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Obsolete concept

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Obsolete marker is a powerful concept that allow mercurial to safely handle

history rewriting operations. It is a new type of relation between Mercurial

changesets that track the result of history rewriting operations.

This concept is simple to define and provides a very solid base to:

- Very fast history rewriting operations,

- auditable and reversible history rewritting process,

- clean final history,

- share and collaborate on mutable part of the history,

- gracefully handle history rewriting conflict,

- allows various history rewriting UI to collaborate with a underlying common API.

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Basic concept

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Every history rewriting operation  stores the information that old rewritten

changesets has newer version available in a set of changeset.

This simple rules allows to express any possible history rewriting operation:

.. figure:: ./figures/example-1-update.png

    *Updating* a changeset

    Create one obsolete marker: ``([A'] obsolete A)``

.. figure:: ./figures/example-2-split.png

    *Splitting* a changeset in multiple one

    Create one obsolete marker ``([B1, B2] obsolete B)]``

.. figure:: ./figures/example-3-merge.png

    *Merging* multiple changeset in a single one

    Create two obsolete markers ``([C] obsolete A), ([C] obsolete B)``

.. figure:: ./figures/example-4-reorder.png

    *Moving* changeset around

    Reordering those two changesets need two obsolete markers:

    ``([A'] obsolete A), ([B'] obsolete B)``

.. figure:: ./figures/example-5-delete.png

    *Removing* a changeset:

    One obselete marker ``([] obsolete B)``

To conclude, a single obsolete marker express a relation from **0..n** new

changesets to **1** old changeset.

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Basic Usage

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Obsolete markers create a perpendicular history: **a versionned version of the

changeset graph**. This means that we can have the same feature we have for

versioned files but applied to changeset:

First: we can display a **coherent view** of the history graph with only a

single version of your changeset are displayed by the UI.

Second, because obsolete changeset content are still **available**. You can 

    * **browse** the content of your obsolete commit,

    * **compare** newer and older version of a changeset,

    * **restore** content of previously obsolete changeset.

Finally, obsolete marker can be **exchanged between repositories**. You are able to

share the result on your history rewriting operation with other and **collaborate

on mutable part of the history**.

Conflicting history rewriting operation can be detected and **resolved** as easily

as conflicting changes on file.

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Detecting and solving tricky situation

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History rewriting can lead to complex situation. Obsolete marker introduce a

simple representation this complex reality. But people using complex workflow

will one day or another you have to face the intrinsics complexity of some

situation.

This section describe possible situations, define precise set of changesets

involved in such situation and explains how error case can we automatically

resolved using available information.

obsolete changesets

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Old changesets left behind by obsolete operation are said **obsolete**.

With current version of mercurial, this *obsolete* part is stripped from the

repository before the end of every rewritting operation.

.. figure:: ./figures/error-obsolete.png

    Rebasing `B` and `C` on `A` (as `B'`, `C'`)

    This rebase operation added two obsolete markers from new changesets to old

    changesets. These Two old changesets are now part of the *obsolete* part of the

    history.

In most case the obsolete set will be fully hidden to both UI and discovery so

user do not have to care about them unless he wants to audit history rewriting

operation.

Unstable changesets

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While exploring obsolete marker possibility a bit further you way end up with

*obsolete* changeset with *non-obsolete* children. There is two common ways to

achieve this:

* Pull a changeset based of an old version of a changeset [#]_.

* Use a partial rewriting operation. For example amend on a changeset with

  childrens.

*Non-obsolete* changeset based on *obsolete* one are said **unstable**

.. figure:: ./figures/error-unstable.png

    Amend `A` into `A'` leaving `B` behind.

    In this situation we can not consider `B` as *obsolete*.  But we have all

    necessary data to detect `B` as an *unstable* branch of the history because

    its parent `A` is *obsolete*. In addition, we have enough data to

    automatically resolve this instability: we know that the new version of `B`

    parent (`A`) is `A'`, We can deduce that we should rebase `B` on `A'` to get

    a stable history again.

Proper warning should be issued when part of the history become unstable. UI

will be able to use the obsolete marker to automatically suggest resolution to

the user of even carry them out for him.

XXX details automatic resolution for

* movement

* handling deletion

* handling split on multiple head

.. [#] For this to happen one needs to explicitly enable exchange of draft

       changeset. See phase help for details.

The two part of the obsolete set

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The previous section show that it could be two kinds of *obsolete* changeset:

* *obsolete* changeset with no or *obsolete* only descendants, said **extinct**.

* *obsolete* changeset with *unstable* descendants, said **suspended**.

.. figure:: ./figures/error-extinct.png

    Amend `A` and `C` leaving `B` behind.

    In this example we have two *obsolete* changesets: `C` with no *unstable*

    children is *extinct*. `A` with *unstable* descendant (`B`) is *suspended*.

    `B` is *unstable* as before.

Because nothing outside the obsolete set default on *extinct* changesets, they

can be safely hidden in the UI and even garbage collected. *Suspended* changeset

have to stay visible and available until they unstable descendant are rewritten

in stable version.

Conflicting rewriting

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If people start to concurrently edit the same part of the history they will

likely meet conflicting situation when a changeset have been rewritten in two

different versions.

.. figure:: ./figures/error-conflicting.png

    Conflicting rewriting of `A` into `A'` and `A''`

This kind of conflict is easy to detect with obsolete marker because an obsolete

changeset have more than one new version. It may be seen as the multiple heads

case Mercurial warn you about on pull. It is resolved the same way by a merge of

A' and A'' that will keep the same parent than `A'` and `A''` with two obsolete

markers pointing to both `A` and `A'`

.. warning::  TODO: Add a schema of the resolution. (merge A' and A'' with A as

              ancestor and graft the result of A^)

Allowing multiple new changesets to obsolete a single one allow to distinct a

splitted changeset from history rewriting conflict.

Reliable history

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Obsolete marker really help to smooth rewriting operation process. However they

do not change the fact that **you should only rewrite the mutable part of the

history**. The phase concept enforce this rules by explicitly defining a

public immutable set of changeset. Rewriting operation refuse to work on

public changeset, but they is still some corner case where changesets

rewritten in the past are made public.

Special rules apply for obsolete marker pointing to public changeset

* Public changesets are excluded from the obsolete set (public changeset are

  never hidden or candidate to garbage collection)

* *newer* version of public changeset are said **latecomer** and highlighted as

  error case.

Solving such error is easy. Because we know what changeset a *latecomer* try to

rewrite, we can easily compute a smaller changeset containing only the change

from the old *public* to the new *latecomer*.

.. warning:: add a schema