
bos@104: \chapter{Advanced uses of Mercurial Queues}
bos@104: 
bos@104: While it's easy to pick up straightforward uses of Mercurial Queues,
bos@104: use of a little discipline and some of MQ's less frequently used
bos@104: capabilities makes it possible to work in complicated development
bos@104: environments.
bos@104: 
bos@104: In this chapter, I will discuss a technique I have developed to manage
bos@104: the development of an Infiniband device driver for the Linux kernel.
bos@104: The driver in question is large (at least as drivers go), with 25,000
bos@104: lines of code spread across 35 source files.  It is maintained by a
bos@104: small team of developers.
bos@104: 
bos@104: While much of the material in this chapter is specific to Linux, the
bos@104: same principles apply to any code base for which you're not the
bos@104: primary owner, and upon which you need to do a lot of development.
bos@104: 
bos@104: \section{The problem of many targets}
bos@104: 
bos@104: The Linux kernel changes rapidly, and has never been internally
bos@104: stable; developers frequently make drastic changes between releases.
bos@104: This means that a version of the driver that works well with a
bos@104: particular released version of the kernel will not even \emph{compile}
bos@104: correctly against, typically, any other version.
bos@104: 
bos@104: To maintain a driver, we have to keep a number of distinct versions of
bos@104: Linux in mind.
bos@104: \begin{itemize}
bos@104: \item One target is the main Linux kernel development tree.
bos@104:   Maintenance of the code is in this case partly shared by other
bos@104:   developers in the kernel community, who make ``drive-by''
bos@104:   modifications to the driver as they develop and refine kernel
bos@104:   subsystems.
bos@104: \item We also maintain a number of ``backports'' to older versions of
bos@104:   the Linux kernel, to support the needs of customers who are running
bos@104:   older Linux distributions that do not incorporate our drivers.
bos@104: \item Finally, we make software releases on a schedule that is
bos@104:   necessarily not aligned with those used by Linux distributors and
bos@104:   kernel developers, so that we can deliver new features to customers
bos@104:   without forcing them to upgrade their entire kernels or
bos@104:   distributions.
bos@104: \end{itemize}
bos@104: 
bos@104: \subsection{Tempting approaches that don't work well}
bos@104: 
bos@104: There are two ``standard'' ways to maintain a piece of software that
bos@104: has to target many different environments.
bos@104: 
bos@104: The first is to maintain a number of branches, each intended for a
bos@104: single target.  The trouble with this approach is that you must
bos@104: maintain iron discipline in the flow of changes between repositories.
bos@104: A new feature or bug fix must start life in a ``pristine'' repository,
bos@104: then percolate out to every backport repository.  Backport changes are
bos@104: more limited in the branches they should propagate to; a backport
bos@104: change that is applied to a branch where it doesn't belong will
bos@104: probably stop the driver from compiling.
bos@104: 
bos@104: The second is to maintain a single source tree filled with conditional
bos@104: statements that turn chunks of code on or off depending on the
bos@104: intended target.  Because these ``ifdefs'' are not allowed in the
bos@104: Linux kernel tree, a manual or automatic process must be followed to
bos@104: strip them out and yield a clean tree.  A code base maintained in this
bos@104: fashion rapidly becomes a rat's nest of conditional blocks that are
bos@104: difficult to understand and maintain.
bos@104: 
bos@104: Neither of these approaches is well suited to a situation where you
bos@104: don't ``own'' the canonical copy of a source tree.  In the case of a
bos@104: Linux driver that is distributed with the standard kernel, Linus's
bos@104: tree contains the copy of the code that will be treated by the world
bos@104: as canonical.  The upstream version of ``my'' driver can be modified
bos@104: by people I don't know, without me even finding out about it until
bos@104: after the changes show up in Linus's tree.  
bos@104: 
bos@104: These approaches have the added weakness of making it difficult to
bos@104: generate well-formed patches to submit upstream.
bos@104: 
bos@104: In principle, Mercurial Queues seems like a good candidate to manage a
bos@104: development scenario such as the above.  While this is indeed the
bos@104: case, MQ contains a few added features that make the job more
bos@104: pleasant.
bos@104: 
bos@104: \section{Conditionally applying patches with guards}
bos@104: 
bos@104: Perhaps the best way to maintain sanity with so many targets is to be
bos@104: able to choose specific patches to apply for a given situation.  MQ
bos@104: provides a feature called ``guards'' (which originates with quilt's
bos@104: \texttt{guards} command) that does just this.  To start off, let's
bos@104: create a simple repository for experimenting in.
bos@104: \interaction{mq.guards.init}
bos@104: This gives us a tiny repository that contains two patches that don't
bos@104: have any dependencies on each other, because they touch different files.
bos@104: 
bos@104: The idea behind conditional application is that you can ``tag'' a
bos@104: patch with a \emph{guard}, which is simply a text string of your
bos@104: choosing, then tell MQ to select specific guards to use when applying
bos@104: patches.  MQ will then either apply, or skip over, a guarded patch,
bos@104: depending on the guards that you have selected.
bos@104: 
bos@104: A patch can have an arbitrary number of guards;
bos@104: each one is \emph{positive} (``apply this patch if this guard is
bos@104: selected'') or \emph{negative} (``skip this patch if this guard is
bos@104: selected'').  A patch with no guards is always applied.
bos@104: 
bos@104: \section{Controlling the guards on a patch}
bos@104: 
bos@104: The \hgcmd{qguard} command lets you determine which guards should
bos@104: apply to a patch, or display the guards that are already in effect.
bos@104: Without any arguments, it displays the guards on the current topmost
bos@104: patch.
bos@104: \interaction{mq.guards.qguard}
bos@104: To set a positive guard on a patch, prefix the name of the guard with
bos@104: a ``\texttt{+}''.
bos@104: \interaction{mq.guards.qguard.pos}
bos@104: To set a negative guard on a patch, prefix the name of the guard with
bos@104: a ``\texttt{-}''.
bos@104: \interaction{mq.guards.qguard.neg}
bos@104: 
bos@104: \begin{note}
bos@104:   The \hgcmd{qguard} command \emph{sets} the guards on a patch; it
bos@104:   doesn't \emph{modify} them.  What this means is that if you run
bos@104:   \hgcmdargs{qguard}{+a +b} on a patch, then \hgcmdargs{qguard}{+c} on
bos@104:   the same patch, the \emph{only} guard that will be set on it
bos@104:   afterwards is \texttt{+c}.
bos@104: \end{note}
bos@104: 
bos@104: Mercurial stores guards in the \sfilename{series} file; the form in
bos@104: which they are stored is easy both to understand and to edit by hand.
bos@104: (In other words, you don't have to use the \hgcmd{qguard} command if
bos@104: you don't want to; it's okay to simply edit the \sfilename{series}
bos@104: file.)
bos@104: \interaction{mq.guards.series}
bos@104: 
bos@104: \section{Selecting the guards to use}
bos@104: 
bos@104: The \hgcmd{qselect} command determines which guards are active at a
bos@104: given time.  The effect of this is to determine which patches MQ will
bos@104: apply the next time you run \hgcmd{qpush}.  It has no other effect; in
bos@104: particular, it doesn't do anything to patches that are already
bos@104: applied.
bos@104: 
bos@104: With no arguments, the \hgcmd{qselect} command lists the guards
bos@104: currently in effect, one per line of output.  Each argument is treated
bos@104: as the name of a guard to apply.
bos@104: \interaction{mq.guards.qselect.foo}
bos@104: In case you're interested, the currently selected guards are stored in
bos@104: the \sfilename{guards} file.
bos@104: \interaction{mq.guards.qselect.cat}
bos@104: We can see the effect the selected guards have when we run
bos@104: \hgcmd{qpush}.
bos@104: \interaction{mq.guards.qselect.qpush}
bos@104: 
bos@104: A guard cannot start with a ``\texttt{+}'' or ``\texttt{-}''
bos@104: character.
bos@104: \interaction{mq.guards.qselect.error}
bos@104: Changing the selected guards changes the patches that are applied.
bos@104: \interaction{mq.guards.qselect.quux}
bos@104: You can see here that negative guards take precedence over positive
bos@104: guards.
bos@104: \interaction{mq.guards.qselect.foobar}
bos@104: 
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