docs/concepts.rst
author Anton Shestakov <av6@dwimlabs.net>
Mon, 03 Dec 2018 14:15:00 +0800
changeset 4302 8e9940f1ae56
parent 1288 c3ecf6871872
child 4618 803d32f4e498
permissions -rw-r--r--
topic: simplify _showlasttouched and _getlasttouched (and its data structure)
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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.. Copyright 2014 Greg Ward <greg@gerg.ca>
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----------------
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Evolve: Concepts
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----------------
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Getting the most out of software requires an accurate understanding of
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the concepts underlying it. For example, you cannot use Mercurial to
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its full potential without understanding the DAG (directed acyclic
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graph) of changesets and the meaning of parent/child relationships
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between nodes in that graph. Mercurial with changeset evolution adds
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some additional concepts to the graph of changesets. Understanding
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those concepts will make you an informed and empowered user of
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``evolve``.
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.. note:: This document contains math! If you have a pathological fear
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          of set theory and the associated notation, you might be
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          better off just reading the `user guide`_. But if you
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          appreciate the theoretical rigour underlying core Mercurial,
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          you will be happy to know that it continues right into
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          changeset evolution.
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.. note:: This document is incomplete! (The formatting of the math
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          isn't quite right yet, and the diagrams are missing for
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          malformatted.)
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This document follows standard set theory notation::
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  x ∈ A: x is a member of A
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  A ∪ B: union of A and B: { x | x ∈ A or x ∈ B }
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  A ∖ B: set difference: { x | x ∈ A and x ∉ B }
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  A ⊇ B: superset: if x ∈ B, then x ∈ A
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.. _`user guide`: user-guide.html
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Phases
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------
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First, every changeset in a Mercurial repository (since 2.3) has a
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*phase*. Phases are independent of ``evolve`` and they affect
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Mercurial usage with or without changeset evolution. However, they
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were implemented in order to support evolution, and are a critical
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foundation of ``evolve``.
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Phases are strictly ordered:
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  secret > draft > public
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Changesets generally only move from a higher phase to a lower phase.
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Typically, changesets start life in *draft* phase, and move to
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*public* phase when they are pushed to a public repository. (You can
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set the default phase of new commits in Mercurial configuration.)
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The purpose of phases is to prevent modifying published history.
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``evolve`` will therefore only let you rewrite changesets in one of
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the two *mutable* phases (secret or draft).
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Run ``hg help phases`` for more information on phases.
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Obsolete changesets
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-------------------
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*Obsolescence* is they key concept at the heart of changeset
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evolution. Everything else in this document depends on understanding
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obsolescence. So: what does it mean for a changeset to be obsolete?
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In implementation terms, there is an *obsolescence marker* associated
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with changesets: every changeset is either obsolete or not.
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The simplest way that a changeset becomes obsolete is by *pruning* it.
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The ``hg prune`` command simply marks the specified changesets
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obsolete, as long as they are mutable.
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More commonly, a changeset *A* becomes obsolete by *amending* it.
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Amendment creates a new changeset *A'* that replaces *A*, which is now
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obsolete. *A'* is the successor of *A*, and *A* the predecessor of *A'*:
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  [diagram: A and A' with pred/succ edge]
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The predecessor/successor relationship forms an additional
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*obsolescence graph* overlaid on top of the traditional DAG formed by
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changesets and their parent/child relationships. In fact, the
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obsolescence graph is second-order version control. Where the
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traditional parent/child DAG tracks changes to your source code, the
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obsolescence graph tracks changes to your changesets. It tracks the
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evolution of your changesets.
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(If you prefer a calculus metaphor to set theory, it might help to
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think of the traditional parent/child DAG as the first derivative of
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your source code, and the obsolescence DAG as the second derivative.)
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Troubled changesets (unstable, bumped, divergent)
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-------------------------------------------------
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Evolving history can introduce problems that need to be solved. For
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example, if you prune a changeset *P* but not its descendants, those
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descendants are now on thin ice. To push a changeset to another
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repository *R*, all of its ancestors must be present in *R* or pushed
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at the same time. But Mercurial does not push obsolete changesets like
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*P*, so it cannot push the descendants of *P*. Any non-obsolete
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changeset that is a descendant of an obsolete changeset is said to be
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*unstable*.
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  [diagram: obsolete cset with non-obsolete descendant]
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Another sort of trouble occurs when two developers, Alice and Bob,
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collaborate via a shared non-publishing repository. (This is how
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developers can safely `share mutable history`_.) Say Alice and Bob
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both start the day with changeset *C* in *draft* phase. If Alice
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pushes *C* to their public repository, then it is now published and
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therefore immutable. But Bob is working from a desert island and
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cannot pull this change in *C*'s phase. For Bob, *C* is still in draft
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phase and therefore mutable. So Bob amends *C*, which marks it
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obsolete and replaces it with *C'*. When he is back online and pulls
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from the public repository, Mercurial learns that *C* is public, which
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means it cannot be obsolete. We say that *C'* is *bumped*, since it is
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the successor of a public changeset.
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.. _`share mutable history`: sharing.html
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(Incidentally, the terminology here comes from airline overbooking: if
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two people have bought tickets for the same seat on a plane and they
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both show up at the airport, only one of them gets on the plane. The
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passenger who is left behind in the airport terminal has been
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"bumped".)
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The third sort of trouble is when Alice and Bob both amend the same
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changeset *C* to have different successors. When this happens, the
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successors are both called *divergent* (unless one of them is in
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public phase; only mutable changesets are divergent).
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The collective term for unstable, bumped, and divergent changeset is
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*troubled*::
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  troubled = unstable ∪ bumped ∪ divergent
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It is possible for a changeset to be in any of the troubled categories
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at the same time: it might be unstable and divergent, or bumped and
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divergent, or whatever.
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  [diagram: Venn diagram of troubled changesets, showing overlap]
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The presence of troubled changesets indicates the need to run ``hg
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evolve``.
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Hidden (and visible) changesets
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-------------------------------
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Some obsolete changesets are *hidden*: deliberately suppressed by
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Mercurial and usually not visible through the UI. (As of Mercurial
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2.9, there are still some commands that inadvertently reveal hidden
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changesets; these are bugs and will be fixed in due course.)
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All hidden changesets are obsolete, and all obsolete changesets are
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part of your repository. Mathematically speaking::
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  repo ⊇ obsolete ⊇ hidden
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Or, putting it visually:
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  [diagram: Venn diagram showing nested strict subsets]
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However, the presence of obsolete but not hidden changesets should be
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temporary. The desired end state for any history mutation operation is
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that all obsolete changesets are hidden, i.e.:
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  repo ⊇ obsolete, obsolete = hidden
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Visually:
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  [diagram: Venn diagram showing obsolete = hidden, subset of repo]
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Why is this changeset visible?
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------------------------------
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Any changeset which is not hidden is *visible*. That is, ::
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  visible = repo ∖ hidden
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(Recall that ∖ means set difference: *visible* is the set of
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changesets that are in *repo* but not in *hidden*.)
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After amending or pruning a changeset, you might expect it to be
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hidden. It doesn't always work out that way. The precise rules are::
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  hideable = obsolete
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  blockers = bookmarks ∪ parents(workingcopy) ∪ localtags
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  hidden = hideable ∖ ancestors((repo ∖ hideable) ∪ blockers)
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This will probably be clearer with a worked example. First, here's a
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repository with some obsolete changesets, some troubled changesets,
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one bookmark, a working copy, and some hidden changesets::
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        x-x
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       /
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  -o-o-o-o
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     \
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      x-x-o
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Here's the computation required to determine which changesets are
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hidden::
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  repo = { 0, 1, 2, 3, 4, 5, 6, 7, 8 }
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  hideable = obsolete = { 2, 4, 5, 8 }
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  blockers = { 6 } ∪ { 4 } ∪ {}
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  blockers = { 4, 6 }
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  hidden = hideable ∖ ancestors((repo ∖ { 2, 4, 5, 8 }) ∪ { 4, 6 })
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  hidden = hideable ∖ ancestors({ 0, 1, 3, 6, 7 } ∪ { 4, 6 })
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  hidden = hideable ∖ ancestors({ 0, 1, 3, 4, 6, 7 })
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  hidden = { 2, 4, 5, 8 } ∖ { 0, 1, 2, 3, 4, 5, 6, 7 }
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  hidden = { 8 }