Chapter 8. Managing releases and branchy development

Table of Contents

8.1. Giving a persistent name to a revision
8.1.1. Handling tag conflicts during a merge
8.1.2. Tags and cloning
8.1.3. When permanent tags are too much
8.2. The flow of changes—big picture vs. little
8.3. Managing big-picture branches in repositories
8.4. Don't repeat yourself: merging across branches
8.5. Naming branches within one repository
8.6. Dealing with multiple named branches in a repository
8.7. Branch names and merging
8.8. Branch naming is generally useful

Mercurial provides several mechanisms for you to manage a project that is making progress on multiple fronts at once. To understand these mechanisms, let's first take a brief look at a fairly normal software project structure.

Many software projects issue periodic major releases that contain substantial new features. In parallel, they may issue minor releases. These are usually identical to the major releases off which they're based, but with a few bugs fixed.

In this chapter, we'll start by talking about how to keep records of project milestones such as releases. We'll then continue on to talk about the flow of work between different phases of a project, and how Mercurial can help you to isolate and manage this work.

8.1. Giving a persistent name to a revision

Once you decide that you'd like to call a particular revision a release, it's a good idea to record the identity of that revision. This will let you reproduce that release at a later date, for whatever purpose you might need at the time (reproducing a bug, porting to a new platform, etc).

$ hg init mytag
$ cd mytag
$ echo hello > myfile
$ hg commit -A -m 'Initial commit'
adding myfile

Mercurial lets you give a permanent name to any revision using the hg tag command. Not surprisingly, these names are called tags.

$ hg tag v1.0

A tag is nothing more than a symbolic name for a revision. Tags exist purely for your convenience, so that you have a handy permanent way to refer to a revision; Mercurial doesn't interpret the tag names you use in any way. Neither does Mercurial place any restrictions on the name of a tag, beyond a few that are necessary to ensure that a tag can be parsed unambiguously. A tag name cannot contain any of the following characters:

  • Colon (ASCII 58, :)

  • Carriage return (ASCII 13, \r)

  • Newline (ASCII 10, \n)

You can use the hg tags command to display the tags present in your repository. In the output, each tagged revision is identified first by its name, then by revision number, and finally by the unique hash of the revision.

$ hg tags
tip                                1:bd27c29b5009
v1.0                               0:de59b5abd767

Notice that tip is listed in the output of hg tags. The tip tag is a special floating tag, which always identifies the newest revision in the repository.

In the output of the hg tags command, tags are listed in reverse order, by revision number. This usually means that recent tags are listed before older tags. It also means that tip is always going to be the first tag listed in the output of hg tags.

When you run hg log, if it displays a revision that has tags associated with it, it will print those tags.

$ hg log
changeset:   1:bd27c29b5009
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:30:12 2010 +0000
summary:     Added tag v1.0 for changeset de59b5abd767

changeset:   0:de59b5abd767
tag:         v1.0
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:30:11 2010 +0000
summary:     Initial commit

Any time you need to provide a revision ID to a Mercurial command, the command will accept a tag name in its place. Internally, Mercurial will translate your tag name into the corresponding revision ID, then use that.

$ echo goodbye > myfile2
$ hg commit -A -m 'Second commit'
adding myfile2
$ hg log -r v1.0
changeset:   0:de59b5abd767
tag:         v1.0
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:30:11 2010 +0000
summary:     Initial commit

There's no limit on the number of tags you can have in a repository, or on the number of tags that a single revision can have. As a practical matter, it's not a great idea to have too many (a number which will vary from project to project), simply because tags are supposed to help you to find revisions. If you have lots of tags, the ease of using them to identify revisions diminishes rapidly.

For example, if your project has milestones as frequent as every few days, it's perfectly reasonable to tag each one of those. But if you have a continuous build system that makes sure every revision can be built cleanly, you'd be introducing a lot of noise if you were to tag every clean build. Instead, you could tag failed builds (on the assumption that they're rare!), or simply not use tags to track buildability.

If you want to remove a tag that you no longer want, use hg tag --remove.

$ hg tag --remove v1.0
$ hg tags
tip                                3:a128e67cfe99

You can also modify a tag at any time, so that it identifies a different revision, by simply issuing a new hg tag command. You'll have to use the -f option to tell Mercurial that you really want to update the tag.

$ hg tag -r 1 v1.1
$ hg tags
tip                                4:3624e1abfcd6
v1.1                               1:bd27c29b5009
$ hg tag -r 2 v1.1
abort: tag 'v1.1' already exists (use -f to force)
$ hg tag -f -r 2 v1.1
$ hg tags
tip                                5:2a43b67fc4f3
v1.1                               2:327eb54d507e

There will still be a permanent record of the previous identity of the tag, but Mercurial will no longer use it. There's thus no penalty to tagging the wrong revision; all you have to do is turn around and tag the correct revision once you discover your error.

Mercurial stores tags in a normal revision-controlled file in your repository. If you've created any tags, you'll find them in a file in the root of your repository named .hgtags. When you run the hg tag command, Mercurial modifies this file, then automatically commits the change to it. This means that every time you run hg tag, you'll see a corresponding changeset in the output of hg log.

$ hg tip
changeset:   5:2a43b67fc4f3
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:30:13 2010 +0000
summary:     Added tag v1.1 for changeset 327eb54d507e

8.1.1. Handling tag conflicts during a merge

You won't often need to care about the .hgtags file, but it sometimes makes its presence known during a merge. The format of the file is simple: it consists of a series of lines. Each line starts with a changeset hash, followed by a space, followed by the name of a tag.

If you're resolving a conflict in the .hgtags file during a merge, there's one twist to modifying the .hgtags file: when Mercurial is parsing the tags in a repository, it never reads the working copy of the .hgtags file. Instead, it reads the most recently committed revision of the file.

An unfortunate consequence of this design is that you can't actually verify that your merged .hgtags file is correct until after you've committed a change. So if you find yourself resolving a conflict on .hgtags during a merge, be sure to run hg tags after you commit. If it finds an error in the .hgtags file, it will report the location of the error, which you can then fix and commit. You should then run hg tags again, just to be sure that your fix is correct.

8.1.2. Tags and cloning

You may have noticed that the hg clone command has a -r option that lets you clone an exact copy of the repository as of a particular changeset. The new clone will not contain any project history that comes after the revision you specified. This has an interaction with tags that can surprise the unwary.

Recall that a tag is stored as a revision to the .hgtags file. When you create a tag, the changeset in which its recorded refers to an older changeset. When you run hg clone -r foo to clone a repository as of tag foo, the new clone will not contain any revision newer than the one the tag refers to, including the revision where the tag was created. The result is that you'll get exactly the right subset of the project's history in the new repository, but not the tag you might have expected.

8.1.3. When permanent tags are too much

Since Mercurial's tags are revision controlled and carried around with a project's history, everyone you work with will see the tags you create. But giving names to revisions has uses beyond simply noting that revision 4237e45506ee is really v2.0.2. If you're trying to track down a subtle bug, you might want a tag to remind you of something like Anne saw the symptoms with this revision.

For cases like this, what you might want to use are local tags. You can create a local tag with the -l option to the hg tag command. This will store the tag in a file called .hg/localtags. Unlike .hgtags, .hg/localtags is not revision controlled. Any tags you create using -l remain strictly local to the repository you're currently working in.

8.2. The flow of changes—big picture vs. little

To return to the outline I sketched at the beginning of the chapter, let's think about a project that has multiple concurrent pieces of work under development at once.

There might be a push for a new main release; a new minor bugfix release to the last main release; and an unexpected hot fix to an old release that is now in maintenance mode.

The usual way people refer to these different concurrent directions of development is as branches. However, we've already seen numerous times that Mercurial treats all of history as a series of branches and merges. Really, what we have here is two ideas that are peripherally related, but which happen to share a name.

  • Big picture branches represent the sweep of a project's evolution; people give them names, and talk about them in conversation.

  • Little picture branches are artefacts of the day-to-day activity of developing and merging changes. They expose the narrative of how the code was developed.

8.3. Managing big-picture branches in repositories

The easiest way to isolate a big picture branch in Mercurial is in a dedicated repository. If you have an existing shared repository—let's call it myproject—that reaches a 1.0 milestone, you can start to prepare for future maintenance releases on top of version 1.0 by tagging the revision from which you prepared the 1.0 release.

$ cd myproject
$ hg tag v1.0

You can then clone a new shared myproject-1.0.1 repository as of that tag.

$ cd ..
$ hg clone myproject myproject-1.0.1
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved

Afterwards, if someone needs to work on a bug fix that ought to go into an upcoming 1.0.1 minor release, they clone the myproject-1.0.1 repository, make their changes, and push them back.

$ hg clone myproject-1.0.1 my-1.0.1-bugfix
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd my-1.0.1-bugfix
$ echo 'I fixed a bug using only echo!' >> myfile
$ hg commit -m 'Important fix for 1.0.1'
$ hg push
pushing to /tmp/branch-repo0tRSKc/myproject-1.0.1
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files

Meanwhile, development for the next major release can continue, isolated and unabated, in the myproject repository.

$ cd ..
$ hg clone myproject my-feature
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd my-feature
$ echo 'This sure is an exciting new feature!' > mynewfile
$ hg commit -A -m 'New feature'
adding mynewfile
$ hg push
pushing to /tmp/branch-repo0tRSKc/myproject
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files

8.4. Don't repeat yourself: merging across branches

In many cases, if you have a bug to fix on a maintenance branch, the chances are good that the bug exists on your project's main branch (and possibly other maintenance branches, too). It's a rare developer who wants to fix the same bug multiple times, so let's look at a few ways that Mercurial can help you to manage these bugfixes without duplicating your work.

In the simplest instance, all you need to do is pull changes from your maintenance branch into your local clone of the target branch.

$ cd ..
$ hg clone myproject myproject-merge
updating to branch default
3 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd myproject-merge
$ hg pull ../myproject-1.0.1
pulling from ../myproject-1.0.1
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg heads' to see heads, 'hg merge' to merge)

You'll then need to merge the heads of the two branches, and push back to the main branch.

$ hg merge
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg commit -m 'Merge bugfix from 1.0.1 branch'
$ hg push
pushing to /tmp/branch-repo0tRSKc/myproject
searching for changes
adding changesets
adding manifests
adding file changes
added 2 changesets with 1 changes to 1 files

8.5. Naming branches within one repository

In most instances, isolating branches in repositories is the right approach. Its simplicity makes it easy to understand; and so it's hard to make mistakes. There's a one-to-one relationship between branches you're working in and directories on your system. This lets you use normal (non-Mercurial-aware) tools to work on files within a branch/repository.

If you're more in the power user category (and your collaborators are too), there is an alternative way of handling branches that you can consider. I've already mentioned the human-level distinction between small picture and big picture branches. While Mercurial works with multiple small picture branches in a repository all the time (for example after you pull changes in, but before you merge them), it can also work with multiple big picture branches.

The key to working this way is that Mercurial lets you assign a persistent name to a branch. There always exists a branch named default. Even before you start naming branches yourself, you can find traces of the default branch if you look for them.

As an example, when you run the hg commit command, and it pops up your editor so that you can enter a commit message, look for a line that contains the text HG: branch default at the bottom. This is telling you that your commit will occur on the branch named default.

To start working with named branches, use the hg branches command. This command lists the named branches already present in your repository, telling you which changeset is the tip of each.

$ hg tip
changeset:   0:0a8046aed745
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:29:19 2010 +0000
summary:     Initial commit

$ hg branches
default                        0:0a8046aed745

Since you haven't created any named branches yet, the only one that exists is default.

To find out what the current branch is, run the hg branch command, giving it no arguments. This tells you what branch the parent of the current changeset is on.

$ hg branch
default

To create a new branch, run the hg branch command again. This time, give it one argument: the name of the branch you want to create.

$ hg branch foo
marked working directory as branch foo
$ hg branch
foo

After you've created a branch, you might wonder what effect the hg branch command has had. What do the hg status and hg tip commands report?

$ hg status
$ hg tip
changeset:   0:0a8046aed745
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:29:19 2010 +0000
summary:     Initial commit

Nothing has changed in the working directory, and there's been no new history created. As this suggests, running the hg branch command has no permanent effect; it only tells Mercurial what branch name to use the next time you commit a changeset.

When you commit a change, Mercurial records the name of the branch on which you committed. Once you've switched from the default branch to another and committed, you'll see the name of the new branch show up in the output of hg log, hg tip, and other commands that display the same kind of output.

$ echo 'hello again' >> myfile
$ hg commit -m 'Second commit'
$ hg tip
changeset:   1:839a6dfa0599
branch:      foo
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:29:20 2010 +0000
summary:     Second commit

The hg log-like commands will print the branch name of every changeset that's not on the default branch. As a result, if you never use named branches, you'll never see this information.

Once you've named a branch and committed a change with that name, every subsequent commit that descends from that change will inherit the same branch name. You can change the name of a branch at any time, using the hg branch command.

$ hg branch
foo
$ hg branch bar
marked working directory as branch bar
$ echo new file > newfile
$ hg commit -A -m 'Third commit'
adding newfile
$ hg tip
changeset:   2:127d6631bc4f
branch:      bar
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:29:20 2010 +0000
summary:     Third commit

In practice, this is something you won't do very often, as branch names tend to have fairly long lifetimes. (This isn't a rule, just an observation.)

8.6. Dealing with multiple named branches in a repository

If you have more than one named branch in a repository, Mercurial will remember the branch that your working directory is on when you start a command like hg update or hg pull -u. It will update the working directory to the tip of this branch, no matter what the repo-wide tip is. To update to a revision that's on a different named branch, you may need to use the -C option to hg update.

This behavior is a little subtle, so let's see it in action. First, let's remind ourselves what branch we're currently on, and what branches are in our repository.

$ hg parents
changeset:   2:127d6631bc4f
branch:      bar
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:29:20 2010 +0000
summary:     Third commit

$ hg branches
bar                            2:127d6631bc4f
foo                            1:839a6dfa0599 (inactive)
default                        0:0a8046aed745 (inactive)

We're on the bar branch, but there also exists an older hg foo branch.

We can hg update back and forth between the tips of the foo and bar branches without needing to use the -C option, because this only involves going backwards and forwards linearly through our change history.

$ hg update foo
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg parents
changeset:   1:839a6dfa0599
branch:      foo
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:29:20 2010 +0000
summary:     Second commit

$ hg update bar
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg parents
changeset:   2:127d6631bc4f
branch:      bar
tag:         tip
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:29:20 2010 +0000
summary:     Third commit

If we go back to the foo branch and then run hg update, it will keep us on foo, not move us to the tip of bar.

$ hg update foo
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg update
0 files updated, 0 files merged, 0 files removed, 0 files unresolved

Committing a new change on the foo branch introduces a new head.

$ echo something > somefile
$ hg commit -A -m 'New file'
adding somefile
$ hg heads
changeset:   3:3704ef55f6ae
branch:      foo
tag:         tip
parent:      1:839a6dfa0599
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:29:21 2010 +0000
summary:     New file

changeset:   2:127d6631bc4f
branch:      bar
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:29:20 2010 +0000
summary:     Third commit

changeset:   0:0a8046aed745
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:29:19 2010 +0000
summary:     Initial commit

8.7. Branch names and merging

As you've probably noticed, merges in Mercurial are not symmetrical. Let's say our repository has two heads, 17 and 23. If I hg update to 17 and then hg merge with 23, Mercurial records 17 as the first parent of the merge, and 23 as the second. Whereas if I hg update to 23 and then hg merge with 17, it records 23 as the first parent, and 17 as the second.

This affects Mercurial's choice of branch name when you merge. After a merge, Mercurial will retain the branch name of the first parent when you commit the result of the merge. If your first parent's branch name is foo, and you merge with bar, the branch name will still be foo after you merge.

It's not unusual for a repository to contain multiple heads, each with the same branch name. Let's say I'm working on the foo branch, and so are you. We commit different changes; I pull your changes; I now have two heads, each claiming to be on the foo branch. The result of a merge will be a single head on the foo branch, as you might hope.

But if I'm working on the bar branch, and I merge work from the foo branch, the result will remain on the bar branch.

$ hg branch
bar
$ hg merge foo
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg commit -m 'Merge'
$ hg tip
changeset:   4:4e6b7026d088
branch:      bar
tag:         tip
parent:      2:127d6631bc4f
parent:      3:3704ef55f6ae
user:        Bryan O'Sullivan <bos@serpentine.com>
date:        Thu Jul 01 21:29:22 2010 +0000
summary:     Merge

To give a more concrete example, if I'm working on the bleeding-edge branch, and I want to bring in the latest fixes from the stable branch, Mercurial will choose the right (bleeding-edge) branch name when I pull and merge from stable.

8.8. Branch naming is generally useful

You shouldn't think of named branches as applicable only to situations where you have multiple long-lived branches cohabiting in a single repository. They're very useful even in the one-branch-per-repository case.

In the simplest case, giving a name to each branch gives you a permanent record of which branch a changeset originated on. This gives you more context when you're trying to follow the history of a long-lived branchy project.

If you're working with shared repositories, you can set up a pretxnchangegroup hook on each that will block incoming changes that have the wrong branch name. This provides a simple, but effective, defence against people accidentally pushing changes from a bleeding edge branch to a stable branch. Such a hook might look like this inside the shared repo's /.hgrc.

[hooks]
pretxnchangegroup.branch = hg heads --template '{branches} ' | grep mybranch