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1 <!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->
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2
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3 <chapter id="chap:branch">
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4 <title>Managing releases and branchy development</title>
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5
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6 <para>Mercurial provides several mechanisms for you to manage a
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7 project that is making progress on multiple fronts at once. To
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8 understand these mechanisms, let's first take a brief look at a
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9 fairly normal software project structure.</para>
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10
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11 <para>Many software projects issue periodic <quote>major</quote>
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12 releases that contain substantial new features. In parallel, they
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13 may issue <quote>minor</quote> releases. These are usually
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14 identical to the major releases off which they're based, but with
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15 a few bugs fixed.</para>
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16
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17 <para>In this chapter, we'll start by talking about how to keep
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18 records of project milestones such as releases. We'll then
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19 continue on to talk about the flow of work between different
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20 phases of a project, and how Mercurial can help you to isolate and
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21 manage this work.</para>
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22
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23 <sect1>
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24 <title>Giving a persistent name to a revision</title>
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25
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26 <para>Once you decide that you'd like to call a particular
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27 revision a <quote>release</quote>, it's a good idea to record
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28 the identity of that revision. This will let you reproduce that
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29 release at a later date, for whatever purpose you might need at
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30 the time (reproducing a bug, porting to a new platform, etc).
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31 <!-- &interaction.tag.init; --></para>
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32
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33 <para>Mercurial lets you give a permanent name to any revision
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34 using the <command role="hg-cmd">hg tag</command> command. Not
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35 surprisingly, these names are called <quote>tags</quote>. <!--
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36 &interaction.tag.tag; --></para>
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37
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38 <para>A tag is nothing more than a <quote>symbolic name</quote>
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39 for a revision. Tags exist purely for your convenience, so that
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40 you have a handy permanent way to refer to a revision; Mercurial
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41 doesn't interpret the tag names you use in any way. Neither
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42 does Mercurial place any restrictions on the name of a tag,
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43 beyond a few that are necessary to ensure that a tag can be
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44 parsed unambiguously. A tag name cannot contain any of the
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45 following characters:</para>
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46 <itemizedlist>
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47 <listitem><para>Colon (ASCII 58,
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48 <quote><literal>:</literal></quote>)</para>
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49 </listitem>
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50 <listitem><para>Carriage return (ASCII 13,
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51 <quote><literal>\r</literal></quote>)</para>
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52 </listitem>
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53 <listitem><para>Newline (ASCII 10,
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54 <quote><literal>\n</literal></quote>)</para>
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55 </listitem></itemizedlist>
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56
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57 <para>You can use the <command role="hg-cmd">hg tags</command>
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58 command to display the tags present in your repository. In the
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59 output, each tagged revision is identified first by its name,
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60 then by revision number, and finally by the unique hash of the
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61 revision. <!-- &interaction.tag.tags; --> Notice that
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62 <literal>tip</literal> is listed in the output of <command
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63 role="hg-cmd">hg tags</command>. The <literal>tip</literal>
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64 tag is a special <quote>floating</quote> tag, which always
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65 identifies the newest revision in the repository.</para>
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66
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67 <para>In the output of the <command role="hg-cmd">hg
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68 tags</command> command, tags are listed in reverse order, by
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69 revision number. This usually means that recent tags are listed
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70 before older tags. It also means that <literal>tip</literal> is
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71 always going to be the first tag listed in the output of
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72 <command role="hg-cmd">hg tags</command>.</para>
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73
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74 <para>When you run <command role="hg-cmd">hg log</command>, if it
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75 displays a revision that has tags associated with it, it will
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76 print those tags. <!-- &interaction.tag.log; --></para>
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77
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78 <para>Any time you need to provide a revision ID to a Mercurial
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79 command, the command will accept a tag name in its place.
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80 Internally, Mercurial will translate your tag name into the
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81 corresponding revision ID, then use that. <!--
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82 &interaction.tag.log.v1.0; --></para>
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83
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84 <para>There's no limit on the number of tags you can have in a
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85 repository, or on the number of tags that a single revision can
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86 have. As a practical matter, it's not a great idea to have
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87 <quote>too many</quote> (a number which will vary from project
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88 to project), simply because tags are supposed to help you to
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89 find revisions. If you have lots of tags, the ease of using
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90 them to identify revisions diminishes rapidly.</para>
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91
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92 <para>For example, if your project has milestones as frequent as
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93 every few days, it's perfectly reasonable to tag each one of
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94 those. But if you have a continuous build system that makes
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95 sure every revision can be built cleanly, you'd be introducing a
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96 lot of noise if you were to tag every clean build. Instead, you
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97 could tag failed builds (on the assumption that they're rare!),
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98 or simply not use tags to track buildability.</para>
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99
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100 <para>If you want to remove a tag that you no longer want, use
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101 <command role="hg-cmd">hg tag --remove</command>. <!--
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102 &interaction.tag.remove; --> You can also modify a tag at any
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103 time, so that it identifies a different revision, by simply
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104 issuing a new <command role="hg-cmd">hg tag</command> command.
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105 You'll have to use the <option role="hg-opt-tag">-f</option>
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106 option to tell Mercurial that you <emphasis>really</emphasis>
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107 want to update the tag. <!-- &interaction.tag.replace; --> There
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108 will still be a permanent record of the previous identity of the
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109 tag, but Mercurial will no longer use it. There's thus no
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110 penalty to tagging the wrong revision; all you have to do is
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111 turn around and tag the correct revision once you discover your
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112 error.</para>
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113
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114 <para>Mercurial stores tags in a normal revision-controlled file
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115 in your repository. If you've created any tags, you'll find
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116 them in a file named <filename
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117 role="special">.hgtags</filename>. When you run the <command
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118 role="hg-cmd">hg tag</command> command, Mercurial modifies
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119 this file, then automatically commits the change to it. This
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120 means that every time you run <command role="hg-cmd">hg
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121 tag</command>, you'll see a corresponding changeset in the
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122 output of <command role="hg-cmd">hg log</command>. <!--
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123 &interaction.tag.tip; --></para>
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124
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125 <sect2>
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126 <title>Handling tag conflicts during a merge</title>
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127
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128 <para>You won't often need to care about the <filename
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129 role="special">.hgtags</filename> file, but it sometimes
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130 makes its presence known during a merge. The format of the
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131 file is simple: it consists of a series of lines. Each line
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132 starts with a changeset hash, followed by a space, followed by
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133 the name of a tag.</para>
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134
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135 <para>If you're resolving a conflict in the <filename
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136 role="special">.hgtags</filename> file during a merge,
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137 there's one twist to modifying the <filename
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138 role="special">.hgtags</filename> file: when Mercurial is
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139 parsing the tags in a repository, it
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140 <emphasis>never</emphasis> reads the working copy of the
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141 <filename role="special">.hgtags</filename> file. Instead, it
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142 reads the <emphasis>most recently committed</emphasis>
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143 revision of the file.</para>
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144
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145 <para>An unfortunate consequence of this design is that you
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146 can't actually verify that your merged <filename
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147 role="special">.hgtags</filename> file is correct until
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148 <emphasis>after</emphasis> you've committed a change. So if
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149 you find yourself resolving a conflict on <filename
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150 role="special">.hgtags</filename> during a merge, be sure to
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151 run <command role="hg-cmd">hg tags</command> after you commit.
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152 If it finds an error in the <filename
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153 role="special">.hgtags</filename> file, it will report the
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154 location of the error, which you can then fix and commit. You
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155 should then run <command role="hg-cmd">hg tags</command>
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156 again, just to be sure that your fix is correct.</para>
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157
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158 </sect2>
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159 <sect2>
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160 <title>Tags and cloning</title>
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161
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162 <para>You may have noticed that the <command role="hg-cmd">hg
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163 clone</command> command has a <option
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164 role="hg-opt-clone">-r</option> option that lets you clone
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165 an exact copy of the repository as of a particular changeset.
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166 The new clone will not contain any project history that comes
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167 after the revision you specified. This has an interaction
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168 with tags that can surprise the unwary.</para>
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169
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170 <para>Recall that a tag is stored as a revision to the <filename
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171 role="special">.hgtags</filename> file, so that when you
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172 create a tag, the changeset in which it's recorded necessarily
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173 refers to an older changeset. When you run <command
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174 role="hg-cmd">hg clone -r foo</command> to clone a
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175 repository as of tag <literal>foo</literal>, the new clone
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176 <emphasis>will not contain the history that created the
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177 tag</emphasis> that you used to clone the repository. The
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178 result is that you'll get exactly the right subset of the
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179 project's history in the new repository, but
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180 <emphasis>not</emphasis> the tag you might have
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181 expected.</para>
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182
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183 </sect2>
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184 <sect2>
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185 <title>When permanent tags are too much</title>
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186
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187 <para>Since Mercurial's tags are revision controlled and carried
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188 around with a project's history, everyone you work with will
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189 see the tags you create. But giving names to revisions has
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190 uses beyond simply noting that revision
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191 <literal>4237e45506ee</literal> is really
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192 <literal>v2.0.2</literal>. If you're trying to track down a
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193 subtle bug, you might want a tag to remind you of something
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194 like <quote>Anne saw the symptoms with this
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195 revision</quote>.</para>
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196
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197 <para>For cases like this, what you might want to use are
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198 <emphasis>local</emphasis> tags. You can create a local tag
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199 with the <option role="hg-opt-tag">-l</option> option to the
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200 <command role="hg-cmd">hg tag</command> command. This will
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201 store the tag in a file called <filename
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202 role="special">.hg/localtags</filename>. Unlike <filename
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203 role="special">.hgtags</filename>, <filename
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204 role="special">.hg/localtags</filename> is not revision
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205 controlled. Any tags you create using <option
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206 role="hg-opt-tag">-l</option> remain strictly local to the
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207 repository you're currently working in.</para>
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208
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209 </sect2>
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210 </sect1>
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211 <sect1>
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212 <title>The flow of changes&emdash;big picture vs. little</title>
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213
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214 <para>To return to the outline I sketched at the beginning of a
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215 chapter, let's think about a project that has multiple
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216 concurrent pieces of work under development at once.</para>
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217
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218 <para>There might be a push for a new <quote>main</quote> release;
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219 a new minor bugfix release to the last main release; and an
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220 unexpected <quote>hot fix</quote> to an old release that is now
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221 in maintenance mode.</para>
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222
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223 <para>The usual way people refer to these different concurrent
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224 directions of development is as <quote>branches</quote>.
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225 However, we've already seen numerous times that Mercurial treats
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226 <emphasis>all of history</emphasis> as a series of branches and
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227 merges. Really, what we have here is two ideas that are
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228 peripherally related, but which happen to share a name.</para>
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229 <itemizedlist>
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230 <listitem><para><quote>Big picture</quote> branches represent
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231 the sweep of a project's evolution; people give them names,
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232 and talk about them in conversation.</para>
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233 </listitem>
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234 <listitem><para><quote>Little picture</quote> branches are
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235 artefacts of the day-to-day activity of developing and
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236 merging changes. They expose the narrative of how the code
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237 was developed.</para>
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238 </listitem></itemizedlist>
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239
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240 </sect1>
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241 <sect1>
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242 <title>Managing big-picture branches in repositories</title>
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243
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244 <para>The easiest way to isolate a <quote>big picture</quote>
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245 branch in Mercurial is in a dedicated repository. If you have
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246 an existing shared repository&emdash;let's call it
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247 <literal>myproject</literal>&emdash;that reaches a
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248 <quote>1.0</quote> milestone, you can start to prepare for
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249 future maintenance releases on top of version 1.0 by tagging the
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250 revision from which you prepared the 1.0 release. <!--
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251 &interaction.branch-repo.tag; --> You can then clone a new
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252 shared <literal>myproject-1.0.1</literal> repository as of that
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253 tag. <!-- &interaction.branch-repo.clone; --></para>
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254
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255 <para>Afterwards, if someone needs to work on a bug fix that ought
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256 to go into an upcoming 1.0.1 minor release, they clone the
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257 <literal>myproject-1.0.1</literal> repository, make their
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258 changes, and push them back. <!--
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259 &interaction.branch-repo.bugfix; --> Meanwhile, development for
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260 the next major release can continue, isolated and unabated, in
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261 the <literal>myproject</literal> repository. <!--
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262 &interaction.branch-repo.new; --></para>
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263
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264 </sect1>
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265 <sect1>
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266 <title>Don't repeat yourself: merging across branches</title>
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267
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268 <para>In many cases, if you have a bug to fix on a maintenance
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269 branch, the chances are good that the bug exists on your
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270 project's main branch (and possibly other maintenance branches,
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271 too). It's a rare developer who wants to fix the same bug
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272 multiple times, so let's look at a few ways that Mercurial can
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273 help you to manage these bugfixes without duplicating your
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274 work.</para>
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275
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276 <para>In the simplest instance, all you need to do is pull changes
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277 from your maintenance branch into your local clone of the target
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278 branch. <!-- &interaction.branch-repo.pull; --> You'll then need
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279 to merge the heads of the two branches, and push back to the
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280 main branch. <!-- &interaction.branch-repo.merge; --></para>
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281
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282 </sect1>
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283 <sect1>
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284 <title>Naming branches within one repository</title>
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285
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286 <para>In most instances, isolating branches in repositories is the
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287 right approach. Its simplicity makes it easy to understand; and
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288 so it's hard to make mistakes. There's a one-to-one
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289 relationship between branches you're working in and directories
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290 on your system. This lets you use normal (non-Mercurial-aware)
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291 tools to work on files within a branch/repository.</para>
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292
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293 <para>If you're more in the <quote>power user</quote> category
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294 (<emphasis>and</emphasis> your collaborators are too), there is
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295 an alternative way of handling branches that you can consider.
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296 I've already mentioned the human-level distinction between
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297 <quote>small picture</quote> and <quote>big picture</quote>
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298 branches. While Mercurial works with multiple <quote>small
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299 picture</quote> branches in a repository all the time (for
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300 example after you pull changes in, but before you merge them),
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301 it can <emphasis>also</emphasis> work with multiple <quote>big
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302 picture</quote> branches.</para>
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303
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304 <para>The key to working this way is that Mercurial lets you
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305 assign a persistent <emphasis>name</emphasis> to a branch.
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306 There always exists a branch named <literal>default</literal>.
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307 Even before you start naming branches yourself, you can find
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308 traces of the <literal>default</literal> branch if you look for
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309 them.</para>
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310
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311 <para>As an example, when you run the <command role="hg-cmd">hg
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312 commit</command> command, and it pops up your editor so that
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313 you can enter a commit message, look for a line that contains
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314 the text <quote><literal>HG: branch default</literal></quote> at
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315 the bottom. This is telling you that your commit will occur on
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316 the branch named <literal>default</literal>.</para>
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317
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318 <para>To start working with named branches, use the <command
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319 role="hg-cmd">hg branches</command> command. This command
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320 lists the named branches already present in your repository,
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321 telling you which changeset is the tip of each. <!--
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322 &interaction.branch-named.branches; --> Since you haven't
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323 created any named branches yet, the only one that exists is
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324 <literal>default</literal>.</para>
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325
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326 <para>To find out what the <quote>current</quote> branch is, run
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327 the <command role="hg-cmd">hg branch</command> command, giving
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328 it no arguments. This tells you what branch the parent of the
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329 current changeset is on. <!-- &interaction.branch-named.branch;
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330 --></para>
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331
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332 <para>To create a new branch, run the <command role="hg-cmd">hg
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333 branch</command> command again. This time, give it one
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334 argument: the name of the branch you want to create. <!--
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335 &interaction.branch-named.create; --></para>
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336
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337 <para>After you've created a branch, you might wonder what effect
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338 the <command role="hg-cmd">hg branch</command> command has had.
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339 What do the <command role="hg-cmd">hg status</command> and
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340 <command role="hg-cmd">hg tip</command> commands report? <!--
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341 &interaction.branch-named.status; --> Nothing has changed in the
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342 working directory, and there's been no new history created. As
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343 this suggests, running the <command role="hg-cmd">hg
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344 branch</command> command has no permanent effect; it only
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345 tells Mercurial what branch name to use the
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346 <emphasis>next</emphasis> time you commit a changeset.</para>
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347
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348 <para>When you commit a change, Mercurial records the name of the
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349 branch on which you committed. Once you've switched from the
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350 <literal>default</literal> branch to another and committed,
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351 you'll see the name of the new branch show up in the output of
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352 <command role="hg-cmd">hg log</command>, <command
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353 role="hg-cmd">hg tip</command>, and other commands that
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354 display the same kind of output. <!--
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355 &interaction.branch-named.commit; --> The <command
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356 role="hg-cmd">hg log</command>-like commands will print the
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357 branch name of every changeset that's not on the
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358 <literal>default</literal> branch. As a result, if you never
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359 use named branches, you'll never see this information.</para>
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360
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361 <para>Once you've named a branch and committed a change with that
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362 name, every subsequent commit that descends from that change
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363 will inherit the same branch name. You can change the name of a
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364 branch at any time, using the <command role="hg-cmd">hg
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365 branch</command> command. <!--
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366 &interaction.branch-named.rebranch; --> In practice, this is
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367 something you won't do very often, as branch names tend to have
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368 fairly long lifetimes. (This isn't a rule, just an
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369 observation.)</para>
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370
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371 </sect1>
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372 <sect1>
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373 <title>Dealing with multiple named branches in a
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374 repository</title>
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375
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376 <para>If you have more than one named branch in a repository,
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377 Mercurial will remember the branch that your working directory
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378 on when you start a command like <command role="hg-cmd">hg
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379 update</command> or <command role="hg-cmd">hg pull
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380 -u</command>. It will update the working directory to the tip
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381 of this branch, no matter what the <quote>repo-wide</quote> tip
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382 is. To update to a revision that's on a different named branch,
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383 you may need to use the <option role="hg-opt-update">-C</option>
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384 option to <command role="hg-cmd">hg update</command>.</para>
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385
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386 <para>This behaviour is a little subtle, so let's see it in
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387 action. First, let's remind ourselves what branch we're
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388 currently on, and what branches are in our repository. <!--
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389 &interaction.branch-named.parents; --> We're on the
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390 <literal>bar</literal> branch, but there also exists an older
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391 <command role="hg-cmd">hg foo</command> branch.</para>
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392
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bos@559
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393 <para>We can <command role="hg-cmd">hg update</command> back and
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394 forth between the tips of the <literal>foo</literal> and
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395 <literal>bar</literal> branches without needing to use the
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396 <option role="hg-opt-update">-C</option> option, because this
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397 only involves going backwards and forwards linearly through our
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398 change history. <!-- &interaction.branch-named.update-switchy;
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399 --></para>
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400
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bos@559
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401 <para>If we go back to the <literal>foo</literal> branch and then
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402 run <command role="hg-cmd">hg update</command>, it will keep us
|
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403 on <literal>foo</literal>, not move us to the tip of
|
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404 <literal>bar</literal>. <!--
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405 &interaction.branch-named.update-nothing; --></para>
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406
|
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bos@559
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407 <para>Committing a new change on the <literal>foo</literal> branch
|
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408 introduces a new head. <!--
|
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bos@559
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409 &interaction.branch-named.foo-commit; --></para>
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410
|
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411 </sect1>
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bos@559
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412 <sect1>
|
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bos@559
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413 <title>Branch names and merging</title>
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414
|
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bos@559
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415 <para>As you've probably noticed, merges in Mercurial are not
|
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416 symmetrical. Let's say our repository has two heads, 17 and 23.
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417 If I <command role="hg-cmd">hg update</command> to 17 and then
|
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418 <command role="hg-cmd">hg merge</command> with 23, Mercurial
|
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419 records 17 as the first parent of the merge, and 23 as the
|
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420 second. Whereas if I <command role="hg-cmd">hg update</command>
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421 to 23 and then <command role="hg-cmd">hg merge</command> with
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422 17, it records 23 as the first parent, and 17 as the
|
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423 second.</para>
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424
|
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bos@559
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425 <para>This affects Mercurial's choice of branch name when you
|
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426 merge. After a merge, Mercurial will retain the branch name of
|
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427 the first parent when you commit the result of the merge. If
|
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428 your first parent's branch name is <literal>foo</literal>, and
|
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429 you merge with <literal>bar</literal>, the branch name will
|
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430 still be <literal>foo</literal> after you merge.</para>
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431
|
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bos@559
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432 <para>It's not unusual for a repository to contain multiple heads,
|
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433 each with the same branch name. Let's say I'm working on the
|
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434 <literal>foo</literal> branch, and so are you. We commit
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435 different changes; I pull your changes; I now have two heads,
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436 each claiming to be on the <literal>foo</literal> branch. The
|
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437 result of a merge will be a single head on the
|
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438 <literal>foo</literal> branch, as you might hope.</para>
|
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439
|
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bos@559
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440 <para>But if I'm working on the <literal>bar</literal> branch, and
|
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bos@559
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441 I merge work from the <literal>foo</literal> branch, the result
|
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bos@559
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442 will remain on the <literal>bar</literal> branch. <!--
|
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bos@559
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443 &interaction.branch-named.merge; --></para>
|
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bos@559
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444
|
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bos@559
|
445 <para>To give a more concrete example, if I'm working on the
|
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bos@559
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446 <literal>bleeding-edge</literal> branch, and I want to bring in
|
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bos@559
|
447 the latest fixes from the <literal>stable</literal> branch,
|
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|
448 Mercurial will choose the <quote>right</quote>
|
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bos@559
|
449 (<literal>bleeding-edge</literal>) branch name when I pull and
|
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450 merge from <literal>stable</literal>.</para>
|
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bos@559
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451
|
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bos@559
|
452 </sect1>
|
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bos@559
|
453 <sect1>
|
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bos@559
|
454 <title>Branch naming is generally useful</title>
|
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bos@559
|
455
|
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bos@559
|
456 <para>You shouldn't think of named branches as applicable only to
|
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|
457 situations where you have multiple long-lived branches
|
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|
458 cohabiting in a single repository. They're very useful even in
|
|
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|
459 the one-branch-per-repository case.</para>
|
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bos@559
|
460
|
|
bos@559
|
461 <para>In the simplest case, giving a name to each branch gives you
|
|
bos@559
|
462 a permanent record of which branch a changeset originated on.
|
|
bos@559
|
463 This gives you more context when you're trying to follow the
|
|
bos@559
|
464 history of a long-lived branchy project.</para>
|
|
bos@559
|
465
|
|
bos@559
|
466 <para>If you're working with shared repositories, you can set up a
|
|
bos@559
|
467 <literal role="hook">pretxnchangegroup</literal> hook on each
|
|
bos@559
|
468 that will block incoming changes that have the
|
|
bos@559
|
469 <quote>wrong</quote> branch name. This provides a simple, but
|
|
bos@559
|
470 effective, defence against people accidentally pushing changes
|
|
bos@559
|
471 from a <quote>bleeding edge</quote> branch to a
|
|
bos@559
|
472 <quote>stable</quote> branch. Such a hook might look like this
|
|
bos@559
|
473 inside the shared repo's <filename role="special">
|
|
bos@559
|
474 /.hgrc</filename>.</para>
|
|
bos@559
|
475 <programlisting>[hooks] pretxnchangegroup.branch = hg heads
|
|
bos@559
|
476 --template '{branches} ' | grep mybranch</programlisting>
|
|
bos@559
|
477
|
|
bos@559
|
478 </sect1>
|
|
bos@559
|
479 </chapter>
|
|
bos@559
|
480
|
|
bos@559
|
481 <!--
|
|
bos@559
|
482 local variables:
|
|
bos@559
|
483 sgml-parent-document: ("00book.xml" "book" "chapter")
|
|
bos@559
|
484 end:
|
|
bos@559
|
485 -->
|
