarios result in

obsolete changesets that are still visible, which indicates your

history modification work is not yet done. We'll see examples of that

later, when we cover advanced usage.

Seeing hidden changesets

========================

TODO

Under the hood: Prune an unwanted changeset

===========================================

``prune`` (example 4 above) is the simplest history modification

command provided by ``evolve``. All it does is mark the specified

changeset(s) obsolete, with no successor/precursor relationships

involved. (If the working directory parent was one of the obsolete

changesets, ``prune`` updates back to a suitable ancestor.)

.. figure:: figures/figure-ug03.svg

   Figure 3: pruning a changeset marks it obsolete with no successors.

Under the hood: Uncommit changes to certain files

=================================================

In one sense, ``uncommit`` is a simplified version of ``amend``. Like

``amend``, it obsoletes one changeset and leaves it with a single

successor. Unlike ``amend``, there is no ugly "temporary amend commit"

cluttering up the repository.

In another sense, ``uncommit`` is the inverse of ``amend``: ``amend``

takes any uncommitted changes in the working dir and “adds”

them to the working directory's parent changeset. (In reality, of

course, it creates a successor changeset, marking the original

obsolete.) In contrast, ``uncommit`` takes some changes in the working

directory's parent and moves them to the working dir, creating a new

successor changeset in the process. Figure 4 illustrates.

.. figure:: figures/figure-ug04.svg

   Figure 4: uncommit moves some of the changes from the working

   directory parent into the working dir, preserving the remaining

   changes as a new successor changeset. (N.B. revision 4 is not shown

   here because it was marked obsolete in the previous example.)

Under the hood: Fold multiple changesets together into one

==========================================================

The last basic example is folding multiple changesets into one, which

marks multiple changesets obsolete, replacing them all with a single

successor.

.. figure:: figures/figure-ug05.svg

   Figure 5: fold combines multiple changesets into a single

   successor, marking the original (folded) changesets obsolete.

Obsolete is not hidden

======================

TODO

Understanding revision numbers

==============================

If you're trying these examples on your own, especially using ``hg

log`` without ``--hidden``, you have probably noticed some funny

business going on with revision numbers: there are now gaps in the

sequence. That's something you don't see with plain vanilla Mercurial;

normally, revision N is always followed by revision N+1.

This is just the visible manifestation of hidden changesets. If

revision 95 is followed by revision 98, that means there are two

hidden changesets, 96 and 97, in between.

Note that changeset IDs are still the permanent, immutable identifier

for changesets. Revision numbers are, as ever, a handy shorthand that

work in your local repository, but cannot be used across repositories.

They also have the useful property of showing when there are hidden

changesets lurking under the covers, which is why this document uses

revision numbers.

Life with ``evolve`` (advanced usage)

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

Now that you've got a solid understanding of how ``evolve`` works in

concert with changeset obsolescence, let's explore some more advanced

scenarios. All of these scenarios will involve *unstable* changesets,

which is an unavoidable consequence of obsolescence. What really sets

``evolve`` apart from other history modification mechanisms is the

fact that it recognizes troubles like unstable changesets and provides

a consistent way for you to get out of trouble.

(Incidentally, there are two other types of trouble that changesets

can get into with ``evolve``: they may be *divergent* or *bumped*.

Both of those states are more likely to occur when `sharing mutable

history`_, so we won't see them in this user guide.)

.. _`sharing mutable history`: sharing.html

Example 7: Amend an older changeset

===================================

Sometimes you don't notice your mistakes until after you have

committed some new changesets on top of them. ::

  $ hg commit -m 'fix bug 17'         # rev 11 (mistake here)

  $ hg commit -m 'cleanup'            # rev 12

  $ hg commit -m 'feature 23'         # rev 13

Traditionally, your only option is to commit an "oops" changeset that

fixes your mistake. That works, of course, but it makes you look bad:

you made a mistake, and the record of that mistake is recorded in

history for all eternity. (If the mistake was in the commit message,

too bad.)

More subtly, there now exist changesets that are *worse* than what

came before—the code no longer builds, the tests don't pass, or

similar. Anyone reviewing these patches will waste time noticing the

error in the earlier patch, and then the correction later on.

You can avoid all this by amending the bad changeset and *evolving*

subsequent history. Here's how it works, assuming you have just

committed revision 13 and noticed the mistake in revision 11::

  $ hg update 11

  [...fix mistake...]

  $ hg amend

At this point, revision 11 is *obsolete* and revisions 12 and 13—the

descendants of 11—are in a funny state: they are *unstable*.

.. figure:: figures/figure-ug06.svg

   Figure 6: amending a changeset with descendants means the amended

   changeset is obsolete but remains visible; its non-obsolete

   descendants are *unstable*. The temporary amend commit, revision

   14, is hidden because it has no non-obsolete descendants.

All non-obsolete descendants of an obsolete changeset are unstable. An

interesting consequence of this is that revision 11 is still visible,

even though it is obsolete. Obsolete changesets with non-obsolete

descendants are not hidden.

The fix is to *evolve* history::

  $ hg evolve --all

This is a separate step, not automatically part of ``hg amend``,

because there might be conflicts. If your amended changeset modifies a

file that one of its descendants also modified, Mercurial has to fire

up your merge tool to resolve the conflict. More importantly, you have

to switch contexts from "writing code" to "resolving conflicts". That

can be an expensive context switch, so Mercurial lets you decide when