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1 \chapter{Finding and fixing your mistakes}
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2 \label{chap:undo}
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3
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4 To err might be human, but to really handle the consequences well
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5 takes a top-notch revision control system. In this chapter, we'll
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6 discuss some of the techniques you can use when you find that a
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7 problem has crept into your project. Mercurial has some highly
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8 capable features that will help you to isolate the sources of
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9 problems, and to handle them appropriately.
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10
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11 \section{Erasing local history}
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12
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13 \subsection{The accidental commit}
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14
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15 I have the occasional but persistent problem of typing rather more
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16 quickly than I can think, which sometimes results in me committing a
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17 changeset that is either incomplete or plain wrong. In my case, the
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18 usual kind of incomplete changeset is one in which I've created a new
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19 source file, but forgotten to \hgcmd{add} it. A ``plain wrong''
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20 changeset is not as common, but no less annoying.
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21
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22 \subsection{Rolling back a transaction}
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23 \label{sec:undo:rollback}
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24
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25 In section~\ref{sec:concepts:txn}, I mentioned that Mercurial treats
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26 each modification of a repository as a \emph{transaction}. Every time
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27 you commit a changeset or pull changes from another repository,
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28 Mercurial remembers what you did. You can undo, or \emph{roll back},
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29 exactly one of these actions using the \hgcmd{rollback} command.
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30
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31 Here's a mistake that I often find myself making: committing a change
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32 in which I've created a new file, but forgotten to \hgcmd{add} it.
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33 \interaction{rollback.commit}
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34 Looking at the output of \hgcmd{status} after the commit immediately
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35 confirms the error.
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36 \interaction{rollback.status}
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37 The commit captured the changes to the file \filename{a}, but not the
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38 new file \filename{b}. If I were to push this changeset to a
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39 repository that I shared with a colleague, the chances are high that
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40 something in \filename{a} would refer to \filename{b}, which would not
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41 be present in their repository when they pulled my changes. I would
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42 thus become the object of some indignation.
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43
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44 However, luck is with me---I've caught my error before I pushed the
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45 changeset. I use the \hgcmd{rollback} command, and Mercurial makes
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46 that last changeset vanish.
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47 \interaction{rollback.rollback}
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48 Notice that the changeset is no longer present in the repository's
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49 history, and the working directory once again thinks that the file
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50 \filename{a} is modified. The commit and rollback have left the
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51 working directory exactly as it was prior to the commit; the changeset
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52 has been completely erased. I can now safely \hgcmd{add} the file
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53 \filename{b}, and rerun my commit.
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54 \interaction{rollback.add}
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55
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56 \subsection{The erroneous pull}
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57
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58 It's common practice with Mercurial to maintain separate development
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59 branches of a project in different repositories. Your development
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60 team might have one shared repository for your project's ``0.9''
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61 release, and another, containing different changes, for the ``1.0''
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62 release.
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63
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64 Given this, you can imagine that the consequences could be messy if
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65 you had a local ``0.9'' repository, and accidentally pulled changes
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66 from the shared ``1.0'' repository into it. At worst, you could be
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67 paying insufficient attention, and push those changes into the shared
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68 ``0.9'' tree, confusing your entire team (but don't worry, we'll
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69 return to this horror scenario later). However, it's more likely that
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70 you'll notice immediately, because Mercurial will display the URL it's
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71 pulling from, or you will see it pull a suspiciously large number of
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72 changes into the repository.
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73
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74 The \hgcmd{rollback} command will work nicely to expunge all of the
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75 changesets that you just pulled. Mercurial groups all changes from
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76 one \hgcmd{pull} into a single transaction, so one \hgcmd{rollback} is
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77 all you need to undo this mistake.
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78
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79 \subsection{Rolling back is useless once you've pushed}
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80
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81 The value of the \hgcmd{rollback} command drops to zero once you've
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82 pushed your changes to another repository. Rolling back a change
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83 makes it disappear entirely, but \emph{only} in the repository in
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84 which you perform the \hgcmd{rollback}. Because a rollback eliminates
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85 history, there's no way for the disappearance of a change to propagate
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86 between repositories.
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87
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88 If you've pushed a change to another repository---particularly if it's
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89 a shared repository---it has essentially ``escaped into the wild,''
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90 and you'll have to recover from your mistake in a different way. What
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91 will happen if you push a changeset somewhere, then roll it back, then
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92 pull from the repository you pushed to, is that the changeset will
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93 reappear in your repository.
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94
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95 (If you absolutely know for sure that the change you want to roll back
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96 is the most recent change in the repository that you pushed to,
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97 \emph{and} you know that nobody else could have pulled it from that
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98 repository, you can roll back the changeset there, too, but you really
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99 should really not rely on this working reliably. If you do this,
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100 sooner or later a change really will make it into a repository that
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101 you don't directly control (or have forgotten about), and come back to
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102 bite you.)
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103
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104 \subsection{You can only roll back once}
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105
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106 Mercurial stores exactly one transaction in its transaction log; that
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107 transaction is the most recent one that occurred in the repository.
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108 This means that you can only roll back one transaction. If you expect
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109 to be able to roll back one transaction, then its predecessor, this is
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110 not the behaviour you will get.
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111 \interaction{rollback.twice}
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112 Once you've rolled back one transaction in a repository, you can't
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113 roll back again in that repository until you perform another commit or
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114 pull.
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115
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116 \section{Reverting the mistaken change}
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117
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118 If you make a modification to a file, and decide that you really
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119 didn't want to change the file at all, and you haven't yet committed
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120 your changes, the \hgcmd{revert} command is the one you'll need. It
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121 looks at the changeset that's the parent of the working directory, and
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122 restores the contents of the file to their state as of that changeset.
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123 (That's a long-winded way of saying that, in the normal case, it
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124 undoes your modifications.)
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125
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126 Let's illustrate how the \hgcmd{revert} command works with yet another
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127 small example. We'll begin by modifying a file that Mercurial is
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128 already tracking.
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129 \interaction{daily.revert.modify}
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130 If we don't want that change, we can simply \hgcmd{revert} the file.
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131 \interaction{daily.revert.unmodify}
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132 The \hgcmd{revert} command provides us with an extra degree of safety
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133 by saving our modified file with a \filename{.orig} extension.
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134 \interaction{daily.revert.status}
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135
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136 Here is a summary of the cases that the \hgcmd{revert} command can
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137 deal with. We will describe each of these in more detail in the
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138 section that follows.
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139 \begin{itemize}
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140 \item If you modify a file, it will restore the file to its unmodified
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141 state.
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142 \item If you \hgcmd{add} a file, it will undo the ``added'' state of
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143 the file, but leave the file itself untouched.
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144 \item If you delete a file without telling Mercurial, it will restore
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145 the file to its unmodified contents.
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146 \item If you use the \hgcmd{remove} command to remove a file, it will
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147 undo the ``removed'' state of the file, and restore the file to its
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148 unmodified contents.
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149 \end{itemize}
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150
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151 \subsection{File management errors}
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152 \label{sec:undo:mgmt}
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153
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154 The \hgcmd{revert} command is useful for more than just modified
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155 files. It lets you reverse the results of all of Mercurial's file
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156 management commands---\hgcmd{add}, \hgcmd{remove}, and so on.
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157
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158 If you \hgcmd{add} a file, then decide that in fact you don't want
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159 Mercurial to track it, use \hgcmd{revert} to undo the add. Don't
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160 worry; Mercurial will not modify the file in any way. It will just
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161 ``unmark'' the file.
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162 \interaction{daily.revert.add}
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163
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164 Similarly, if you ask Mercurial to \hgcmd{remove} a file, you can use
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165 \hgcmd{revert} to restore it to the contents it had as of the parent
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166 of the working directory.
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167 \interaction{daily.revert.remove}
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168 This works just as well for a file that you deleted by hand, without
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169 telling Mercurial (recall that in Mercurial terminology, this kind of
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170 file is called ``missing'').
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171 \interaction{daily.revert.missing}
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172
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173 If you revert a \hgcmd{copy}, the copied-to file remains in your
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174 working directory afterwards, untracked. Since a copy doesn't affect
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175 the copied-from file in any way, Mercurial doesn't do anything with
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176 the copied-from file.
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177 \interaction{daily.revert.copy}
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178
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179 \subsubsection{A slightly special case: reverting a rename}
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180
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181 If you \hgcmd{rename} a file, there is one small detail that
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182 you should remember. When you \hgcmd{revert} a rename, it's not
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183 enough to provide the name of the renamed-to file, as you can see
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184 here.
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185 \interaction{daily.revert.rename}
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186 As you can see from the output of \hgcmd{status}, the renamed-to file
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187 is no longer identified as added, but the renamed-\emph{from} file is
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188 still removed! This is counter-intuitive (at least to me), but at
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189 least it's easy to deal with.
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190 \interaction{daily.revert.rename-orig}
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191 So remember, to revert a \hgcmd{rename}, you must provide \emph{both}
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192 the source and destination names.
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193
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194 (By the way, if you rename a file, then modify the renamed-to file,
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195 then revert both components of the rename, when Mercurial restores the
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196 file that was removed as part of the rename, it will be unmodified.
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197 If you need the modifications in the renamed-to file to show up in the
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198 renamed-from file, don't forget to copy them over.)
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199
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200 These fiddly aspects of reverting a rename arguably constitute a small
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201 bug in Mercurial.
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202
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203 \section{Dealing with committed changes}
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204
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205 Consider a case where you have committed a change $a$, and another
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206 change $b$ on top of it; you then realise that change $a$ was
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207 incorrect. Mercurial lets you ``back out'' an entire changeset
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208 automatically, and building blocks that let you reverse part of a
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209 changeset by hand.
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210
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211 Before you read this section, here's something to keep in mind: the
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212 \hgcmd{backout} command undoes changes by \emph{adding} history, not
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213 by modifying or erasing it. It's the right tool to use if you're
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214 fixing bugs, but not if you're trying to undo some change that has
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215 catastrophic consequences. To deal with those, see
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216 section~\ref{sec:undo:aaaiiieee}.
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217
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218 \subsection{Backing out a changeset}
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219
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220 The \hgcmd{backout} command lets you ``undo'' the effects of an entire
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221 changeset in an automated fashion. Because Mercurial's history is
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222 immutable, this command \emph{does not} get rid of the changeset you
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223 want to undo. Instead, it creates a new changeset that
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224 \emph{reverses} the effect of the to-be-undone changeset.
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225
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226 The operation of the \hgcmd{backout} command is a little intricate, so
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227 let's illustrate it with some examples. First, we'll create a
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228 repository with some simple changes.
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229 \interaction{backout.init}
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230
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231 The \hgcmd{backout} command takes a single changeset ID as its
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232 argument; this is the changeset to back out. Normally,
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233 \hgcmd{backout} will drop you into a text editor to write a commit
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234 message, so you can record why you're backing the change out. In this
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235 example, we provide a commit message on the command line using the
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236 \hgopt{backout}{-m} option.
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237
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238 \subsection{Backing out the tip changeset}
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239
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240 We're going to start by backing out the last changeset we committed.
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241 \interaction{backout.simple}
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242 You can see that the second line from \filename{myfile} is no longer
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243 present. Taking a look at the output of \hgcmd{log} gives us an idea
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244 of what the \hgcmd{backout} command has done.
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245 \interaction{backout.simple.log}
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246 Notice that the new changeset that \hgcmd{backout} has created is a
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247 child of the changeset we backed out. It's easier to see this in
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248 figure~\ref{fig:undo:backout}, which presents a graphical view of the
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249 change history. As you can see, the history is nice and linear.
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250
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251 \begin{figure}[htb]
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252 \centering
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253 \grafix{undo-simple}
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254 \caption{Backing out a change using the \hgcmd{backout} command}
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255 \label{fig:undo:backout}
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256 \end{figure}
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257
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258 \subsection{Backing out a non-tip change}
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259
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260 If you want to back out a change other than the last one you
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261 committed, pass the \hgopt{backout}{--merge} option to the
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262 \hgcmd{backout} command.
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263 \interaction{backout.non-tip.clone}
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264 This makes backing out any changeset a ``one-shot'' operation that's
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265 usually simple and fast.
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266 \interaction{backout.non-tip.backout}
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267
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268 If you take a look at the contents of \filename{myfile} after the
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269 backout finishes, you'll see that the first and third changes are
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270 present, but not the second.
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271 \interaction{backout.non-tip.cat}
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272
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273 As the graphical history in figure~\ref{fig:undo:backout-non-tip}
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274 illustrates, Mercurial actually commits \emph{two} changes in this
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275 kind of situation (the box-shaped nodes are the ones that Mercurial
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276 commits automatically). Before Mercurial begins the backout process,
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277 it first remembers what the current parent of the working directory
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278 is. It then backs out the target changeset, and commits that as a
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279 changeset. Finally, it merges back to the previous parent of the
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280 working directory, and commits the result of the merge.
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281
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282 \begin{figure}[htb]
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283 \centering
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284 \grafix{undo-non-tip}
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285 \caption{Automated backout of a non-tip change using the \hgcmd{backout} command}
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286 \label{fig:undo:backout-non-tip}
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287 \end{figure}
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288
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289 The result is that you end up ``back where you were'', only with some
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290 extra history that undoes the effect of the changeset you wanted to
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291 back out.
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292
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293 \subsubsection{Always use the \hgopt{backout}{--merge} option}
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294
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295 In fact, since the \hgopt{backout}{--merge} option will do the ``right
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296 thing'' whether or not the changeset you're backing out is the tip
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297 (i.e.~it won't try to merge if it's backing out the tip, since there's
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298 no need), you should \emph{always} use this option when you run the
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299 \hgcmd{backout} command.
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300
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301 \subsection{Gaining more control of the backout process}
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302
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303 While I've recommended that you always use the
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304 \hgopt{backout}{--merge} option when backing out a change, the
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305 \hgcmd{backout} command lets you decide how to merge a backout
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306 changeset. Taking control of the backout process by hand is something
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307 you will rarely need to do, but it can be useful to understand what
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308 the \hgcmd{backout} command is doing for you automatically. To
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309 illustrate this, let's clone our first repository, but omit the
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310 backout change that it contains.
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311
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312 \interaction{backout.manual.clone}
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313 As with our earlier example, We'll commit a third changeset, then back
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314 out its parent, and see what happens.
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315 \interaction{backout.manual.backout}
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316 Our new changeset is again a descendant of the changeset we backout
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317 out; it's thus a new head, \emph{not} a descendant of the changeset
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318 that was the tip. The \hgcmd{backout} command was quite explicit in
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319 telling us this.
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320 \interaction{backout.manual.log}
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321
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322 Again, it's easier to see what has happened by looking at a graph of
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323 the revision history, in figure~\ref{fig:undo:backout-manual}. This
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324 makes it clear that when we use \hgcmd{backout} to back out a change
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325 other than the tip, Mercurial adds a new head to the repository (the
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326 change it committed is box-shaped).
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327
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328 \begin{figure}[htb]
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329 \centering
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330 \grafix{undo-manual}
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331 \caption{Backing out a change using the \hgcmd{backout} command}
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332 \label{fig:undo:backout-manual}
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333 \end{figure}
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334
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335 After the \hgcmd{backout} command has completed, it leaves the new
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336 ``backout'' changeset as the parent of the working directory.
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337 \interaction{backout.manual.parents}
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338 Now we have two isolated sets of changes.
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339 \interaction{backout.manual.heads}
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340
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341 Let's think about what we expect to see as the contents of
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342 \filename{myfile} now. The first change should be present, because
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343 we've never backed it out. The second change should be missing, as
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344 that's the change we backed out. Since the history graph shows the
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345 third change as a separate head, we \emph{don't} expect to see the
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346 third change present in \filename{myfile}.
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347 \interaction{backout.manual.cat}
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348 To get the third change back into the file, we just do a normal merge
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349 of our two heads.
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350 \interaction{backout.manual.merge}
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351 Afterwards, the graphical history of our repository looks like
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352 figure~\ref{fig:undo:backout-manual-merge}.
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353
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354 \begin{figure}[htb]
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355 \centering
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356 \grafix{undo-manual-merge}
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357 \caption{Manually merging a backout change}
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358 \label{fig:undo:backout-manual-merge}
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359 \end{figure}
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360
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361 \subsection{Why \hgcmd{backout} works as it does}
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362
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363 Here's a brief description of how the \hgcmd{backout} command works.
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364 \begin{enumerate}
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365 \item It ensures that the working directory is ``clean'', i.e.~that
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366 the output of \hgcmd{status} would be empty.
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367 \item It remembers the current parent of the working directory. Let's
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368 call this changeset \texttt{orig}
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369 \item It does the equivalent of a \hgcmd{update} to sync the working
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370 directory to the changeset you want to back out. Let's call this
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371 changeset \texttt{backout}
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372 \item It finds the parent of that changeset. Let's call that
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373 changeset \texttt{parent}.
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374 \item For each file that the \texttt{backout} changeset affected, it
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375 does the equivalent of a \hgcmdargs{revert}{-r parent} on that file,
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376 to restore it to the contents it had before that changeset was
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377 committed.
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378 \item It commits the result as a new changeset. This changeset has
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379 \texttt{backout} as its parent.
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380 \item If you specify \hgopt{backout}{--merge} on the command line, it
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381 merges with \texttt{orig}, and commits the result of the merge.
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382 \end{enumerate}
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383
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384 An alternative way to implement the \hgcmd{backout} command would be
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385 to \hgcmd{export} the to-be-backed-out changeset as a diff, then use
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386 the \cmdopt{patch}{--reverse} option to the \command{patch} command to
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387 reverse the effect of the change without fiddling with the working
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388 directory. This sounds much simpler, but it would not work nearly as
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389 well.
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390
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391 The reason that \hgcmd{backout} does an update, a commit, a merge, and
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392 another commit is to give the merge machinery the best chance to do a
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393 good job when dealing with all the changes \emph{between} the change
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394 you're backing out and the current tip.
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395
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396 If you're backing out a changeset that's~100 revisions back in your
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397 project's history, the chances that the \command{patch} command will
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398 be able to apply a reverse diff cleanly are not good, because
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399 intervening changes are likely to have ``broken the context'' that
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400 \command{patch} uses to determine whether it can apply a patch (if
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401 this sounds like gibberish, see \ref{sec:mq:patch} for a
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402 discussion of the \command{patch} command). Also, Mercurial's merge
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403 machinery will handle files and directories being renamed, permission
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404 changes, and modifications to binary files, none of which
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405 \command{patch} can deal with.
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406
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407 \section{Changes that should never have been}
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408 \label{sec:undo:aaaiiieee}
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409
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410 Most of the time, the \hgcmd{backout} command is exactly what you need
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411 if you want to undo the effects of a change. It leaves a permanent
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412 record of exactly what you did, both when committing the original
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413 changeset and when you cleaned up after it.
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414
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415 On rare occasions, though, you may find that you've committed a change
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416 that really should not be present in the repository at all. For
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417 example, it would be very unusual, and usually considered a mistake,
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418 to commit a software project's object files as well as its source
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419 files. Object files have almost no intrinsic value, and they're
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420 \emph{big}, so they increase the size of the repository and the amount
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421 of time it takes to clone or pull changes.
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422
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423 Before I discuss the options that you have if you commit a ``brown
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424 paper bag'' change (the kind that's so bad that you want to pull a
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425 brown paper bag over your head), let me first discuss some approaches
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426 that probably won't work.
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427
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428 Since Mercurial treats history as accumulative---every change builds
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429 on top of all changes that preceded it---you generally can't just make
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430 disastrous changes disappear. The one exception is when you've just
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431 committed a change, and it hasn't been pushed or pulled into another
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432 repository. That's when you can safely use the \hgcmd{rollback}
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433 command, as I detailed in section~\ref{sec:undo:rollback}.
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434
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435 After you've pushed a bad change to another repository, you
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436 \emph{could} still use \hgcmd{rollback} to make your local copy of the
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437 change disappear, but it won't have the consequences you want. The
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438 change will still be present in the remote repository, so it will
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439 reappear in your local repository the next time you pull.
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440
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441 If a situation like this arises, and you know which repositories your
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442 bad change has propagated into, you can \emph{try} to get rid of the
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443 changeefrom \emph{every} one of those repositories. This is, of
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444 course, not a satisfactory solution: if you miss even a single
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445 repository while you're expunging, the change is still ``in the
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446 wild'', and could propagate further.
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447
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448 If you've committed one or more changes \emph{after} the change that
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449 you'd like to see disappear, your options are further reduced.
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450 Mercurial doesn't provide a way to ``punch a hole'' in history,
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451 leaving changesets intact.
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452
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453 XXX This needs filling out. The \texttt{hg-replay} script in the
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454 \texttt{examples} directory works, but doesn't handle merge
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455 changesets. Kind of an important omission.
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456
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457 %%% Local Variables:
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458 %%% mode: latex
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459 %%% TeX-master: "00book"
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460 %%% End:
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