docs/obs-concept.rst
author Pierre-Yves David <pierre-yves.david@ens-lyon.org>
Sat, 25 Aug 2012 14:43:34 +0200
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.. Copyright 2011 Pierre-Yves David <pierre-yves.david@ens-lyon.org>
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..                Logilab SA        <contact@logilab.fr>
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Why Do We Need a New Concept
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Current DVCSes are great tools for forging a series of flawless
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changesets on your own. But they perform poorly when it comes to
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**sharing** some work in progress and **collaborating** on such work
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in progress.
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When people forge a new version of a changeset they actually create a
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new changeset and get rid of the original changeset. Difficulties to
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collaborate mostly came from the way old content is *removed* from
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a repository.
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Mercurial Approach: Strip
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-----------------------------------------------------
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With the current version of mercurial, every changeset that exists in
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your repository is *visible* and *meaningful*. To delete old
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(rewritten) changesets, mercurial removes them from the repository
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storage with an operation called *strip*. After the *stripping*, the
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repository looks as if the changeset never existed.
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This approach is simple and effective except for one big
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drawback: you can remove changesets from **your repository only**. If
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a stripped changeset exists in another repository it touches, it will
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show up again. This is because a shared changeset becomes
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part of a shared global history. Stripping a changeset from all
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repositories is at best impractical and in most case impossible.
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As consequence, **you can not rewrite something once you exchange it with
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others**. The old version will still exist along side the new one [#]_.
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Moreover stripping changesets creates backup bundles. This allows
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restoration of the deleted changesets, but the process is painful.
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Finally, as the repository format is not optimized for deletion. stripping a
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changeset may be slow in some situations.
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To sum up, the strip approach is very simple but does not handle
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interaction with the outer world, which is very unfortunate for a
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*Distributed* VCS.
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.. [#] various work around exists but they require their own workflows
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   which are distinct from the very elegant basic workflow of
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   Mercurial.
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Git Approach: Overwrite Reference
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The Git approach to repository structure is a bit more complex: there
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can be any amount of unrelated changesets in a repository, and **only
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changesets referenced by a git branch** are *visible* and
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*meaningful*.
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.. warning:: add a schema::
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        C
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        | B---<foo>
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        |/
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        |
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        A
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    Only B and A are visible.
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This simplifies the process of getting rid of old changesets. You can
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just leave them in place and move the reference on the new one. You
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can then propagate this change by moving the git-branch on remote host
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with the newer version of the marker overwriting the older one.
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This approach goes a bit further but still has a major drawback:
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Because you **overwrite** the git-branch, you have no conflict
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resolution. The last to act wins. This makes collaboration on multiple
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changesets difficult because you can't merge concurrent updates on a
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changeset.
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Every overwrite is a forced operation where the operator says, "yes I
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want this to replace that". In highly distributed environments, a user
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may end up with conflicting references and no proper way to choose.
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Because of this way to visualize a repository, git-branches are a core
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part of git, which makes the user interface more complicated and
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constrains moving through history.
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Finally, even if all older changesets still exist in the repository,
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accesing them is still painful.
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The Obsolete Marker Concept
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As none of the concepts was powerful enough to fulfill the need of
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safely rewriting history, including easy sharing and collaboration on
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mutable history, we needed another one.
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Basic concept
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Every history rewriting operation stores the information that old rewritten
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changeset is replaced by newer version in a given set of changesets.
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All basic history rewriting operation can create an appropriate obsolete marker.
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.. figure:: ./figures/example-1-update.*
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    *Updating* a changeset
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    Create one obsolete marker: ``([A'] obsolete A)``
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.. figure:: ./figures/example-2-split.*
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    *Splitting* a changeset in multiple one
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    Create one obsolete marker ``([B1, B2] obsolete B)]``
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.. figure:: ./figures/simple-3-merge.*
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    *Merging* multiple changeset in a single one
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    Create two obsolete markers ``([C] obsolete A), ([C] obsolete B)``
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.. figure:: ./figures/simple-4-reorder.*
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    *Moving* changeset around
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    Reordering those two changesets need two obsolete markers:
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    ``([A'] obsolete A), ([B'] obsolete B)``
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.. figure:: ./figures/simple-5-delete.*
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    *Removing* a changeset:
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    One obselete marker ``([] obsolete B)``
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To conclude, a single obsolete marker express a relation from **0..n** new
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changesets to **1** old changeset.
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Basic Usage
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-----------------------------------------------------
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Obsolete markers create a perpendicular history: **a versioned
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changeset graph**. This means that offers the same features we have
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for versioned files but applied to changeset:
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First: we can display a **coherent view** of the history graph in which only a
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single version of your changesets is displayed by the UI.
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Second, because obsolete changeset content is still **available**. You can 
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you can
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    * **browse** the content of your obsolete commits,
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    * **compare** newer and older versions of a changeset,
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    * **restore** content of previously obsolete changesets.
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Finally, the obsolete marker can be **exchanged between
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repositories**. You are able to share the result on your history
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rewriting operations with other prople and **collaborate on the
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mutable part of the history**.
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Conflicting history rewriting operation can be detected and
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**resolved** as easily as conflicting changes on a file.
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Detecting and solving tricky situations
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-----------------------------------------------------
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History rewriting can lead to complex situations. The obsolete marker
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introduces a simple representation for this complex reality. But
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people using complex workflows will one day or another have to face
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the intrinsic complexity of some real-world situation.
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This section describes possible situations, defines precise sets of
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changesets involved in such situations and explains how the error
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cases can be resolved automatically using the available information.
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Obsolete changesets
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````````````````````
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Old changesets left behind by obsolete operation are called **obsolete**.
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With the current version of mercurial, this *obsolete* part is stripped from the
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repository before the end of every rewriting operation.
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.. figure:: ./figures/error-obsolete.*
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    Rebasing `B` and `C` on `A` (as `B'`, `C'`)
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    This rebase operation added two obsolete markers from new
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    changesets to old changesets. These two old changesets are now
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    part of the *obsolete* part of the history.
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In most cases, the obsolete set will be fully hidden to both the UI and
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discovery, hence users do not have to care about them unless they want to
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audit history rewriting operations.
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Unstable changesets
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```````````````````
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While exploring the possibilities of the obsolete marker a bit
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further, you may end up with *obsolete* changesets which have
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*non-obsolete* children. There is two common ways to achieve this:
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* Pull a changeset based of an old version of a changeset [#]_.
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* Use a partial rewriting operation. For example amend on a changeset with
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  children.
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*Non-obsolete* changeset based on *obsolete* one are called **unstable**
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.. figure:: ./figures/error-unstable.*
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    Amend `A` into `A'` leaving `B` behind.
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    In this situation we cannot consider `B` as *obsolete*. But we
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    have all the necessary data to detect `B` as an *unstable* branch
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    of the history because its parent `A` is *obsolete*. In addition,
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    we have enough data to automatically resolve this instability: we
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    know that the new version of `B` parent (`A`) is `A'`. We can
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    deduce that we should rebase `B` on `A'` to get a stable history
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    again.
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Proper warnings should be issued when part of the history becomes
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unstable. The UI will be able to use the obsolete marker to
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automatically suggest a resolution to the user of even carry them out
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for them.
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XXX details on automatic resolution for
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* movement
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* handling deletion
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* handling split on multiple head
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.. [#] For this to happen one needs to explicitly enable exchange of draft
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       changesets. See phase help for details.
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The two parts of the obsolete set
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``````````````````````````````````````
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The previous section shows that there could be two kinds of *obsolete*
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changesets:
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* an *obsolete* changeset with no or *obsolete* only descendants is called **extinct**.
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* an *obsolete* changeset with *unstable* descendants is called **suspended**.
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.. figure:: ./figures/error-extinct.*
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    Amend `A` and `C` leaving `B` behind.
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    In this example we have two *obsolete* changesets: `C` with no *unstable*
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    children is *extinct*. `A` with *unstable* descendant (`B`) is *suspended*.
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    `B` is *unstable* as before.
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Because nothing outside the obsolete set default on *extinct*
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changesets, they can be safely hidden in the UI and even garbage
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collected. *Suspended* changesets have to stay visible and available
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until their unstable descendant are rewritten into stable version.
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Conflicting rewrites
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````````````````````
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If people start to concurrently edit the same part of the history they will
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likely meet conflicting situations when a changeset has been rewritten in two
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different ways.
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.. figure:: ./figures/error-conflicting.*
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    Conflicting rewrite of `A` into `A'` and `A''`
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This kind of conflict is easy to detect with an obsolete marker
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because an obsolete changeset can have more than one new version. It
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may be seen as the multiple heads case. Mercurial warns you about this
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on pull. It is resolved the same way by a merge of A' and A'' that
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will keep the same parent than `A'` and `A''` with two obsolete
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markers pointing to both `A` and `A'`
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.. warning::  TODO: Add a schema of the resolution. (merge A' and A'' with A as
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              ancestor and graft the result of A^)
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Allowing multiple new changesets to obsolete a single one allows to
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distinguish a split changeset from a history rewriting conflict.
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Reliable history
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``````````````````````
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Obsolete markers help to smooth rewriting operation process. However
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they do not change the fact that **you should only rewrite the mutable
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part of the history**. The phase concept enforces this rule by
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explicitly defining a public immutable set of changesets. Rewriting
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operations refuse to work on public changesets, but there are still
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some corner cases where previously rewritten changesets are made
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public.
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Special rules apply for obsolete markers pointing to public changesets:
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* Public changesets are excluded from the obsolete set (public
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  changesets are never hidden or candidate to garbage collection)
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* *newer* version of a public changeset are called **latecomer** and
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  highlighted as an error case.
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Solving such an error is easy. Because we know what changeset a
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*latecomer* tries to rewrite, we can easily compute a smaller
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changeset containing only the change from the old *public* to the new
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*latecomer*.
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.. warning:: add a schema
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Conclusion
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----------------
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The obsolete marker is a powerful concept that allows mercurial to safely handle
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history rewriting operations. It is a new type of relation between Mercurial
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changesets which tracks the result of history rewriting operations.
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This concept is simple to define and provides a very solid base for:
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- Very fast history rewriting operations,
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- auditable and reversible history rewriting process,
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- clean final history,
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- sharing and collaborating on the mutable part of the history,
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- gracefully handling history rewriting conflicts,
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- various history rewriting UI's collaborating with an underlying common API.
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.. list-table:: Comparison on solution [#]_
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   :header-rows: 1
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   * - Solution
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     - Remove changeset locally
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     - Works on any point of your history
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     - Propagation
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     - Collaboration
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     - Speed
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     - Access to older version
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   * - Strip
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     - `+`
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     - `+`
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     - \
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     - \ 
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     - \ 
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     - `- -`
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   * - Reference
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     - `+`
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     - \ 
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     - `+`
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     - \ 
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     - `+`
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     - `-`
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   * - Obsolete
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Pierre-Yves David <pierre-yves.david@logilab.fr>
parents: 163
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   385
     - `+`
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Pierre-Yves David <pierre-yves.david@logilab.fr>
parents: 163
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   386
     - `+`
8f8a52cd0b9f big doc update
Pierre-Yves David <pierre-yves.david@logilab.fr>
parents: 163
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   387
     - `++`
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Pierre-Yves David <pierre-yves.david@logilab.fr>
parents: 163
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   388
     - `++`
8f8a52cd0b9f big doc update
Pierre-Yves David <pierre-yves.david@logilab.fr>
parents: 163
diff changeset
   389
     - `+`
8f8a52cd0b9f big doc update
Pierre-Yves David <pierre-yves.david@logilab.fr>
parents: 163
diff changeset
   390
     - `+`
8f8a52cd0b9f big doc update
Pierre-Yves David <pierre-yves.david@logilab.fr>
parents: 163
diff changeset
   391
8f8a52cd0b9f big doc update
Pierre-Yves David <pierre-yves.david@logilab.fr>
parents: 163
diff changeset
   392
8f8a52cd0b9f big doc update
Pierre-Yves David <pierre-yves.david@logilab.fr>
parents: 163
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   393
8f8a52cd0b9f big doc update
Pierre-Yves David <pierre-yves.david@logilab.fr>
parents: 163
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   394
.. [#] To preserve good tradition in comparison table, an overwhelming advantage
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       goes to the defended solution.