.. Copyright 2014 Greg Ward <greg@gerg.ca>

----------------

Evolve: Concepts

----------------

Getting the most out of software requires an accurate understanding of

the concepts underlying it. For example, you cannot use Mercurial to

its full potential without understanding the DAG (directed acyclic

graph) of changesets and the meaning of parent/child relationships

between nodes in that graph. Mercurial with changeset evolution adds

some additional concepts to the graph of changesets. Understanding

those concepts will make you an informed and empowered user of

``evolve``.

.. note:: This document contains math! If you have a pathological fear

          of set theory and the associated notation, you might be

          better off just reading the `user guide`_. But if you

          appreciate the theoretical rigour underlying core Mercurial,

          you will be happy to know that it continues right into

          changeset evolution.

.. note:: This document is incomplete! (The formatting of the math

          isn't quite right yet, and the diagrams are missing for

          malformatted.)

This document follows standard set theory notation:

  x ∈ A: x is a member of A

  A ∪ B: union of A and B: { x | x ∈ A or x ∈ B }

  A ∖ B: set difference: { x | x ∈ A and x ∉ B }

  A ⊇ B: superset: if x ∈ B, then x ∈ A

.. _`user guide`: user-guide.html

Phases

------

First, every changeset in a Mercurial repository (since 2.3) has a

*phase*. Phases are independent of ``evolve`` and they affect

Mercurial usage with or without changeset evolution. However, they

were implemented in order to support evolution, and are a critical

foundation of ``evolve``.

Phases are strictly ordered:

  secret > draft > public

Changesets generally only move from a higher phase to a lower phase.

Typically, changesets start life in *draft* phase, and move to

*public* phase when they are pushed to a public repository. (You can

set the default phase of new commits in Mercurial configuration.)

The purpose of phases is to prevent modifying published history.

``evolve`` will therefore only let you rewrite changesets in one of

the two *mutable* phases (secret or draft).

Run ``hg help phases`` for more information on phases.

Obsolete changesets

-------------------

*Obsolescence* is they key concept at the heart of changeset

evolution. Everything else in this document depends on understanding

obsolescence. So: what does it mean for a changeset to be obsolete?

In implementation terms, there is an *obsolescence marker* associated

with changesets: every changeset is either obsolete or not.

The simplest way that a changeset becomes obsolete is by *pruning* it.

The ``hg prune`` command simply marks the specified changesets

obsolete, as long as they are mutable.

More commonly, a changeset *A* becomes obsolete by *amending* it.

Amendment creates a new changeset *A'* that replaces *A*, which is now

obsolete. *A'* is the successor of *A*, and *A* the predecessor of *A'*:

  [diagram: A and A' with pred/succ edge]

The predecessor/successor relationship forms an additional

*obsolescence graph* overlaid on top of the traditional DAG formed by

changesets and their parent/child relationships. In fact, the

obsolescence graph is second-order version control. Where the

traditional parent/child DAG tracks changes to your source code, the

obsolescence graph tracks changes to your changesets. It tracks the

evolution of your changesets.

(If you prefer a calculus metaphor to set theory, it might help to

think of the traditional parent/child DAG as the first derivative of

your source code, and the obsolescence DAG as the second derivative.)

Troubled changesets (unstable, bumped, divergent)

-------------------------------------------------

Evolving history can introduce problems that need to be solved. For

example, if you prune a changeset *P* but not its descendants, those

descendants are now on thin ice. To push a changeset to another

repository *R*, all of its ancestors must be present in *R* or pushed

at the same time. But Mercurial does not push obsolete changesets like

*P*, so it cannot push the descendants of *P*. Any non-obsolete

changeset that is a descendant of an obsolete changeset is said to be

*unstable*.

  [diagram: obsolete cset with non-obsolete descendant]

Another sort of trouble occurs when two developers, Alice and Bob,

collaborate via a shared non-publishing repository. (This is how

developers can safely `share mutable history`_.) Say Alice and Bob

both start the day with changeset *C* in *draft* phase. If Alice

pushes *C* to their public repository, then it is now published and

therefore immutable. But Bob is working from a desert island and

cannot pull this change in *C*'s phase. For Bob, *C* is still in draft

phase and therefore mutable. So Bob amends *C*, which marks it

obsolete and replaces it with *C'*. When he is back online and pulls

from the public repository, Mercurial learns that *C* is public, which

means it cannot be obsolete. We say that *C'* is *bumped*, since it is

the successor of a public changeset.

.. _`share mutable history`: sharing.html

(Incidentally, the terminology here comes from airline overbooking: if

two people have bought tickets for the same seat on a plane and they

both show up at the airport, only one of them gets on the plane. The

passenger who is left behind in the airport terminal has been

"bumped".)

The third sort of trouble is when Alice and Bob both amend the same

changeset *C* to have different successors. When this happens, the

successors are both called *divergent* (unless one of them is in

public phase; only mutable changesets are divergent).

The collective term for unstable, bumped, and divergent changeset is

*troubled*:

  troubled = unstable ∪ bumped ∪ divergent

It is possible for a changeset to be in any of the troubled categories

at the same time: it might be unstable and divergent, or bumped and

divergent, or whatever.

  [diagram: Venn diagram of troubled changesets, showing overlap]

The presence of troubled changesets indicates the need to run ``hg

evolve``.

Hidden (and visible) changesets

-------------------------------

Some obsolete changesets are *hidden*: deliberately suppressed by

Mercurial and usually not visible through the UI. (As of Mercurial

2.9, there are still some commands that inadvertently reveal hidden

changesets; these are bugs and will be fixed in due course.)

All hidden changesets are obsolete, and all obsolete changesets are

part of your repository. Mathematically speaking:

  repo ⊇ obsolete ⊇ hidden

Or, putting it visually:

  [diagram: Venn diagram showing nested strict subsets]

However, the presence of obsolete but not hidden changesets should be

temporary. The desired end state for any history mutation operation is

that all obsolete changesets are hidden, i.e.:

  repo ⊇ obsolete, obsolete = hidden

Visually:

  [diagram: Venn diagram showing obsolete = hidden, subset of repo]

Why is this changeset visible?

------------------------------

Any changeset which is not hidden is *visible*. That is,

  visible = repo ∖ hidden

(Recall that ∖ means set difference: *visible* is the set of

changesets that are in *repo* but not in *hidden*.)

After amending or pruning a changeset, you might expect it to be

hidden. It doesn't always work out that way. The precise rules are:

  hideable = obsolete

  blockers = bookmarks ∪ parents(workingcopy) ∪ localtags

  hidden = hideable ∖ ancestors((repo ∖ hideable) ∪ blockers)

This will probably be clearer with a worked example. First, here's a

repository with some obsolete changesets, some troubled changesets,

one bookmark, a working copy, and some hidden changesets:

        x-x

       /

  -o-o-o-o

     \

      x-x-o

Here's the computation required to determine which changesets are

hidden:

  repo = { 0, 1, 2, 3, 4, 5, 6, 7, 8 }

  hideable = obsolete = { 2, 4, 5, 8 }

  blockers = { 6 } ∪ { 4 } ∪ {}

  blockers = { 4, 6 }

  hidden = hideable ∖ ancestors((repo ∖ { 2, 4, 5, 8 }) ∪ { 4, 6 })

  hidden = hideable ∖ ancestors({ 0, 1, 3, 6, 7 } ∪ { 4, 6 })

  hidden = hideable ∖ ancestors({ 0, 1, 3, 4, 6, 7 })

  hidden = { 2, 4, 5, 8 } ∖ { 0, 1, 2, 3, 4, 5, 6, 7 }

  hidden = { 8 }