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Obsolescence markers make it possible to mark changesets that have been
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deleted or superseded in a new version of the changeset.
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Unlike the previous way of handling such changes, by stripping the old
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changesets from the repository, obsolescence markers can be propagated
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between repositories. This allows for a safe and simple way of exchanging
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mutable history and altering it after the fact. Changeset phases are
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respected, such that only draft and secret changesets can be altered (see
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:hg:`help phases` for details).
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Obsolescence is tracked using "obsolescence markers", a piece of metadata
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tracking which changesets have been made obsolete, potential successors for
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a given changeset, the moment the changeset was marked as obsolete, and the
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user who performed the rewriting operation. The markers are stored
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separately from standard changeset data can be exchanged without any of the
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precursor changesets, preventing unnecessary exchange of obsolescence data.
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The complete set of obsolescence markers describes a history of changeset
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modifications that is orthogonal to the repository history of file
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modifications. This changeset history allows for detection and automatic
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resolution of edge cases arising from multiple users rewriting the same part
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of history concurrently.
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Current feature status
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======================
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This feature is still in development.
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Instability
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===========
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Rewriting changesets might introduce instability.
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There are two main kinds of instability: orphaning and diverging.
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Orphans are changesets left behind when their ancestors are rewritten.
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Divergence has two variants:
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* Content-divergence occurs when independent rewrites of the same changesets
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lead to different results.
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* Phase-divergence occurs when the old (obsolete) version of a changeset
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becomes public.
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It is possible to prevent local creation of orphans by using the following config::
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[experimental]
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evolution.createmarkers = true
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evolution.exchange = true
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You can also enable that option explicitly::
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[experimental]
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evolution.createmarkers = true
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evolution.exchange = true
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evolution.allowunstable = true
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