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1 \chapter{Administración de Versiones y desarrollo ramificado}
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2 \label{chap:branch}
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3
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4 Mercurial ofrece varios mecanismos que le permitirán administrar un
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5 proyecto que avanza en múltiples frentes simultáneamente. Para
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6 entender estos mecanismos, demos un vistazo a la estructura usual de
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7 un proyecto de software.
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8
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9 Muchos proyectos de software liberan una versión``mayor'' que contiene
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10 nuevas características substanciales. En paralelo, pueden liberar
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11 versiones ``menores''. Estas usualmente son idénticas a las
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12 versiones mayores en las cuales están basadas, pero con arreglo de
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13 algunos fallos.
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14
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15 En este capítulo, comenzaremos hablando de cómo mantener registro de
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16 las etapas del proyecto como las liberaciones de una
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17 versión. Continuaremos hablando del flujo de trabajo entre las
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18 diferentes fases de un proyecto, y como puede ayudar Mercurial a
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19 independizar y administrar tal trabajo.
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20
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21 \section{Dar un nombre persistente a una revisión}
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22
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23 Cuando se decide a otorgar a una revisión el nombre particular de una
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24 ``versión'', es buena idea grabar la identidad para tal revisión.
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25 Lo cual permitirá reproducir tal versión en una fecha posterior, o el
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26 propósito que se considere en ese momento (reproducir un fallo, portar
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27 a una nueva plataforma, etc).
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28 \interaction{tag.init}
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29
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30 Mercurial le permite dar un nombre permanente a cualquier revisión
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31 usando la orden \hgcmd{tag}. Sin causa de sorpresa, esos nombres se llaman
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32 ``tags''(etiquetas).
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33 \interaction{tag.tag}
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34
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35 Un tag no es más que un ``nombre simbólico'' para una revisión. Los
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36 tags existen únicamente para su conveniencia, dotándolo de una forma
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37 permanente y sencilla para referirse a una revisión; Mercurial no
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38 interpreta de ninguna manera los nombres de los tags que usted use.
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39 Mercurial tampoco impone restricción alguna al nombre de un tag, más
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40 allá de lo necesario para asegurar que un tag puede parsearse sin
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41 ambigüedades. El nombre de un tag no puede tener ninguno de los
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42 caracteres siguientes:
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43 \begin{itemize}
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44 \item Dos puntos (ASCII 58, ``\texttt{:}'')
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45 \item Retroceso (return) (ASCII 13, ``\Verb+\r+'')
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46 \item Nueva línea (ASCII 10, ``\Verb+\n+'')
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47 \end{itemize}
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48
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49 Puede usar la orden \hgcmd{tags} para observar los tags presentes en
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50 su repositorio. Al desplegarse, cada revisión marcada se identifica
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51 primero con su nombre, después el número de revisión y finalmente con
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52 un hash único de la revisión.
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53 \interaction{tag.tags}
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54 Note que \texttt{tip} aparece en la lista de \hgcmd{tags}. El tag
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55 \texttt{tip} es un tag ``flotante'' especial, que identifica siempre
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56 la revisión más nueva en el repositorio.
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57
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58 Al desplegar la orden \hgcmd{tags}, los tags se listan en orden
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59 inverso, por número de revisión. Lo que significa usualmente que los
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60 tags más recientes se listan antes que los más antiguos. También
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61 significa que el tag \texttt{tip} siempre aparecerá como primer tag
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62 listado al desplegar la orden \hgcmd{tags}.
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63
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64 Cuando ejecuta \hgcmd{log}, se desplegará la revisión que tenga los
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65 tags asociados a ella, se imprimirán tales tags.
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66 \interaction{tag.log}
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67
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68 Siempre que requiera indicar un ~ID de revisión a una Orden de
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69 Mercurial, aceptará un nombre de tag en su lugar. Internamente,
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70 Mercurial traducirá su nombre de tag en el ~ID de revisión
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71 correspondiente, y lo usará.
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72 \interaction{tag.log.v1.0}
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73
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74 No hay límites en la cantidad de tags por reposirorio, o la cantidad
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75 de tags que una misma revisión pueda tener. Siendo prácticos, no es
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76 muy buena idea tener ``demasiados'' (la cantidad variará de un
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77 proyecto a otro), debido a que la intención es ayudarle a encontrar
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78 revisiones. Si tiene demasiados tags, la facilidad para identificar
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79 revisiones disminuirá rápidamente.
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80
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81 Por ejemplo, si su proyecto tiene etapas(milestones) frecuentes en pocos
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82 días, es perfectamente razonable asignarle un tag a cada una de
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83 ellas. Pero si tiene un sistema de construcción automática de binarios
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84 que asegura que cada revisión puede generarse limpiamente, estaría
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85 introduciendo mucho ruido si se usara un tag para cada generación
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86 exitosa. Más bien, podría usar tags para generaciones fallidas (en
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87 caso de que estas sean raras!), o simplemente evitar los tags para
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88 llevar cuenta de la posibilidad de generación de binarios.
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89
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90
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91 If you want to remove a tag that you no longer want, use
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92 \hgcmdargs{tag}{--remove}.
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93 \interaction{tag.remove}
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94 You can also modify a tag at any time, so that it identifies a
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95 different revision, by simply issuing a new \hgcmd{tag} command.
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96 You'll have to use the \hgopt{tag}{-f} option to tell Mercurial that
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97 you \emph{really} want to update the tag.
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98 \interaction{tag.replace}
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99 There will still be a permanent record of the previous identity of the
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100 tag, but Mercurial will no longer use it. There's thus no penalty to
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101 tagging the wrong revision; all you have to do is turn around and tag
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102 the correct revision once you discover your error.
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103
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104 Mercurial stores tags in a normal revision-controlled file in your
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105 repository. If you've created any tags, you'll find them in a file
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106 named \sfilename{.hgtags}. When you run the \hgcmd{tag} command,
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107 Mercurial modifies this file, then automatically commits the change to
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108 it. This means that every time you run \hgcmd{tag}, you'll see a
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109 corresponding changeset in the output of \hgcmd{log}.
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110 \interaction{tag.tip}
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111
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112 \subsection{Handling tag conflicts during a merge}
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113
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114 You won't often need to care about the \sfilename{.hgtags} file, but
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115 it sometimes makes its presence known during a merge. The format of
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116 the file is simple: it consists of a series of lines. Each line
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117 starts with a changeset hash, followed by a space, followed by the
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118 name of a tag.
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119
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120 If you're resolving a conflict in the \sfilename{.hgtags} file during
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121 a merge, there's one twist to modifying the \sfilename{.hgtags} file:
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122 when Mercurial is parsing the tags in a repository, it \emph{never}
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123 reads the working copy of the \sfilename{.hgtags} file. Instead, it
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124 reads the \emph{most recently committed} revision of the file.
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125
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126 An unfortunate consequence of this design is that you can't actually
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127 verify that your merged \sfilename{.hgtags} file is correct until
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128 \emph{after} you've committed a change. So if you find yourself
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129 resolving a conflict on \sfilename{.hgtags} during a merge, be sure to
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130 run \hgcmd{tags} after you commit. If it finds an error in the
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131 \sfilename{.hgtags} file, it will report the location of the error,
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132 which you can then fix and commit. You should then run \hgcmd{tags}
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133 again, just to be sure that your fix is correct.
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134
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135 \subsection{Tags and cloning}
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136
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137 You may have noticed that the \hgcmd{clone} command has a
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138 \hgopt{clone}{-r} option that lets you clone an exact copy of the
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139 repository as of a particular changeset. The new clone will not
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140 contain any project history that comes after the revision you
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141 specified. This has an interaction with tags that can surprise the
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142 unwary.
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143
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144 Recall that a tag is stored as a revision to the \sfilename{.hgtags}
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145 file, so that when you create a tag, the changeset in which it's
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146 recorded necessarily refers to an older changeset. When you run
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147 \hgcmdargs{clone}{-r foo} to clone a repository as of tag
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148 \texttt{foo}, the new clone \emph{will not contain the history that
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149 created the tag} that you used to clone the repository. The result
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150 is that you'll get exactly the right subset of the project's history
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151 in the new repository, but \emph{not} the tag you might have expected.
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152
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153 \subsection{When permanent tags are too much}
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154
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155 Since Mercurial's tags are revision controlled and carried around with
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156 a project's history, everyone you work with will see the tags you
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157 create. But giving names to revisions has uses beyond simply noting
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158 that revision \texttt{4237e45506ee} is really \texttt{v2.0.2}. If
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159 you're trying to track down a subtle bug, you might want a tag to
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160 remind you of something like ``Anne saw the symptoms with this
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161 revision''.
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162
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163 For cases like this, what you might want to use are \emph{local} tags.
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164 You can create a local tag with the \hgopt{tag}{-l} option to the
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165 \hgcmd{tag} command. This will store the tag in a file called
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166 \sfilename{.hg/localtags}. Unlike \sfilename{.hgtags},
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167 \sfilename{.hg/localtags} is not revision controlled. Any tags you
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168 create using \hgopt{tag}{-l} remain strictly local to the repository
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169 you're currently working in.
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170
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171 \section{The flow of changes---big picture vs. little}
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172
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173 To return to the outline I sketched at the beginning of a chapter,
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174 let's think about a project that has multiple concurrent pieces of
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175 work under development at once.
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176
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177 There might be a push for a new ``main'' release; a new minor bugfix
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178 release to the last main release; and an unexpected ``hot fix'' to an
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179 old release that is now in maintenance mode.
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180
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181 The usual way people refer to these different concurrent directions of
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182 development is as ``branches''. However, we've already seen numerous
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183 times that Mercurial treats \emph{all of history} as a series of
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184 branches and merges. Really, what we have here is two ideas that are
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185 peripherally related, but which happen to share a name.
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186 \begin{itemize}
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187 \item ``Big picture'' branches represent the sweep of a project's
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188 evolution; people give them names, and talk about them in
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189 conversation.
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190 \item ``Little picture'' branches are artefacts of the day-to-day
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191 activity of developing and merging changes. They expose the
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192 narrative of how the code was developed.
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193 \end{itemize}
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194
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195 \section{Managing big-picture branches in repositories}
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196
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197 The easiest way to isolate a ``big picture'' branch in Mercurial is in
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198 a dedicated repository. If you have an existing shared
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199 repository---let's call it \texttt{myproject}---that reaches a ``1.0''
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200 milestone, you can start to prepare for future maintenance releases on
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201 top of version~1.0 by tagging the revision from which you prepared
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202 the~1.0 release.
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203 \interaction{branch-repo.tag}
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204 You can then clone a new shared \texttt{myproject-1.0.1} repository as
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205 of that tag.
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206 \interaction{branch-repo.clone}
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207
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208 Afterwards, if someone needs to work on a bug fix that ought to go
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209 into an upcoming~1.0.1 minor release, they clone the
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210 \texttt{myproject-1.0.1} repository, make their changes, and push them
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211 back.
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212 \interaction{branch-repo.bugfix}
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213 Meanwhile, development for the next major release can continue,
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214 isolated and unabated, in the \texttt{myproject} repository.
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215 \interaction{branch-repo.new}
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216
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217 \section{Don't repeat yourself: merging across branches}
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218
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219 In many cases, if you have a bug to fix on a maintenance branch, the
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220 chances are good that the bug exists on your project's main branch
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221 (and possibly other maintenance branches, too). It's a rare developer
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222 who wants to fix the same bug multiple times, so let's look at a few
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223 ways that Mercurial can help you to manage these bugfixes without
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224 duplicating your work.
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225
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226 In the simplest instance, all you need to do is pull changes from your
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227 maintenance branch into your local clone of the target branch.
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228 \interaction{branch-repo.pull}
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229 You'll then need to merge the heads of the two branches, and push back
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230 to the main branch.
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231 \interaction{branch-repo.merge}
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232
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233 \section{Naming branches within one repository}
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234
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235 In most instances, isolating branches in repositories is the right
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236 approach. Its simplicity makes it easy to understand; and so it's
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237 hard to make mistakes. There's a one-to-one relationship between
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238 branches you're working in and directories on your system. This lets
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239 you use normal (non-Mercurial-aware) tools to work on files within a
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240 branch/repository.
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241
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242 If you're more in the ``power user'' category (\emph{and} your
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243 collaborators are too), there is an alternative way of handling
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244 branches that you can consider. I've already mentioned the
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245 human-level distinction between ``small picture'' and ``big picture''
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246 branches. While Mercurial works with multiple ``small picture''
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247 branches in a repository all the time (for example after you pull
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248 changes in, but before you merge them), it can \emph{also} work with
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249 multiple ``big picture'' branches.
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250
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251 The key to working this way is that Mercurial lets you assign a
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252 persistent \emph{name} to a branch. There always exists a branch
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253 named \texttt{default}. Even before you start naming branches
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254 yourself, you can find traces of the \texttt{default} branch if you
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255 look for them.
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256
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257 As an example, when you run the \hgcmd{commit} command, and it pops up
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258 your editor so that you can enter a commit message, look for a line
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259 that contains the text ``\texttt{HG: branch default}'' at the bottom.
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260 This is telling you that your commit will occur on the branch named
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261 \texttt{default}.
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262
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263 To start working with named branches, use the \hgcmd{branches}
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264 command. This command lists the named branches already present in
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265 your repository, telling you which changeset is the tip of each.
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266 \interaction{branch-named.branches}
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267 Since you haven't created any named branches yet, the only one that
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268 exists is \texttt{default}.
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269
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270 To find out what the ``current'' branch is, run the \hgcmd{branch}
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271 command, giving it no arguments. This tells you what branch the
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272 parent of the current changeset is on.
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273 \interaction{branch-named.branch}
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274
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275 To create a new branch, run the \hgcmd{branch} command again. This
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276 time, give it one argument: the name of the branch you want to create.
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277 \interaction{branch-named.create}
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278
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279 After you've created a branch, you might wonder what effect the
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280 \hgcmd{branch} command has had. What do the \hgcmd{status} and
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281 \hgcmd{tip} commands report?
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282 \interaction{branch-named.status}
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283 Nothing has changed in the working directory, and there's been no new
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284 history created. As this suggests, running the \hgcmd{branch} command
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285 has no permanent effect; it only tells Mercurial what branch name to
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286 use the \emph{next} time you commit a changeset.
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287
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288 When you commit a change, Mercurial records the name of the branch on
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289 which you committed. Once you've switched from the \texttt{default}
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290 branch to another and committed, you'll see the name of the new branch
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291 show up in the output of \hgcmd{log}, \hgcmd{tip}, and other commands
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292 that display the same kind of output.
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293 \interaction{branch-named.commit}
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294 The \hgcmd{log}-like commands will print the branch name of every
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295 changeset that's not on the \texttt{default} branch. As a result, if
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296 you never use named branches, you'll never see this information.
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297
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298 Once you've named a branch and committed a change with that name,
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299 every subsequent commit that descends from that change will inherit
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300 the same branch name. You can change the name of a branch at any
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301 time, using the \hgcmd{branch} command.
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302 \interaction{branch-named.rebranch}
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303 In practice, this is something you won't do very often, as branch
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304 names tend to have fairly long lifetimes. (This isn't a rule, just an
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305 observation.)
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306
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307 \section{Dealing with multiple named branches in a repository}
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308
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309 If you have more than one named branch in a repository, Mercurial will
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310 remember the branch that your working directory on when you start a
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311 command like \hgcmd{update} or \hgcmdargs{pull}{-u}. It will update
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312 the working directory to the tip of this branch, no matter what the
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313 ``repo-wide'' tip is. To update to a revision that's on a different
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314 named branch, you may need to use the \hgopt{update}{-C} option to
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315 \hgcmd{update}.
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316
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317 This behaviour is a little subtle, so let's see it in action. First,
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318 let's remind ourselves what branch we're currently on, and what
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319 branches are in our repository.
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320 \interaction{branch-named.parents}
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321 We're on the \texttt{bar} branch, but there also exists an older
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322 \hgcmd{foo} branch.
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323
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324 We can \hgcmd{update} back and forth between the tips of the
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325 \texttt{foo} and \texttt{bar} branches without needing to use the
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326 \hgopt{update}{-C} option, because this only involves going backwards
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327 and forwards linearly through our change history.
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328 \interaction{branch-named.update-switchy}
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329
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330 If we go back to the \texttt{foo} branch and then run \hgcmd{update},
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331 it will keep us on \texttt{foo}, not move us to the tip of
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332 \texttt{bar}.
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333 \interaction{branch-named.update-nothing}
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334
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335 Committing a new change on the \texttt{foo} branch introduces a new
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336 head.
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337 \interaction{branch-named.foo-commit}
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338
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339 \section{Branch names and merging}
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340
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341 As you've probably noticed, merges in Mercurial are not symmetrical.
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342 Let's say our repository has two heads, 17 and 23. If I
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343 \hgcmd{update} to 17 and then \hgcmd{merge} with 23, Mercurial records
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344 17 as the first parent of the merge, and 23 as the second. Whereas if
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345 I \hgcmd{update} to 23 and then \hgcmd{merge} with 17, it records 23
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346 as the first parent, and 17 as the second.
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347
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348 This affects Mercurial's choice of branch name when you merge. After
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349 a merge, Mercurial will retain the branch name of the first parent
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350 when you commit the result of the merge. If your first parent's
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351 branch name is \texttt{foo}, and you merge with \texttt{bar}, the
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352 branch name will still be \texttt{foo} after you merge.
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353
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354 It's not unusual for a repository to contain multiple heads, each with
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355 the same branch name. Let's say I'm working on the \texttt{foo}
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356 branch, and so are you. We commit different changes; I pull your
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357 changes; I now have two heads, each claiming to be on the \texttt{foo}
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358 branch. The result of a merge will be a single head on the
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359 \texttt{foo} branch, as you might hope.
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360
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361 But if I'm working on the \texttt{bar} branch, and I merge work from
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362 the \texttt{foo} branch, the result will remain on the \texttt{bar}
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363 branch.
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364 \interaction{branch-named.merge}
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365
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366 To give a more concrete example, if I'm working on the
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367 \texttt{bleeding-edge} branch, and I want to bring in the latest fixes
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368 from the \texttt{stable} branch, Mercurial will choose the ``right''
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369 (\texttt{bleeding-edge}) branch name when I pull and merge from
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370 \texttt{stable}.
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371
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372 \section{Branch naming is generally useful}
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373
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374 You shouldn't think of named branches as applicable only to situations
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375 where you have multiple long-lived branches cohabiting in a single
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376 repository. They're very useful even in the one-branch-per-repository
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377 case.
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378
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379 In the simplest case, giving a name to each branch gives you a
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380 permanent record of which branch a changeset originated on. This
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381 gives you more context when you're trying to follow the history of a
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382 long-lived branchy project.
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383
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384 If you're working with shared repositories, you can set up a
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385 \hook{pretxnchangegroup} hook on each that will block incoming changes
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386 that have the ``wrong'' branch name. This provides a simple, but
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387 effective, defence against people accidentally pushing changes from a
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388 ``bleeding edge'' branch to a ``stable'' branch. Such a hook might
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389 look like this inside the shared repo's \hgrc.
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390 \begin{codesample2}
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391 [hooks]
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392 pretxnchangegroup.branch = hg heads --template '{branches} ' | grep mybranch
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393 \end{codesample2}
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394
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395 %%% Local Variables:
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396 %%% mode: latex
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397 %%% TeX-master: "00book"
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398 %%% End:
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