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Implementation of Obsolete Marker
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.. warning:: This document is still in heavy work in progress

Main questions about Obsolete Marker Implementation
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What data should be contained in a Marker ?
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There are two critical pieces of information that **must** be stored
in an obsolete Marker.

:object:
    the old obsoleted changeset

:replacements:
    list of new changeset. list size can be anything, including 0 (0..N)

Everybody agreed on this point.

 ---

This is probably a good idea to have an unique Identifier, for UI, transfer and
access.

    :id: same as changeset but for marker.

The field below will depend on the way we exchange obsolete marker between
changesets.

 ---

Having audit data will be very useful. When it gets messy you need all the
information available to understand the situation.

I have the feeling that we are versioning history. Therefor we will probably
need the same kind of information than when versioning Files.

:date: date of the marker creation

:user: ui.username

To go further:

:description: "Optional reason for the rewrite (generated by the user)"

:tool: the automated tool that made this

:operation: Kind of rewritting operation that created the marker (delete,
            update, split, fold, reordering), to help conflict resolution.

Matt said this is "too complicated". I'll wait for him to meet a very hairy
situation to agree that they are needed.

Leaving the door open to any addition data is an option too.

How shall we store Marker on disk
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Requirement
.............

We need to quickly load the 'object' to know the "obsolete" set.
We need quick access by object and replacements to travels along the graph.

Common Part
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The file is store in `.hg/store/obsmarkers`. It is a binary files:

The files starts with a Format Version string


Minimalistic proposal
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The core of a Marker will we stored as:

* number of replacement (8-Bytes integer)
* node id of the obsolete changeset (20-Bytes hash)
* node id of replacement changeset (20-Bytes hash x number of remplacement)

Version with ID
.........................

This version add a node id computed from the marker content. It will be present
*before* other data:

* node id of the maker (20-Bytes hash)


Version with Metadata proposal
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An extra files is used to old metadata (date, user, etc) `.hg/store/obs-extra`:.

The format of this field is undefined yet. This will add the following
field at the end of a marker

* offset of the metadata in obs-extra (8-Bytes integer)


How shall we exchange Marker over the Wire ?
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We can have a lot of markers. We do not want to exchange data for the one we
already know. Listkey() is not very appropriate there as you get everything.

Moreover, we might want to only hear about Marker that impact changeset we are
pulling.

pushkey is not batchable yet (could be fixed)

A dedicated discovery and exchange protocol seems mandatory here.


Various technical details
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Some stuff that worse to note. some may deserve their own section later.

storing old changeset
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The new general delta format allows a very efficient storage of two very similar
changesets. Storing obsolete children using general delta takes no more place
than storing the obsolete diff. Reverted file will even we reused. The whole
operation will take much less space the strip backup.


Abstraction from history rewriting UI
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How Mercurial handles obsolete marker is independent from what decides
to create them and what actual operation solves the error case. Any of
the existing history rewriting UI (rebase, mq, histedit) can lay
obsolete markers and resolve situation created by others. To go
further, a hook system of obsolete marker creation would allow each
mechanism to collaborate with other though a standard and central
mechanism.


Obsolete marker storage
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The Obsolete marker will most likely be stored outside standard
history. They are multiple reasons for this:

First, obsolete markers are really perpendicular to standard history
there is no strong reason to include it here other than convenience.

Second, storing obsolete marker inside standard history means:

* A changeset must be created every time an obsolete relation is added. Very
  inconvenient for delete operation.

* Obsolete marker must be forged at the creation of the new changeset. This
  is very inconvenient for split operation. And in general it becomes
  complicated to fix history afterward in particular when working with older
  clients.

Storing obsolete marker outside history have several pros:

* It eases Exchange of obsolete markers without unnecessary obsolete
  changeset contents.

* It allows tuning the actual storage and protocol exchange while maintaining
  compatibility with older clients through the wire (as we do the repository
  format).

* It eases the exchange of obsolete related information during
  discovery to exchange obsolete changeset relevant to conflict
  resolution. Exchanging such information deserves a dedicated
  protocol.

Persistent
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*Extinct* changeset and obsolete marker will most likely be garbage collected as
some point. However, archive server may decide to keep them forever in order to
keep a fully auditable history in its finest conception.


Current status
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An experimental implementatione exists. What have been done so far.


* 1-1 obsolete marker stored outside history,

* compute obsolete-tip

* obsolete marker exchange through pushkey,

* compute obsolete, unstable, extinct and suspended set.

* hidden extinct changesets for UI.

* Use secret phase to remove from discovery obsolete and unstable changesets (to
  be improved soon)

* alter rebase to use obsolete markers instead of stripping.

* Have an experimental mq-like extension to rewrite history (more on that later)

* Have an extension to update and mq repository according evolution of
  standard (more on that later)