
bos@196: \chapter{Managing releases and branchy development}
bos@187: \label{chap:branch}
bos@187: 
bos@197: Mercurial provides several mechanisms for you to manage a project that
bos@197: is making progress on multiple fronts at once.  To understand these
bos@197: mechanisms, let's first take a brief look at a fairly normal software
bos@197: project structure.
bos@187: 
bos@196: Many software projects issue periodic ``major'' releases that contain
bos@196: substantial new features.  In parallel, they may issue ``minor''
bos@196: releases.  These are usually identical to the major releases off which
bos@196: they're based, but with a few bugs fixed.
bos@196: 
bos@197: In this chapter, we'll start by talking about how to keep records of
bos@197: project milestones such as releases.  We'll then continue on to talk
bos@197: about the flow of work between different phases of a project, and how
bos@197: Mercurial can help you to isolate and manage this work.
bos@197: 
bos@196: \section{Giving a persistent name to a revision}
bos@196: 
bos@196: Once you decide that you'd like to call a particular revision a
bos@196: ``release'', it's a good idea to record the identity of that revision.
bos@196: This will let you reproduce that release at a later date, for whatever
bos@196: purpose you might need at the time (reproducing a bug, porting to a
bos@196: new platform, etc).
bos@196: \interaction{tag.init}
bos@196: 
bos@196: Mercurial lets you give a permanent name to any revision using the
bos@196: \hgcmd{tag} command.  Not surprisingly, these names are called
bos@196: ``tags''.
bos@196: \interaction{tag.tag}
bos@196: 
bos@196: A tag is nothing more than a ``symbolic name'' for a revision.  Tags
bos@196: exist purely for your convenience, so that you have a handy permanent
bos@196: way to refer to a revision; Mercurial doesn't interpret the tag names
bos@196: you use in any way.  Neither does Mercurial place any restrictions on
bos@196: the name of a tag, beyond a few that are necessary to ensure that a
bos@196: tag can be parsed unambiguously.  A tag name cannot contain any of the
bos@196: following characters:
bos@196: \begin{itemize}
bos@196: \item Colon (ASCII 58, ``\texttt{:}'')
bos@261: \item Carriage return (ASCII 13, ``\Verb+\r+'')
bos@261: \item Newline (ASCII 10, ``\Verb+\n+'')
bos@196: \end{itemize}
bos@196: 
bos@196: You can use the \hgcmd{tags} command to display the tags present in
bos@196: your repository.  In the output, each tagged revision is identified
bos@196: first by its name, then by revision number, and finally by the unique
bos@196: hash of the revision.  
bos@196: \interaction{tag.tags}
bos@196: Notice that \texttt{tip} is listed in the output of \hgcmd{tags}.  The
bos@196: \texttt{tip} tag is a special ``floating'' tag, which always
bos@196: identifies the newest revision in the repository.
bos@196: 
bos@196: In the output of the \hgcmd{tags} command, tags are listed in reverse
bos@196: order, by revision number.  This usually means that recent tags are
bos@196: listed before older tags.  It also means that \texttt{tip} is always
bos@196: going to be the first tag listed in the output of \hgcmd{tags}.
bos@196: 
bos@196: When you run \hgcmd{log}, if it displays a revision that has tags
bos@196: associated with it, it will print those tags.
bos@196: \interaction{tag.log}
bos@196: 
bos@196: Any time you need to provide a revision~ID to a Mercurial command, the
bos@196: command will accept a tag name in its place.  Internally, Mercurial
bos@196: will translate your tag name into the corresponding revision~ID, then
bos@196: use that.
bos@196: \interaction{tag.log.v1.0}
bos@196: 
bos@196: There's no limit on the number of tags you can have in a repository,
bos@196: or on the number of tags that a single revision can have.  As a
bos@196: practical matter, it's not a great idea to have ``too many'' (a number
bos@196: which will vary from project to project), simply because tags are
bos@196: supposed to help you to find revisions.  If you have lots of tags, the
bos@196: ease of using them to identify revisions diminishes rapidly.
bos@196: 
bos@196: For example, if your project has milestones as frequent as every few
bos@196: days, it's perfectly reasonable to tag each one of those.  But if you
bos@196: have a continuous build system that makes sure every revision can be
bos@196: built cleanly, you'd be introducing a lot of noise if you were to tag
bos@196: every clean build.  Instead, you could tag failed builds (on the
bos@196: assumption that they're rare!), or simply not use tags to track
bos@196: buildability.
bos@196: 
bos@196: If you want to remove a tag that you no longer want, use
bos@196: \hgcmdargs{tag}{--remove}.  
bos@196: \interaction{tag.remove}
bos@196: You can also modify a tag at any time, so that it identifies a
bos@196: different revision, by simply issuing a new \hgcmd{tag} command.
bos@196: You'll have to use the \hgopt{tag}{-f} option to tell Mercurial that
bos@196: you \emph{really} want to update the tag.
bos@196: \interaction{tag.replace}
bos@196: There will still be a permanent record of the previous identity of the
bos@197: tag, but Mercurial will no longer use it.  There's thus no penalty to
bos@197: tagging the wrong revision; all you have to do is turn around and tag
bos@197: the correct revision once you discover your error.
bos@196: 
bos@196: Mercurial stores tags in a normal revision-controlled file in your
bos@196: repository.  If you've created any tags, you'll find them in a file
bos@196: named \sfilename{.hgtags}.  When you run the \hgcmd{tag} command,
bos@196: Mercurial modifies this file, then automatically commits the change to
bos@196: it.  This means that every time you run \hgcmd{tag}, you'll see a
bos@196: corresponding changeset in the output of \hgcmd{log}.
bos@196: \interaction{tag.tip}
bos@196: 
bos@196: \subsection{Handling tag conflicts during a merge}
bos@196: 
bos@196: You won't often need to care about the \sfilename{.hgtags} file, but
bos@196: it sometimes makes its presence known during a merge.  The format of
bos@196: the file is simple: it consists of a series of lines.  Each line
bos@196: starts with a changeset hash, followed by a space, followed by the
bos@196: name of a tag.
bos@196: 
bos@196: If you're resolving a conflict in the \sfilename{.hgtags} file during
bos@196: a merge, there's one twist to modifying the \sfilename{.hgtags} file:
bos@196: when Mercurial is parsing the tags in a repository, it \emph{never}
bos@196: reads the working copy of the \sfilename{.hgtags} file.  Instead, it
bos@196: reads the \emph{most recently committed} revision of the file.
bos@196: 
bos@196: An unfortunate consequence of this design is that you can't actually
bos@196: verify that your merged \sfilename{.hgtags} file is correct until
bos@196: \emph{after} you've committed a change.  So if you find yourself
bos@196: resolving a conflict on \sfilename{.hgtags} during a merge, be sure to
bos@196: run \hgcmd{tags} after you commit.  If it finds an error in the
bos@196: \sfilename{.hgtags} file, it will report the location of the error,
bos@196: which you can then fix and commit.  You should then run \hgcmd{tags}
bos@196: again, just to be sure that your fix is correct.
bos@187: 
bos@198: \subsection{Tags and cloning}
bos@198: 
bos@198: You may have noticed that the \hgcmd{clone} command has a
wbunaarfubss@259: \hgopt{clone}{-r} option that lets you clone an exact copy of the
bos@198: repository as of a particular changeset.  The new clone will not
bos@198: contain any project history that comes after the revision you
bos@198: specified.  This has an interaction with tags that can surprise the
bos@198: unwary.
bos@198: 
bos@198: Recall that a tag is stored as a revision to the \sfilename{.hgtags}
bos@198: file, so that when you create a tag, the changeset in which it's
bos@198: recorded necessarily refers to an older changeset.  When you run
bos@198: \hgcmdargs{clone}{-r foo} to clone a repository as of tag
bos@198: \texttt{foo}, the new clone \emph{will not contain the history that
bos@198:   created the tag} that you used to clone the repository.  The result
bos@198: is that you'll get exactly the right subset of the project's history
bos@198: in the new repository, but \emph{not} the tag you might have expected.
bos@198: 
bos@197: \subsection{When permanent tags are too much}
bos@197: 
bos@197: Since Mercurial's tags are revision controlled and carried around with
bos@197: a project's history, everyone you work with will see the tags you
bos@197: create.  But giving names to revisions has uses beyond simply noting
bos@197: that revision \texttt{4237e45506ee} is really \texttt{v2.0.2}.  If
bos@197: you're trying to track down a subtle bug, you might want a tag to
bos@197: remind you of something like ``Anne saw the symptoms with this
bos@197: revision''.
bos@197: 
bos@197: For cases like this, what you might want to use are \emph{local} tags.
bos@197: You can create a local tag with the \hgopt{tag}{-l} option to the
bos@197: \hgcmd{tag} command.  This will store the tag in a file called
bos@197: \sfilename{.hg/localtags}.  Unlike \sfilename{.hgtags},
bos@197: \sfilename{.hg/localtags} is not revision controlled.  Any tags you
bos@197: create using \hgopt{tag}{-l} remain strictly local to the repository
bos@197: you're currently working in.
bos@197: 
bos@198: \section{The flow of changes---big picture vs. little}
bos@198: 
bos@198: To return to the outline I sketched at the beginning of a chapter,
bos@198: let's think about a project that has multiple concurrent pieces of
bos@198: work under development at once.
bos@198: 
bos@198: There might be a push for a new ``main'' release; a new minor bugfix
bos@198: release to the last main release; and an unexpected ``hot fix'' to an
bos@198: old release that is now in maintenance mode.
bos@198: 
bos@198: The usual way people refer to these different concurrent directions of
bos@198: development is as ``branches''.  However, we've already seen numerous
bos@198: times that Mercurial treats \emph{all of history} as a series of
bos@198: branches and merges.  Really, what we have here is two ideas that are
bos@198: peripherally related, but which happen to share a name.
bos@198: \begin{itemize}
bos@198: \item ``Big picture'' branches represent the sweep of a project's
bos@198:   evolution; people give them names, and talk about them in
bos@198:   conversation.
bos@198: \item ``Little picture'' branches are artefacts of the day-to-day
bos@198:   activity of developing and merging changes.  They expose the
bos@198:   narrative of how the code was developed.
bos@198: \end{itemize}
bos@198: 
bos@198: \section{Managing big-picture branches in repositories}
bos@198: 
bos@198: The easiest way to isolate a ``big picture'' branch in Mercurial is in
bos@198: a dedicated repository.  If you have an existing shared
bos@198: repository---let's call it \texttt{myproject}---that reaches a ``1.0''
bos@198: milestone, you can start to prepare for future maintenance releases on
bos@198: top of version~1.0 by tagging the revision from which you prepared
bos@198: the~1.0 release.
bos@198: \interaction{branch-repo.tag}
bos@198: You can then clone a new shared \texttt{myproject-1.0.1} repository as
bos@198: of that tag.
bos@198: \interaction{branch-repo.clone}
bos@198: 
bos@198: Afterwards, if someone needs to work on a bug fix that ought to go
bos@198: into an upcoming~1.0.1 minor release, they clone the
bos@198: \texttt{myproject-1.0.1} repository, make their changes, and push them
bos@198: back.
bos@198: \interaction{branch-repo.bugfix}
bos@198: Meanwhile, development for the next major release can continue,
bos@198: isolated and unabated, in the \texttt{myproject} repository.
bos@198: \interaction{branch-repo.new}
bos@198: 
bos@199: \section{Don't repeat yourself: merging across branches}
bos@199: 
bos@199: In many cases, if you have a bug to fix on a maintenance branch, the
bos@199: chances are good that the bug exists on your project's main branch
bos@199: (and possibly other maintenance branches, too).  It's a rare developer
bos@199: who wants to fix the same bug multiple times, so let's look at a few
bos@199: ways that Mercurial can help you to manage these bugfixes without
bos@199: duplicating your work.
bos@199: 
bos@199: In the simplest instance, all you need to do is pull changes from your
bos@199: maintenance branch into your local clone of the target branch.
bos@199: \interaction{branch-repo.pull}
bos@199: You'll then need to merge the heads of the two branches, and push back
bos@199: to the main branch.
bos@199: \interaction{branch-repo.merge}
bos@197: 
bos@202: \section{Naming branches within one repository}
bos@202: 
bos@202: In most instances, isolating branches in repositories is the right
bos@202: approach.  Its simplicity makes it easy to understand; and so it's
bos@202: hard to make mistakes.  There's a one-to-one relationship between
bos@202: branches you're working in and directories on your system.  This lets
bos@202: you use normal (non-Mercurial-aware) tools to work on files within a
bos@202: branch/repository.
bos@202: 
bos@202: If you're more in the ``power user'' category (\emph{and} your
bos@202: collaborators are too), there is an alternative way of handling
bos@202: branches that you can consider.  I've already mentioned the
bos@202: human-level distinction between ``small picture'' and ``big picture''
bos@202: branches.  While Mercurial works with multiple ``small picture''
bos@202: branches in a repository all the time (for example after you pull
bos@202: changes in, but before you merge them), it can \emph{also} work with
bos@202: multiple ``big picture'' branches.
bos@202: 
bos@202: The key to working this way is that Mercurial lets you assign a
bos@202: persistent \emph{name} to a branch.  There always exists a branch
bos@202: named \texttt{default}.  Even before you start naming branches
bos@202: yourself, you can find traces of the \texttt{default} branch if you
bos@202: look for them.
bos@202: 
bos@202: As an example, when you run the \hgcmd{commit} command, and it pops up
bos@202: your editor so that you can enter a commit message, look for a line
bos@202: that contains the text ``\texttt{HG: branch default}'' at the bottom.
bos@202: This is telling you that your commit will occur on the branch named
bos@202: \texttt{default}.
bos@202: 
bos@202: To start working with named branches, use the \hgcmd{branches}
bos@202: command.  This command lists the named branches already present in
bos@203: your repository, telling you which changeset is the tip of each.
bos@202: \interaction{branch-named.branches}
bos@203: Since you haven't created any named branches yet, the only one that
bos@203: exists is \texttt{default}.
bos@203: 
bos@203: To find out what the ``current'' branch is, run the \hgcmd{branch}
bos@203: command, giving it no arguments.  This tells you what branch the
bos@203: parent of the current changeset is on.
bos@203: \interaction{branch-named.branch}
bos@203: 
bos@203: To create a new branch, run the \hgcmd{branch} command again.  This
bos@203: time, give it one argument: the name of the branch you want to create.
bos@203: \interaction{branch-named.create}
bos@203: 
bos@203: After you've created a branch, you might wonder what effect the
bos@203: \hgcmd{branch} command has had.  What do the \hgcmd{status} and
bos@203: \hgcmd{tip} commands report?
bos@203: \interaction{branch-named.status}
bos@203: Nothing has changed in the working directory, and there's been no new
bos@203: history created.  As this suggests, running the \hgcmd{branch} command
bos@203: has no permanent effect; it only tells Mercurial what branch name to
bos@203: use the \emph{next} time you commit a changeset.
bos@203: 
bos@203: When you commit a change, Mercurial records the name of the branch on
bos@203: which you committed.  Once you've switched from the \texttt{default}
bos@203: branch to another and committed, you'll see the name of the new branch
bos@203: show up in the output of \hgcmd{log}, \hgcmd{tip}, and other commands
bos@203: that display the same kind of output.
bos@203: \interaction{branch-named.commit}
bos@203: The \hgcmd{log}-like commands will print the branch name of every
bos@205: changeset that's not on the \texttt{default} branch.  As a result, if
bos@205: you never use named branches, you'll never see this information.
bos@205: 
bos@205: Once you've named a branch and committed a change with that name,
bos@205: every subsequent commit that descends from that change will inherit
bos@206: the same branch name.  You can change the name of a branch at any
bos@206: time, using the \hgcmd{branch} command.  
bos@206: \interaction{branch-named.rebranch}
bos@206: In practice, this is something you won't do very often, as branch
bos@206: names tend to have fairly long lifetimes.  (This isn't a rule, just an
bos@206: observation.)
bos@206: 
bos@206: \section{Dealing with multiple named branches in a repository}
bos@206: 
bos@206: If you have more than one named branch in a repository, Mercurial will
bos@206: remember the branch that your working directory on when you start a
bos@206: command like \hgcmd{update} or \hgcmdargs{pull}{-u}.  It will update
bos@206: the working directory to the tip of this branch, no matter what the
bos@206: ``repo-wide'' tip is.  To update to a revision that's on a different
bos@206: named branch, you may need to use the \hgopt{update}{-C} option to
bos@206: \hgcmd{update}.
bos@206: 
bos@206: This behaviour is a little subtle, so let's see it in action.  First,
bos@206: let's remind ourselves what branch we're currently on, and what
bos@206: branches are in our repository.
bos@206: \interaction{branch-named.parents}
bos@206: We're on the \texttt{bar} branch, but there also exists an older
bos@206: \hgcmd{foo} branch.
bos@206: 
bos@206: We can \hgcmd{update} back and forth between the tips of the
bos@206: \texttt{foo} and \texttt{bar} branches without needing to use the
bos@206: \hgopt{update}{-C} option, because this only involves going backwards
bos@206: and forwards linearly through our change history.
bos@206: \interaction{branch-named.update-switchy}
bos@206: 
bos@206: If we go back to the \texttt{foo} branch and then run \hgcmd{update},
bos@206: it will keep us on \texttt{foo}, not move us to the tip of
bos@206: \texttt{bar}.
bos@206: \interaction{branch-named.update-nothing}
bos@206: 
bos@206: Committing a new change on the \texttt{foo} branch introduces a new
bos@206: head.
bos@206: \interaction{branch-named.foo-commit}
bos@205: 
bos@205: \section{Branch names and merging}
bos@205: 
bos@205: As you've probably noticed, merges in Mercurial are not symmetrical.
bos@205: Let's say our repository has two heads, 17 and 23.  If I
bos@205: \hgcmd{update} to 17 and then \hgcmd{merge} with 23, Mercurial records
bos@205: 17 as the first parent of the merge, and 23 as the second.  Whereas if
bos@205: I \hgcmd{update} to 23 and then \hgcmd{merge} with 17, it records 23
bos@205: as the first parent, and 17 as the second.
bos@205: 
bos@206: This affects Mercurial's choice of branch name when you merge.  After
bos@206: a merge, Mercurial will retain the branch name of the first parent
bos@206: when you commit the result of the merge.  If your first parent's
bos@206: branch name is \texttt{foo}, and you merge with \texttt{bar}, the
bos@206: branch name will still be \texttt{foo} after you merge.
bos@206: 
bos@206: It's not unusual for a repository to contain multiple heads, each with
bos@206: the same branch name.  Let's say I'm working on the \texttt{foo}
bos@206: branch, and so are you.  We commit different changes; I pull your
bos@206: changes; I now have two heads, each claiming to be on the \texttt{foo}
bos@206: branch.  The result of a merge will be a single head on the
bos@206: \texttt{foo} branch, as you might hope.
bos@206: 
bos@206: But if I'm working on the \texttt{bar} branch, and I merge work from
bos@206: the \texttt{foo} branch, the result will remain on the \texttt{bar}
bos@206: branch.
bos@206: \interaction{branch-named.merge}
bos@206: 
bos@206: To give a more concrete example, if I'm working on the
bos@206: \texttt{bleeding-edge} branch, and I want to bring in the latest fixes
bos@206: from the \texttt{stable} branch, Mercurial will choose the ``right''
bos@206: (\texttt{bleeding-edge}) branch name when I pull and merge from
bos@206: \texttt{stable}.
bos@206: 
bos@206: \section{Branch naming is generally useful}
bos@206: 
bos@206: You shouldn't think of named branches as applicable only to situations
bos@206: where you have multiple long-lived branches cohabiting in a single
bos@206: repository.  They're very useful even in the one-branch-per-repository
bos@206: case.  
bos@206: 
bos@206: In the simplest case, giving a name to each branch gives you a
bos@206: permanent record of which branch a changeset originated on.  This
bab@266: gives you more context when you're trying to follow the history of a
bos@206: long-lived branchy project.
bos@206: 
bos@207: If you're working with shared repositories, you can set up a
bos@207: \hook{pretxnchangegroup} hook on each that will block incoming changes
bos@207: that have the ``wrong'' branch name.  This provides a simple, but
bos@207: effective, defence against people accidentally pushing changes from a
bos@207: ``bleeding edge'' branch to a ``stable'' branch.  Such a hook might
bos@207: look like this inside the shared repo's \hgrc.
bos@207: \begin{codesample2}
bos@207:   [hooks]
bos@207:   pretxnchangegroup.branch = hg heads --template '{branches} ' | grep mybranch
bos@207: \end{codesample2}
bos@202: 
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