docs/user-guide.rst
author Pierre-Yves David <pierre-yves.david@octobus.net>
Tue, 22 Jan 2019 10:43:44 -0500
branchmercurial-4.8
changeset 4355 75db6a9d0b54
parent 2931 83d2c9637e89
child 4615 8406d9b06130
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branching: start branch for test compat with Mercurial 4.8
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.. Copyright © 2014 Greg Ward <greg@gerg.ca>
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------------------
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Evolve: User Guide
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------------------
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.. contents::
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Life without ``evolve``
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-----------------------
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Before we dive into learning about ``evolve``, let's look into some
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features of core Mercurial that interact with ``evolve``. ``commit``
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affects ``evolve``, and ``evolve`` modifies how ``commit --amend``
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works.
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Example 1: Commit a new changeset
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=================================
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To create a new changeset, simply run ``hg commit`` as usual.
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``evolve`` does not change the behaviour of ``commit`` at all.
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However, it's important to understand that new changesets are in the
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*draft* phase by default: they are mutable. This means that they can
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be modified by Mercurial's existing history-editing commands
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(``rebase``, ``histedit``, etc.), and also by the ``evolve``
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extension. Specifically, ``evolve`` adds a number of commands that can
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be used to modify history: ``amend``, ``uncommit``, ``prune``,
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``fold``, and ``evolve``. ::
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  $ hg commit -m 'implement feature X'
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  $ hg phase -r .
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  1: draft
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Generally speaking, changesets remain in *draft* phase until they are
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pushed to another repository, at which point they enter the *public*
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phase. (Strictly speaking, changesets only become public when they are
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pushed to a *publishing* repository. But all repositories are publishing
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by default; you have to explicitly configure repositories to be
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*non-publishing*. Non-publishing repositories are an advanced topic
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which we'll see when we get to `sharing mutable history`_.)
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.. _`sharing mutable history`: sharing.html
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Example 2: Amend a changeset (traditional)
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==========================================
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Imagine you've just committed a new changeset, and then you discover a
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mistake. Maybe you forgot to run the tests and a failure slipped in.
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You want to modify history so that you push one perfect changeset,
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rather than one flawed changeset followed by an "oops" commit. (Or
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perhaps you made a typo in the commit message—this is really feature
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*Y*, not feature X. You can't fix that with a followup commit.)
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This is actually trivial with plain vanilla Mercurial since 2.2: fix
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your mistake and run ::
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  $ hg commit --amend -m 'implement feature Y'
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to create a new, amended changeset. The drawback of doing this with
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vanilla Mercurial is that your original, flawed, changeset is removed
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from the repository. This is *unsafe* history editing. It's probably
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not too serious if all you did was fix a syntax error, but for deeper
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changes there can be more serious consequences to unsafe history editing.
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.. figure:: figures/figure-ug01.svg
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   Figure 1: unsafe history modification with core Mercurial (not
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   using ``evolve``): the original revision 1 is destroyed.
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(Incidentally, Mercurial's traditional history modification mechanism
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isn't *really* unsafe: any changeset(s) removed from the repository
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are kept in a backup directory, so you can manually restore them later
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if you change your mind. However, this mechanism is very awkward and
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inconvenient compared to the features provided by ``evolve`` and
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changeset obsolescence.)
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Life with ``evolve`` (basic usage)
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----------------------------------
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Once you enable the ``evolve`` extension, a number of features are
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available to you. First, we're going to explore several examples of
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painless, trouble-free history modification.
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Example 3: Amend a changeset (with ``evolve``)
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==============================================
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Outwardly, amending a changeset with ``evolve`` can look exactly the
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same as it does with core Mercurial (example 2)::
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  $ hg commit --amend -m 'implement feature Y'
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Alternately, you can use the new ``amend`` command added by
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``evolve``::
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  $ hg amend -m 'implement feature Y'
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(``hg amend`` is nearly synonymous with ``hg commit --amend``. The
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difference is that ``hg amend`` reuses the existing commit message by
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default, whereas ``hg commit --amend`` runs your editor if you don't
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pass ``-m`` or ``-l``.)
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Under the hood, though, things are quite different. Mercurial has
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simply marked the old changeset as *obsolete*, replacing it with a new
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one. We'll explore what this means in detail later, after working
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through a few more examples.
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Example 4: Prune an unwanted changeset
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======================================
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Sometimes you make a change, and then decide it was such a bad idea
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that you don't want anyone to know about it. Or maybe it was a
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debugging hack that you needed to keep around for a while, but do not
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intend to ever push publicly. ::
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  $ echo 'debug hack' >> file1.c
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  $ hg commit -m 'debug hack'
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In either case, ``hg prune`` is the answer. ``prune`` simply marks
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changesets obsolete without creating any new changesets to replace
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them::
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  $ hg prune .
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  1 changesets pruned
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  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
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  working directory now at 934359450037
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Outwardly, it appears that your “debug hack” commit never happened;
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we're right back where we started::
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  $ hg parents --template '{rev}:{node|short}  {desc|firstline}\n'
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  3:934359450037  implement feature Y
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In reality, though, the “debug hack” is still there, obsolete and hidden.
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Example 5: Uncommit changes to certain files
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============================================
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Occasionally you commit more than you intended: perhaps you made
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unrelated changes to different files, and thus intend to commit
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different files separately. ::
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  $ echo 'relevant' >> file1.c
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  $ echo 'irrelevant' >> file2.c
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If you forget to specify filenames on the ``commit`` command line,
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Mercurial commits all those changes together::
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  $ hg commit -m 'fix bug 234'          # oops: too many files
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Luckily, this mistake is easy to fix with ``uncommit``::
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  $ hg uncommit file2.c
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  $ hg status
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  M file2.c
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Let's verify that the replacement changeset looks right (i.e.,
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modifies only ``file1.c``)::
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  $ hg parents --template '{rev}:{node|short}  {desc|firstline}\n{files}\n'
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  6:c8defeecf7a4  fix bug 234
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  file1.c
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As before, the original flawed changeset is still there, but obsolete
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and hidden. It won't be exchanged with other repositories by ``push``,
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``pull``, or ``clone``.
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Example 6: Fold multiple changesets together into one
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=====================================================
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If you're making extensive changes to fragile source code, you might
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commit more frequently than normal so that you can fallback on a
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known good state if one step goes badly. ::
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  $ echo step1 >> file1.c
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  $ hg commit -m 'step 1'               # revision 7
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  $ echo step2 >> file1.c
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  $ hg commit -m 'step 2'               # revision 8
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  $ echo step3 >> file2.c
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  $ hg commit -m 'step 3'               # revision 9
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At the end of such a sequence, you often end up with a series of small
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changesets that are tedious to review individually. It might make more
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sense to combine them into a single changeset using the ``fold``
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command.
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To make sure we pass the right revisions to ``fold``, let's review the
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changesets we just created, from revision 7::
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  $ hg log --template '{rev}:{node|short}  {desc|firstline}\n' -r 7::
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  7:05e61aab8294  step 1
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  8:be6d5bc8e4cc  step 2
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  9:35f432d9f7c1  step 3
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and fold them::
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  $ hg fold -m 'fix bug 64' -r 7:: --exact
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  3 changesets folded
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  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
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This time, Mercurial marks three changesets obsolete, replacing them
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all with a single *successor*.
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(You might be familiar with this operation under other names, like
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*squash* or *collapse*.)
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Changeset obsolescence under the hood
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-------------------------------------
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So far, everything has gone just fine: we haven't run into merge
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conflicts or other trouble. Before we start exploring advanced usage
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that can run into trouble, let's step back and see what happens when
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Mercurial marks changesets obsolete. That will make it much easier to
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understand the more advanced use cases we'll see later.
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When you have the ``evolve`` extension enabled, all history
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modification uses the same underlying mechanism: the original
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changesets are marked *obsolete* and replaced by zero or more
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*successors*. The obsolete changesets are the *predecessors* of their
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successors. This applies equally to built-in commands (``commit
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--amend``), commands added by ``evolve`` (``amend``, ``prune``,
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``uncommit``, ``fold``), and commands provided by other extensions
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(``rebase``, ``histedit``).
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Another way of looking at it is that obsolescence is second-order
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version control, i.e. the history of your history. We'll cover this in
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more detail (and mathematical precision) in the `concepts`_ guide.
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.. _`concepts`: concepts.html
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Under the hood: Amend a changeset
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=================================
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Consider Example 2, amending a changeset with ``evolve``. We saw above
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that you can do this using the exact same command-line syntax as core
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Mercurial, namely ``hg commit --amend``. But the implementation is
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quite different, as Figure 2 shows.
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.. figure:: figures/figure-ug02.svg
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   Figure 2: safe history modification using ``evolve``: the original
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   revision 1 is preserved as an obsolete changeset. (The "temporary
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   amend commit", marked with T, is an implementation detail stemming
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   from limitations in Mercurial's current merge machinery. Future
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   versions of Mercurial will not create them.)
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In this case, the obsolete changesets are also *hidden*. That is the
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usual end state for obsolete changesets. However, many scenarios result
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in obsolete changesets that are still visible, which indicates your
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history modification work is not yet done. We'll see examples of that
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later, when we cover advanced usage.
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Understanding revision numbers and hidden changesets
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====================================================
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As the name implies, hidden changesets are normally not visible. If
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you run ``hg log`` on the repository from Figure 2, Mercurial will
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show revisions 0 and 3, but not 1 and 2. That's something you don't
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see with plain vanilla Mercurial—normally, revision *N* is always
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followed by revision *N* + 1.
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This is just the visible manifestation of hidden changesets. If
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revision 0 is followed by revision 3, that means there are two hidden
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changesets, 1 and 2, in between.
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To see those hidden changesets, use the ``--hidden`` option::
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  $ hg --hidden log --graph --template '{rev}:{node|short}  {desc|firstline}\n'
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  @  3:934359450037  implement feature Y
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  |
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  | x  2:6c5f78d5d467  temporary amend commit for fe0ecd3bd2a4
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  | |
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  | x  1:fe0ecd3bd2a4  implement feature Y
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  |/
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  o  0:08c4b6f4efc8  init
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Note that changeset IDs are still the permanent, immutable identifier
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for changesets. Revision numbers are, as ever, a handy shorthand that
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work in your local repository, but cannot be used across repositories.
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They also have the useful property of showing when there are hidden
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changesets lurking under the covers, which is why this document uses
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revision numbers.
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Under the hood: Prune an unwanted changeset
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===========================================
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``prune`` (example 4 above) is the simplest history modification
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command provided by ``evolve``. All it does is mark the specified
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changeset(s) obsolete, with no successor/precursor relationships
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involved. (If the working directory parent was one of the obsoleted
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changesets, ``prune`` updates back to a suitable ancestor.)
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.. figure:: figures/figure-ug03.svg
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   Figure 3: pruning a changeset marks it obsolete with no successors.
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Under the hood: Uncommit changes to certain files
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=================================================
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In one sense, ``uncommit`` is a simplified version of ``amend``. Like
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``amend``, it obsoletes one changeset and leaves it with a single
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successor. In another sense, ``uncommit`` is the inverse of ``amend``:
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``amend`` takes any uncommitted changes in the working dir and “adds”
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them to the working directory's parent changeset. (In reality, of
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course, it creates a successor changeset, marking the original
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obsolete.) In contrast, ``uncommit`` takes some changes in the working
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directory's parent and moves them to the working dir, creating a new
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successor changeset in the process. Figure 4 illustrates.
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.. figure:: figures/figure-ug04.svg
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   Figure 4: uncommit moves some of the changes from the working
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   directory parent into the working dir, preserving the remaining
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   changes as a new successor changeset. (N.B. revision 4 is not shown
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   here because it was marked obsolete in the previous example.)
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Under the hood: Fold multiple changesets together into one
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==========================================================
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The last basic example is folding multiple changesets into one, which
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marks multiple changesets obsolete, replacing them all with a single
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successor.
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.. figure:: figures/figure-ug05.svg
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   Figure 5: fold combines multiple changesets into a single
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   successor, marking the original (folded) changesets obsolete.
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Obsolete is not hidden
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======================
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1269
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So far, every obsolete changeset we have seen is also hidden. However,
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these are *not* the same thing—that's why they have different names.
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It's entirely possible to have obsolete changesets that are not
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hidden. We'll see examples of that soon, when we create *unstable*
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changesets.
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Note that all hidden changesets are obsolete: hidden is a subset of
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obsolete. This is explained in more depth in the `concepts`_ section.
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.. _`concepts`: concepts.html
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Life with ``evolve`` (advanced usage)
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-------------------------------------
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Now that you've got a solid understanding of how ``evolve`` works in
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concert with changeset obsolescence, let's explore some more advanced
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scenarios. All of these scenarios will involve *unstable* changesets,
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which is an unavoidable consequence of obsolescence. What really sets
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``evolve`` apart from other history modification mechanisms is the
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fact that it recognizes troubles like unstable changesets and provides
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a consistent way for you to get back to a stable repository.
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(Incidentally, there are two other types of troubles that changesets
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can get into with ``evolve``: they may be *divergent* or
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*bumped*. Both of those states are more likely to occur when
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`sharing mutable history`_, so we won't cover them in this user guide.)
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.. _`sharing mutable history`: sharing.html
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Example 7: Amend an older changeset
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===================================
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Sometimes you don't notice a mistake until after you have committed
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new changesets on top of the changeset with the mistake. ::
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  $ hg commit -m 'fix bug 17'         # rev 11 (mistake here)
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  $ hg commit -m 'cleanup'            # rev 12
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  $ hg commit -m 'feature 23'         # rev 13
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Traditionally, your only option is to commit an "oops" changeset that
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fixes your mistake. That works, of course, but it makes you look bad:
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you made a mistake, and the record of that mistake is recorded in
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history for all eternity. (If the mistake was in the commit message,
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too bad: you cannot fix it.)
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More subtly, there now exist changesets that are *worse* than what
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came before—the code no longer builds, the tests don't pass, or
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similar. Anyone reviewing these patches will waste time on the error
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in the earlier patch, and then the correction later on.
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You can avoid all this by amending the bad changeset and *evolving*
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subsequent history. Here's how it works, assuming you have just
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committed revision 13 and noticed the mistake in revision 11::
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  $ hg update 11
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  [...fix mistake...]
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  $ hg amend
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At this point, revision 11 is *obsolete* and revisions 12 and 13—the
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descendants of 11—are in a funny state: they are *unstable*.
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.. figure:: figures/figure-ug06.svg
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   Figure 6: amending a changeset with descendants means the amended
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   changeset is obsolete but remains visible; its non-obsolete
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   descendants are *unstable*. The temporary amend commit, revision
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   14, is hidden because it has no non-obsolete descendants.
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All non-obsolete descendants of an obsolete changeset are considered
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unstable. An interesting consequence of this is that revision 11 is
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still visible, even though it is obsolete. Obsolete changesets with
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non-obsolete descendants are not hidden.
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The fix is to *evolve* history::
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  $ hg evolve --all
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This is a separate step, not automatically part of ``hg amend``,
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because there might be conflicts. If your amended changeset modifies a
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file that one of its descendants also modified, Mercurial has to fire
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up your merge tool to resolve the conflict. More importantly, you have
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to switch from "writing code" to "resolving conflicts". That can be an
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expensive context switch, so Mercurial lets you decide when to do it.
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The end state, after ``evolve`` finishes, is that the original
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revisions (11-13) are obsolete and hidden. Their successor revisions
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(15-17) replace them.
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.. figure:: figures/figure-ug07.svg
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   Figure 7: evolve your repository (``hg evolve --all``) to take care
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   of instability. Unstable changesets become obsolete, and are
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   replaced by successors just like the amended changeset was.
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Example 8: Prune an older changeset
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===================================
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Let's say you've just committed the following changesets::
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  $ hg commit -m 'useful work'       # rev 18
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  $ hg commit -m 'debug hack'        # rev 19
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  $ hg commit -m 'more work'         # rev 20
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You want to drop revision 19, but keep 18 and 20. No problem::
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  $ hg prune 19
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  1 changesets pruned
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  1 new unstable changesets
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As above, this leaves your repository in a funny intermediate state:
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revision 20 is the non-obsolete descendant of obsolete revision 19.
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That is, revision 20 is unstable.
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.. figure:: figures/figure-ug08.svg
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   Figure 8: ``hg prune`` marks a changeset obsolete without creating
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   a successor. Just like with ``hg amend``, non-obsolete descendants
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   of the pruned changeset are now unstable.
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As before, the solution to unstable changesets is to evolve your
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repository::
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  $ hg evolve --all
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This rebases revision 20 on top of 18 as the new revision 21, leaving
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19 and 20 obsolete and hidden:
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.. figure:: figures/figure-ug09.svg
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   Figure 9: once again, ``hg evolve --all`` takes care of instability.
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Example 9: Uncommit files from an older changeset (discard changes)
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=======================================================================
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As in example 5, let's say you accidentally commit some unrelated
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changes together. Unlike example 5, you don't notice your mistake
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   473
immediately, but commit a new changeset on top of the bad one. ::
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   474
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   475
  $ echo 'this fixes bug 53' >> file1.c
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   476
  $ echo 'debug hack' >> file2.c
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   477
  $ hg commit -m 'fix bug 53'                     # rev 22 (oops)
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   478
  $ echo 'and this handles bug 67' >> file1.c
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   479
  $ hg commit -m 'fix bug 67'                     # rev 23 (fine)
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   480
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   481
As with ``amend``, you need to travel back in time and repair revision
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   482
22, leaving your changes to ``file2.c`` back in the working
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   483
directory::
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   484
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   485
  $ hg update 22
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   486
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   487
  $ hg uncommit file2.c
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   488
  1 new unstable changesets
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   489
  $ hg status
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   490
  M file2.c
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   491
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   492
Now your repository has unstable changesets, so you need to evolve it.
2931
83d2c9637e89 user-guide: edit pass
Ryan McElroy <rmcelroy@fb.com>
parents: 1270
diff changeset
   493
However, ``hg evolve`` requires a clean working directory to resolve merge
978
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   494
conflicts, so you need to decide what to do with ``file2.c``.
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   495
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   496
In this case, the change to ``file2.c`` was a temporary debugging
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   497
hack, so we can discard it and immediately evolve the instability away::
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   498
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   499
  $ hg revert file2.c
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   500
  $ hg evolve --all
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   501
  move:[23] fix bug 67
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   502
  atop:[24] fix bug 53
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   503
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   504
Figure 10 illustrates the whole process.
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   505
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   506
.. figure:: figures/figure-ug10.svg
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   507
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   508
   Figure 10: ``hg uncommit`` of a changeset with descendants results
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   509
   in instability *and* a dirty working directory, both of which must
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   510
   be dealt with.
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   511
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   512
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   513
Example 10: Uncommit files to an older changeset (keep changes)
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   514
===================================================================
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   515
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   516
This is very similar to example 9. The difference that this time, our
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   517
change to ``file2.c`` is valuable enough to commit, making things a
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   518
bit more complicated. The setup is nearly identical::
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   519
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   520
  $ echo 'fix a bug' >> file1.c
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   521
  $ echo 'useful but unrelated' >> file2.c
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   522
  $ hg commit -u dan -d '11 0' -m 'fix a bug'     # rev 26 (oops)
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   523
  $ echo 'new feature' >> file1.c
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   524
  $ hg commit -u dan -d '12 0' -m 'new feature'   # rev 27 (fine)
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   525
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   526
As before, we update back to the flawed changeset (this time,
2931
83d2c9637e89 user-guide: edit pass
Ryan McElroy <rmcelroy@fb.com>
parents: 1270
diff changeset
   527
revision 26) and use ``uncommit``, leaving uncommitted changes to
978
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   528
``file2.c`` in the working dir::
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   529
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   530
  $ hg update -q 26
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   531
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
2931
83d2c9637e89 user-guide: edit pass
Ryan McElroy <rmcelroy@fb.com>
parents: 1270
diff changeset
   532
  $ hg uncommit -q file2.c  # obsoletes rev 26, creates rev 28
978
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   533
  1 new unstable changesets
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   534
  $ hg status
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   535
  M file2.c
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   536
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   537
This time, let's save that useful change before evolving::
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   538
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   539
  $ hg commit -m 'useful tweak'                   # rev 29
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   540
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   541
Figure 11 shows the story so far: ``uncommit`` obsoleted revision 26
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   542
and created revision 28, the successor of 26. Then we committed
2931
83d2c9637e89 user-guide: edit pass
Ryan McElroy <rmcelroy@fb.com>
parents: 1270
diff changeset
   543
revision 29, a child of 28. We still have to deal with the revision 27,
83d2c9637e89 user-guide: edit pass
Ryan McElroy <rmcelroy@fb.com>
parents: 1270
diff changeset
   544
which is an unstable changeset.
978
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   545
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   546
.. figure:: figures/figure-ug11.svg
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   547
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   548
   Figure 11: Uncommitting a file and then committing that change
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   549
   separately will soon result in a two-headed repository.
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   550
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   551
This is where things get tricky. As usual when a repository has
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   552
unstable changesets, we want to evolve it::
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   553
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   554
  $ hg evolve --all
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   555
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   556
The problem is that ``hg evolve`` rebases revision 27 onto revision
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   557
28, creating 30 (the successor of 27). This is entirely logical: 27
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   558
was the child of 26, and 26's successor is 28. So of course 27's
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   559
successor (30) should be the child of 26's successor (28).
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   560
Unfortunately, that leaves us with a two-headed repository:
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   561
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   562
.. figure:: figures/figure-ug12.svg
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   563
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   564
   Figure 12: ``evolve`` takes care of unstable changesets; it does
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   565
   not solve all the world's problems.
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   566
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   567
As usual when faced with a two-headed repository, you can either merge
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   568
or rebase. It's up to you.
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   569
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   570
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   571
Example 11: Recover an obsolete changeset
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   572
=========================================
8328337d23b2 docs: add new user guide
Greg Ward <greg@gerg.ca>
parents:
diff changeset
   573
1270
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   574
Sometimes you might obsolete a changeset, and then change your mind. You'll
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   575
probably start looking for an “unobsolete” command to restore a changeset
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   576
to normal state. For complicated implementation reasons, that command
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   577
doesn't exist. (If you have already pushed an obsolescence marker to
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   578
another repo, then Mercurial would need a way to revoke that remote
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   579
obsolesence marker. That's a hard problem.)
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   580
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   581
Instead, ``evolve`` provides a ``touch`` command to resurrect an
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   582
obsolete changeset. An unexpected quirk: you almost certainly need to
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   583
use ``--hidden``, since obsolete changesets tend to be hidden, and you
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   584
can't reference a hidden changeset otherwise. Typical usage thus looks
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   585
like ::
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   586
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   587
  $ hg --hidden touch REV
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   588
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   589
This creates a new, normal changeset which is the same as ``REV``—except
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   590
with a different changeset ID. The new changeset will have the same parent
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   591
as ``REV``, and will be a successor of ``REV``.
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   592
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   593
The current implementation of ``hg touch`` is not ideal, and is likely to
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   594
change in the future. Consider the history in Figure 12, where revision 27
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   595
is obsolete and the child of 26, also obsolete. If we ``hg touch 27``, that
2931
83d2c9637e89 user-guide: edit pass
Ryan McElroy <rmcelroy@fb.com>
parents: 1270
diff changeset
   596
creates a new revision which is a non-obsolete child of 26—i.e., it is an
1270
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   597
unstable. It's also *divergent*, another type of trouble that we'll learn
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   598
about in the `next section`_.
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   599
0683e3030316 docs: cover 'touch' command
Greg Ward <greg@gerg.ca>
parents: 1269
diff changeset
   600
.. _`next section`: sharing.html