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1 \chapter{Managing releases and branchy development}
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2 \label{chap:branch}
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3
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4 Mercurial provides several mechanisms for you to manage a project that
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5 is making progress on multiple fronts at once. To understand these
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6 mechanisms, let's first take a brief look at a fairly normal software
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7 project structure.
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8
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9 Many software projects issue periodic ``major'' releases that contain
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10 substantial new features. In parallel, they may issue ``minor''
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11 releases. These are usually identical to the major releases off which
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12 they're based, but with a few bugs fixed.
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13
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14 In this chapter, we'll start by talking about how to keep records of
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15 project milestones such as releases. We'll then continue on to talk
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16 about the flow of work between different phases of a project, and how
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17 Mercurial can help you to isolate and manage this work.
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18
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19 \section{Giving a persistent name to a revision}
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20
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21 Once you decide that you'd like to call a particular revision a
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22 ``release'', it's a good idea to record the identity of that revision.
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23 This will let you reproduce that release at a later date, for whatever
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24 purpose you might need at the time (reproducing a bug, porting to a
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25 new platform, etc).
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26 \interaction{tag.init}
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27
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28 Mercurial lets you give a permanent name to any revision using the
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29 \hgcmd{tag} command. Not surprisingly, these names are called
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30 ``tags''.
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31 \interaction{tag.tag}
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32
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33 A tag is nothing more than a ``symbolic name'' for a revision. Tags
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34 exist purely for your convenience, so that you have a handy permanent
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35 way to refer to a revision; Mercurial doesn't interpret the tag names
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36 you use in any way. Neither does Mercurial place any restrictions on
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37 the name of a tag, beyond a few that are necessary to ensure that a
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38 tag can be parsed unambiguously. A tag name cannot contain any of the
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39 following characters:
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40 \begin{itemize}
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41 \item Colon (ASCII 58, ``\texttt{:}'')
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42 \item Carriage return (ASCII 13, ``\Verb+\r+'')
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43 \item Newline (ASCII 10, ``\Verb+\n+'')
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44 \end{itemize}
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45
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46 You can use the \hgcmd{tags} command to display the tags present in
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47 your repository. In the output, each tagged revision is identified
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48 first by its name, then by revision number, and finally by the unique
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49 hash of the revision.
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50 \interaction{tag.tags}
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51 Notice that \texttt{tip} is listed in the output of \hgcmd{tags}. The
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52 \texttt{tip} tag is a special ``floating'' tag, which always
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53 identifies the newest revision in the repository.
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54
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55 In the output of the \hgcmd{tags} command, tags are listed in reverse
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56 order, by revision number. This usually means that recent tags are
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57 listed before older tags. It also means that \texttt{tip} is always
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58 going to be the first tag listed in the output of \hgcmd{tags}.
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59
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60 When you run \hgcmd{log}, if it displays a revision that has tags
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61 associated with it, it will print those tags.
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62 \interaction{tag.log}
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63
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64 Any time you need to provide a revision~ID to a Mercurial command, the
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65 command will accept a tag name in its place. Internally, Mercurial
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66 will translate your tag name into the corresponding revision~ID, then
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67 use that.
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68 \interaction{tag.log.v1.0}
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69
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70 There's no limit on the number of tags you can have in a repository,
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71 or on the number of tags that a single revision can have. As a
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72 practical matter, it's not a great idea to have ``too many'' (a number
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73 which will vary from project to project), simply because tags are
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74 supposed to help you to find revisions. If you have lots of tags, the
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75 ease of using them to identify revisions diminishes rapidly.
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76
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77 For example, if your project has milestones as frequent as every few
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78 days, it's perfectly reasonable to tag each one of those. But if you
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79 have a continuous build system that makes sure every revision can be
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80 built cleanly, you'd be introducing a lot of noise if you were to tag
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81 every clean build. Instead, you could tag failed builds (on the
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82 assumption that they're rare!), or simply not use tags to track
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83 buildability.
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84
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85 If you want to remove a tag that you no longer want, use
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86 \hgcmdargs{tag}{--remove}.
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87 \interaction{tag.remove}
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88 You can also modify a tag at any time, so that it identifies a
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89 different revision, by simply issuing a new \hgcmd{tag} command.
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90 You'll have to use the \hgopt{tag}{-f} option to tell Mercurial that
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91 you \emph{really} want to update the tag.
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92 \interaction{tag.replace}
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93 There will still be a permanent record of the previous identity of the
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94 tag, but Mercurial will no longer use it. There's thus no penalty to
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95 tagging the wrong revision; all you have to do is turn around and tag
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96 the correct revision once you discover your error.
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97
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98 Mercurial stores tags in a normal revision-controlled file in your
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99 repository. If you've created any tags, you'll find them in a file
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100 named \sfilename{.hgtags}. When you run the \hgcmd{tag} command,
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101 Mercurial modifies this file, then automatically commits the change to
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102 it. This means that every time you run \hgcmd{tag}, you'll see a
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103 corresponding changeset in the output of \hgcmd{log}.
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104 \interaction{tag.tip}
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105
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106 \subsection{Handling tag conflicts during a merge}
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107
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108 You won't often need to care about the \sfilename{.hgtags} file, but
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109 it sometimes makes its presence known during a merge. The format of
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110 the file is simple: it consists of a series of lines. Each line
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111 starts with a changeset hash, followed by a space, followed by the
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112 name of a tag.
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113
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114 If you're resolving a conflict in the \sfilename{.hgtags} file during
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115 a merge, there's one twist to modifying the \sfilename{.hgtags} file:
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116 when Mercurial is parsing the tags in a repository, it \emph{never}
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117 reads the working copy of the \sfilename{.hgtags} file. Instead, it
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118 reads the \emph{most recently committed} revision of the file.
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119
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120 An unfortunate consequence of this design is that you can't actually
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121 verify that your merged \sfilename{.hgtags} file is correct until
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122 \emph{after} you've committed a change. So if you find yourself
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123 resolving a conflict on \sfilename{.hgtags} during a merge, be sure to
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124 run \hgcmd{tags} after you commit. If it finds an error in the
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125 \sfilename{.hgtags} file, it will report the location of the error,
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126 which you can then fix and commit. You should then run \hgcmd{tags}
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127 again, just to be sure that your fix is correct.
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128
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129 \subsection{Tags and cloning}
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130
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131 You may have noticed that the \hgcmd{clone} command has a
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132 \hgopt{clone}{-r} option that lets you clone an exact copy of the
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133 repository as of a particular changeset. The new clone will not
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134 contain any project history that comes after the revision you
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135 specified. This has an interaction with tags that can surprise the
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136 unwary.
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137
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138 Recall that a tag is stored as a revision to the \sfilename{.hgtags}
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139 file, so that when you create a tag, the changeset in which it's
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140 recorded necessarily refers to an older changeset. When you run
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141 \hgcmdargs{clone}{-r foo} to clone a repository as of tag
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142 \texttt{foo}, the new clone \emph{will not contain the history that
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143 created the tag} that you used to clone the repository. The result
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144 is that you'll get exactly the right subset of the project's history
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145 in the new repository, but \emph{not} the tag you might have expected.
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146
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147 \subsection{When permanent tags are too much}
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148
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149 Since Mercurial's tags are revision controlled and carried around with
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150 a project's history, everyone you work with will see the tags you
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151 create. But giving names to revisions has uses beyond simply noting
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152 that revision \texttt{4237e45506ee} is really \texttt{v2.0.2}. If
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153 you're trying to track down a subtle bug, you might want a tag to
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154 remind you of something like ``Anne saw the symptoms with this
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155 revision''.
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156
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157 For cases like this, what you might want to use are \emph{local} tags.
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158 You can create a local tag with the \hgopt{tag}{-l} option to the
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159 \hgcmd{tag} command. This will store the tag in a file called
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160 \sfilename{.hg/localtags}. Unlike \sfilename{.hgtags},
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161 \sfilename{.hg/localtags} is not revision controlled. Any tags you
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162 create using \hgopt{tag}{-l} remain strictly local to the repository
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163 you're currently working in.
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164
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165 \section{The flow of changes---big picture vs. little}
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166
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167 To return to the outline I sketched at the beginning of a chapter,
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168 let's think about a project that has multiple concurrent pieces of
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169 work under development at once.
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170
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171 There might be a push for a new ``main'' release; a new minor bugfix
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172 release to the last main release; and an unexpected ``hot fix'' to an
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173 old release that is now in maintenance mode.
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174
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175 The usual way people refer to these different concurrent directions of
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176 development is as ``branches''. However, we've already seen numerous
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177 times that Mercurial treats \emph{all of history} as a series of
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178 branches and merges. Really, what we have here is two ideas that are
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179 peripherally related, but which happen to share a name.
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180 \begin{itemize}
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181 \item ``Big picture'' branches represent the sweep of a project's
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182 evolution; people give them names, and talk about them in
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183 conversation.
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184 \item ``Little picture'' branches are artefacts of the day-to-day
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185 activity of developing and merging changes. They expose the
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186 narrative of how the code was developed.
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187 \end{itemize}
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188
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189 \section{Managing big-picture branches in repositories}
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190
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191 The easiest way to isolate a ``big picture'' branch in Mercurial is in
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192 a dedicated repository. If you have an existing shared
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193 repository---let's call it \texttt{myproject}---that reaches a ``1.0''
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194 milestone, you can start to prepare for future maintenance releases on
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195 top of version~1.0 by tagging the revision from which you prepared
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196 the~1.0 release.
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197 \interaction{branch-repo.tag}
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198 You can then clone a new shared \texttt{myproject-1.0.1} repository as
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199 of that tag.
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200 \interaction{branch-repo.clone}
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201
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202 Afterwards, if someone needs to work on a bug fix that ought to go
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203 into an upcoming~1.0.1 minor release, they clone the
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204 \texttt{myproject-1.0.1} repository, make their changes, and push them
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205 back.
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206 \interaction{branch-repo.bugfix}
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207 Meanwhile, development for the next major release can continue,
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208 isolated and unabated, in the \texttt{myproject} repository.
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209 \interaction{branch-repo.new}
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210
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211 \section{Don't repeat yourself: merging across branches}
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212
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213 In many cases, if you have a bug to fix on a maintenance branch, the
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214 chances are good that the bug exists on your project's main branch
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215 (and possibly other maintenance branches, too). It's a rare developer
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216 who wants to fix the same bug multiple times, so let's look at a few
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217 ways that Mercurial can help you to manage these bugfixes without
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218 duplicating your work.
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219
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220 In the simplest instance, all you need to do is pull changes from your
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221 maintenance branch into your local clone of the target branch.
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222 \interaction{branch-repo.pull}
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223 You'll then need to merge the heads of the two branches, and push back
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224 to the main branch.
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225 \interaction{branch-repo.merge}
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226
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227 \section{Naming branches within one repository}
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228
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229 In most instances, isolating branches in repositories is the right
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230 approach. Its simplicity makes it easy to understand; and so it's
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231 hard to make mistakes. There's a one-to-one relationship between
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232 branches you're working in and directories on your system. This lets
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233 you use normal (non-Mercurial-aware) tools to work on files within a
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234 branch/repository.
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235
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236 If you're more in the ``power user'' category (\emph{and} your
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237 collaborators are too), there is an alternative way of handling
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238 branches that you can consider. I've already mentioned the
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239 human-level distinction between ``small picture'' and ``big picture''
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240 branches. While Mercurial works with multiple ``small picture''
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241 branches in a repository all the time (for example after you pull
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242 changes in, but before you merge them), it can \emph{also} work with
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243 multiple ``big picture'' branches.
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244
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245 The key to working this way is that Mercurial lets you assign a
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246 persistent \emph{name} to a branch. There always exists a branch
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247 named \texttt{default}. Even before you start naming branches
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248 yourself, you can find traces of the \texttt{default} branch if you
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249 look for them.
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250
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251 As an example, when you run the \hgcmd{commit} command, and it pops up
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252 your editor so that you can enter a commit message, look for a line
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253 that contains the text ``\texttt{HG: branch default}'' at the bottom.
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254 This is telling you that your commit will occur on the branch named
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255 \texttt{default}.
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256
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257 To start working with named branches, use the \hgcmd{branches}
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258 command. This command lists the named branches already present in
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259 your repository, telling you which changeset is the tip of each.
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260 \interaction{branch-named.branches}
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261 Since you haven't created any named branches yet, the only one that
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262 exists is \texttt{default}.
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263
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264 To find out what the ``current'' branch is, run the \hgcmd{branch}
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265 command, giving it no arguments. This tells you what branch the
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266 parent of the current changeset is on.
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267 \interaction{branch-named.branch}
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268
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269 To create a new branch, run the \hgcmd{branch} command again. This
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270 time, give it one argument: the name of the branch you want to create.
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271 \interaction{branch-named.create}
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272
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273 After you've created a branch, you might wonder what effect the
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274 \hgcmd{branch} command has had. What do the \hgcmd{status} and
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275 \hgcmd{tip} commands report?
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276 \interaction{branch-named.status}
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277 Nothing has changed in the working directory, and there's been no new
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278 history created. As this suggests, running the \hgcmd{branch} command
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279 has no permanent effect; it only tells Mercurial what branch name to
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280 use the \emph{next} time you commit a changeset.
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281
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282 When you commit a change, Mercurial records the name of the branch on
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283 which you committed. Once you've switched from the \texttt{default}
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284 branch to another and committed, you'll see the name of the new branch
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285 show up in the output of \hgcmd{log}, \hgcmd{tip}, and other commands
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286 that display the same kind of output.
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287 \interaction{branch-named.commit}
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288 The \hgcmd{log}-like commands will print the branch name of every
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289 changeset that's not on the \texttt{default} branch. As a result, if
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290 you never use named branches, you'll never see this information.
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291
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292 Once you've named a branch and committed a change with that name,
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293 every subsequent commit that descends from that change will inherit
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294 the same branch name. You can change the name of a branch at any
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295 time, using the \hgcmd{branch} command.
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296 \interaction{branch-named.rebranch}
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297 In practice, this is something you won't do very often, as branch
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298 names tend to have fairly long lifetimes. (This isn't a rule, just an
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299 observation.)
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300
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301 \section{Dealing with multiple named branches in a repository}
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302
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303 If you have more than one named branch in a repository, Mercurial will
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304 remember the branch that your working directory on when you start a
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305 command like \hgcmd{update} or \hgcmdargs{pull}{-u}. It will update
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306 the working directory to the tip of this branch, no matter what the
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307 ``repo-wide'' tip is. To update to a revision that's on a different
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308 named branch, you may need to use the \hgopt{update}{-C} option to
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309 \hgcmd{update}.
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310
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311 This behaviour is a little subtle, so let's see it in action. First,
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312 let's remind ourselves what branch we're currently on, and what
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313 branches are in our repository.
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314 \interaction{branch-named.parents}
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315 We're on the \texttt{bar} branch, but there also exists an older
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316 \hgcmd{foo} branch.
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317
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318 We can \hgcmd{update} back and forth between the tips of the
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319 \texttt{foo} and \texttt{bar} branches without needing to use the
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320 \hgopt{update}{-C} option, because this only involves going backwards
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321 and forwards linearly through our change history.
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322 \interaction{branch-named.update-switchy}
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323
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324 If we go back to the \texttt{foo} branch and then run \hgcmd{update},
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325 it will keep us on \texttt{foo}, not move us to the tip of
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326 \texttt{bar}.
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327 \interaction{branch-named.update-nothing}
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328
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329 Committing a new change on the \texttt{foo} branch introduces a new
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330 head.
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331 \interaction{branch-named.foo-commit}
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332
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333 \section{Branch names and merging}
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334
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335 As you've probably noticed, merges in Mercurial are not symmetrical.
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336 Let's say our repository has two heads, 17 and 23. If I
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337 \hgcmd{update} to 17 and then \hgcmd{merge} with 23, Mercurial records
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338 17 as the first parent of the merge, and 23 as the second. Whereas if
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339 I \hgcmd{update} to 23 and then \hgcmd{merge} with 17, it records 23
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340 as the first parent, and 17 as the second.
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341
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342 This affects Mercurial's choice of branch name when you merge. After
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343 a merge, Mercurial will retain the branch name of the first parent
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344 when you commit the result of the merge. If your first parent's
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345 branch name is \texttt{foo}, and you merge with \texttt{bar}, the
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346 branch name will still be \texttt{foo} after you merge.
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347
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348 It's not unusual for a repository to contain multiple heads, each with
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349 the same branch name. Let's say I'm working on the \texttt{foo}
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350 branch, and so are you. We commit different changes; I pull your
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351 changes; I now have two heads, each claiming to be on the \texttt{foo}
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352 branch. The result of a merge will be a single head on the
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353 \texttt{foo} branch, as you might hope.
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354
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355 But if I'm working on the \texttt{bar} branch, and I merge work from
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356 the \texttt{foo} branch, the result will remain on the \texttt{bar}
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357 branch.
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358 \interaction{branch-named.merge}
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359
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360 To give a more concrete example, if I'm working on the
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361 \texttt{bleeding-edge} branch, and I want to bring in the latest fixes
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362 from the \texttt{stable} branch, Mercurial will choose the ``right''
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363 (\texttt{bleeding-edge}) branch name when I pull and merge from
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364 \texttt{stable}.
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365
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366 \section{Branch naming is generally useful}
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367
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368 You shouldn't think of named branches as applicable only to situations
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369 where you have multiple long-lived branches cohabiting in a single
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370 repository. They're very useful even in the one-branch-per-repository
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371 case.
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372
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373 In the simplest case, giving a name to each branch gives you a
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374 permanent record of which branch a changeset originated on. This
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375 gives you more context when you're trying to follow the history of a
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376 long-lived branchy project.
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377
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378 If you're working with shared repositories, you can set up a
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379 \hook{pretxnchangegroup} hook on each that will block incoming changes
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380 that have the ``wrong'' branch name. This provides a simple, but
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381 effective, defence against people accidentally pushing changes from a
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382 ``bleeding edge'' branch to a ``stable'' branch. Such a hook might
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383 look like this inside the shared repo's \hgrc.
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384 \begin{codesample2}
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385 [hooks]
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386 pretxnchangegroup.branch = hg heads --template '{branches} ' | grep mybranch
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387 \end{codesample2}
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388
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389 %%% Local Variables:
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390 %%% mode: latex
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391 %%% TeX-master: "00book"
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392 %%% End:
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