
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@196: \item Carriage return (ASCII 13, ``\texttt{$\backslash$r}'')
bos@196: \item Newline (ASCII 10, ``\texttt{$\backslash$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
bos@198: \hgopt{clone}{-r} option that lets you clone an exact copy of
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@198: 
bos@197: 
bos@187: %%% Local Variables: 
bos@187: %%% mode: latex
bos@187: %%% TeX-master: "00book"
bos@187: %%% End: 




