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1 \chapter{Adding functionality with extensions}
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2 \label{chap:hgext}
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3
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4 While the core of Mercurial is quite complete from a functionality
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5 standpoint, it's deliberately shorn of fancy features. This approach
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6 of preserving simplicity keeps the software easy to deal with for both
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7 maintainers and users.
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8
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9 However, Mercurial doesn't box you in with an inflexible command set:
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10 you can add features to it as \emph{extensions} (sometimes known as
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11 \emph{plugins}). We've already discussed a few of these extensions in
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12 earlier chapters.
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13 \begin{itemize}
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14 \item Section~\ref{sec:tour-merge:fetch} covers the \hgext{fetch}
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15 extension; this combines pulling new changes and merging them with
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16 local changes into a single command, \hgcmd{fetch}.
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17 \item The \hgext{bisect} extension adds an efficient pruning search
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18 for changes that introduced bugs, and we documented it in
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19 chapter~\ref{sec:undo:bisect}.
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20 \item In chapter~\ref{chap:hook}, we covered several extensions that
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21 are useful for hook-related functionality: \hgext{acl} adds access
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22 control lists; \hgext{bugzilla} adds integration with the Bugzilla
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23 bug tracking system; and \hgext{notify} sends notification emails on
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24 new changes.
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25 \item The Mercurial Queues patch management extension is so invaluable
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26 that it merits two chapters and an appendix all to itself.
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27 Chapter~\ref{chap:mq} covers the basics;
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28 chapter~\ref{chap:mq-collab} discusses advanced topics; and
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29 appendix~\ref{chap:mqref} goes into detail on each command.
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30 \end{itemize}
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31
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32 In this chapter, we'll cover some of the other extensions that are
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33 available for Mercurial, and briefly touch on some of the machinery
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34 you'll need to know about if you want to write an extension of your
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35 own.
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36 \begin{itemize}
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37 \item In section~\ref{sec:hgext:inotify}, we'll discuss the
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38 possibility of \emph{huge} performance improvements using the
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39 \hgext{inotify} extension.
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40 \end{itemize}
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41
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42 \section{Improve performance with the \hgext{inotify} extension}
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43 \label{sec:hgext:inotify}
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44
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45 Are you interested in having some of the most common Mercurial
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46 operations run as much as a hundred times faster? Read on!
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47
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48 Mercurial has great performance under normal circumstances. For
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49 example, when you run the \hgcmd{status} command, Mercurial has to
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50 scan almost every directory and file in your repository so that it can
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51 display file status. Many other Mercurial commands need to do the
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52 same work behind the scenes; for example, the \hgcmd{diff} command
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53 uses the status machinery to avoid doing an expensive comparison
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54 operation on files that obviously haven't changed.
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55
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56 Because obtaining file status is crucial to good performance, the
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57 authors of Mercurial have optimised this code to within an inch of its
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58 life. However, there's no avoiding the fact that when you run
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59 \hgcmd{status}, Mercurial is going to have to perform at least one
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60 expensive system call for each managed file to determine whether it's
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61 changed since the last time Mercurial checked. For a sufficiently
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62 large repository, this can take a long time.
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63
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64 To put a number on the magnitude of this effect, I created a
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65 repository containing 150,000 managed files. I timed \hgcmd{status}
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66 as taking ten seconds to run, even when \emph{none} of those files had
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67 been modified.
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68
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69 Many modern operating systems contain a file notification facility.
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70 If a program signs up to an appropriate service, the operating system
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71 will notify it every time a file of interest is created, modified, or
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72 deleted. On Linux systems, the kernel component that does this is
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73 called \texttt{inotify}.
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74
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75 Mercurial's \hgext{inotify} extension talks to the kernel's
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76 \texttt{inotify} component to optimise \hgcmd{status} commands. The
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77 extension has two components. A daemon sits in the background and
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78 receives notifications from the \texttt{inotify} subsystem. It also
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79 listens for connections from a regular Mercurial command. The
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80 extension modifies Mercurial's behaviour so that instead of scanning
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81 the filesystem, it queries the daemon. Since the daemon has perfect
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82 information about the state of the repository, it can respond with a
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83 result instantaneously, avoiding the need to scan every directory and
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84 file in the repository.
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85
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86 Recall the ten seconds that I measured plain Mercurial as taking to
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87 run \hgcmd{status} on a 150,000 file repository. With the
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88 \hgext{inotify} extension enabled, the time dropped to 0.1~seconds, a
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89 factor of \emph{one hundred} faster.
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90
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91 Before we continue, please pay attention to some caveats.
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92 \begin{itemize}
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93 \item The \hgext{inotify} extension is Linux-specific. Because it
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94 interfaces directly to the Linux kernel's \texttt{inotify}
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95 subsystem, it does not work on other operating systems.
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96 \item It should work on any Linux distribution that was released after
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97 early~2005. Older distributions are likely to have a kernel that
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98 lacks \texttt{inotify}, or a version of \texttt{glibc} that does not
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99 have the necessary interfacing support.
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100 \item Not all filesystems are suitable for use with the
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101 \hgext{inotify} extension. Network filesystems such as NFS are a
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102 non-starter, for example, particularly if you're running Mercurial
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103 on several systems, all mounting the same network filesystem. The
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104 kernel's \texttt{inotify} system has no way of knowing about changes
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105 made on another system. Most local filesystems (e.g.~ext3, XFS,
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106 ReiserFS) should work fine.
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107 \end{itemize}
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108
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109 The \hgext{inotify} extension is not yet shipped with Mercurial as of
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110 May~2007, so it's a little more involved to set up than other
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111 extensions. But the performance improvement is worth it!
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112
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113 The extension currently comes in two parts: a set of patches to the
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114 Mercurial source code, and a library of Python bindings to the
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115 \texttt{inotify} subsystem.
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116 \begin{note}
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117 There are \emph{two} Python \texttt{inotify} binding libraries. One
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118 of them is called \texttt{pyinotify}, and is packaged by some Linux
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119 distributions as \texttt{python-inotify}. This is \emph{not} the
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120 one you'll need, as it is too buggy and inefficient to be practical.
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121 \end{note}
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122 To get going, it's best to already have a functioning copy of
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123 Mercurial installed.
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124 \begin{note}
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125 If you follow the instructions below, you'll be \emph{replacing} and
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126 overwriting any existing installation of Mercurial that you might
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127 already have, using the latest ``bleeding edge'' Mercurial code.
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128 Don't say you weren't warned!
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129 \end{note}
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130 \begin{enumerate}
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131 \item Clone the Python \texttt{inotify} binding repository. Build and
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132 install it.
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133 \begin{codesample4}
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134 hg clone http://hg.kublai.com/python/inotify
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135 cd inotify
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136 python setup.py build --force
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137 sudo python setup.py install --skip-build
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138 \end{codesample4}
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139 \item Clone the \dirname{crew} Mercurial repository. Clone the
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140 \hgext{inotify} patch repository so that Mercurial Queues will be
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141 able to apply patches to your cope of the \dirname{crew} repository.
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142 \begin{codesample4}
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143 hg clone http://hg.intevation.org/mercurial/crew
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144 hg clone crew inotify
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145 hg clone http://hg.kublai.com/mercurial/patches/inotify inotify/.hg/patches
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146 \end{codesample4}
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147 \item Make sure that you have the Mercurial Queues extension,
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148 \hgext{mq}, enabled. If you've never used MQ, read
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149 section~\ref{sec:mq:start} to get started quickly.
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150 \item Go into the \dirname{inotify} repo, and apply all of the
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151 \hgext{inotify} patches using the \hgopt{qpush}{-a} option to the
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152 \hgcmd{qpush} command.
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153 \begin{codesample4}
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154 cd inotify
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155 hg qpush -a
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156 \end{codesample4}
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157 If you get an error message from \hgcmd{qpush}, you should not
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158 continue. Instead, ask for help.
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159 \item Build and install the patched version of Mercurial.
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160 \begin{codesample4}
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161 python setup.py build --force
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162 sudo python setup.py install --skip-build
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163 \end{codesample4}
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164 \end{enumerate}
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165 Once you've build a suitably patched version of Mercurial, all you
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166 need to do to enable the \hgext{inotify} extension is add an entry to
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167 your \hgrc.
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168 \begin{codesample2}
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169 [extensions]
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170 inotify =
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171 \end{codesample2}
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172 When the \hgext{inotify} extension is enabled, Mercurial will
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173 automatically and transparently start the status daemon the first time
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174 you run a command that needs status in a repository. It runs one
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175 status daemon per repository.
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176
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177 The status daemon is started silently, and runs in the background. If
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178 you look at a list of running processes after you've enabled the
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179 \hgext{inotify} extension and run a few commands in different
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180 repositories, you'll thus see a few \texttt{hg} processes sitting
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181 around, waiting for updates from the kernel and queries from
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182 Mercurial.
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183
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184 The first time you run a Mercurial command in a repository when you
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185 have the \hgext{inotify} extension enabled, it will run with about the
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186 same performance as a normal Mercurial command. This is because the
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187 status daemon needs to perform a normal status scan so that it has a
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188 baseline against which to apply later updates from the kernel.
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189 However, \emph{every} subsequent command that does any kind of status
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190 check should be noticeably faster on repositories of even fairly
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191 modest size. Better yet, the bigger your repository is, the greater a
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192 performance advantage you'll see. The \hgext{inotify} daemon makes
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193 status operations almost instantaneous on repositories of all sizes!
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194
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195 If you like, you can manually start a status daemon using the
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196 \hgcmd{inserve} command. This gives you slightly finer control over
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197 how the daemon ought to run. This command will of course only be
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198 available when the \hgext{inotify} extension is enabled.
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199
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200 When you're using the \hgext{inotify} extension, you should notice
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201 \emph{no difference at all} in Mercurial's behaviour, with the sole
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202 exception of status-related commands running a whole lot faster than
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203 they used to. You should specifically expect that commands will not
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204 print different output; neither should they give different results.
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205 If either of these situations occurs, please report a bug.
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206
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207 \section{Flexible diff support with the \hgext{extdiff} extension}
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208 \label{sec:hgext:extdiff}
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209
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210 Mercurial's built-in \hgcmd{diff} command outputs plaintext unified
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211 diffs.
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212 \interaction{extdiff.diff}
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213 If you would like to use an external tool to display modifications,
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214 you'll want to use the \hgext{extdiff} extension. This will let you
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215 use, for example, a graphical diff tool.
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216
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217 The \hgext{extdiff} extension is bundled with Mercurial, so it's easy
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218 to set up. In the \rcsection{extensions} section of your \hgrc,
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219 simply add a one-line entry to enable the extension.
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220 \begin{codesample2}
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221 [extensions]
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222 extdiff =
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223 \end{codesample2}
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224 This introduces a command named \hgcmd{extdiff}, which by default uses
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225 your system's \command{diff} command to generate a unified diff in the
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226 same form as the built-in \hgcmd{diff} command.
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227 \interaction{extdiff.extdiff}
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228 The result won't be exactly the same as with the built-in \hgcmd{diff}
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229 variations, because the output of \command{diff} varies from one
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230 system to another, even when passed the same options.
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231
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232 As the ``\texttt{making snapshot}'' lines of output above imply, the
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233 \hgcmd{extdiff} command works by creating two snapshots of your source
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234 tree. The first snapshot is of the source revision; the second, of
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235 the target revision or working directory. The \hgcmd{extdiff} command
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236 generates these snapshots in a temporary directory, passes the name of
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237 each directory to an external diff viewer, then deletes the temporary
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238 directory. For efficiency, it only snapshots the directories and
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239 files that have changed between the two revisions.
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240
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241 Snapshot directory names have the same base name as your repository.
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242 If your repository path is \dirname{/quux/bar/foo}, then \dirname{foo}
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243 will be the name of each snapshot directory. Each snapshot directory
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244 name has its changeset ID appended, if appropriate. If a snapshot is
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245 of revision \texttt{a631aca1083f}, the directory will be named
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246 \dirname{foo.a631aca1083f}. A snapshot of the working directory won't
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247 have a changeset ID appended, so it would just be \dirname{foo} in
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248 this example. To see what this looks like in practice, look again at
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249 the \hgcmd{extdiff} example above. Notice that the diff has the
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250 snapshot directory names embedded in its header.
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251
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252 The \hgcmd{extdiff} command accepts two important options. The
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253 \hgopt{extdiff}{-p} option lets you choose a program to view
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254 differences with, instead of \command{diff}. With the
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255 \hgopt{extdiff}{-o} option, you can change the options that
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256 \hgcmd{extdiff} passes to the program (by default, these options are
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257 ``\texttt{-Npru}'', which only make sense if you're running
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258 \command{diff}). In other respects, the \hgcmd{extdiff} acts
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259 similarly to the built-in \hgcmd{diff} command: you use the same
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260 option names, syntax, and arguments to specify the revisions you want,
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261 the files you want, and so on.
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262
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263 As an example, here's how to run the normal system \command{diff}
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264 command, getting it to generate context diffs (using the
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265 \cmdopt{diff}{-c} option) instead of unified diffs, and five lines of
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266 context instead of the default three (passing \texttt{5} as the
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267 argument to the \cmdopt{diff}{-C} option).
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268 \interaction{extdiff.extdiff-ctx}
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269
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270 Launching a visual diff tool is just as easy. Here's how to launch
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271 the \command{kdiff3} viewer.
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272 \begin{codesample2}
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273 hg extdiff -p kdiff3 -o ''
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274 \end{codesample2}
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275
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276 If your diff viewing command can't deal with directories, you can
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277 easily work around this with a little scripting. For an example of
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278 such scripting in action with the \hgext{mq} extension and the
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279 \command{interdiff} command, see
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280 section~\ref{mq-collab:tips:interdiff}.
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281
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282 \subsection{Defining command aliases}
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283
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284 It can be cumbersome to remember the options to both the
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285 \hgcmd{extdiff} command and the diff viewer you want to use, so the
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286 \hgext{extdiff} extension lets you define \emph{new} commands that
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287 will invoke your diff viewer with exactly the right options.
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288
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289 All you need to do is edit your \hgrc, and add a section named
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290 \rcsection{extdiff}. Inside this section, you can define multiple
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291 commands. Here's how to add a \texttt{kdiff3} command. Once you've
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292 defined this, you can type ``\texttt{hg kdiff3}'' and the
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293 \hgext{extdiff} extension will run \command{kdiff3} for you.
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294 \begin{codesample2}
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295 [extdiff]
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296 cmd.kdiff3 =
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297 \end{codesample2}
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298 If you leave the right hand side of the definition empty, as above,
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299 the \hgext{extdiff} extension uses the name of the command you defined
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300 as the name of the external program to run. But these names don't
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301 have to be the same. Here, we define a command named ``\texttt{hg
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302 wibble}'', which runs \command{kdiff3}.
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303 \begin{codesample2}
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304 [extdiff]
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305 cmd.wibble = kdiff3
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306 \end{codesample2}
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307
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308 You can also specify the default options that you want to invoke your
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309 diff viewing program with. The prefix to use is ``\texttt{opts.}'',
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310 followed by the name of the command to which the options apply. This
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311 example defines a ``\texttt{hg vimdiff}'' command that runs the
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312 \command{vim} editor's \texttt{DirDiff} extension.
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313 \begin{codesample2}
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314 [extdiff]
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315 cmd.vimdiff = vim
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316 opts.vimdiff = -f '+next' '+execute "DirDiff" argv(0) argv(1)'
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317 \end{codesample2}
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318
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319 %%% Local Variables:
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320 %%% mode: latex
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321 %%% TeX-master: "00book"
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322 %%% End:
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