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1 \chapter{Advanced uses of Mercurial Queues}
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2 \label{chap:mq-collab}
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3
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4 While it's easy to pick up straightforward uses of Mercurial Queues,
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5 use of a little discipline and some of MQ's less frequently used
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6 capabilities makes it possible to work in complicated development
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7 environments.
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8
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9 In this chapter, I will use as an example a technique I have used to
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10 manage the development of an Infiniband device driver for the Linux
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11 kernel. The driver in question is large (at least as drivers go),
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12 with 25,000 lines of code spread across 35 source files. It is
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13 maintained by a small team of developers.
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14
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15 While much of the material in this chapter is specific to Linux, the
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16 same principles apply to any code base for which you're not the
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17 primary owner, and upon which you need to do a lot of development.
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18
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19 \section{The problem of many targets}
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20
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21 The Linux kernel changes rapidly, and has never been internally
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22 stable; developers frequently make drastic changes between releases.
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23 This means that a version of the driver that works well with a
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24 particular released version of the kernel will not even \emph{compile}
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25 correctly against, typically, any other version.
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26
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27 To maintain a driver, we have to keep a number of distinct versions of
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28 Linux in mind.
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29 \begin{itemize}
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30 \item One target is the main Linux kernel development tree.
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31 Maintenance of the code is in this case partly shared by other
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32 developers in the kernel community, who make ``drive-by''
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33 modifications to the driver as they develop and refine kernel
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34 subsystems.
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35 \item We also maintain a number of ``backports'' to older versions of
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36 the Linux kernel, to support the needs of customers who are running
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37 older Linux distributions that do not incorporate our drivers. (To
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38 \emph{backport} a piece of code is to modify it to work in an older
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39 version of its target environment than the version it was developed
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40 for.)
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41 \item Finally, we make software releases on a schedule that is
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42 necessarily not aligned with those used by Linux distributors and
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43 kernel developers, so that we can deliver new features to customers
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44 without forcing them to upgrade their entire kernels or
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45 distributions.
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46 \end{itemize}
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47
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48 \subsection{Tempting approaches that don't work well}
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49
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50 There are two ``standard'' ways to maintain a piece of software that
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51 has to target many different environments.
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52
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53 The first is to maintain a number of branches, each intended for a
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54 single target. The trouble with this approach is that you must
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55 maintain iron discipline in the flow of changes between repositories.
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56 A new feature or bug fix must start life in a ``pristine'' repository,
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57 then percolate out to every backport repository. Backport changes are
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58 more limited in the branches they should propagate to; a backport
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59 change that is applied to a branch where it doesn't belong will
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60 probably stop the driver from compiling.
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61
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62 The second is to maintain a single source tree filled with conditional
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63 statements that turn chunks of code on or off depending on the
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64 intended target. Because these ``ifdefs'' are not allowed in the
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65 Linux kernel tree, a manual or automatic process must be followed to
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66 strip them out and yield a clean tree. A code base maintained in this
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67 fashion rapidly becomes a rat's nest of conditional blocks that are
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68 difficult to understand and maintain.
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69
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70 Neither of these approaches is well suited to a situation where you
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71 don't ``own'' the canonical copy of a source tree. In the case of a
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72 Linux driver that is distributed with the standard kernel, Linus's
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73 tree contains the copy of the code that will be treated by the world
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74 as canonical. The upstream version of ``my'' driver can be modified
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75 by people I don't know, without me even finding out about it until
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76 after the changes show up in Linus's tree.
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77
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78 These approaches have the added weakness of making it difficult to
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79 generate well-formed patches to submit upstream.
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80
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81 In principle, Mercurial Queues seems like a good candidate to manage a
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82 development scenario such as the above. While this is indeed the
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83 case, MQ contains a few added features that make the job more
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84 pleasant.
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85
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86 \section{Conditionally applying patches with
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87 guards}
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88
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89 Perhaps the best way to maintain sanity with so many targets is to be
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90 able to choose specific patches to apply for a given situation. MQ
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91 provides a feature called ``guards'' (which originates with quilt's
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92 \texttt{guards} command) that does just this. To start off, let's
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93 create a simple repository for experimenting in.
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94 \interaction{mq.guards.init}
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95 This gives us a tiny repository that contains two patches that don't
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96 have any dependencies on each other, because they touch different files.
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97
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98 The idea behind conditional application is that you can ``tag'' a
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99 patch with a \emph{guard}, which is simply a text string of your
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100 choosing, then tell MQ to select specific guards to use when applying
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101 patches. MQ will then either apply, or skip over, a guarded patch,
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102 depending on the guards that you have selected.
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103
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104 A patch can have an arbitrary number of guards;
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105 each one is \emph{positive} (``apply this patch if this guard is
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106 selected'') or \emph{negative} (``skip this patch if this guard is
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107 selected''). A patch with no guards is always applied.
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108
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109 \section{Controlling the guards on a patch}
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110
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111 The \hgxcmd{mq}{qguard} command lets you determine which guards should
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112 apply to a patch, or display the guards that are already in effect.
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113 Without any arguments, it displays the guards on the current topmost
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114 patch.
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115 \interaction{mq.guards.qguard}
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116 To set a positive guard on a patch, prefix the name of the guard with
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117 a ``\texttt{+}''.
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118 \interaction{mq.guards.qguard.pos}
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119 To set a negative guard on a patch, prefix the name of the guard with
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120 a ``\texttt{-}''.
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121 \interaction{mq.guards.qguard.neg}
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122
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123 \begin{note}
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124 The \hgxcmd{mq}{qguard} command \emph{sets} the guards on a patch; it
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125 doesn't \emph{modify} them. What this means is that if you run
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126 \hgcmdargs{qguard}{+a +b} on a patch, then \hgcmdargs{qguard}{+c} on
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127 the same patch, the \emph{only} guard that will be set on it
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128 afterwards is \texttt{+c}.
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129 \end{note}
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130
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131 Mercurial stores guards in the \sfilename{series} file; the form in
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132 which they are stored is easy both to understand and to edit by hand.
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133 (In other words, you don't have to use the \hgxcmd{mq}{qguard} command if
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134 you don't want to; it's okay to simply edit the \sfilename{series}
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135 file.)
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136 \interaction{mq.guards.series}
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137
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138 \section{Selecting the guards to use}
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139
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140 The \hgxcmd{mq}{qselect} command determines which guards are active at a
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141 given time. The effect of this is to determine which patches MQ will
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142 apply the next time you run \hgxcmd{mq}{qpush}. It has no other effect; in
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143 particular, it doesn't do anything to patches that are already
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144 applied.
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145
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146 With no arguments, the \hgxcmd{mq}{qselect} command lists the guards
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147 currently in effect, one per line of output. Each argument is treated
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148 as the name of a guard to apply.
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149 \interaction{mq.guards.qselect.foo}
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150 In case you're interested, the currently selected guards are stored in
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151 the \sfilename{guards} file.
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152 \interaction{mq.guards.qselect.cat}
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153 We can see the effect the selected guards have when we run
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154 \hgxcmd{mq}{qpush}.
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155 \interaction{mq.guards.qselect.qpush}
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156
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157 A guard cannot start with a ``\texttt{+}'' or ``\texttt{-}''
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158 character. The name of a guard must not contain white space, but most
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159 other characters are acceptable. If you try to use a guard with an
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160 invalid name, MQ will complain:
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161 \interaction{mq.guards.qselect.error}
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162 Changing the selected guards changes the patches that are applied.
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163 \interaction{mq.guards.qselect.quux}
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164 You can see in the example below that negative guards take precedence
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165 over positive guards.
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166 \interaction{mq.guards.qselect.foobar}
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167
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168 \section{MQ's rules for applying patches}
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169
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170 The rules that MQ uses when deciding whether to apply a patch
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171 are as follows.
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172 \begin{itemize}
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173 \item A patch that has no guards is always applied.
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174 \item If the patch has any negative guard that matches any currently
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175 selected guard, the patch is skipped.
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176 \item If the patch has any positive guard that matches any currently
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177 selected guard, the patch is applied.
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178 \item If the patch has positive or negative guards, but none matches
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179 any currently selected guard, the patch is skipped.
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180 \end{itemize}
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181
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182 \section{Trimming the work environment}
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183
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184 In working on the device driver I mentioned earlier, I don't apply the
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185 patches to a normal Linux kernel tree. Instead, I use a repository
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186 that contains only a snapshot of the source files and headers that are
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187 relevant to Infiniband development. This repository is~1\% the size
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188 of a kernel repository, so it's easier to work with.
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189
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190 I then choose a ``base'' version on top of which the patches are
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191 applied. This is a snapshot of the Linux kernel tree as of a revision
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192 of my choosing. When I take the snapshot, I record the changeset ID
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193 from the kernel repository in the commit message. Since the snapshot
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194 preserves the ``shape'' and content of the relevant parts of the
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195 kernel tree, I can apply my patches on top of either my tiny
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196 repository or a normal kernel tree.
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197
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198 Normally, the base tree atop which the patches apply should be a
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199 snapshot of a very recent upstream tree. This best facilitates the
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200 development of patches that can easily be submitted upstream with few
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201 or no modifications.
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202
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203 \section{Dividing up the \sfilename{series} file}
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204
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205 I categorise the patches in the \sfilename{series} file into a number
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206 of logical groups. Each section of like patches begins with a block
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207 of comments that describes the purpose of the patches that follow.
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208
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209 The sequence of patch groups that I maintain follows. The ordering of
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210 these groups is important; I'll describe why after I introduce the
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211 groups.
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212 \begin{itemize}
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213 \item The ``accepted'' group. Patches that the development team has
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214 submitted to the maintainer of the Infiniband subsystem, and which
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215 he has accepted, but which are not present in the snapshot that the
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216 tiny repository is based on. These are ``read only'' patches,
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217 present only to transform the tree into a similar state as it is in
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218 the upstream maintainer's repository.
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219 \item The ``rework'' group. Patches that I have submitted, but that
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220 the upstream maintainer has requested modifications to before he
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221 will accept them.
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222 \item The ``pending'' group. Patches that I have not yet submitted to
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223 the upstream maintainer, but which we have finished working on.
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224 These will be ``read only'' for a while. If the upstream maintainer
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225 accepts them upon submission, I'll move them to the end of the
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226 ``accepted'' group. If he requests that I modify any, I'll move
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227 them to the beginning of the ``rework'' group.
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228 \item The ``in progress'' group. Patches that are actively being
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229 developed, and should not be submitted anywhere yet.
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230 \item The ``backport'' group. Patches that adapt the source tree to
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231 older versions of the kernel tree.
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232 \item The ``do not ship'' group. Patches that for some reason should
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233 never be submitted upstream. For example, one such patch might
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234 change embedded driver identification strings to make it easier to
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235 distinguish, in the field, between an out-of-tree version of the
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236 driver and a version shipped by a distribution vendor.
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237 \end{itemize}
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238
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239 Now to return to the reasons for ordering groups of patches in this
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240 way. We would like the lowest patches in the stack to be as stable as
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241 possible, so that we will not need to rework higher patches due to
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242 changes in context. Putting patches that will never be changed first
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243 in the \sfilename{series} file serves this purpose.
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244
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245 We would also like the patches that we know we'll need to modify to be
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246 applied on top of a source tree that resembles the upstream tree as
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247 closely as possible. This is why we keep accepted patches around for
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248 a while.
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249
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250 The ``backport'' and ``do not ship'' patches float at the end of the
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251 \sfilename{series} file. The backport patches must be applied on top
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252 of all other patches, and the ``do not ship'' patches might as well
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253 stay out of harm's way.
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254
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255 \section{Maintaining the patch series}
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256
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257 In my work, I use a number of guards to control which patches are to
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258 be applied.
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259
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260 \begin{itemize}
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261 \item ``Accepted'' patches are guarded with \texttt{accepted}. I
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262 enable this guard most of the time. When I'm applying the patches
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263 on top of a tree where the patches are already present, I can turn
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264 this patch off, and the patches that follow it will apply cleanly.
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265 \item Patches that are ``finished'', but not yet submitted, have no
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266 guards. If I'm applying the patch stack to a copy of the upstream
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267 tree, I don't need to enable any guards in order to get a reasonably
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268 safe source tree.
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269 \item Those patches that need reworking before being resubmitted are
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270 guarded with \texttt{rework}.
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271 \item For those patches that are still under development, I use
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272 \texttt{devel}.
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273 \item A backport patch may have several guards, one for each version
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274 of the kernel to which it applies. For example, a patch that
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275 backports a piece of code to~2.6.9 will have a~\texttt{2.6.9} guard.
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276 \end{itemize}
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277 This variety of guards gives me considerable flexibility in
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278 qdetermining what kind of source tree I want to end up with. For most
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279 situations, the selection of appropriate guards is automated during
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280 the build process, but I can manually tune the guards to use for less
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281 common circumstances.
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282
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283 \subsection{The art of writing backport patches}
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284
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285 Using MQ, writing a backport patch is a simple process. All such a
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286 patch has to do is modify a piece of code that uses a kernel feature
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287 not present in the older version of the kernel, so that the driver
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288 continues to work correctly under that older version.
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289
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290 A useful goal when writing a good backport patch is to make your code
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291 look as if it was written for the older version of the kernel you're
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292 targeting. The less obtrusive the patch, the easier it will be to
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293 understand and maintain. If you're writing a collection of backport
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294 patches to avoid the ``rat's nest'' effect of lots of
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295 \texttt{\#ifdef}s (hunks of source code that are only used
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296 conditionally) in your code, don't introduce version-dependent
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297 \texttt{\#ifdef}s into the patches. Instead, write several patches,
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298 each of which makes unconditional changes, and control their
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299 application using guards.
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300
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301 There are two reasons to divide backport patches into a distinct
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302 group, away from the ``regular'' patches whose effects they modify.
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303 The first is that intermingling the two makes it more difficult to use
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304 a tool like the \hgext{patchbomb} extension to automate the process of
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305 submitting the patches to an upstream maintainer. The second is that
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306 a backport patch could perturb the context in which a subsequent
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307 regular patch is applied, making it impossible to apply the regular
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308 patch cleanly \emph{without} the earlier backport patch already being
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309 applied.
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310
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311 \section{Useful tips for developing with MQ}
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312
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313 \subsection{Organising patches in directories}
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314
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315 If you're working on a substantial project with MQ, it's not difficult
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316 to accumulate a large number of patches. For example, I have one
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317 patch repository that contains over 250 patches.
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318
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319 If you can group these patches into separate logical categories, you
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320 can if you like store them in different directories; MQ has no
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321 problems with patch names that contain path separators.
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322
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323 \subsection{Viewing the history of a patch}
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324 \label{mq-collab:tips:interdiff}
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325
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326 If you're developing a set of patches over a long time, it's a good
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327 idea to maintain them in a repository, as discussed in
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328 section~\ref{sec:mq:repo}. If you do so, you'll quickly discover that
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329 using the \hgcmd{diff} command to look at the history of changes to a
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330 patch is unworkable. This is in part because you're looking at the
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331 second derivative of the real code (a diff of a diff), but also
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332 because MQ adds noise to the process by modifying time stamps and
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333 directory names when it updates a patch.
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334
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335 However, you can use the \hgext{extdiff} extension, which is bundled
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336 with Mercurial, to turn a diff of two versions of a patch into
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337 something readable. To do this, you will need a third-party package
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338 called \package{patchutils}~\cite{web:patchutils}. This provides a
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339 command named \command{interdiff}, which shows the differences between
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340 two diffs as a diff. Used on two versions of the same diff, it
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341 generates a diff that represents the diff from the first to the second
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342 version.
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343
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344 You can enable the \hgext{extdiff} extension in the usual way, by
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345 adding a line to the \rcsection{extensions} section of your \hgrc.
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346 \begin{codesample2}
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347 [extensions]
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348 extdiff =
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349 \end{codesample2}
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350 The \command{interdiff} command expects to be passed the names of two
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351 files, but the \hgext{extdiff} extension passes the program it runs a
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352 pair of directories, each of which can contain an arbitrary number of
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353 files. We thus need a small program that will run \command{interdiff}
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354 on each pair of files in these two directories. This program is
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355 available as \sfilename{hg-interdiff} in the \dirname{examples}
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356 directory of the source code repository that accompanies this book.
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357 \excode{hg-interdiff}
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358
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359 With the \sfilename{hg-interdiff} program in your shell's search path,
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360 you can run it as follows, from inside an MQ patch directory:
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361 \begin{codesample2}
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362 hg extdiff -p hg-interdiff -r A:B my-change.patch
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363 \end{codesample2}
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364 Since you'll probably want to use this long-winded command a lot, you
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365 can get \hgext{hgext} to make it available as a normal Mercurial
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366 command, again by editing your \hgrc.
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367 \begin{codesample2}
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368 [extdiff]
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369 cmd.interdiff = hg-interdiff
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370 \end{codesample2}
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371 This directs \hgext{hgext} to make an \texttt{interdiff} command
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372 available, so you can now shorten the previous invocation of
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373 \hgxcmd{extdiff}{extdiff} to something a little more wieldy.
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374 \begin{codesample2}
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375 hg interdiff -r A:B my-change.patch
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376 \end{codesample2}
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377
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378 \begin{note}
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379 The \command{interdiff} command works well only if the underlying
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380 files against which versions of a patch are generated remain the
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381 same. If you create a patch, modify the underlying files, and then
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382 regenerate the patch, \command{interdiff} may not produce useful
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383 output.
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384 \end{note}
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385
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386 The \hgext{extdiff} extension is useful for more than merely improving
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387 the presentation of MQ~patches. To read more about it, go to
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388 section~\ref{sec:hgext:extdiff}.
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389
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390 %%% Local Variables:
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391 %%% mode: latex
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392 %%% TeX-master: "00book"
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393 %%% End:
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