docs/obs-implementation.rst
author Pierre-Yves David <pierre-yves.david@logilab.fr>
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Implementation of Obsolete Marker
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.. warning:: This document is still in heavy work in progress
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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
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already know. Listkey() is not very appropriate there as you get everything.
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Moreover, we might want to only hear about Marker that impact changeset we are
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pulling.
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pushkey is not batchable yet (could be fixed)
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A dedicated discovery and exchange protocol seems mandatory here.
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Various technical details
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Some stuff that worse to note. some may deserve their own section later.
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storing old changeset
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The new general delta format allows a very efficient storage of two very similar
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changesets. Storing obsolete children using general delta takes no more place
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than storing the obsolete diff. Reverted file will even we reused. The whole
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operation will take much less space the strip backup.
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Abstraction from history rewriting UI
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How Mercurial handles obsolete marker is independent from what decides
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to create them and what actual operation solves the error case. Any of
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the existing history rewriting UI (rebase, mq, histedit) can lay
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obsolete markers and resolve situation created by others. To go
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further, a hook system of obsolete marker creation would allow each
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mechanism to collaborate with other though a standard and central
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mechanism.
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Obsolete marker storage
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The Obsolete marker will most likely be stored outside standard
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history. They are multiple reasons for this:
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First, obsolete markers are really perpendicular to standard history
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there is no strong reason to include it here other than convenience.
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Second, storing obsolete marker inside standard history means:
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* A changeset must be created every time an obsolete relation is added. Very
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  inconvenient for delete operation.
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* Obsolete marker must be forged at the creation of the new changeset. This
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  is very inconvenient for split operation. And in general it becomes
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  complicated to fix history afterward in particular when working with older
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  clients.
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Storing obsolete marker outside history have several pros:
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* It eases Exchange of obsolete markers without unnecessary obsolete
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  changeset contents.
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* It allows tuning the actual storage and protocol exchange while maintaining
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  compatibility with older clients through the wire (as we do the repository
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  format).
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* It eases the exchange of obsolete related information during
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  discovery to exchange obsolete changeset relevant to conflict
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  resolution. Exchanging such information deserves a dedicated
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  protocol.
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Persistent
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*Extinct* changeset and obsolete marker will most likely be garbage collected as
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some point. However, archive server may decide to keep them forever in order to
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keep a fully auditable history in its finest conception.
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Current status
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Obsolete marker are partialy in core.
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2.3:
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- storage over obsolete marker
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- exchange suing pushkey
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- extinct changeset are properly hidden
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- extinct changeset are excluded from exchange