
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@105: In this chapter, I will use as an example a technique I have used to
bos@105: manage the development of an Infiniband device driver for the Linux
bos@105: kernel.  The driver in question is large (at least as drivers go),
bos@105: with 25,000 lines of code spread across 35 source files.  It is
bos@105: maintained by a 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@105:   older Linux distributions that do not incorporate our drivers.  (To
bos@105:   \emph{backport} a piece of code is to modify it to work in an older
bos@105:   version of its target environment than the version it was developed
bos@105:   for.)
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@105: \section{Conditionally applying patches with 
bos@105:   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@105: character.  The name of a guard must start with an alphabetic
bos@105: character (upper or lower case) or an underscore.  The rest of the
bos@105: guard's name can contain any of these characters, or a digit.  These
bos@105: rules are similar to those used for variable naming in most popular
bos@105: programming languages.  If you try to use a guard with an invalid
bos@105: name, MQ will complain:
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@105: You can see in the example below that negative guards take precedence
bos@105: over positive guards.
bos@104: \interaction{mq.guards.qselect.foobar}
bos@104: 
bos@105: \section{MQ's rules for applying patches}
bos@105: 
bos@105: The rules that MQ uses when deciding whether to apply a patch
bos@105: are as follows.
bos@105: \begin{itemize}
bos@105: \item A patch that has no guards is always applied.
bos@105: \item If the patch has any negative guard that matches any currently
bos@105:   selected guard, the patch is skipped.
bos@105: \item If the patch has any positive guard that matches any currently
bos@105:   selected guard, the patch is applied.
bos@105: \item If the patch has positive or negative guards, but none matches
bos@105:   any currently selected guard, the patch is skipped.
bos@105: \end{itemize}
bos@105: 
bos@105: \section{Trimming the work environment}
bos@105: 
bos@105: In working on the device driver I mentioned earlier, I don't apply the
bos@105: patches to a normal Linux kernel tree.  Instead, I use a repository
bos@105: that contains only a snapshot of the source files and headers that are
bos@105: relevant to Infiniband development.  This repository is~1\% the size
bos@105: of a kernel repository, so it's easier to work with.
bos@105: 
bos@105: I then choose a ``base'' version on top of which the patches are
bos@105: applied.  This is a snapshot of the Linux kernel tree as of a revision
bos@105: of my choosing.  When I take the snapshot, I record the changeset ID
bos@105: from the kernel repository in the commit message.  Since the snapshot
bos@105: preserves the ``shape'' and content of the relevant parts of the
bos@105: kernel tree, I can apply my patches on top of either my tiny
bos@105: repository or a normal kernel tree.
bos@105: 
bos@105: Normally, the base tree atop which the patches apply should be a
bos@105: snapshot of a very recent upstream tree.  This best facilitates the
bos@105: development of patches that can easily be submitted upstream with few
bos@105: or no modifications.
bos@105: 
bos@105: \section{Dividing up the \sfilename{series} file}
bos@105: 
bos@105: I categorise the patches in the \sfilename{series} file into a number
bos@105: of logical groups.  Each section of like patches begins with a block
bos@105: of comments that describes the purpose of the patches that follow.
bos@105: 
bos@105: The sequence of patch groups that I maintain follows.  The ordering of
bos@105: these groups is important; I'll describe why after I introduce the
bos@105: groups.
bos@105: \begin{itemize}
bos@105: \item The ``accepted'' group.  Patches that the development team has
bos@105:   submitted to the maintainer of the Infiniband subsystem, and which
bos@105:   he has accepted, but which are not present in the snapshot that the
bos@105:   tiny repository is based on.  These are ``read only'' patches,
bos@105:   present only to transform the tree into a similar state as it is in
bos@105:   the upstream maintainer's repository.
bos@105: \item The ``rework'' group.  Patches that I have submitted, but that
bos@105:   the upstream maintainer has requested modifications to before he
bos@105:   will accept them.
bos@105: \item The ``pending'' group.  Patches that I have not yet submitted to
bos@105:   the upstream maintainer, but which we have finished working on.
bos@105:   These will be ``read only'' for a while.  If the upstream maintainer
bos@105:   accepts them upon submission, I'll move them to the end of the
bos@105:   ``accepted'' group.  If he requests that I modify any, I'll move
bos@105:   them to the beginning of the ``rework'' group.
bos@105: \item The ``in progress'' group.  Patches that are actively being
bos@105:   developed, and should not be submitted anywhere yet.
bos@105: \item The ``backport'' group.  Patches that adapt the source tree to
bos@105:   older versions of the kernel tree.
bos@105: \item The ``do not ship'' group.  Patches that for some reason should
bos@105:   never be submitted upstream.  For example, one such patch might
bos@105:   change embedded driver identification strings to make it easier to
bos@105:   distinguish, in the field, between an out-of-tree version of the
bos@105:   driver and a version shipped by a distribution vendor.
bos@105: \end{itemize}
bos@105: 
bos@105: Now to return to the reasons for ordering groups of patches in this
bos@105: way.  We would like the lowest patches in the stack to be as stable as
bos@105: possible, so that we will not need to rework higher patches due to
bos@105: changes in context.  Putting patches that will never be changed first
bos@105: in the \sfilename{series} file serves this purpose.
bos@105: 
bos@105: We would also like the patches that we know we'll need to modify to be
bos@105: applied on top of a source tree that resembles the upstream tree as
bos@105: closely as possible.  This is why we keep accepted patches around for
bos@105: a while.
bos@105: 
bos@105: The ``backport'' and ``do not ship'' patches float at the end of the
bos@105: \sfilename{series} file in part because they'll never be shipped
bos@105: upstream.  Additionally, the backport patches must be applied on top
bos@105: of all other patches.
bos@105: 
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