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: bos@104: %%% Local Variables: bos@104: %%% mode: latex bos@104: %%% TeX-master: "00book" bos@104: %%% End: