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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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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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