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1 \chapter{Tras bambalinas}
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2 \label{chap:concepts}
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3
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4 A diferencia de varios sistemas de control de revisiones, los
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5 conceptos en los que se fundamenta Mercurial son lo suficientemente
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6 simples como para entender fácilmente cómo funciona el software.
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7 Saber esto no es necesario, pero considero útil tener un ``modelo
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8 mental'' de qué es lo que sucede.
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9
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10 Comprender esto me da la confianza de que Mercurial ha sido
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11 cuidadosamente diseñado para ser tanto \emph{seguro} como
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12 \emph{eficiente}. Y tal vez con la misma importancia, si es fácil
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13 para mí hacerme a una idea adecuada de qué está haciendo el software
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14 cuando llevo a cabo una tarea relacionada con control de revisiones,
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15 es menos probable que me sosprenda su comportamiento.
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16
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17 En este capítulo, cubriremos inicialmente los conceptos centrales
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18 del diseño de Mercurial, y luego discutiremos algunos detalles
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19 interesantes de su implementación.
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20
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21 \section{Registro del historial de Mercurial}
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22
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23 \subsection{Seguir el historial de un único fichero}
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24
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25 Cuando Mercurial sigue las modificaciones a un fichero, guarda el
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26 historial de dicho fichero en un objeto de metadatos llamado
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27 of that file in a metadata object called a \emph{filelog}. Each entry
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28 in the filelog contains enough information to reconstruct one revision
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29 of the file that is being tracked. Filelogs are stored as files in
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30 the \sdirname{.hg/store/data} directory. A filelog contains two kinds
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31 of information: revision data, and an index to help Mercurial to find
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32 a revision efficiently.
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33
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34 A file that is large, or has a lot of history, has its filelog stored
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35 in separate data (``\texttt{.d}'' suffix) and index (``\texttt{.i}''
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36 suffix) files. For small files without much history, the revision
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37 data and index are combined in a single ``\texttt{.i}'' file. The
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38 correspondence between a file in the working directory and the filelog
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39 that tracks its history in the repository is illustrated in
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40 figure~\ref{fig:concepts:filelog}.
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41
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42 \begin{figure}[ht]
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43 \centering
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44 \grafix{filelog}
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45 \caption{Relationships between files in working directory and
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46 filelogs in repository}
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47 \label{fig:concepts:filelog}
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48 \end{figure}
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49
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50 \subsection{Managing tracked files}
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51
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52 Mercurial uses a structure called a \emph{manifest} to collect
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53 together information about the files that it tracks. Each entry in
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54 the manifest contains information about the files present in a single
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55 changeset. An entry records which files are present in the changeset,
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56 the revision of each file, and a few other pieces of file metadata.
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57
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58 \subsection{Recording changeset information}
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59
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60 The \emph{changelog} contains information about each changeset. Each
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61 revision records who committed a change, the changeset comment, other
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62 pieces of changeset-related information, and the revision of the
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63 manifest to use.
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64
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65 \subsection{Relationships between revisions}
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66
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67 Within a changelog, a manifest, or a filelog, each revision stores a
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68 pointer to its immediate parent (or to its two parents, if it's a
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69 merge revision). As I mentioned above, there are also relationships
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70 between revisions \emph{across} these structures, and they are
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71 hierarchical in nature.
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72
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73 For every changeset in a repository, there is exactly one revision
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74 stored in the changelog. Each revision of the changelog contains a
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75 pointer to a single revision of the manifest. A revision of the
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76 manifest stores a pointer to a single revision of each filelog tracked
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77 when that changeset was created. These relationships are illustrated
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78 in figure~\ref{fig:concepts:metadata}.
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79
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80 \begin{figure}[ht]
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81 \centering
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82 \grafix{metadata}
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83 \caption{Metadata relationships}
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84 \label{fig:concepts:metadata}
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85 \end{figure}
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86
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87 As the illustration shows, there is \emph{not} a ``one to one''
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88 relationship between revisions in the changelog, manifest, or filelog.
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89 If the manifest hasn't changed between two changesets, the changelog
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90 entries for those changesets will point to the same revision of the
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91 manifest. If a file that Mercurial tracks hasn't changed between two
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92 changesets, the entry for that file in the two revisions of the
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93 manifest will point to the same revision of its filelog.
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94
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95 \section{Safe, efficient storage}
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96
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97 The underpinnings of changelogs, manifests, and filelogs are provided
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98 by a single structure called the \emph{revlog}.
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99
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100 \subsection{Efficient storage}
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101
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102 The revlog provides efficient storage of revisions using a
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103 \emph{delta} mechanism. Instead of storing a complete copy of a file
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104 for each revision, it stores the changes needed to transform an older
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105 revision into the new revision. For many kinds of file data, these
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106 deltas are typically a fraction of a percent of the size of a full
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107 copy of a file.
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108
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109 Some obsolete revision control systems can only work with deltas of
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110 text files. They must either store binary files as complete snapshots
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111 or encoded into a text representation, both of which are wasteful
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112 approaches. Mercurial can efficiently handle deltas of files with
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113 arbitrary binary contents; it doesn't need to treat text as special.
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114
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115 \subsection{Safe operation}
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116 \label{sec:concepts:txn}
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117
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118 Mercurial only ever \emph{appends} data to the end of a revlog file.
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119 It never modifies a section of a file after it has written it. This
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120 is both more robust and efficient than schemes that need to modify or
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121 rewrite data.
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122
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123 In addition, Mercurial treats every write as part of a
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124 \emph{transaction} that can span a number of files. A transaction is
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125 \emph{atomic}: either the entire transaction succeeds and its effects
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126 are all visible to readers in one go, or the whole thing is undone.
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127 This guarantee of atomicity means that if you're running two copies of
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128 Mercurial, where one is reading data and one is writing it, the reader
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129 will never see a partially written result that might confuse it.
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130
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131 The fact that Mercurial only appends to files makes it easier to
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132 provide this transactional guarantee. The easier it is to do stuff
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133 like this, the more confident you should be that it's done correctly.
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134
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135 \subsection{Fast retrieval}
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136
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137 Mercurial cleverly avoids a pitfall common to all earlier
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138 revision control systems: the problem of \emph{inefficient retrieval}.
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139 Most revision control systems store the contents of a revision as an
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140 incremental series of modifications against a ``snapshot''. To
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141 reconstruct a specific revision, you must first read the snapshot, and
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142 then every one of the revisions between the snapshot and your target
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143 revision. The more history that a file accumulates, the more
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144 revisions you must read, hence the longer it takes to reconstruct a
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145 particular revision.
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146
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147 \begin{figure}[ht]
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148 \centering
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149 \grafix{snapshot}
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150 \caption{Snapshot of a revlog, with incremental deltas}
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151 \label{fig:concepts:snapshot}
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152 \end{figure}
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153
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154 The innovation that Mercurial applies to this problem is simple but
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155 effective. Once the cumulative amount of delta information stored
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156 since the last snapshot exceeds a fixed threshold, it stores a new
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157 snapshot (compressed, of course), instead of another delta. This
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158 makes it possible to reconstruct \emph{any} revision of a file
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159 quickly. This approach works so well that it has since been copied by
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160 several other revision control systems.
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161
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162 Figure~\ref{fig:concepts:snapshot} illustrates the idea. In an entry
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163 in a revlog's index file, Mercurial stores the range of entries from
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164 the data file that it must read to reconstruct a particular revision.
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165
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166 \subsubsection{Aside: the influence of video compression}
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167
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168 If you're familiar with video compression or have ever watched a TV
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169 feed through a digital cable or satellite service, you may know that
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170 most video compression schemes store each frame of video as a delta
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171 against its predecessor frame. In addition, these schemes use
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172 ``lossy'' compression techniques to increase the compression ratio, so
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173 visual errors accumulate over the course of a number of inter-frame
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174 deltas.
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175
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176 Because it's possible for a video stream to ``drop out'' occasionally
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177 due to signal glitches, and to limit the accumulation of artefacts
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178 introduced by the lossy compression process, video encoders
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179 periodically insert a complete frame (called a ``key frame'') into the
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180 video stream; the next delta is generated against that frame. This
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181 means that if the video signal gets interrupted, it will resume once
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182 the next key frame is received. Also, the accumulation of encoding
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183 errors restarts anew with each key frame.
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184
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185 \subsection{Identification and strong integrity}
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186
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187 Along with delta or snapshot information, a revlog entry contains a
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188 cryptographic hash of the data that it represents. This makes it
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189 difficult to forge the contents of a revision, and easy to detect
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190 accidental corruption.
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191
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192 Hashes provide more than a mere check against corruption; they are
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193 used as the identifiers for revisions. The changeset identification
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194 hashes that you see as an end user are from revisions of the
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195 changelog. Although filelogs and the manifest also use hashes,
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196 Mercurial only uses these behind the scenes.
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197
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198 Mercurial verifies that hashes are correct when it retrieves file
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199 revisions and when it pulls changes from another repository. If it
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200 encounters an integrity problem, it will complain and stop whatever
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201 it's doing.
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202
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203 In addition to the effect it has on retrieval efficiency, Mercurial's
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204 use of periodic snapshots makes it more robust against partial data
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205 corruption. If a revlog becomes partly corrupted due to a hardware
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206 error or system bug, it's often possible to reconstruct some or most
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207 revisions from the uncorrupted sections of the revlog, both before and
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208 after the corrupted section. This would not be possible with a
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209 delta-only storage model.
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210
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211 \section{Revision history, branching,
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212 and merging}
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213
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214 Every entry in a Mercurial revlog knows the identity of its immediate
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215 ancestor revision, usually referred to as its \emph{parent}. In fact,
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216 a revision contains room for not one parent, but two. Mercurial uses
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217 a special hash, called the ``null ID'', to represent the idea ``there
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218 is no parent here''. This hash is simply a string of zeroes.
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219
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220 In figure~\ref{fig:concepts:revlog}, you can see an example of the
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221 conceptual structure of a revlog. Filelogs, manifests, and changelogs
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222 all have this same structure; they differ only in the kind of data
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223 stored in each delta or snapshot.
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224
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225 The first revision in a revlog (at the bottom of the image) has the
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226 null ID in both of its parent slots. For a ``normal'' revision, its
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227 first parent slot contains the ID of its parent revision, and its
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228 second contains the null ID, indicating that the revision has only one
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229 real parent. Any two revisions that have the same parent ID are
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230 branches. A revision that represents a merge between branches has two
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231 normal revision IDs in its parent slots.
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232
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233 \begin{figure}[ht]
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234 \centering
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235 \grafix{revlog}
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236 \caption{}
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237 \label{fig:concepts:revlog}
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238 \end{figure}
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239
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240 \section{The working directory}
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241
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242 In the working directory, Mercurial stores a snapshot of the files
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243 from the repository as of a particular changeset.
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244
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245 The working directory ``knows'' which changeset it contains. When you
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246 update the working directory to contain a particular changeset,
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247 Mercurial looks up the appropriate revision of the manifest to find
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248 out which files it was tracking at the time that changeset was
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249 committed, and which revision of each file was then current. It then
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250 recreates a copy of each of those files, with the same contents it had
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251 when the changeset was committed.
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252
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253 The \emph{dirstate} contains Mercurial's knowledge of the working
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254 directory. This details which changeset the working directory is
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255 updated to, and all of the files that Mercurial is tracking in the
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256 working directory.
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257
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258 Just as a revision of a revlog has room for two parents, so that it
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259 can represent either a normal revision (with one parent) or a merge of
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260 two earlier revisions, the dirstate has slots for two parents. When
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261 you use the \hgcmd{update} command, the changeset that you update to
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262 is stored in the ``first parent'' slot, and the null ID in the second.
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263 When you \hgcmd{merge} with another changeset, the first parent
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264 remains unchanged, and the second parent is filled in with the
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265 changeset you're merging with. The \hgcmd{parents} command tells you
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266 what the parents of the dirstate are.
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267
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268 \subsection{What happens when you commit}
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269
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270 The dirstate stores parent information for more than just book-keeping
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271 purposes. Mercurial uses the parents of the dirstate as \emph{the
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272 parents of a new changeset} when you perform a commit.
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273
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274 \begin{figure}[ht]
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275 \centering
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276 \grafix{wdir}
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277 \caption{The working directory can have two parents}
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278 \label{fig:concepts:wdir}
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279 \end{figure}
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280
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281 Figure~\ref{fig:concepts:wdir} shows the normal state of the working
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282 directory, where it has a single changeset as parent. That changeset
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283 is the \emph{tip}, the newest changeset in the repository that has no
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284 children.
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285
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286 \begin{figure}[ht]
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287 \centering
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288 \grafix{wdir-after-commit}
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289 \caption{The working directory gains new parents after a commit}
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290 \label{fig:concepts:wdir-after-commit}
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291 \end{figure}
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292
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293 It's useful to think of the working directory as ``the changeset I'm
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294 about to commit''. Any files that you tell Mercurial that you've
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295 added, removed, renamed, or copied will be reflected in that
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296 changeset, as will modifications to any files that Mercurial is
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297 already tracking; the new changeset will have the parents of the
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298 working directory as its parents.
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299
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300 After a commit, Mercurial will update the parents of the working
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301 directory, so that the first parent is the ID of the new changeset,
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302 and the second is the null ID. This is shown in
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303 figure~\ref{fig:concepts:wdir-after-commit}. Mercurial doesn't touch
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304 any of the files in the working directory when you commit; it just
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305 modifies the dirstate to note its new parents.
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306
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307 \subsection{Creating a new head}
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308
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309 It's perfectly normal to update the working directory to a changeset
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310 other than the current tip. For example, you might want to know what
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311 your project looked like last Tuesday, or you could be looking through
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312 changesets to see which one introduced a bug. In cases like this, the
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313 natural thing to do is update the working directory to the changeset
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314 you're interested in, and then examine the files in the working
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315 directory directly to see their contents as they werea when you
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316 committed that changeset. The effect of this is shown in
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317 figure~\ref{fig:concepts:wdir-pre-branch}.
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318
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319 \begin{figure}[ht]
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320 \centering
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321 \grafix{wdir-pre-branch}
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322 \caption{The working directory, updated to an older changeset}
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323 \label{fig:concepts:wdir-pre-branch}
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324 \end{figure}
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325
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326 Having updated the working directory to an older changeset, what
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327 happens if you make some changes, and then commit? Mercurial behaves
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328 in the same way as I outlined above. The parents of the working
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329 directory become the parents of the new changeset. This new changeset
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330 has no children, so it becomes the new tip. And the repository now
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331 contains two changesets that have no children; we call these
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332 \emph{heads}. You can see the structure that this creates in
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333 figure~\ref{fig:concepts:wdir-branch}.
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334
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335 \begin{figure}[ht]
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336 \centering
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337 \grafix{wdir-branch}
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338 \caption{After a commit made while synced to an older changeset}
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339 \label{fig:concepts:wdir-branch}
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340 \end{figure}
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341
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342 \begin{note}
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343 If you're new to Mercurial, you should keep in mind a common
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344 ``error'', which is to use the \hgcmd{pull} command without any
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345 options. By default, the \hgcmd{pull} command \emph{does not}
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346 update the working directory, so you'll bring new changesets into
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347 your repository, but the working directory will stay synced at the
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348 same changeset as before the pull. If you make some changes and
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349 commit afterwards, you'll thus create a new head, because your
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350 working directory isn't synced to whatever the current tip is.
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351
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352 I put the word ``error'' in quotes because all that you need to do
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353 to rectify this situation is \hgcmd{merge}, then \hgcmd{commit}. In
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354 other words, this almost never has negative consequences; it just
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355 surprises people. I'll discuss other ways to avoid this behaviour,
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356 and why Mercurial behaves in this initially surprising way, later
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357 on.
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358 \end{note}
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359
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360 \subsection{Merging heads}
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361
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362 When you run the \hgcmd{merge} command, Mercurial leaves the first
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363 parent of the working directory unchanged, and sets the second parent
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364 to the changeset you're merging with, as shown in
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365 figure~\ref{fig:concepts:wdir-merge}.
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366
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367 \begin{figure}[ht]
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368 \centering
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369 \grafix{wdir-merge}
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370 \caption{Merging two heads}
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371 \label{fig:concepts:wdir-merge}
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372 \end{figure}
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373
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374 Mercurial also has to modify the working directory, to merge the files
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375 managed in the two changesets. Simplified a little, the merging
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376 process goes like this, for every file in the manifests of both
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377 changesets.
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378 \begin{itemize}
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379 \item If neither changeset has modified a file, do nothing with that
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380 file.
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381 \item If one changeset has modified a file, and the other hasn't,
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382 create the modified copy of the file in the working directory.
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383 \item If one changeset has removed a file, and the other hasn't (or
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384 has also deleted it), delete the file from the working directory.
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385 \item If one changeset has removed a file, but the other has modified
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386 the file, ask the user what to do: keep the modified file, or remove
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387 it?
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388 \item If both changesets have modified a file, invoke an external
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389 merge program to choose the new contents for the merged file. This
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390 may require input from the user.
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391 \item If one changeset has modified a file, and the other has renamed
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392 or copied the file, make sure that the changes follow the new name
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393 of the file.
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394 \end{itemize}
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395 There are more details---merging has plenty of corner cases---but
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396 these are the most common choices that are involved in a merge. As
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397 you can see, most cases are completely automatic, and indeed most
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398 merges finish automatically, without requiring your input to resolve
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399 any conflicts.
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400
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401 When you're thinking about what happens when you commit after a merge,
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402 once again the working directory is ``the changeset I'm about to
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403 commit''. After the \hgcmd{merge} command completes, the working
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404 directory has two parents; these will become the parents of the new
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405 changeset.
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406
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407 Mercurial lets you perform multiple merges, but you must commit the
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408 results of each individual merge as you go. This is necessary because
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409 Mercurial only tracks two parents for both revisions and the working
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410 directory. While it would be technically possible to merge multiple
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411 changesets at once, the prospect of user confusion and making a
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412 terrible mess of a merge immediately becomes overwhelming.
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413
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414 \section{Other interesting design features}
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415
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416 In the sections above, I've tried to highlight some of the most
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417 important aspects of Mercurial's design, to illustrate that it pays
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418 careful attention to reliability and performance. However, the
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419 attention to detail doesn't stop there. There are a number of other
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420 aspects of Mercurial's construction that I personally find
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421 interesting. I'll detail a few of them here, separate from the ``big
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422 ticket'' items above, so that if you're interested, you can gain a
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423 better idea of the amount of thinking that goes into a well-designed
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424 system.
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425
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426 \subsection{Clever compression}
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427
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428 When appropriate, Mercurial will store both snapshots and deltas in
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429 compressed form. It does this by always \emph{trying to} compress a
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430 snapshot or delta, but only storing the compressed version if it's
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431 smaller than the uncompressed version.
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432
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433 This means that Mercurial does ``the right thing'' when storing a file
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434 whose native form is compressed, such as a \texttt{zip} archive or a
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435 JPEG image. When these types of files are compressed a second time,
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436 the resulting file is usually bigger than the once-compressed form,
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437 and so Mercurial will store the plain \texttt{zip} or JPEG.
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438
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439 Deltas between revisions of a compressed file are usually larger than
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440 snapshots of the file, and Mercurial again does ``the right thing'' in
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441 these cases. It finds that such a delta exceeds the threshold at
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442 which it should store a complete snapshot of the file, so it stores
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443 the snapshot, again saving space compared to a naive delta-only
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444 approach.
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445
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446 \subsubsection{Network recompression}
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447
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448 When storing revisions on disk, Mercurial uses the ``deflate''
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449 compression algorithm (the same one used by the popular \texttt{zip}
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450 archive format), which balances good speed with a respectable
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451 compression ratio. However, when transmitting revision data over a
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452 network connection, Mercurial uncompresses the compressed revision
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453 data.
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454
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455 If the connection is over HTTP, Mercurial recompresses the entire
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456 stream of data using a compression algorithm that gives a better
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457 compression ratio (the Burrows-Wheeler algorithm from the widely used
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458 \texttt{bzip2} compression package). This combination of algorithm
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459 and compression of the entire stream (instead of a revision at a time)
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460 substantially reduces the number of bytes to be transferred, yielding
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461 better network performance over almost all kinds of network.
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462
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463 (If the connection is over \command{ssh}, Mercurial \emph{doesn't}
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464 recompress the stream, because \command{ssh} can already do this
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465 itself.)
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466
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467 \subsection{Read/write ordering and atomicity}
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468
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469 Appending to files isn't the whole story when it comes to guaranteeing
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470 that a reader won't see a partial write. If you recall
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471 figure~\ref{fig:concepts:metadata}, revisions in the changelog point to
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472 revisions in the manifest, and revisions in the manifest point to
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473 revisions in filelogs. This hierarchy is deliberate.
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474
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475 A writer starts a transaction by writing filelog and manifest data,
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476 and doesn't write any changelog data until those are finished. A
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477 reader starts by reading changelog data, then manifest data, followed
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478 by filelog data.
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479
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480 Since the writer has always finished writing filelog and manifest data
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481 before it writes to the changelog, a reader will never read a pointer
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482 to a partially written manifest revision from the changelog, and it will
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483 never read a pointer to a partially written filelog revision from the
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484 manifest.
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485
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486 \subsection{Concurrent access}
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487
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488 The read/write ordering and atomicity guarantees mean that Mercurial
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489 never needs to \emph{lock} a repository when it's reading data, even
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490 if the repository is being written to while the read is occurring.
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491 This has a big effect on scalability; you can have an arbitrary number
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492 of Mercurial processes safely reading data from a repository safely
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493 all at once, no matter whether it's being written to or not.
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494
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495 The lockless nature of reading means that if you're sharing a
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496 repository on a multi-user system, you don't need to grant other local
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497 users permission to \emph{write} to your repository in order for them
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498 to be able to clone it or pull changes from it; they only need
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499 \emph{read} permission. (This is \emph{not} a common feature among
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500 revision control systems, so don't take it for granted! Most require
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501 readers to be able to lock a repository to access it safely, and this
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502 requires write permission on at least one directory, which of course
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503 makes for all kinds of nasty and annoying security and administrative
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504 problems.)
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505
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506 Mercurial uses locks to ensure that only one process can write to a
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507 repository at a time (the locking mechanism is safe even over
|
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508 filesystems that are notoriously hostile to locking, such as NFS). If
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509 a repository is locked, a writer will wait for a while to retry if the
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510 repository becomes unlocked, but if the repository remains locked for
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511 too long, the process attempting to write will time out after a while.
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512 This means that your daily automated scripts won't get stuck forever
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513 and pile up if a system crashes unnoticed, for example. (Yes, the
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514 timeout is configurable, from zero to infinity.)
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515
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516 \subsubsection{Safe dirstate access}
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517
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518 As with revision data, Mercurial doesn't take a lock to read the
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519 dirstate file; it does acquire a lock to write it. To avoid the
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520 possibility of reading a partially written copy of the dirstate file,
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521 Mercurial writes to a file with a unique name in the same directory as
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522 the dirstate file, then renames the temporary file atomically to
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523 \filename{dirstate}. The file named \filename{dirstate} is thus
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524 guaranteed to be complete, not partially written.
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525
|
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526 \subsection{Avoiding seeks}
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527
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528 Critical to Mercurial's performance is the avoidance of seeks of the
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529 disk head, since any seek is far more expensive than even a
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530 comparatively large read operation.
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531
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532 This is why, for example, the dirstate is stored in a single file. If
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533 there were a dirstate file per directory that Mercurial tracked, the
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534 disk would seek once per directory. Instead, Mercurial reads the
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535 entire single dirstate file in one step.
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536
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537 Mercurial also uses a ``copy on write'' scheme when cloning a
|
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538 repository on local storage. Instead of copying every revlog file
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539 from the old repository into the new repository, it makes a ``hard
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540 link'', which is a shorthand way to say ``these two names point to the
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541 same file''. When Mercurial is about to write to one of a revlog's
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542 files, it checks to see if the number of names pointing at the file is
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543 greater than one. If it is, more than one repository is using the
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544 file, so Mercurial makes a new copy of the file that is private to
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545 this repository.
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546
|
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547 A few revision control developers have pointed out that this idea of
|
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548 making a complete private copy of a file is not very efficient in its
|
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549 use of storage. While this is true, storage is cheap, and this method
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550 gives the highest performance while deferring most book-keeping to the
|
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551 operating system. An alternative scheme would most likely reduce
|
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552 performance and increase the complexity of the software, each of which
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553 is much more important to the ``feel'' of day-to-day use.
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554
|
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555 \subsection{Other contents of the dirstate}
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556
|
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557 Because Mercurial doesn't force you to tell it when you're modifying a
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558 file, it uses the dirstate to store some extra information so it can
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559 determine efficiently whether you have modified a file. For each file
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560 in the working directory, it stores the time that it last modified the
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561 file itself, and the size of the file at that time.
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562
|
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563 When you explicitly \hgcmd{add}, \hgcmd{remove}, \hgcmd{rename} or
|
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564 \hgcmd{copy} files, Mercurial updates the dirstate so that it knows
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565 what to do with those files when you commit.
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566
|
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567 When Mercurial is checking the states of files in the working
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568 directory, it first checks a file's modification time. If that has
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569 not changed, the file must not have been modified. If the file's size
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570 has changed, the file must have been modified. If the modification
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571 time has changed, but the size has not, only then does Mercurial need
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572 to read the actual contents of the file to see if they've changed.
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573 Storing these few extra pieces of information dramatically reduces the
|
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574 amount of data that Mercurial needs to read, which yields large
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575 performance improvements compared to other revision control systems.
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576
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577 %%% Local Variables:
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578 %%% mode: latex
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579 %%% TeX-master: "00book"
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580 %%% End:
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