Managing releases and branchy development#
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.
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)…
Show code cell content
export HGRCPATH=$PWD/../hgrc4book
mkdir -p /tmp/tmp_mercurial_book
cd /tmp/tmp_mercurial_book
rm -rf /tmp/tmp_mercurial_book/*
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
”)Null (ASCII 0, “
\0
”)
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 1:3c5442d66145
v1.0 0:c087d36a8f85
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: 1:3c5442d66145
tag: tip
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:21 2023 +0000
summary: Added tag v1.0 for changeset c087d36a8f85
changeset: 0:c087d36a8f85
tag: v1.0
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:20 2023 +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'
hg log -r v1.0
adding myfile2
changeset: 0:c087d36a8f85
tag: v1.0
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:20 2023 +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:bc04caf6e6eb
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
hg tag -r 2 v1.1
hg tag -f -r 2 v1.1
hg tags
tip 4:983a1df0b004
v1.1 1:3c5442d66145
abort: tag 'v1.1' already exists (use -f to force)
tip 5:55ca7ed2b78a
v1.1 2:73fa7ee2412b
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: 5:55ca7ed2b78a
tag: tip
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:24 2023 +0000
summary: Added tag v1.1 for changeset 73fa7ee2412b
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.
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.
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 /tmp/tmp_mercurial_book
hg init myproject
cd myproject
echo hello > myfile
hg commit -A -m 'Initial commit'
adding myfile
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
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
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
pushing to /tmp/tmp_mercurial_book/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
cd my-feature
echo 'This sure is an exciting new feature!' > mynewfile
hg commit -A -m 'New feature'
hg push
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding mynewfile
pushing to /tmp/tmp_mercurial_book/myproject
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
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
cd myproject-merge
hg pull ../myproject-1.0.1
updating to branch default
3 files updated, 0 files merged, 0 files removed, 0 files unresolved
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)
new changesets 1222a88d16c2
(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
hg commit -m 'Merge bugfix from 1.0.1 branch'
hg push
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
pushing to /tmp/tmp_mercurial_book/myproject
searching for changes
adding changesets
adding manifests
adding file changes
added 2 changesets with 1 changes to 1 files
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
hg branches
changeset: 4:65b6b73c91ec
tag: tip
parent: 2:a6c2dbcf1078
parent: 3:1222a88d16c2
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:29 2023 +0000
summary: Merge bugfix from 1.0.1 branch
default 4:65b6b73c91ec
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
hg branch
marked working directory as branch foo
(branches are permanent and global, did you want a bookmark?)
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: 4:65b6b73c91ec
tag: tip
parent: 2:a6c2dbcf1078
parent: 3:1222a88d16c2
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:29 2023 +0000
summary: Merge bugfix from 1.0.1 branch
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: 5:8d7c7fda2410
branch: foo
tag: tip
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:31 2023 +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
hg branch bar
echo new file > newfile
hg commit -A -m 'Third commit'
hg tip
foo
marked working directory as branch bar
adding newfile
changeset: 6:dca955765eb3
branch: bar
tag: tip
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:32 2023 +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.)
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
(or -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
hg branches
changeset: 6:dca955765eb3
branch: bar
tag: tip
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:32 2023 +0000
summary: Third commit
bar 6:dca955765eb3
foo 5:8d7c7fda2410 (inactive)
default 4:65b6b73c91ec (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
hg parents
hg update bar
hg parents
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
changeset: 5:8d7c7fda2410
branch: foo
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:31 2023 +0000
summary: Second commit
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
changeset: 6:dca955765eb3
branch: bar
tag: tip
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:32 2023 +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
hg update
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
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'
hg heads
adding somefile
changeset: 7:f0a1941f404c
branch: foo
tag: tip
parent: 5:8d7c7fda2410
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:35 2023 +0000
summary: New file
changeset: 6:dca955765eb3
branch: bar
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:32 2023 +0000
summary: Third commit
changeset: 4:65b6b73c91ec
parent: 2:a6c2dbcf1078
parent: 3:1222a88d16c2
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:29 2023 +0000
summary: Merge bugfix from 1.0.1 branch
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
hg merge foo
hg commit -m 'Merge'
hg tip
foo
abort: merging with a working directory ancestor has no effect
nothing changed
changeset: 7:f0a1941f404c
branch: foo
tag: tip
parent: 5:8d7c7fda2410
user: Pierre Augier <pa@example.com>
date: Wed Sep 13 16:15:35 2023 +0000
summary: New file
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
.
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