States Plan

This page is intended for developers

A set of proposed Mercurial features to cleanly and safely handle mutable history.

1. Introduction

This page aims to define features connected with the 'liquid hg' discussion in enough detail that they can be fine-tuned and implemented.

They are intentionally presented in a proposed order of implementation.

2. Hidden changesets

Various features will require changesets to be hidden by default. So it makes sense to implement a centralized method of dealing with this property.

Usage:

Hidden changesets can be shown with the --hidden flag, eg 'hg log --hidden' will show all changesets.

Implementation:

The changelog object contains a _hidden set (using revision numbers) that log and other commands consult. To hide a changeset, add it to the set at startup. Contexts are given a .hidden() predicate that consults the set.

3. Changeset states

A 'changeset state' is an indicator that tells us how a changeset is manipulated and communicated. The details of each state is described below, here we describe their shared properties.

Like bookmarks, states are not stored in history and thus are not permanent and leave no audit trail.

First, no changeset can be in two states at once. States are also ordered, so they can be considered from lowest to highest. The default, lowest state is 'frozen' - this is the normal state of existing changesets. No child changeset can be in a lower state than its parents.

The proposed states are:

frozen < liquid < local < dead

These states share a hierarchy of traits:

These names are subject to change. Some of them have initials that collide with each other and with options like force and they don't have an obvious progression.

3.1. Usage

States are manipulated via the 'hg state' command, ie 'hg state -f x' to mark x as frozen. Each state puts its own constraints on how it can be manipulated. The current state of changesets is displayed in the log with a 'state:' header. The frozen state is not displayed at all.

There are corresponding revset predicates for each state as well.

3.2. Implementation

Contexts provide a state() method that returns the current state as a string. It may be useful to represent the frozen state with the empty string.

4. Frozen changesets

Frozen changesets are changesets that are considered permanently immutable. This matches the history model presented by legacy Mercurial: changesets cannot be changed or removed without using history editing features from extensions. This state can be thought of as 'published': once a changeset is published, it becomes very difficult to remove it from distributed history.

4.1. Usage

Changeset are moved to the frozen state via 'hg state --frozen X'.

4.2. Implementation

N/A

4.3. Legacy clients

N/A

5. Liquid changesets

Liquid changesets are changesets that the user is permitted to use history-modifying operations like rebase or mq on. They may not be tagged. They may also be thought of as 'unpublished'.

Various operations such as pushing to certain public servers will move changesets into the frozen state. Changesets cannot be moved from frozen to liquid without a forcing operation.

The liquidity of changesets can be communicated between compatible servers and clients. This allows people to collaborate on work in progress before it becomes finalized.

This should generally be engineered such that users don't have to give any additional thought to liquid vs frozen in their day-to-day usage.

5.1. Usage

Changesets must be in a liquid state when they are created. So new commits will start in a liquid state by default.

5.2. Implementation

The set of liquid changesets is stored as a list of liquid roots. All descendants of these roots are liquid (or in a higher state). This set is known as the 'liquid barrier' and defines the 'liquid set'. This barrier is intended only to advance.

The liquid barrier is communicated via the pushkey protocol to servers that support it. The client is responsible for advancing the barrier on both the client and server sides. On each operation, the client reduces the liquid set on both sides to the intersection of the sets on the client and server. That is, if a changeset is frozen on either side, it becomes frozen on both sides.

Some servers are configured as 'publishing servers'. Legacy servers are publishing servers by default. These are recognized by not having the 'liquid' pushkey namespace. When pushing to a publishing server, all pushed changesets are moved into the frozen state on the client. Similarly, all changesets pulled from a publishing server are treated as frozen.

5.3. Legacy clients

See above for pushing to legacy clients. Legacy clients are allowed to pull liquid changesets, though a round trip will make them frozen.

6. Local changesets

Local changesets are changesets that are not visible to remote clients. This is useful to mark work private and to avoid inadvertently publishing changesets.

6.1. Usage

Local changesets (and their descendents) can be marked with 'hg state --local'.

6.2. Implementation

Like liquid changesets, local changesets are implemented via a local barrier set. This set is used to filter changesets from remote clients for push/pull/hgweb. This is probably best implemented via a _local set on changelog.

6.3. Legacy clients

Legacy clients cannot see local changesets so will not pull them. New clients will not push local changesets by default

7. Dead changesets

Dead changesets are changesets that are hidden and are eligible for garbage collection.

Dead changesets themselves are never pushed or pulled between clients (they are a subset of local) but deadness of changesets can be communicated between clients.

If a client has a dead changeset X and happens to pull a remote changeset Y that is a descendant of it, changeset X becomes liquid or frozen as appropriate.

Garbage collection consists of stripping all dead changesets. When garbage collection occurs is currently undefined.

7.1. Usage

Commits (and their descendents) can be manually marked dead with 'hg state --dead'. Some operations like strip, qpop, and rebase may choose to mark changesets dead rather than actually stripping them.

7.2. Implementation

The dead set is stored as a complete list of all revisions. This allows dead status to be propagated after garbage collection occurs. Dead status is communicated via pushkey.

7.3. Legacy clients

Like local changesets, old clients cannot see dead changesets.

8. Abandoned changesets

Abandoned changesets are changesets that have been marked as 'no longer relevant'. Like dead changesets, they are hidden, but abandoned changesets are implemented via markers in history so they are not part of the states concept.

Abandoned changesets are not pushed or pulled (see the implementation of local) but are not subject to garbage collection.

Abandoned changesets can be unabandoned by committing new descendants.

8.1. Usage

To mark a branch as abandoned, use 'hg commit --abandon' to abandon the working directory and its descendants.

8.2. Implementation

The abandon commit contains an explicit list of abandoned changesets.

8.3. Legacy clients

Old clients see abandoned changesets and their marker commits as normal changesets.

9. Obsolete changesets

Obsolete changesets are part of an advanced concept ('changeset evolution') used to automatically resolve and combine refactoring operations between collaborators using liquid changesets. Like dead changessets, these are also hidden, but obsolete markers are implemented as pointers from X' (new changeset) to X (obsolete changeset) so are again not part of the state concept.

10. naming states

constraints:

• all be on an obvious continuum
• not have conflicting initials (liquid vs local)
• not conflict with common options (frozen vs force)

Current proposal:

a < b < c < d

• published < review < draft < trash

• published < mutable < private < discarded

Discarded name:

• local(ambiguous, too wide, may conflict with liquid)
• frozen (conflict with -f --force)
• dead (too definitive)

Still available noon:

• immutable (a)
• mutable (b)
• liquid (b)
• private (c)
• review (b)
• draft (c)
• trash (d)
• discarded (d)

How to name the transition:

• don't verb the nouns, use a different command/option for viewing versus changing (hg log -r 'trash()' vs hg setstate --trash)

CategoryDeveloper