Arcanist User Guide: Commit Ranges

Phorge Administrator and User Documentation (Application User Guides)

Explains how commit ranges work in Arcanist.

This is an advanced user guide which covers a complicated topic in detail. If you're just getting started, you don't need to look at this yet. Instead, start with the Arcanist User Guide.

Overview

In Subversion, arc commands always operate on the uncommitted changes in the working copy. If you use Subversion, this document is not relevant to you.

In Git and Mercurial, many arc commands (notably, arc diff) operate on a range of commits beginning with some commit you specify and ending with the working copy state.

Since the end of the range is fixed (the working copy state), you only need to specify the beginning of the range. This is called the "base commit". You can do this explicitly when you run commands:

$ arc diff HEAD^ # git: just the most recent commit
$ arc diff .^    #  hg: just the most recent commit

You can also configure arc so it defaults to some base commit, or figures out the base commit using a (potentially sophisticated) ruleset.

Configuring Base Commit Rules

Base commit rule configuration may be more complicated than you expect. This is because people use many different workflows in Git and Mercurial, and have very different expectations about what base commit arc should pick when run. To make matters worse, some of the most common ways of thinking about which commits represent a change are incompatible when selecting defaults.

Historically, we tried to use a number of heuristics and simpler approaches to determine the base commit, but there is so much diversity in how people think about version control and what they expect to happen that some users were always unhappy.

Although ruleset configuration is fairly complex, it's powerful enough that you should be able to get exactly the behavior you want.

To determine the base commit, arc processes rules one at a time until it gets a match (a rule which identifies a valid commit). The first match is the base commit that is used to determine the beginning of the commit range.

A rule looks like this:

arc:upstream

A rule may match, meaning that it identifies some valid commit in the working copy, or fail, meaning that it does not identify a valid commit. For instance, the rule arc:upstream will match if the current Git branch tracks an upstream branch, but fail if the current Git branch does not track an upstream branch, or the working copy isn't a Git working copy. When a rule fails, processing continues with the next rule. Some rules can never match but produce useful side effects instead. These are described below.

A ruleset is a comma-separated list of rules:

arc:upstream, arc:prompt

arc reads five rulesets:

1. args, specified with --base <ruleset> on the command line when you run a command. This ruleset is processed first.
2. local, specified with arc set-config --local base <ruleset>. This ruleset is local to the working copy where it is set, and is processed second.
3. project, specified by setting the "base" key in .arcconfig. This ruleset is bound to the project where it is configured, and is processed third.
4. global, specified with arc set-config base <ruleset>. This ruleset is global for the current user, and is processed fourth.
5. system, specified in a system-wide configuration file. This ruleset is global for all users on the system, and is processed last.

The rules in each ruleset are processed one at a time until a valid base commit is found. Valid rules are listed below. In this list, "*" means "any string".

• git:* Use the specified symbolic commit, if it exists.
• git:merge-base(*) Use the merge-base of HEAD and the specified symbolic commit, if it exists.
• git:branch-unique(*) Attempt to select changes unique to this branch (that is, changes between the branch point and HEAD). This rule is complicated and has limitations, see below for a detailed description.
• hg:* Use the specified symbolic commit, if it exists.
• hg:gca(*) Use the greatest common ancestor of . and the specified symbolic commit, if it exists.
• arc:upstream Use the merge-base of the current branch's upstream and HEAD, if it exists. (git-only)
• arc:outgoing Use the most recent non-outgoing ancestor of the working copy parent. (hg-only)
• arc:exec(*) Execute the specified command. The command should determine the base revision to use and print it to stdout, then exit with return code 0. If the command exits with another return code, the rule will fail. The command will be executed with the root directory of the working copy as the current working directory.
• arc:bookmark Use the most recent non-outgoing ancestor of the working copy parent or the most recent bookmark, whichever is more recent. This rule is complicated and has limitations, see below for a detailed description.
• arc:amended Use the current commit (HEAD in Git, or . in Mercurial) if it has been amended to include a "Differential Revision:" field. Otherwise, fail.
• arc:prompt Prompt the user to provide a commit.
• arc:empty Use the empty state (as though the repository were completely empty, the range will include every commit that is an ancestor of the working copy).
• arc:this Use the current commit. This means . under Mercurial and HEAD under Git.

Rules are also available which change the processing order of rulesets:

• arc:args, arc:local, arc:project, arc:global, arc:system Stop processing the current ruleset and begin processing the specified ruleset. The current ruleset will resume processing after the specified ruleset is exhausted.
• arc:yield Stop processing the current ruleset and begin processing the next ruleset. The current ruleset will resume processing after other rulesets have processed or when it next appears in the processing order, whichever comes first.
• arc:halt Stops processing all rules. This will cause the command you ran to fail, but can be used to avoid running rules which would otherwise be processed later.

Additionally, there are some rules which are probably useful mostly for testing or debugging rulesets:

• arc:verbose Turns on verbose logging of rule processing.
• arc:skip This rule has no effect.
• literal:* Use the specified commit literally. Almost certainly wrong in production rules.

Examples

Diff against origin/master if it exists, and prompt if it doesn't:

git:merge-base(origin/master), arc:prompt

Diff against the upstream if it exists, or just use the last commit if it doesn't:

arc:upstream, git:HEAD^

As a user, ignore project rules and always use my rules:

(local) arc:global, arc:halt

As a project maintainer, respect user rules over project rules:

(project) arc:yield, <defaults>

Debug your rules:

$ arc diff --base arc:verbose

Understand rules processing:

$ arc which
$ arc which --base '<ruleset>'
$ arc which --base 'arc:verbose, <ruleset>'

Detailed Rule Descriptions

Some rules have complex operation, described here in more detail. These rules are advanced features for expert users wishing to optimize their workflow and save a little typing. You do not need to understand the behavior of these rules to use arc (you can always specify a base commit explicitly).

git:branch-unique(*)

This rule only works in Git.

This rule tries to find commits that are unique to the current branch. It is most likely to be useful if you develop using one branch per feature, update changes by amending commits (instead of stacking commits) and merge changes by rebasing (instead of merging).

The rule operates by first determining the merge-base of the specified commit and HEAD, if it exists. If no such commit exists, the rule fails. If such a commit exists, the rule counts how many branches contain HEAD, then walks from HEAD to the merge-base commit, counting how many branches contain each commit. It stops when it finds a commit which appears on more branches than HEAD, or when it reaches the merge-base commit.

This rule works well for trees that look like this:

|   *  Commit B1, on branch "subfeature" (HEAD)
|  /
| *    Commit A1, on branch "feature"
|/
*      Commit M1, on branch "master"
|

This tree represents using feature branches to develop one feature ("feature"), and then creating a sub-branch to develop a dependent feature ("subfeature").

Normally, if you run arc diff on branch "subfeature" (with HEAD at B1), a rule like arc:merge-base(master) will select M1 as the base commit and thus incorrectly include A1 in the commit range.

For trees like this, git:branch-unique(master) will instead select A1 as the base commit (because it is the first commit between B1 and M1 which appears on more branches than B1 -- B1 appears on only "subfeature" while A1 appears on "subfeature" and "feature") and include only B1 in the commit range.

The rule will also do the right thing when run from "feature" in this case.

However, this rule will select the wrong commit range in some cases. For instance, it will do the wrong thing in this tree:

|
| *    Commit A2, on branch "feature" (HEAD)
| |
| | *  Commit B1, on branch "subfeature"
| |/
| *    Commit A1, on branch "feature"
|/
*      Commit M1, on branch "master"
|

This tree represents making another commit (A2) on "feature", on top of A1.

Here, when arc diff is run from branch "feature" (with HEAD at A2), this rule will incorrectly select only A2 because A2 (which is HEAD) appears on one branch ("feature") while A1 appears on two branches ("feature", "subfeature").

You can avoid this problem by amending changes into A1 instead of adding new commits, or by rebasing "subfeature" before running arc diff.

This rule will also select the wrong commit range in a tree like this:

|
| *    Commit A1', on branch "feature", created by amending A1
| |
| | *  Commit B1, on branch "subfeature" (HEAD)
| |/
| o    Commit A1, no longer on "feature" but still on "subfeature"
|/
*      Commit M1, on branch "master"
|

This tree represents amending A1 without rebasing "subfeature", so that A1 is no longer on "feature" (replaced with A1') but still on "subfeature". In this case, running arc diff from "subfeature" will incorrectly select both B1 and A1, because they now are contained by the same number of branches.

You can avoid this problem by rebasing sub-branches before running arc diff, or by using a rule like arc:amended before git:branch-unique(*).

arc:bookmark

This rule only works in Mercurial.

This rule finds outgoing changes, but stops when it encounters a bookmark. It is most likely to be useful if you use one bookmark per feature.

This rule operates like arc:outgoing, but then walks the commits between . and the selected base commit. It stops when it encounters a bookmark. For example, if you have a tree like this:

|
| *  C4 (outgoing, bookmark: stripes)
| |
| *  C3 (outgoing, bookmark: zebra)
| |
| *  C2 (outgoing, no bookmark)
|/
*    C1 (pushed, no bookmark)
|

When run from C4, this rule will select just C4, stopping on C3 because it has a different bookmark. When run from C3, it will select C2 and C3.

However, this rule will select the wrong commit range in some cases (for example, if the "zebra" bookmark has moved on, the rule will no longer stop on C3 and will select C2, C3 and C4 when run from C4).

arc:exec(*)

This rule runs some external script or shell command. It is intended for advanced users who want specialized behavior that can't be expressed with other rules.

To use this rule, provide some script or shell command. For example:

arc:exec(git merge-base origin/master HEAD)
arc:exec(/path/to/some/script.sh)

The command will be executed with the working copy as its working directory, and passed no arguments. To match, it should print the name of a base commit on stdout and then exit with return code 0. To fail, it should exit with any other return code.

Next Steps

Continue by:

• learning about arc diff in more detail with Arcanist User Guide: arc diff; or
• returning to Arcanist User Guide.