Mercurial provides several mechanisms for you to manage a project that is progressing 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.
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 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:
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:ccd961c70ae0 v1.0 0:c78e12f9da76
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:ccd961c70ae0 tag: tip user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:45 2009 +0000 summary: Added tag v1.0 for changeset c78e12f9da76 changeset: 0:c78e12f9da76 tag: v1.0 user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:45 2009 +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:c78e12f9da76 tag: v1.0 user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:45 2009 +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 that 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 3:6a30b02be96a
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 4:2f612e0b083a v1.1 1:ccd961c70ae0$
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:dd9e7a899c13 v1.1 2:9ac23a7cc526
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:dd9e7a899c13 tag: tip user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:45 2009 +0000 summary: Added tag v1.1 for changeset 9ac23a7cc526
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.
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 it is 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.
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.
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 bug fix 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 are two ideas that are peripherally related, but which happen to share a name.
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 working directory 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 working directory 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-repo8ztZpS/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 working directory 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-repo8ztZpS/myproject searching for changes adding changesets adding manifests adding file changes added 1 changesets with 1 changes to 1 files
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 bug fixes 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 working directory 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-repo8ztZpS/myproject searching for changes adding changesets adding manifests adding file changes added 2 changesets with 1 changes to 1 files
In most instances, isolating branches in repositories is the right approach. Its simplicity makes it easy to understand, 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 “little 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: 0:9a972e4b5a97 tag: tip user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:25 2009 +0000 summary: Initial commit$
hg branches
default 0:9a972e4b5a97
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: 0:9a972e4b5a97 tag: tip user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:25 2009 +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: 1:8928355fee43 branch: foo tag: tip user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:25 2009 +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:f32855c6764f branch: bar tag: tip user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:25 2009 +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.)
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:f32855c6764f branch: bar tag: tip user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:25 2009 +0000 summary: Third commit$
hg branches
bar 2:f32855c6764f foo 1:8928355fee43 (inactive) default 0:9a972e4b5a97 (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: 1:8928355fee43 branch: foo user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:25 2009 +0000 summary: Second commit$
hg update bar
1 files updated, 0 files merged, 0 files removed, 0 files unresolved$
hg parents
changeset: 2:f32855c6764f branch: bar tag: tip user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:25 2009 +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 created new head$
hg heads
changeset: 3:2e55e6a73143 branch: foo tag: tip parent: 1:8928355fee43 user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:26 2009 +0000 summary: New file changeset: 2:f32855c6764f branch: bar user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:25 2009 +0000 summary: Third commit
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: 4:00d606298226 branch: bar tag: tip parent: 2:f32855c6764f parent: 3:2e55e6a73143 user: Bryan O'Sullivan <[email protected]> date: Tue May 05 06:44:26 2009 +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
.
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, defense 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
18.117.99.71