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Implementation of Obsolete Marker

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.. warning:: This document is still in heavy work in progress

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 allow a very efficient storage of two very similar

changesets. Storing obsolete childrens 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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rewriting UI (rebase, mq, histedit) can lay obsolete marker and resolve

situation created by other. 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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Obsolete marker will most likely be stored outside standard history. They are

multiple reasons for that:

First, obsolete markers are really perpendicular to standard history there is not

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 become

  complicated to fix history afterward in particular when working with older

  client.

Storing obsolete marker outside history have several pro:

* It ease Exchange of obsolete marker without unnecessary obsolete changeset content

* It allow tuning the actual storage and protocol exchange while maintaining

  compatibility with older client through the wire (as we do the repository

  format)

* ease the exchange of obsolete related information during discovery to exchange

  obsolete changeset relevant to conflict resolution. Exchanging such

  information deserve 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 it's finest conception.

Current status

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

* compute obsolete, unstable, extinct and suspended set.

* hidden extinct changesets for UI.

* alter rebase to use obsolete marker instead of stripping. (XXX break --keep for now)

* 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)