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     1.4 +\chapter{Managing releases and branchy development}
     1.5 +\label{chap:branch}
     1.6 +
     1.7 +Mercurial provides several mechanisms for you to manage a project that
     1.8 +is making progress on multiple fronts at once.  To understand these
     1.9 +mechanisms, let's first take a brief look at a fairly normal software
    1.10 +project structure.
    1.11 +
    1.12 +Many software projects issue periodic ``major'' releases that contain
    1.13 +substantial new features.  In parallel, they may issue ``minor''
    1.14 +releases.  These are usually identical to the major releases off which
    1.15 +they're based, but with a few bugs fixed.
    1.16 +
    1.17 +In this chapter, we'll start by talking about how to keep records of
    1.18 +project milestones such as releases.  We'll then continue on to talk
    1.19 +about the flow of work between different phases of a project, and how
    1.20 +Mercurial can help you to isolate and manage this work.
    1.21 +
    1.22 +\section{Giving a persistent name to a revision}
    1.23 +
    1.24 +Once you decide that you'd like to call a particular revision a
    1.25 +``release'', it's a good idea to record the identity of that revision.
    1.26 +This will let you reproduce that release at a later date, for whatever
    1.27 +purpose you might need at the time (reproducing a bug, porting to a
    1.28 +new platform, etc).
    1.29 +\interaction{tag.init}
    1.30 +
    1.31 +Mercurial lets you give a permanent name to any revision using the
    1.32 +\hgcmd{tag} command.  Not surprisingly, these names are called
    1.33 +``tags''.
    1.34 +\interaction{tag.tag}
    1.35 +
    1.36 +A tag is nothing more than a ``symbolic name'' for a revision.  Tags
    1.37 +exist purely for your convenience, so that you have a handy permanent
    1.38 +way to refer to a revision; Mercurial doesn't interpret the tag names
    1.39 +you use in any way.  Neither does Mercurial place any restrictions on
    1.40 +the name of a tag, beyond a few that are necessary to ensure that a
    1.41 +tag can be parsed unambiguously.  A tag name cannot contain any of the
    1.42 +following characters:
    1.43 +\begin{itemize}
    1.44 +\item Colon (ASCII 58, ``\texttt{:}'')
    1.45 +\item Carriage return (ASCII 13, ``\Verb+\r+'')
    1.46 +\item Newline (ASCII 10, ``\Verb+\n+'')
    1.47 +\end{itemize}
    1.48 +
    1.49 +You can use the \hgcmd{tags} command to display the tags present in
    1.50 +your repository.  In the output, each tagged revision is identified
    1.51 +first by its name, then by revision number, and finally by the unique
    1.52 +hash of the revision.  
    1.53 +\interaction{tag.tags}
    1.54 +Notice that \texttt{tip} is listed in the output of \hgcmd{tags}.  The
    1.55 +\texttt{tip} tag is a special ``floating'' tag, which always
    1.56 +identifies the newest revision in the repository.
    1.57 +
    1.58 +In the output of the \hgcmd{tags} command, tags are listed in reverse
    1.59 +order, by revision number.  This usually means that recent tags are
    1.60 +listed before older tags.  It also means that \texttt{tip} is always
    1.61 +going to be the first tag listed in the output of \hgcmd{tags}.
    1.62 +
    1.63 +When you run \hgcmd{log}, if it displays a revision that has tags
    1.64 +associated with it, it will print those tags.
    1.65 +\interaction{tag.log}
    1.66 +
    1.67 +Any time you need to provide a revision~ID to a Mercurial command, the
    1.68 +command will accept a tag name in its place.  Internally, Mercurial
    1.69 +will translate your tag name into the corresponding revision~ID, then
    1.70 +use that.
    1.71 +\interaction{tag.log.v1.0}
    1.72 +
    1.73 +There's no limit on the number of tags you can have in a repository,
    1.74 +or on the number of tags that a single revision can have.  As a
    1.75 +practical matter, it's not a great idea to have ``too many'' (a number
    1.76 +which will vary from project to project), simply because tags are
    1.77 +supposed to help you to find revisions.  If you have lots of tags, the
    1.78 +ease of using them to identify revisions diminishes rapidly.
    1.79 +
    1.80 +For example, if your project has milestones as frequent as every few
    1.81 +days, it's perfectly reasonable to tag each one of those.  But if you
    1.82 +have a continuous build system that makes sure every revision can be
    1.83 +built cleanly, you'd be introducing a lot of noise if you were to tag
    1.84 +every clean build.  Instead, you could tag failed builds (on the
    1.85 +assumption that they're rare!), or simply not use tags to track
    1.86 +buildability.
    1.87 +
    1.88 +If you want to remove a tag that you no longer want, use
    1.89 +\hgcmdargs{tag}{--remove}.  
    1.90 +\interaction{tag.remove}
    1.91 +You can also modify a tag at any time, so that it identifies a
    1.92 +different revision, by simply issuing a new \hgcmd{tag} command.
    1.93 +You'll have to use the \hgopt{tag}{-f} option to tell Mercurial that
    1.94 +you \emph{really} want to update the tag.
    1.95 +\interaction{tag.replace}
    1.96 +There will still be a permanent record of the previous identity of the
    1.97 +tag, but Mercurial will no longer use it.  There's thus no penalty to
    1.98 +tagging the wrong revision; all you have to do is turn around and tag
    1.99 +the correct revision once you discover your error.
   1.100 +
   1.101 +Mercurial stores tags in a normal revision-controlled file in your
   1.102 +repository.  If you've created any tags, you'll find them in a file
   1.103 +named \sfilename{.hgtags}.  When you run the \hgcmd{tag} command,
   1.104 +Mercurial modifies this file, then automatically commits the change to
   1.105 +it.  This means that every time you run \hgcmd{tag}, you'll see a
   1.106 +corresponding changeset in the output of \hgcmd{log}.
   1.107 +\interaction{tag.tip}
   1.108 +
   1.109 +\subsection{Handling tag conflicts during a merge}
   1.110 +
   1.111 +You won't often need to care about the \sfilename{.hgtags} file, but
   1.112 +it sometimes makes its presence known during a merge.  The format of
   1.113 +the file is simple: it consists of a series of lines.  Each line
   1.114 +starts with a changeset hash, followed by a space, followed by the
   1.115 +name of a tag.
   1.116 +
   1.117 +If you're resolving a conflict in the \sfilename{.hgtags} file during
   1.118 +a merge, there's one twist to modifying the \sfilename{.hgtags} file:
   1.119 +when Mercurial is parsing the tags in a repository, it \emph{never}
   1.120 +reads the working copy of the \sfilename{.hgtags} file.  Instead, it
   1.121 +reads the \emph{most recently committed} revision of the file.
   1.122 +
   1.123 +An unfortunate consequence of this design is that you can't actually
   1.124 +verify that your merged \sfilename{.hgtags} file is correct until
   1.125 +\emph{after} you've committed a change.  So if you find yourself
   1.126 +resolving a conflict on \sfilename{.hgtags} during a merge, be sure to
   1.127 +run \hgcmd{tags} after you commit.  If it finds an error in the
   1.128 +\sfilename{.hgtags} file, it will report the location of the error,
   1.129 +which you can then fix and commit.  You should then run \hgcmd{tags}
   1.130 +again, just to be sure that your fix is correct.
   1.131 +
   1.132 +\subsection{Tags and cloning}
   1.133 +
   1.134 +You may have noticed that the \hgcmd{clone} command has a
   1.135 +\hgopt{clone}{-r} option that lets you clone an exact copy of the
   1.136 +repository as of a particular changeset.  The new clone will not
   1.137 +contain any project history that comes after the revision you
   1.138 +specified.  This has an interaction with tags that can surprise the
   1.139 +unwary.
   1.140 +
   1.141 +Recall that a tag is stored as a revision to the \sfilename{.hgtags}
   1.142 +file, so that when you create a tag, the changeset in which it's
   1.143 +recorded necessarily refers to an older changeset.  When you run
   1.144 +\hgcmdargs{clone}{-r foo} to clone a repository as of tag
   1.145 +\texttt{foo}, the new clone \emph{will not contain the history that
   1.146 +  created the tag} that you used to clone the repository.  The result
   1.147 +is that you'll get exactly the right subset of the project's history
   1.148 +in the new repository, but \emph{not} the tag you might have expected.
   1.149 +
   1.150 +\subsection{When permanent tags are too much}
   1.151 +
   1.152 +Since Mercurial's tags are revision controlled and carried around with
   1.153 +a project's history, everyone you work with will see the tags you
   1.154 +create.  But giving names to revisions has uses beyond simply noting
   1.155 +that revision \texttt{4237e45506ee} is really \texttt{v2.0.2}.  If
   1.156 +you're trying to track down a subtle bug, you might want a tag to
   1.157 +remind you of something like ``Anne saw the symptoms with this
   1.158 +revision''.
   1.159 +
   1.160 +For cases like this, what you might want to use are \emph{local} tags.
   1.161 +You can create a local tag with the \hgopt{tag}{-l} option to the
   1.162 +\hgcmd{tag} command.  This will store the tag in a file called
   1.163 +\sfilename{.hg/localtags}.  Unlike \sfilename{.hgtags},
   1.164 +\sfilename{.hg/localtags} is not revision controlled.  Any tags you
   1.165 +create using \hgopt{tag}{-l} remain strictly local to the repository
   1.166 +you're currently working in.
   1.167 +
   1.168 +\section{The flow of changes---big picture vs. little}
   1.169 +
   1.170 +To return to the outline I sketched at the beginning of a chapter,
   1.171 +let's think about a project that has multiple concurrent pieces of
   1.172 +work under development at once.
   1.173 +
   1.174 +There might be a push for a new ``main'' release; a new minor bugfix
   1.175 +release to the last main release; and an unexpected ``hot fix'' to an
   1.176 +old release that is now in maintenance mode.
   1.177 +
   1.178 +The usual way people refer to these different concurrent directions of
   1.179 +development is as ``branches''.  However, we've already seen numerous
   1.180 +times that Mercurial treats \emph{all of history} as a series of
   1.181 +branches and merges.  Really, what we have here is two ideas that are
   1.182 +peripherally related, but which happen to share a name.
   1.183 +\begin{itemize}
   1.184 +\item ``Big picture'' branches represent the sweep of a project's
   1.185 +  evolution; people give them names, and talk about them in
   1.186 +  conversation.
   1.187 +\item ``Little picture'' branches are artefacts of the day-to-day
   1.188 +  activity of developing and merging changes.  They expose the
   1.189 +  narrative of how the code was developed.
   1.190 +\end{itemize}
   1.191 +
   1.192 +\section{Managing big-picture branches in repositories}
   1.193 +
   1.194 +The easiest way to isolate a ``big picture'' branch in Mercurial is in
   1.195 +a dedicated repository.  If you have an existing shared
   1.196 +repository---let's call it \texttt{myproject}---that reaches a ``1.0''
   1.197 +milestone, you can start to prepare for future maintenance releases on
   1.198 +top of version~1.0 by tagging the revision from which you prepared
   1.199 +the~1.0 release.
   1.200 +\interaction{branch-repo.tag}
   1.201 +You can then clone a new shared \texttt{myproject-1.0.1} repository as
   1.202 +of that tag.
   1.203 +\interaction{branch-repo.clone}
   1.204 +
   1.205 +Afterwards, if someone needs to work on a bug fix that ought to go
   1.206 +into an upcoming~1.0.1 minor release, they clone the
   1.207 +\texttt{myproject-1.0.1} repository, make their changes, and push them
   1.208 +back.
   1.209 +\interaction{branch-repo.bugfix}
   1.210 +Meanwhile, development for the next major release can continue,
   1.211 +isolated and unabated, in the \texttt{myproject} repository.
   1.212 +\interaction{branch-repo.new}
   1.213 +
   1.214 +\section{Don't repeat yourself: merging across branches}
   1.215 +
   1.216 +In many cases, if you have a bug to fix on a maintenance branch, the
   1.217 +chances are good that the bug exists on your project's main branch
   1.218 +(and possibly other maintenance branches, too).  It's a rare developer
   1.219 +who wants to fix the same bug multiple times, so let's look at a few
   1.220 +ways that Mercurial can help you to manage these bugfixes without
   1.221 +duplicating your work.
   1.222 +
   1.223 +In the simplest instance, all you need to do is pull changes from your
   1.224 +maintenance branch into your local clone of the target branch.
   1.225 +\interaction{branch-repo.pull}
   1.226 +You'll then need to merge the heads of the two branches, and push back
   1.227 +to the main branch.
   1.228 +\interaction{branch-repo.merge}
   1.229 +
   1.230 +\section{Naming branches within one repository}
   1.231 +
   1.232 +In most instances, isolating branches in repositories is the right
   1.233 +approach.  Its simplicity makes it easy to understand; and so it's
   1.234 +hard to make mistakes.  There's a one-to-one relationship between
   1.235 +branches you're working in and directories on your system.  This lets
   1.236 +you use normal (non-Mercurial-aware) tools to work on files within a
   1.237 +branch/repository.
   1.238 +
   1.239 +If you're more in the ``power user'' category (\emph{and} your
   1.240 +collaborators are too), there is an alternative way of handling
   1.241 +branches that you can consider.  I've already mentioned the
   1.242 +human-level distinction between ``small picture'' and ``big picture''
   1.243 +branches.  While Mercurial works with multiple ``small picture''
   1.244 +branches in a repository all the time (for example after you pull
   1.245 +changes in, but before you merge them), it can \emph{also} work with
   1.246 +multiple ``big picture'' branches.
   1.247 +
   1.248 +The key to working this way is that Mercurial lets you assign a
   1.249 +persistent \emph{name} to a branch.  There always exists a branch
   1.250 +named \texttt{default}.  Even before you start naming branches
   1.251 +yourself, you can find traces of the \texttt{default} branch if you
   1.252 +look for them.
   1.253 +
   1.254 +As an example, when you run the \hgcmd{commit} command, and it pops up
   1.255 +your editor so that you can enter a commit message, look for a line
   1.256 +that contains the text ``\texttt{HG: branch default}'' at the bottom.
   1.257 +This is telling you that your commit will occur on the branch named
   1.258 +\texttt{default}.
   1.259 +
   1.260 +To start working with named branches, use the \hgcmd{branches}
   1.261 +command.  This command lists the named branches already present in
   1.262 +your repository, telling you which changeset is the tip of each.
   1.263 +\interaction{branch-named.branches}
   1.264 +Since you haven't created any named branches yet, the only one that
   1.265 +exists is \texttt{default}.
   1.266 +
   1.267 +To find out what the ``current'' branch is, run the \hgcmd{branch}
   1.268 +command, giving it no arguments.  This tells you what branch the
   1.269 +parent of the current changeset is on.
   1.270 +\interaction{branch-named.branch}
   1.271 +
   1.272 +To create a new branch, run the \hgcmd{branch} command again.  This
   1.273 +time, give it one argument: the name of the branch you want to create.
   1.274 +\interaction{branch-named.create}
   1.275 +
   1.276 +After you've created a branch, you might wonder what effect the
   1.277 +\hgcmd{branch} command has had.  What do the \hgcmd{status} and
   1.278 +\hgcmd{tip} commands report?
   1.279 +\interaction{branch-named.status}
   1.280 +Nothing has changed in the working directory, and there's been no new
   1.281 +history created.  As this suggests, running the \hgcmd{branch} command
   1.282 +has no permanent effect; it only tells Mercurial what branch name to
   1.283 +use the \emph{next} time you commit a changeset.
   1.284 +
   1.285 +When you commit a change, Mercurial records the name of the branch on
   1.286 +which you committed.  Once you've switched from the \texttt{default}
   1.287 +branch to another and committed, you'll see the name of the new branch
   1.288 +show up in the output of \hgcmd{log}, \hgcmd{tip}, and other commands
   1.289 +that display the same kind of output.
   1.290 +\interaction{branch-named.commit}
   1.291 +The \hgcmd{log}-like commands will print the branch name of every
   1.292 +changeset that's not on the \texttt{default} branch.  As a result, if
   1.293 +you never use named branches, you'll never see this information.
   1.294 +
   1.295 +Once you've named a branch and committed a change with that name,
   1.296 +every subsequent commit that descends from that change will inherit
   1.297 +the same branch name.  You can change the name of a branch at any
   1.298 +time, using the \hgcmd{branch} command.  
   1.299 +\interaction{branch-named.rebranch}
   1.300 +In practice, this is something you won't do very often, as branch
   1.301 +names tend to have fairly long lifetimes.  (This isn't a rule, just an
   1.302 +observation.)
   1.303 +
   1.304 +\section{Dealing with multiple named branches in a repository}
   1.305 +
   1.306 +If you have more than one named branch in a repository, Mercurial will
   1.307 +remember the branch that your working directory on when you start a
   1.308 +command like \hgcmd{update} or \hgcmdargs{pull}{-u}.  It will update
   1.309 +the working directory to the tip of this branch, no matter what the
   1.310 +``repo-wide'' tip is.  To update to a revision that's on a different
   1.311 +named branch, you may need to use the \hgopt{update}{-C} option to
   1.312 +\hgcmd{update}.
   1.313 +
   1.314 +This behaviour is a little subtle, so let's see it in action.  First,
   1.315 +let's remind ourselves what branch we're currently on, and what
   1.316 +branches are in our repository.
   1.317 +\interaction{branch-named.parents}
   1.318 +We're on the \texttt{bar} branch, but there also exists an older
   1.319 +\hgcmd{foo} branch.
   1.320 +
   1.321 +We can \hgcmd{update} back and forth between the tips of the
   1.322 +\texttt{foo} and \texttt{bar} branches without needing to use the
   1.323 +\hgopt{update}{-C} option, because this only involves going backwards
   1.324 +and forwards linearly through our change history.
   1.325 +\interaction{branch-named.update-switchy}
   1.326 +
   1.327 +If we go back to the \texttt{foo} branch and then run \hgcmd{update},
   1.328 +it will keep us on \texttt{foo}, not move us to the tip of
   1.329 +\texttt{bar}.
   1.330 +\interaction{branch-named.update-nothing}
   1.331 +
   1.332 +Committing a new change on the \texttt{foo} branch introduces a new
   1.333 +head.
   1.334 +\interaction{branch-named.foo-commit}
   1.335 +
   1.336 +\section{Branch names and merging}
   1.337 +
   1.338 +As you've probably noticed, merges in Mercurial are not symmetrical.
   1.339 +Let's say our repository has two heads, 17 and 23.  If I
   1.340 +\hgcmd{update} to 17 and then \hgcmd{merge} with 23, Mercurial records
   1.341 +17 as the first parent of the merge, and 23 as the second.  Whereas if
   1.342 +I \hgcmd{update} to 23 and then \hgcmd{merge} with 17, it records 23
   1.343 +as the first parent, and 17 as the second.
   1.344 +
   1.345 +This affects Mercurial's choice of branch name when you merge.  After
   1.346 +a merge, Mercurial will retain the branch name of the first parent
   1.347 +when you commit the result of the merge.  If your first parent's
   1.348 +branch name is \texttt{foo}, and you merge with \texttt{bar}, the
   1.349 +branch name will still be \texttt{foo} after you merge.
   1.350 +
   1.351 +It's not unusual for a repository to contain multiple heads, each with
   1.352 +the same branch name.  Let's say I'm working on the \texttt{foo}
   1.353 +branch, and so are you.  We commit different changes; I pull your
   1.354 +changes; I now have two heads, each claiming to be on the \texttt{foo}
   1.355 +branch.  The result of a merge will be a single head on the
   1.356 +\texttt{foo} branch, as you might hope.
   1.357 +
   1.358 +But if I'm working on the \texttt{bar} branch, and I merge work from
   1.359 +the \texttt{foo} branch, the result will remain on the \texttt{bar}
   1.360 +branch.
   1.361 +\interaction{branch-named.merge}
   1.362 +
   1.363 +To give a more concrete example, if I'm working on the
   1.364 +\texttt{bleeding-edge} branch, and I want to bring in the latest fixes
   1.365 +from the \texttt{stable} branch, Mercurial will choose the ``right''
   1.366 +(\texttt{bleeding-edge}) branch name when I pull and merge from
   1.367 +\texttt{stable}.
   1.368 +
   1.369 +\section{Branch naming is generally useful}
   1.370 +
   1.371 +You shouldn't think of named branches as applicable only to situations
   1.372 +where you have multiple long-lived branches cohabiting in a single
   1.373 +repository.  They're very useful even in the one-branch-per-repository
   1.374 +case.  
   1.375 +
   1.376 +In the simplest case, giving a name to each branch gives you a
   1.377 +permanent record of which branch a changeset originated on.  This
   1.378 +gives you more context when you're trying to follow the history of a
   1.379 +long-lived branchy project.
   1.380 +
   1.381 +If you're working with shared repositories, you can set up a
   1.382 +\hook{pretxnchangegroup} hook on each that will block incoming changes
   1.383 +that have the ``wrong'' branch name.  This provides a simple, but
   1.384 +effective, defence against people accidentally pushing changes from a
   1.385 +``bleeding edge'' branch to a ``stable'' branch.  Such a hook might
   1.386 +look like this inside the shared repo's \hgrc.
   1.387 +\begin{codesample2}
   1.388 +  [hooks]
   1.389 +  pretxnchangegroup.branch = hg heads --template '{branches} ' | grep mybranch
   1.390 +\end{codesample2}
   1.391 +
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