Differences between revisions 22 and 27 (spanning 5 versions)
Revision 22 as of 2015-06-10 20:02:21
Size: 9064
Editor: AugieFackler
Comment:
Revision 27 as of 2015-10-23 18:43:25
Size: 11239
Comment:
Deletions are marked like this. Additions are marked like this.
Line 19: Line 19:
=== Goals ===

The main challenges to get such a feature right are:

- '''Life cycle''':
  Grouping changes and naming the topic branch is usually only relevant while working to get the feature done. We need a natural/efficient way for the topic to fade away when feature are complete. [augie notes that it might be interesting/useful to be able to query the topic name after the topic is done]

- '''Distributed''':
  A good solution for topic branch should work well in a distributed environment, where users may be pulling from each other in arbitrary ways. Topic information must be exchanged at the same time as the commits they reference.

- '''Clearly defined set of changes''':
  A topic is usually composed of multiple changesets. Being able to easily define what is in the set is important for commands that handle topics as a whole (eg histedit and email.) Having a good mechanism for this will probably help produce a better UI in evolution as well. Having a defined set of changesets is also needed when only part of the topic is pushed or pulled. A topic for a change can't be derived from its topological branch and a bookmark because sometimes topics share a root, and sometimes exploration leads to multiple heads on a topic. [TODO(marmoute: work with augie to explain this sentence) It can also be confusing when a topic is rebased.]

- '''Anonymous branching''':
  Anonymous branches is a useful feature of Mercurial that adds flexibility and improves productivity. We should keep this strength available within each topic.

- '''name conflict handling''':
  Fixing a bad name (or people fighting over a name) should not result in a very complicated situation (particularly around divergence.)

=== Goals Under Debate ===

- '''Tracking/Target''':
  People typically make changes with the goal of getting them integrated into a specific line of development (default, stable, version 4.2, staging, etc). This "target" should be the default destination for merge, rebase, update, etc. This aspect can probably use (or maybe need) integration with the life cycle.

    ''augie notes that he doesn't like the conceptual complexity this introduces''


=== Current shortcomings of bookmarks ===

This plan is not meant as "bookmarks are doomed, lets do something else." The idea here is more driven by "we have some long standing issues with bookmarks, lets think again from scratch and see what emerges." We'll reconcile topics with bookmarks at a later date, once we know what we want. It's entirely possible that bookmarks can grow bits of new functionality and become topics.

There is currently an experimentation around the idea of "remote bookmarks" [TODO(augie): link to a remote bookmarks wiki page] and wider workflow changes, it will be "evaluated" independently. (it's a wiki, please feel free to update the content of this section)

||requirements || in-core || remote ||
||Life cycle || poor || poor ||
||distributed || okay || bad ||
||defined set || poor || poor ||
||partial exch || bad || okay ||
||branching || bad || bad ||
||conflict || okay || okay ||
||tracking || bad || good ||

The lifecycle of bookmarks is problematic because we still have not figured out a good way to handle deletion and renaming. Once in the wild, it is very hard to get rid of a feature related bookmark. [augie and marmoute are not sure if remote-bookmarks improves that.]

Bookmark behavior in the '''distributed''' area is okay-ish for the in-core bookmark. The current behavior on pull works in a distributed way: it will exchange any bookmark along with the matching changeset. Behavior on push is problematic as the local bookmark may not be pushed (Mercurial may warn about it, but this is not reliable.)

[TODO(marmoute): work with augie to clarify this sentence] The 'remote-boomarks' change is more problematic in this aspect as remote name are not propagated so changesets can get exchanged without there topic information.

'''tracking''' is not covered at all by current core version. It is introduced with 'remote-bookmark' but only covers rebase. Additionally, the UI to configure and observe tracking is unclear to [marmoute] yet.

A bookmark can implicitly '''define a set of revisions''' since everything 'only' under that bookmark it can be considered in the topic. This has issues:

 * Requires the use of bookmarks for the main branches
 * Misbehaves if a some part of the topic is shared with another bookmark
 * No way of handling of extra anonymous heads on the same topic

Because they refer to a single changeset at the top of the stack, bookmark are bad at '''partial exchange'''. It is often practical to push or to pull only a part of the topic because the rest is not ready yet. Because the bookmark have no "start" the shared changesets are pulled anonymously in this case.

For the same reason (referring to a single revision), bookmarks do not allow for experimental branches within a topic label.

Divergent bookmarks provide a solution for '''conflict'''. However it does not handle rewinding a bookmark or deletion/recreation cycles.
== Current Target ==

This describe the target semantic and behavior for topics. Of course some adjustement can be built.

=== General semantics ===

TL;DR; topic are an extra "light-branch" data relevant to draft changesets.

 * Topic is a ''name'' explicitly attached to changesets,

 * This Topic data is primarily meant to categorize ''draft changeset'' and fade out when things become public,

 * Changeset have both a ''topic'' and a 'branch'. The ''topic'' allow to gather related in progress work, while the ''branch'' data refer to the long terms line of development.

 * Behaviors focus on ensuring any ''name'' have a single head.

 * Behaviors related to named branches behave mostly as if the draft-with topic are not on the branch (yet).

 * Behaviors within a topic are similar (with minor sensible difference) to named branches one within the topic.

=== General effect on named branch ===

Changeset with topic are ''only aspiring'' to be part of the named branch, but
not fully in that branch yet. When you have branch `foo` name `foo` is resolved
to the heads of `foo` with no topic.

There is a couple of examples:

{{{#!dot
digraph {
  rankdir=LR
  node [shape=box,style=filled]
  A -> B -> C;
  B -> X -> Y;
  A [label="A\n(b: foo)"];
  B [label="B\n(b: foo)"];
  C [label="C\n(b: foo)"];
  X [label="X\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
  Y [label="Y\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
}
}}}

Multiple heads are not longer ambiguous,

 * The name `foo` resolve to C,
 * The name `bar` resolve to Y,

----

{{{#!dot
digraph {
  rankdir=LR
  node [shape=box,style=filled]
  A -> B;
  B -> X -> Y;
  A [label="A\n(b: foo)"];
  B [label="B\n(b: foo)"];
  X [label="X\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
  Y [label="Y\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
}
}}}

Partial data does not have radical change on the definition,

 * The name `foo` resolve to B,
 * The name `var` resolve to Y,

----

{{{#!dot
digraph {
  rankdir=LR
  node [shape=box,style=filled]
  A -> X -> B;
  A [label="A\n(b: foo)"];
  X [label="X\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
  B [label="B\n(b: foo)"];
}
}}}

Usual traversal rules apply:

 * The name `foo` resolve to B,
 * The name `var` resolve to X,

=== Behavior for update ===

(This implies change in hg update behavior (but are not super relevant))


==== Case 1: active branch ====

{{{#!dot
digraph {
  rankdir=LR
  node [shape=box,style=filled]
  A -> B -> C;
  B -> X -> Y;
  A [label="A\n(b: foo)\n(wdir)",fillcolor="red"];
  B [label="B\n(b: foo)"];
  C [label="C\n(b: foo)"];
  X [label="X\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
  Y [label="Y\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
}
}}}


Currently active: branch `foo`', no active topic. Running `hg update` bring you on the head of foo.

* Currently active branch: `foo`
* Currently active topic: ø
* Running `hg update` bring you on the head of `foo` branch (untopiced-head).

{{{#!dot
digraph {
  rankdir=LR
  node [shape=box,style=filled]
  A -> B -> C;
  B -> X -> Y;
  A [label="A\n(b: foo)"];
  B [label="B\n(b: foo)"];
  C [label="C\n(b: foo)\n(wdir)",fillcolor="red"];
  X [label="X\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
  Y [label="Y\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
}
}}}

----

==== Case 2: active topic ====

{{{#!dot
digraph {
  rankdir=LR
  node [shape=box,style=filled]
  A -> B -> C;
  B -> X -> Y;
  A [label="A\n(b: foo)"];
  B [label="B\n(b: foo)"];
  C [label="C\n(b: foo)"];
  X [label="X\n(b: foo)\n(t: bar)\n(wdir)",fillcolor="red"];
  Y [label="Y\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
}
}}}


* Currently active branch `foo`
* Currently active topic: `bar`
* Running `hg update` bring you on the head of `bar` topic.

{{{#!dot
digraph {
  rankdir=LR
  node [shape=box,style=filled]
  A -> B -> C;
  B -> X -> Y;
  A [label="A\n(b: foo)\n"];
  B [label="B\n(b: foo)"];
  C [label="C\n(b: foo)(wdir)",fillcolor="red"];
  X [label="X\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
  Y [label="Y\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
}
}}}

==== Case 3: active topic, lagging behind ====

`W` was on topic `bar`, but it is now public so `bar` does not apply to `W`. However, the
working copy is still having an active topic.

{{{#!dot
digraph {
  rankdir=LR
  node [shape=box,style=filled]
  W -> B -> C;
  B -> X -> Y;
  W [label="W\n(b: foo)\n(faded-t: bar)\n(wdir)",fillcolor="red"];
  B [label="B\n(b: foo)"];
  C [label="C\n(b: foo)"];
  X [label="X\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
  Y [label="Y\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
}
}}}

* Currently active branch: `foo`
* Currently active topic: `bar`
* Running `hg update` bring you on the head of `bar` topic.

{{{#!dot
digraph {
  rankdir=LR
  node [shape=box,style=filled]
  W -> B -> C;
  B -> X -> Y;
  W [label="W\n(b: foo)"];
  B [label="B\n(b: foo)"];
  C [label="C\n(b: foo)"];
  X [label="X\n(b: foo)\n(t: bar)",fillcolor="lightblue"];
  Y [label="Y\n(b: foo)\n(t: bar)\n(wdir)",fillcolor="red"];
}
}}}

Classical case for getting in that situation is to `hg up bar` that bring you on
`W` with `bar` active, then pull that bring you `B`, `C`, `X`, `Y` and turn `W`
public.

The intent here is to work on topic `bar` and `hg update` should make
you up-to-date in the `bar` context.

'''Alternatively''' we could requires more data from the user.

==== Case 4: active topic, topic is closed ====

`W` was on topic `bar`, but it is now public so `bar` does not apply to `W`
anymore. However, the working copy is still having an active topic.

{{{#!dot
digraph {
  rankdir=LR
  node [shape=box,style=filled]
  X -> B -> C;
  W [label="W\n(b: foo)\n(faded-t: bar)\n(wdir)",fillcolor="red"];
  B [label="B\n(b: foo)"];
  C [label="C\n(b: foo)"];
}
}}}

* Currently active branch: `foo`
* Currently active topic: `bar`
* Running `hg update` bring you on the head of `foo` branch.
* The `bar`is de-activated

{{{#!dot
digraph {
  rankdir=LR
  node [shape=box,style=filled]
  W -> B -> C;
  W [label="W\n(b: foo)"];
  B [label="B\n(b: foo)"];
  C [label="C\n(b: foo)\n(wdir)",fillcolor="red"];
}
}}}

Classical case for getting in that situation is to be working on topic `bar`
then `bar` got 'accepted' and get public. As you former `bar` is not the new
head of `foo`, people merge/rebase and publish more changesets on top of that.

In that case inferring that you topic is "done" and being up to date mean "bring
me to the latest of my branch seems to make sense.

A possible issue, `hg update; hg pull` can provide different from `hg pull; hg
update`, in the case where `hg update` is bringing changeset on the `bar` topic
again.

'''Alternatively''' we could not update, pointing at a command to disable the
topic.

=== Behavior for merge ===

This assume `hg rebase` and `hg merge` to be identical,

=== Behavior for push to publishing (default repo) ===

=== Behavior for push to non-publisehd repo ===

=== Stacked diffs workflow ===

=== User Transition ===

== Pro and Cons ==

== Other questions ==
Line 83: Line 293:
This is a list of idea that emerged while brainstorming. This is a list of idea that emerged while brainstorming. This served as base for the current things.
Line 128: Line 338:
== See also ==

 * FeatureBranchesStruggle

Note:

This page is primarily intended for developers of Mercurial.

Topic Plan

A (speculative) plan for topic branching that would work more seamlessly with common Mercurial workflows. Still very early prototype stage. Everything is subject to change.

1. Problem Statement

The Mercurial community has been struggling for years to define a nice way to handle 'topic' branches (sometimes also called 'feature' branches), especially when it comes to sharing them with other people (mainly for code review or other collaboration.)

Bookmarks are a clone of git's refs, which seems to work more poorly in Mercurial than they do in Git, in part because the synchronization parts of bookmarks aren't really done. Adding the remaining bits of git's refs to Mercurial has been controversial, and may represent enough of a behavior change that it's infeasible.

Named branches are visible forever in the revision history, which makes them unsuitable for feature branch work as the feature branch names rapidly pollute the output of things like hg branches.

2. Current Target

This describe the target semantic and behavior for topics. Of course some adjustement can be built.

2.1. General semantics

TL;DR; topic are an extra "light-branch" data relevant to draft changesets.

  • Topic is a name explicitly attached to changesets,

  • This Topic data is primarily meant to categorize draft changeset and fade out when things become public,

  • Changeset have both a topic and a 'branch'. The topic allow to gather related in progress work, while the branch data refer to the long terms line of development.

  • Behaviors focus on ensuring any name have a single head.

  • Behaviors related to named branches behave mostly as if the draft-with topic are not on the branch (yet).
  • Behaviors within a topic are similar (with minor sensible difference) to named branches one within the topic.

2.2. General effect on named branch

Changeset with topic are only aspiring to be part of the named branch, but not fully in that branch yet. When you have branch foo name foo is resolved to the heads of foo with no topic.

There is a couple of examples:

Multiple heads are not longer ambiguous,

  • The name foo resolve to C,

  • The name bar resolve to Y,


Partial data does not have radical change on the definition,

  • The name foo resolve to B,

  • The name var resolve to Y,


Usual traversal rules apply:

  • The name foo resolve to B,

  • The name var resolve to X,

2.3. Behavior for update

(This implies change in hg update behavior (but are not super relevant))

2.3.1. Case 1: active branch

Currently active: branch foo', no active topic. Running hg update bring you on the head of foo.

* Currently active branch: foo * Currently active topic: ø * Running hg update bring you on the head of foo branch (untopiced-head).


2.3.2. Case 2: active topic

* Currently active branch foo * Currently active topic: bar * Running hg update bring you on the head of bar topic.

2.3.3. Case 3: active topic, lagging behind

W was on topic bar, but it is now public so bar does not apply to W. However, the working copy is still having an active topic.

* Currently active branch: foo * Currently active topic: bar * Running hg update bring you on the head of bar topic.

Classical case for getting in that situation is to hg up bar that bring you on W with bar active, then pull that bring you B, C, X, Y and turn W public.

The intent here is to work on topic bar and hg update should make you up-to-date in the bar context.

Alternatively we could requires more data from the user.

2.3.4. Case 4: active topic, topic is closed

W was on topic bar, but it is now public so bar does not apply to W anymore. However, the working copy is still having an active topic.

* Currently active branch: foo * Currently active topic: bar * Running hg update bring you on the head of foo branch. * The baris de-activated

Classical case for getting in that situation is to be working on topic bar then bar got 'accepted' and get public. As you former bar is not the new head of foo, people merge/rebase and publish more changesets on top of that.

In that case inferring that you topic is "done" and being up to date mean "bring me to the latest of my branch seems to make sense.

A possible issue, hg update; hg pull can provide different from `hg pull; hg update, in the case where hg update is bringing changeset on the bar` topic again.

Alternatively we could not update, pointing at a command to disable the topic.

2.4. Behavior for merge

This assume hg rebase and hg merge to be identical,

2.5. Behavior for push to publishing (default repo)

2.6. Behavior for push to non-publisehd repo

2.7. Stacked diffs workflow

2.8. User Transition

3. Pro and Cons

4. Other questions

5. Open ideas

This is a list of idea that emerged while brainstorming. This served as base for the current things.

  • Topic could be a property attached to each changeset (grouping them by similar topic)
  • Topic could fade away when changesets become public (either archived or plain dropped)
    • A benefit of archiving them is that users can query for topics, eg you could say hg log -r topic(issue123) which would help

  • Tracking could be achieved through the naming scheme. eg:
    • 'default//feature-foo' would be a topic 'feature-foo' tracking the 'default' branch.
    • 'stable//issue4700' would be a topic 'issue4700' tracking branch stable.
    • '@//feature-bar' would be a topic 'feature-bar' tracking bookmark '@' ?
    • 'stable//issue4689//issue4700' would be a topic 'issue4700' tracking the topic 'stable//issue4689'. When topic 'issue4686' face away (because published), the tracking fallback to 'stable'.
  • Topics could be non contiguous (mpm idea) feature-foo -> fix-bar -> feature-foo. Allowing a streamlined work that is automatically split apart after that.

  • Topics could be hierarchical 'issue4700.test' 'issue4700.preparation', activation//reference could be done at any level 'issue4700' or 'issue4700' (this could help handle branching/different approach)
  • pushing a new head on a new topic to a non-publishing server would be allowed.

    • that is, it'd be legal to have one head per topic on a non-publishing server.
  • A changeset could maybe have multiple topic.
    • Augie doesn't feel great about this option just because of UI complexity.
  • Users can name patches (in a sense) without mq
    • One of the major complaints about evolve from veteran mq users is that their patches no longer have explicit names. Topics provide a potential way to name patches again.

6. Current Implementation

Assign topics to non-public changesets. A topic is like a named branch, in that it is a label stored in a changeset's extra, but that topics just disappear when the change moves to public phase (the data still exists, it's just not shown.)

Code is available at http://hg.durin42.com/hg-topic-experiment/.

6.0.1. Non-Goals

  • Topics are not suitable for long term branches. We have named branches for that (and possibly also bookmarks, depending on workflow.)
  • Topics are not suitable for tracking a moving point in public history. This seems to be a perfect fit for bookmarks.

6.1. Open Questions

  • Right now we use changeset extra for storing the topic. That might lead to bonus divergence problems. They might be easily fixed, but should we avoid that?
  • Should changesets be allowed multiple topics?
  • How permissive should we be on topic names?

7. See also


CategoryDeveloper and CategoryNewFeatures

TopicPlan (last edited 2020-02-13 17:58:39 by Pierre-YvesDavid)