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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.undo">
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4 <?dbhtml filename="finding-and-fixing-mistakes.html"?>
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5 <title>Finding and fixing mistakes</title>
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6
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7 <para>To err might be human, but to really handle the consequences
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8 well takes a top-notch revision control system. In this chapter,
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9 we'll discuss some of the techniques you can use when you find
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10 that a problem has crept into your project. Mercurial has some
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11 highly capable features that will help you to isolate the sources
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12 of problems, and to handle them appropriately.</para>
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13
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14 <sect1>
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15 <title>Erasing local history</title>
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16
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17 <sect2>
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18 <title>The accidental commit</title>
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19
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20 <para>I have the occasional but persistent problem of typing
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21 rather more quickly than I can think, which sometimes results
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22 in me committing a changeset that is either incomplete or
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23 plain wrong. In my case, the usual kind of incomplete
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24 changeset is one in which I've created a new source file, but
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25 forgotten to <command role="hg-cmd">hg add</command> it. A
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26 <quote>plain wrong</quote> changeset is not as common, but no
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27 less annoying.</para>
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28
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29 </sect2>
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30 <sect2 id="sec.undo.rollback">
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31 <title>Rolling back a transaction</title>
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32
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33 <para>In section <xref linkend="sec.concepts.txn"/>, I mentioned
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34 that Mercurial treats each modification of a repository as a
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35 <emphasis>transaction</emphasis>. Every time you commit a
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36 changeset or pull changes from another repository, Mercurial
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37 remembers what you did. You can undo, or <emphasis>roll
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38 back</emphasis>, exactly one of these actions using the
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39 <command role="hg-cmd">hg rollback</command> command. (See
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40 section <xref linkend="sec.undo.rollback-after-push"/> for an
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41 important caveat about the use of this command.)</para>
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42
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43 <para>Here's a mistake that I often find myself making:
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44 committing a change in which I've created a new file, but
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45 forgotten to <command role="hg-cmd">hg add</command>
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46 it.</para>
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47
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48 &interaction.rollback.commit;
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49
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50 <para>Looking at the output of <command role="hg-cmd">hg
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51 status</command> after the commit immediately confirms the
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52 error.</para>
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53
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54 &interaction.rollback.status;
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55
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56 <para>The commit captured the changes to the file
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57 <filename>a</filename>, but not the new file
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58 <filename>b</filename>. If I were to push this changeset to a
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59 repository that I shared with a colleague, the chances are
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60 high that something in <filename>a</filename> would refer to
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61 <filename>b</filename>, which would not be present in their
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62 repository when they pulled my changes. I would thus become
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63 the object of some indignation.</para>
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64
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65 <para>However, luck is with me&emdash;I've caught my error
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66 before I pushed the changeset. I use the <command
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67 role="hg-cmd">hg rollback</command> command, and Mercurial
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68 makes that last changeset vanish.</para>
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69
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70 &interaction.rollback.rollback;
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71
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72 <para>Notice that the changeset is no longer present in the
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73 repository's history, and the working directory once again
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74 thinks that the file <filename>a</filename> is modified. The
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75 commit and rollback have left the working directory exactly as
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76 it was prior to the commit; the changeset has been completely
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77 erased. I can now safely <command role="hg-cmd">hg
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78 add</command> the file <filename>b</filename>, and rerun my
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79 commit.</para>
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80
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81 &interaction.rollback.add;
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82
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83 </sect2>
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84 <sect2>
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85 <title>The erroneous pull</title>
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86
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87 <para>It's common practice with Mercurial to maintain separate
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88 development branches of a project in different repositories.
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89 Your development team might have one shared repository for
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90 your project's <quote>0.9</quote> release, and another,
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91 containing different changes, for the <quote>1.0</quote>
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92 release.</para>
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93
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94 <para>Given this, you can imagine that the consequences could be
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95 messy if you had a local <quote>0.9</quote> repository, and
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96 accidentally pulled changes from the shared <quote>1.0</quote>
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97 repository into it. At worst, you could be paying
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98 insufficient attention, and push those changes into the shared
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99 <quote>0.9</quote> tree, confusing your entire team (but don't
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100 worry, we'll return to this horror scenario later). However,
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101 it's more likely that you'll notice immediately, because
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102 Mercurial will display the URL it's pulling from, or you will
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103 see it pull a suspiciously large number of changes into the
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104 repository.</para>
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105
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106 <para>The <command role="hg-cmd">hg rollback</command> command
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107 will work nicely to expunge all of the changesets that you
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108 just pulled. Mercurial groups all changes from one <command
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109 role="hg-cmd">hg pull</command> into a single transaction,
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110 so one <command role="hg-cmd">hg rollback</command> is all you
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111 need to undo this mistake.</para>
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112
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113 </sect2>
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114 <sect2 id="sec.undo.rollback-after-push">
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115 <title>Rolling back is useless once you've pushed</title>
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116
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117 <para>The value of the <command role="hg-cmd">hg
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118 rollback</command> command drops to zero once you've pushed
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119 your changes to another repository. Rolling back a change
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120 makes it disappear entirely, but <emphasis>only</emphasis> in
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121 the repository in which you perform the <command
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122 role="hg-cmd">hg rollback</command>. Because a rollback
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123 eliminates history, there's no way for the disappearance of a
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124 change to propagate between repositories.</para>
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125
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126 <para>If you've pushed a change to another
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127 repository&emdash;particularly if it's a shared
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128 repository&emdash;it has essentially <quote>escaped into the
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129 wild,</quote> and you'll have to recover from your mistake
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130 in a different way. What will happen if you push a changeset
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131 somewhere, then roll it back, then pull from the repository
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132 you pushed to, is that the changeset will reappear in your
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133 repository.</para>
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134
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135 <para>(If you absolutely know for sure that the change you want
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136 to roll back is the most recent change in the repository that
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137 you pushed to, <emphasis>and</emphasis> you know that nobody
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138 else could have pulled it from that repository, you can roll
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139 back the changeset there, too, but you really should really
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140 not rely on this working reliably. If you do this, sooner or
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141 later a change really will make it into a repository that you
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142 don't directly control (or have forgotten about), and come
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143 back to bite you.)</para>
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144
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145 </sect2>
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146 <sect2>
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147 <title>You can only roll back once</title>
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148
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149 <para>Mercurial stores exactly one transaction in its
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150 transaction log; that transaction is the most recent one that
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151 occurred in the repository. This means that you can only roll
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152 back one transaction. If you expect to be able to roll back
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153 one transaction, then its predecessor, this is not the
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154 behaviour you will get.</para>
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155
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156 &interaction.rollback.twice;
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157
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158 <para>Once you've rolled back one transaction in a repository,
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159 you can't roll back again in that repository until you perform
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160 another commit or pull.</para>
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161
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162 </sect2>
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163 </sect1>
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164 <sect1>
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165 <title>Reverting the mistaken change</title>
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166
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167 <para>If you make a modification to a file, and decide that you
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168 really didn't want to change the file at all, and you haven't
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169 yet committed your changes, the <command role="hg-cmd">hg
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170 revert</command> command is the one you'll need. It looks at
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171 the changeset that's the parent of the working directory, and
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172 restores the contents of the file to their state as of that
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173 changeset. (That's a long-winded way of saying that, in the
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174 normal case, it undoes your modifications.)</para>
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175
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176 <para>Let's illustrate how the <command role="hg-cmd">hg
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177 revert</command> command works with yet another small example.
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178 We'll begin by modifying a file that Mercurial is already
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179 tracking.</para>
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180
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181 &interaction.daily.revert.modify;
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182
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183 <para>If we don't
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184 want that change, we can simply <command role="hg-cmd">hg
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185 revert</command> the file.</para>
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186
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187 &interaction.daily.revert.unmodify;
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188
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189 <para>The <command role="hg-cmd">hg revert</command> command
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190 provides us with an extra degree of safety by saving our
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191 modified file with a <filename>.orig</filename>
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192 extension.</para>
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193
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194 &interaction.daily.revert.status;
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195
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196 <para>Here is a summary of the cases that the <command
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197 role="hg-cmd">hg revert</command> command can deal with. We
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198 will describe each of these in more detail in the section that
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199 follows.</para>
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200 <itemizedlist>
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201 <listitem><para>If you modify a file, it will restore the file
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202 to its unmodified state.</para>
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203 </listitem>
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204 <listitem><para>If you <command role="hg-cmd">hg add</command> a
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205 file, it will undo the <quote>added</quote> state of the
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206 file, but leave the file itself untouched.</para>
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207 </listitem>
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208 <listitem><para>If you delete a file without telling Mercurial,
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209 it will restore the file to its unmodified contents.</para>
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210 </listitem>
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211 <listitem><para>If you use the <command role="hg-cmd">hg
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212 remove</command> command to remove a file, it will undo
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213 the <quote>removed</quote> state of the file, and restore
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214 the file to its unmodified contents.</para>
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215 </listitem></itemizedlist>
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216
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217 <sect2 id="sec.undo.mgmt">
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218 <title>File management errors</title>
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219
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220 <para>The <command role="hg-cmd">hg revert</command> command is
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221 useful for more than just modified files. It lets you reverse
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222 the results of all of Mercurial's file management
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223 commands&emdash;<command role="hg-cmd">hg add</command>,
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224 <command role="hg-cmd">hg remove</command>, and so on.</para>
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225
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226 <para>If you <command role="hg-cmd">hg add</command> a file,
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227 then decide that in fact you don't want Mercurial to track it,
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228 use <command role="hg-cmd">hg revert</command> to undo the
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229 add. Don't worry; Mercurial will not modify the file in any
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230 way. It will just <quote>unmark</quote> the file.</para>
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231
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232 &interaction.daily.revert.add;
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233
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234 <para>Similarly, if you ask Mercurial to <command
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235 role="hg-cmd">hg remove</command> a file, you can use
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236 <command role="hg-cmd">hg revert</command> to restore it to
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237 the contents it had as of the parent of the working directory.
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238 &interaction.daily.revert.remove; This works just as
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239 well for a file that you deleted by hand, without telling
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240 Mercurial (recall that in Mercurial terminology, this kind of
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241 file is called <quote>missing</quote>).</para>
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242
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243 &interaction.daily.revert.missing;
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244
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245 <para>If you revert a <command role="hg-cmd">hg copy</command>,
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246 the copied-to file remains in your working directory
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247 afterwards, untracked. Since a copy doesn't affect the
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248 copied-from file in any way, Mercurial doesn't do anything
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249 with the copied-from file.</para>
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250
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251 &interaction.daily.revert.copy;
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252
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253 <sect3>
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254 <title>A slightly special case: reverting a rename</title>
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255
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256 <para>If you <command role="hg-cmd">hg rename</command> a
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257 file, there is one small detail that you should remember.
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258 When you <command role="hg-cmd">hg revert</command> a
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259 rename, it's not enough to provide the name of the
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260 renamed-to file, as you can see here.</para>
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261
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262 &interaction.daily.revert.rename;
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263
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264 <para>As you can see from the output of <command
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265 role="hg-cmd">hg status</command>, the renamed-to file is
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266 no longer identified as added, but the
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267 renamed-<emphasis>from</emphasis> file is still removed!
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268 This is counter-intuitive (at least to me), but at least
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269 it's easy to deal with.</para>
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270
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271 &interaction.daily.revert.rename-orig;
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272
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273 <para>So remember, to revert a <command role="hg-cmd">hg
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274 rename</command>, you must provide
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275 <emphasis>both</emphasis> the source and destination
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276 names.</para>
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277
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278 <para>% TODO: the output doesn't look like it will be
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279 removed!</para>
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280
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281 <para>(By the way, if you rename a file, then modify the
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282 renamed-to file, then revert both components of the rename,
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283 when Mercurial restores the file that was removed as part of
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284 the rename, it will be unmodified. If you need the
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285 modifications in the renamed-to file to show up in the
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286 renamed-from file, don't forget to copy them over.)</para>
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287
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288 <para>These fiddly aspects of reverting a rename arguably
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289 constitute a small bug in Mercurial.</para>
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290
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291 </sect3>
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292 </sect2>
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293 </sect1>
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294 <sect1>
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295 <title>Dealing with committed changes</title>
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296
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297 <para>Consider a case where you have committed a change $a$, and
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298 another change $b$ on top of it; you then realise that change
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299 $a$ was incorrect. Mercurial lets you <quote>back out</quote>
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300 an entire changeset automatically, and building blocks that let
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301 you reverse part of a changeset by hand.</para>
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302
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303 <para>Before you read this section, here's something to keep in
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304 mind: the <command role="hg-cmd">hg backout</command> command
|
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305 undoes changes by <emphasis>adding</emphasis> history, not by
|
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306 modifying or erasing it. It's the right tool to use if you're
|
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307 fixing bugs, but not if you're trying to undo some change that
|
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308 has catastrophic consequences. To deal with those, see section
|
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309 <xref linkend="sec.undo.aaaiiieee"/>.</para>
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310
|
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bos@559
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311 <sect2>
|
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bos@559
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312 <title>Backing out a changeset</title>
|
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313
|
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bos@559
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314 <para>The <command role="hg-cmd">hg backout</command> command
|
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315 lets you <quote>undo</quote> the effects of an entire
|
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316 changeset in an automated fashion. Because Mercurial's
|
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317 history is immutable, this command <emphasis>does
|
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bos@559
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318 not</emphasis> get rid of the changeset you want to undo.
|
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|
319 Instead, it creates a new changeset that
|
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320 <emphasis>reverses</emphasis> the effect of the to-be-undone
|
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321 changeset.</para>
|
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bos@559
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322
|
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bos@559
|
323 <para>The operation of the <command role="hg-cmd">hg
|
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bos@559
|
324 backout</command> command is a little intricate, so let's
|
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bos@559
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325 illustrate it with some examples. First, we'll create a
|
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326 repository with some simple changes.</para>
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327
|
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bos@567
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328 &interaction.backout.init;
|
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329
|
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bos@559
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330 <para>The <command role="hg-cmd">hg backout</command> command
|
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bos@559
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331 takes a single changeset ID as its argument; this is the
|
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bos@559
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332 changeset to back out. Normally, <command role="hg-cmd">hg
|
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333 backout</command> will drop you into a text editor to write
|
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334 a commit message, so you can record why you're backing the
|
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335 change out. In this example, we provide a commit message on
|
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336 the command line using the <option
|
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337 role="hg-opt-backout">-m</option> option.</para>
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338
|
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bos@559
|
339 </sect2>
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bos@559
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340 <sect2>
|
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bos@559
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341 <title>Backing out the tip changeset</title>
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342
|
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bos@559
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343 <para>We're going to start by backing out the last changeset we
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344 committed.</para>
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345
|
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bos@567
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346 &interaction.backout.simple;
|
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347
|
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bos@567
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348 <para>You can see that the second line from
|
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bos@567
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349 <filename>myfile</filename> is no longer present. Taking a
|
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|
350 look at the output of <command role="hg-cmd">hg log</command>
|
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bos@567
|
351 gives us an idea of what the <command role="hg-cmd">hg
|
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bos@567
|
352 backout</command> command has done.
|
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bos@567
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353 &interaction.backout.simple.log; Notice that the new changeset
|
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bos@567
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354 that <command role="hg-cmd">hg backout</command> has created
|
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bos@567
|
355 is a child of the changeset we backed out. It's easier to see
|
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bos@567
|
356 this in figure <xref
|
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dongsheng@625
|
357 linkend="fig.undo.backout"/>, which presents a graphical
|
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|
358 view of the change history. As you can see, the history is
|
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bos@559
|
359 nice and linear.</para>
|
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bos@559
|
360
|
|
dongsheng@625
|
361 <informalfigure id="fig.undo.backout">
|
|
bos@559
|
362 <mediaobject><imageobject><imagedata
|
|
dongsheng@625
|
363 fileref="images/undo-simple.png"/></imageobject><textobject><phrase>XXX
|
|
bos@559
|
364 add text</phrase></textobject><caption><para>Backing out
|
|
bos@559
|
365 a change using the <command role="hg-cmd">hg
|
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bos@559
|
366 backout</command>
|
|
bos@559
|
367 command</para></caption></mediaobject>
|
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bos@559
|
368
|
|
bos@559
|
369 </informalfigure>
|
|
bos@559
|
370
|
|
bos@559
|
371 </sect2>
|
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bos@559
|
372 <sect2>
|
|
bos@559
|
373 <title>Backing out a non-tip change</title>
|
|
bos@559
|
374
|
|
bos@559
|
375 <para>If you want to back out a change other than the last one
|
|
bos@559
|
376 you committed, pass the <option
|
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bos@559
|
377 role="hg-opt-backout">--merge</option> option to the
|
|
bos@567
|
378 <command role="hg-cmd">hg backout</command> command.</para>
|
|
bos@567
|
379
|
|
bos@567
|
380 &interaction.backout.non-tip.clone;
|
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bos@567
|
381
|
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bos@567
|
382 <para>This makes backing out any changeset a
|
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bos@567
|
383 <quote>one-shot</quote> operation that's usually simple and
|
|
bos@567
|
384 fast.</para>
|
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bos@567
|
385
|
|
bos@567
|
386 &interaction.backout.non-tip.backout;
|
|
bos@559
|
387
|
|
bos@559
|
388 <para>If you take a look at the contents of
|
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bos@559
|
389 <filename>myfile</filename> after the backout finishes, you'll
|
|
bos@559
|
390 see that the first and third changes are present, but not the
|
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bos@567
|
391 second.</para>
|
|
bos@567
|
392
|
|
bos@567
|
393 &interaction.backout.non-tip.cat;
|
|
bos@559
|
394
|
|
bos@559
|
395 <para>As the graphical history in figure <xref
|
|
dongsheng@625
|
396 linkend="fig.undo.backout-non-tip"/> illustrates, Mercurial
|
|
bos@559
|
397 actually commits <emphasis>two</emphasis> changes in this kind
|
|
bos@559
|
398 of situation (the box-shaped nodes are the ones that Mercurial
|
|
bos@559
|
399 commits automatically). Before Mercurial begins the backout
|
|
bos@559
|
400 process, it first remembers what the current parent of the
|
|
bos@559
|
401 working directory is. It then backs out the target changeset,
|
|
bos@559
|
402 and commits that as a changeset. Finally, it merges back to
|
|
bos@559
|
403 the previous parent of the working directory, and commits the
|
|
bos@559
|
404 result of the merge.</para>
|
|
bos@559
|
405
|
|
bos@559
|
406 <para>% TODO: to me it looks like mercurial doesn't commit the
|
|
bos@559
|
407 second merge automatically!</para>
|
|
bos@559
|
408
|
|
dongsheng@625
|
409 <informalfigure id="fig.undo.backout-non-tip">
|
|
bos@559
|
410 <mediaobject><imageobject><imagedata
|
|
dongsheng@625
|
411 fileref="images/undo-non-tip.png"/></imageobject><textobject><phrase>XXX
|
|
bos@559
|
412 add text</phrase></textobject><caption><para>Automated
|
|
bos@559
|
413 backout of a non-tip change using the <command
|
|
bos@559
|
414 role="hg-cmd">hg backout</command>
|
|
bos@559
|
415 command</para></caption></mediaobject>
|
|
bos@559
|
416 </informalfigure>
|
|
bos@559
|
417
|
|
bos@559
|
418 <para>The result is that you end up <quote>back where you
|
|
bos@559
|
419 were</quote>, only with some extra history that undoes the
|
|
bos@559
|
420 effect of the changeset you wanted to back out.</para>
|
|
bos@559
|
421
|
|
bos@559
|
422 <sect3>
|
|
bos@559
|
423 <title>Always use the <option
|
|
bos@559
|
424 role="hg-opt-backout">--merge</option> option</title>
|
|
bos@559
|
425
|
|
bos@559
|
426 <para>In fact, since the <option
|
|
bos@559
|
427 role="hg-opt-backout">--merge</option> option will do the
|
|
bos@559
|
428 <quote>right thing</quote> whether or not the changeset
|
|
bos@559
|
429 you're backing out is the tip (i.e. it won't try to merge if
|
|
bos@559
|
430 it's backing out the tip, since there's no need), you should
|
|
bos@559
|
431 <emphasis>always</emphasis> use this option when you run the
|
|
bos@559
|
432 <command role="hg-cmd">hg backout</command> command.</para>
|
|
bos@559
|
433
|
|
bos@559
|
434 </sect3>
|
|
bos@559
|
435 </sect2>
|
|
bos@559
|
436 <sect2>
|
|
bos@559
|
437 <title>Gaining more control of the backout process</title>
|
|
bos@559
|
438
|
|
bos@559
|
439 <para>While I've recommended that you always use the <option
|
|
bos@559
|
440 role="hg-opt-backout">--merge</option> option when backing
|
|
bos@559
|
441 out a change, the <command role="hg-cmd">hg backout</command>
|
|
bos@559
|
442 command lets you decide how to merge a backout changeset.
|
|
bos@559
|
443 Taking control of the backout process by hand is something you
|
|
bos@559
|
444 will rarely need to do, but it can be useful to understand
|
|
bos@559
|
445 what the <command role="hg-cmd">hg backout</command> command
|
|
bos@559
|
446 is doing for you automatically. To illustrate this, let's
|
|
bos@559
|
447 clone our first repository, but omit the backout change that
|
|
bos@559
|
448 it contains.</para>
|
|
bos@559
|
449
|
|
bos@567
|
450 &interaction.backout.manual.clone;
|
|
bos@567
|
451
|
|
bos@567
|
452 <para>As with our
|
|
bos@559
|
453 earlier example, We'll commit a third changeset, then back out
|
|
bos@567
|
454 its parent, and see what happens.</para>
|
|
bos@567
|
455
|
|
bos@567
|
456 &interaction.backout.manual.backout;
|
|
bos@567
|
457
|
|
bos@567
|
458 <para>Our new changeset is again a descendant of the changeset
|
|
bos@567
|
459 we backout out; it's thus a new head, <emphasis>not</emphasis>
|
|
bos@567
|
460 a descendant of the changeset that was the tip. The <command
|
|
bos@567
|
461 role="hg-cmd">hg backout</command> command was quite
|
|
bos@567
|
462 explicit in telling us this.</para>
|
|
bos@567
|
463
|
|
bos@567
|
464 &interaction.backout.manual.log;
|
|
bos@559
|
465
|
|
bos@559
|
466 <para>Again, it's easier to see what has happened by looking at
|
|
bos@559
|
467 a graph of the revision history, in figure <xref
|
|
dongsheng@625
|
468 linkend="fig.undo.backout-manual"/>. This makes it clear
|
|
bos@559
|
469 that when we use <command role="hg-cmd">hg backout</command>
|
|
bos@559
|
470 to back out a change other than the tip, Mercurial adds a new
|
|
bos@559
|
471 head to the repository (the change it committed is
|
|
bos@559
|
472 box-shaped).</para>
|
|
bos@559
|
473
|
|
dongsheng@625
|
474 <informalfigure id="fig.undo.backout-manual">
|
|
bos@559
|
475 <mediaobject><imageobject><imagedata
|
|
dongsheng@625
|
476 fileref="images/undo-manual.png"/></imageobject><textobject><phrase>XXX
|
|
bos@559
|
477 add text</phrase></textobject><caption><para>Backing out
|
|
bos@559
|
478 a change using the <command role="hg-cmd">hg
|
|
bos@559
|
479 backout</command>
|
|
bos@559
|
480 command</para></caption></mediaobject>
|
|
bos@559
|
481
|
|
bos@559
|
482 </informalfigure>
|
|
bos@559
|
483
|
|
bos@559
|
484 <para>After the <command role="hg-cmd">hg backout</command>
|
|
bos@559
|
485 command has completed, it leaves the new
|
|
bos@559
|
486 <quote>backout</quote> changeset as the parent of the working
|
|
bos@567
|
487 directory.</para>
|
|
bos@567
|
488
|
|
bos@567
|
489 &interaction.backout.manual.parents;
|
|
bos@567
|
490
|
|
bos@567
|
491 <para>Now we have two isolated sets of changes.</para>
|
|
bos@567
|
492
|
|
bos@567
|
493 &interaction.backout.manual.heads;
|
|
bos@559
|
494
|
|
bos@559
|
495 <para>Let's think about what we expect to see as the contents of
|
|
bos@559
|
496 <filename>myfile</filename> now. The first change should be
|
|
bos@559
|
497 present, because we've never backed it out. The second change
|
|
bos@559
|
498 should be missing, as that's the change we backed out. Since
|
|
bos@559
|
499 the history graph shows the third change as a separate head,
|
|
bos@559
|
500 we <emphasis>don't</emphasis> expect to see the third change
|
|
bos@567
|
501 present in <filename>myfile</filename>.</para>
|
|
bos@567
|
502
|
|
bos@567
|
503 &interaction.backout.manual.cat;
|
|
bos@567
|
504
|
|
bos@567
|
505 <para>To get the third change back into the file, we just do a
|
|
bos@567
|
506 normal merge of our two heads.</para>
|
|
bos@567
|
507
|
|
bos@567
|
508 &interaction.backout.manual.merge;
|
|
bos@567
|
509
|
|
bos@567
|
510 <para>Afterwards, the graphical history of our repository looks
|
|
bos@567
|
511 like figure
|
|
dongsheng@625
|
512 <xref linkend="fig.undo.backout-manual-merge"/>.</para>
|
|
dongsheng@625
|
513
|
|
dongsheng@625
|
514 <informalfigure id="fig.undo.backout-manual-merge">
|
|
bos@559
|
515 <mediaobject><imageobject><imagedata
|
|
dongsheng@625
|
516 fileref="images/undo-manual-merge.png"/></imageobject><textobject><phrase>XXX
|
|
bos@559
|
517 add text</phrase></textobject><caption><para>Manually
|
|
bos@559
|
518 merging a backout change</para></caption></mediaobject>
|
|
bos@559
|
519
|
|
bos@559
|
520 </informalfigure>
|
|
bos@559
|
521
|
|
bos@559
|
522 </sect2>
|
|
bos@559
|
523 <sect2>
|
|
bos@559
|
524 <title>Why <command role="hg-cmd">hg backout</command> works as
|
|
bos@559
|
525 it does</title>
|
|
bos@559
|
526
|
|
bos@559
|
527 <para>Here's a brief description of how the <command
|
|
bos@559
|
528 role="hg-cmd">hg backout</command> command works.</para>
|
|
bos@559
|
529 <orderedlist>
|
|
bos@559
|
530 <listitem><para>It ensures that the working directory is
|
|
bos@559
|
531 <quote>clean</quote>, i.e. that the output of <command
|
|
bos@559
|
532 role="hg-cmd">hg status</command> would be empty.</para>
|
|
bos@559
|
533 </listitem>
|
|
bos@559
|
534 <listitem><para>It remembers the current parent of the working
|
|
bos@559
|
535 directory. Let's call this changeset
|
|
bos@559
|
536 <literal>orig</literal></para>
|
|
bos@559
|
537 </listitem>
|
|
bos@559
|
538 <listitem><para>It does the equivalent of a <command
|
|
bos@559
|
539 role="hg-cmd">hg update</command> to sync the working
|
|
bos@559
|
540 directory to the changeset you want to back out. Let's
|
|
bos@559
|
541 call this changeset <literal>backout</literal></para>
|
|
bos@559
|
542 </listitem>
|
|
bos@559
|
543 <listitem><para>It finds the parent of that changeset. Let's
|
|
bos@559
|
544 call that changeset <literal>parent</literal>.</para>
|
|
bos@559
|
545 </listitem>
|
|
bos@559
|
546 <listitem><para>For each file that the
|
|
bos@559
|
547 <literal>backout</literal> changeset affected, it does the
|
|
bos@559
|
548 equivalent of a <command role="hg-cmd">hg revert -r
|
|
bos@559
|
549 parent</command> on that file, to restore it to the
|
|
bos@559
|
550 contents it had before that changeset was
|
|
bos@559
|
551 committed.</para>
|
|
bos@559
|
552 </listitem>
|
|
bos@559
|
553 <listitem><para>It commits the result as a new changeset.
|
|
bos@559
|
554 This changeset has <literal>backout</literal> as its
|
|
bos@559
|
555 parent.</para>
|
|
bos@559
|
556 </listitem>
|
|
bos@559
|
557 <listitem><para>If you specify <option
|
|
bos@559
|
558 role="hg-opt-backout">--merge</option> on the command
|
|
bos@559
|
559 line, it merges with <literal>orig</literal>, and commits
|
|
bos@559
|
560 the result of the merge.</para>
|
|
bos@559
|
561 </listitem></orderedlist>
|
|
bos@559
|
562
|
|
bos@559
|
563 <para>An alternative way to implement the <command
|
|
bos@559
|
564 role="hg-cmd">hg backout</command> command would be to
|
|
bos@559
|
565 <command role="hg-cmd">hg export</command> the
|
|
bos@559
|
566 to-be-backed-out changeset as a diff, then use the <option
|
|
bos@559
|
567 role="cmd-opt-patch">--reverse</option> option to the
|
|
bos@559
|
568 <command>patch</command> command to reverse the effect of the
|
|
bos@559
|
569 change without fiddling with the working directory. This
|
|
bos@559
|
570 sounds much simpler, but it would not work nearly as
|
|
bos@559
|
571 well.</para>
|
|
bos@559
|
572
|
|
bos@559
|
573 <para>The reason that <command role="hg-cmd">hg
|
|
bos@559
|
574 backout</command> does an update, a commit, a merge, and
|
|
bos@559
|
575 another commit is to give the merge machinery the best chance
|
|
bos@559
|
576 to do a good job when dealing with all the changes
|
|
bos@559
|
577 <emphasis>between</emphasis> the change you're backing out and
|
|
bos@559
|
578 the current tip.</para>
|
|
bos@559
|
579
|
|
bos@559
|
580 <para>If you're backing out a changeset that's 100 revisions
|
|
bos@559
|
581 back in your project's history, the chances that the
|
|
bos@559
|
582 <command>patch</command> command will be able to apply a
|
|
bos@559
|
583 reverse diff cleanly are not good, because intervening changes
|
|
bos@559
|
584 are likely to have <quote>broken the context</quote> that
|
|
bos@559
|
585 <command>patch</command> uses to determine whether it can
|
|
bos@559
|
586 apply a patch (if this sounds like gibberish, see <xref
|
|
dongsheng@625
|
587 linkend="sec.mq.patch"/> for a
|
|
bos@559
|
588 discussion of the <command>patch</command> command). Also,
|
|
bos@559
|
589 Mercurial's merge machinery will handle files and directories
|
|
bos@559
|
590 being renamed, permission changes, and modifications to binary
|
|
bos@559
|
591 files, none of which <command>patch</command> can deal
|
|
bos@559
|
592 with.</para>
|
|
bos@559
|
593
|
|
bos@559
|
594 </sect2>
|
|
bos@559
|
595 </sect1>
|
|
dongsheng@625
|
596 <sect1 id="sec.undo.aaaiiieee">
|
|
bos@559
|
597 <title>Changes that should never have been</title>
|
|
bos@559
|
598
|
|
bos@559
|
599 <para>Most of the time, the <command role="hg-cmd">hg
|
|
bos@559
|
600 backout</command> command is exactly what you need if you want
|
|
bos@559
|
601 to undo the effects of a change. It leaves a permanent record
|
|
bos@559
|
602 of exactly what you did, both when committing the original
|
|
bos@559
|
603 changeset and when you cleaned up after it.</para>
|
|
bos@559
|
604
|
|
bos@559
|
605 <para>On rare occasions, though, you may find that you've
|
|
bos@559
|
606 committed a change that really should not be present in the
|
|
bos@559
|
607 repository at all. For example, it would be very unusual, and
|
|
bos@559
|
608 usually considered a mistake, to commit a software project's
|
|
bos@559
|
609 object files as well as its source files. Object files have
|
|
bos@559
|
610 almost no intrinsic value, and they're <emphasis>big</emphasis>,
|
|
bos@559
|
611 so they increase the size of the repository and the amount of
|
|
bos@559
|
612 time it takes to clone or pull changes.</para>
|
|
bos@559
|
613
|
|
bos@559
|
614 <para>Before I discuss the options that you have if you commit a
|
|
bos@559
|
615 <quote>brown paper bag</quote> change (the kind that's so bad
|
|
bos@559
|
616 that you want to pull a brown paper bag over your head), let me
|
|
bos@559
|
617 first discuss some approaches that probably won't work.</para>
|
|
bos@559
|
618
|
|
bos@559
|
619 <para>Since Mercurial treats history as accumulative&emdash;every
|
|
bos@559
|
620 change builds on top of all changes that preceded it&emdash;you
|
|
bos@559
|
621 generally can't just make disastrous changes disappear. The one
|
|
bos@559
|
622 exception is when you've just committed a change, and it hasn't
|
|
bos@559
|
623 been pushed or pulled into another repository. That's when you
|
|
bos@559
|
624 can safely use the <command role="hg-cmd">hg rollback</command>
|
|
bos@559
|
625 command, as I detailed in section <xref
|
|
dongsheng@625
|
626 linkend="sec.undo.rollback"/>.</para>
|
|
bos@559
|
627
|
|
bos@559
|
628 <para>After you've pushed a bad change to another repository, you
|
|
bos@559
|
629 <emphasis>could</emphasis> still use <command role="hg-cmd">hg
|
|
bos@559
|
630 rollback</command> to make your local copy of the change
|
|
bos@559
|
631 disappear, but it won't have the consequences you want. The
|
|
bos@559
|
632 change will still be present in the remote repository, so it
|
|
bos@559
|
633 will reappear in your local repository the next time you
|
|
bos@559
|
634 pull.</para>
|
|
bos@559
|
635
|
|
bos@559
|
636 <para>If a situation like this arises, and you know which
|
|
bos@559
|
637 repositories your bad change has propagated into, you can
|
|
bos@559
|
638 <emphasis>try</emphasis> to get rid of the changeefrom
|
|
bos@559
|
639 <emphasis>every</emphasis> one of those repositories. This is,
|
|
bos@559
|
640 of course, not a satisfactory solution: if you miss even a
|
|
bos@559
|
641 single repository while you're expunging, the change is still
|
|
bos@559
|
642 <quote>in the wild</quote>, and could propagate further.</para>
|
|
bos@559
|
643
|
|
bos@559
|
644 <para>If you've committed one or more changes
|
|
bos@559
|
645 <emphasis>after</emphasis> the change that you'd like to see
|
|
bos@559
|
646 disappear, your options are further reduced. Mercurial doesn't
|
|
bos@559
|
647 provide a way to <quote>punch a hole</quote> in history, leaving
|
|
bos@559
|
648 changesets intact.</para>
|
|
bos@559
|
649
|
|
bos@559
|
650 <para>XXX This needs filling out. The
|
|
bos@559
|
651 <literal>hg-replay</literal> script in the
|
|
bos@559
|
652 <literal>examples</literal> directory works, but doesn't handle
|
|
bos@559
|
653 merge changesets. Kind of an important omission.</para>
|
|
bos@559
|
654
|
|
bos@559
|
655 <sect2>
|
|
bos@559
|
656 <title>Protect yourself from <quote>escaped</quote>
|
|
bos@559
|
657 changes</title>
|
|
bos@559
|
658
|
|
bos@559
|
659 <para>If you've committed some changes to your local repository
|
|
bos@559
|
660 and they've been pushed or pulled somewhere else, this isn't
|
|
bos@559
|
661 necessarily a disaster. You can protect yourself ahead of
|
|
bos@559
|
662 time against some classes of bad changeset. This is
|
|
bos@559
|
663 particularly easy if your team usually pulls changes from a
|
|
bos@559
|
664 central repository.</para>
|
|
bos@559
|
665
|
|
bos@559
|
666 <para>By configuring some hooks on that repository to validate
|
|
dongsheng@625
|
667 incoming changesets (see chapter <xref linkend="chap.hook"/>),
|
|
bos@559
|
668 you can
|
|
bos@559
|
669 automatically prevent some kinds of bad changeset from being
|
|
bos@559
|
670 pushed to the central repository at all. With such a
|
|
bos@559
|
671 configuration in place, some kinds of bad changeset will
|
|
bos@559
|
672 naturally tend to <quote>die out</quote> because they can't
|
|
bos@559
|
673 propagate into the central repository. Better yet, this
|
|
bos@559
|
674 happens without any need for explicit intervention.</para>
|
|
bos@559
|
675
|
|
bos@559
|
676 <para>For instance, an incoming change hook that verifies that a
|
|
bos@559
|
677 changeset will actually compile can prevent people from
|
|
bos@559
|
678 inadvertantly <quote>breaking the build</quote>.</para>
|
|
bos@559
|
679
|
|
bos@559
|
680 </sect2>
|
|
bos@559
|
681 </sect1>
|
|
dongsheng@625
|
682 <sect1 id="sec.undo.bisect">
|
|
bos@559
|
683 <title>Finding the source of a bug</title>
|
|
bos@559
|
684
|
|
bos@559
|
685 <para>While it's all very well to be able to back out a changeset
|
|
bos@559
|
686 that introduced a bug, this requires that you know which
|
|
bos@559
|
687 changeset to back out. Mercurial provides an invaluable
|
|
bos@559
|
688 command, called <command role="hg-cmd">hg bisect</command>, that
|
|
bos@559
|
689 helps you to automate this process and accomplish it very
|
|
bos@559
|
690 efficiently.</para>
|
|
bos@559
|
691
|
|
bos@559
|
692 <para>The idea behind the <command role="hg-cmd">hg
|
|
bos@559
|
693 bisect</command> command is that a changeset has introduced
|
|
bos@559
|
694 some change of behaviour that you can identify with a simple
|
|
bos@559
|
695 binary test. You don't know which piece of code introduced the
|
|
bos@559
|
696 change, but you know how to test for the presence of the bug.
|
|
bos@559
|
697 The <command role="hg-cmd">hg bisect</command> command uses your
|
|
bos@559
|
698 test to direct its search for the changeset that introduced the
|
|
bos@559
|
699 code that caused the bug.</para>
|
|
bos@559
|
700
|
|
bos@559
|
701 <para>Here are a few scenarios to help you understand how you
|
|
bos@559
|
702 might apply this command.</para>
|
|
bos@559
|
703 <itemizedlist>
|
|
bos@559
|
704 <listitem><para>The most recent version of your software has a
|
|
bos@559
|
705 bug that you remember wasn't present a few weeks ago, but
|
|
bos@559
|
706 you don't know when it was introduced. Here, your binary
|
|
bos@559
|
707 test checks for the presence of that bug.</para>
|
|
bos@559
|
708 </listitem>
|
|
bos@559
|
709 <listitem><para>You fixed a bug in a rush, and now it's time to
|
|
bos@559
|
710 close the entry in your team's bug database. The bug
|
|
bos@559
|
711 database requires a changeset ID when you close an entry,
|
|
bos@559
|
712 but you don't remember which changeset you fixed the bug in.
|
|
bos@559
|
713 Once again, your binary test checks for the presence of the
|
|
bos@559
|
714 bug.</para>
|
|
bos@559
|
715 </listitem>
|
|
bos@559
|
716 <listitem><para>Your software works correctly, but runs 15%
|
|
bos@559
|
717 slower than the last time you measured it. You want to know
|
|
bos@559
|
718 which changeset introduced the performance regression. In
|
|
bos@559
|
719 this case, your binary test measures the performance of your
|
|
bos@559
|
720 software, to see whether it's <quote>fast</quote> or
|
|
bos@559
|
721 <quote>slow</quote>.</para>
|
|
bos@559
|
722 </listitem>
|
|
bos@559
|
723 <listitem><para>The sizes of the components of your project that
|
|
bos@559
|
724 you ship exploded recently, and you suspect that something
|
|
bos@559
|
725 changed in the way you build your project.</para>
|
|
bos@559
|
726 </listitem></itemizedlist>
|
|
bos@559
|
727
|
|
bos@559
|
728 <para>From these examples, it should be clear that the <command
|
|
bos@559
|
729 role="hg-cmd">hg bisect</command> command is not useful only
|
|
bos@559
|
730 for finding the sources of bugs. You can use it to find any
|
|
bos@559
|
731 <quote>emergent property</quote> of a repository (anything that
|
|
bos@559
|
732 you can't find from a simple text search of the files in the
|
|
bos@559
|
733 tree) for which you can write a binary test.</para>
|
|
bos@559
|
734
|
|
bos@559
|
735 <para>We'll introduce a little bit of terminology here, just to
|
|
bos@559
|
736 make it clear which parts of the search process are your
|
|
bos@559
|
737 responsibility, and which are Mercurial's. A
|
|
bos@559
|
738 <emphasis>test</emphasis> is something that
|
|
bos@559
|
739 <emphasis>you</emphasis> run when <command role="hg-cmd">hg
|
|
bos@559
|
740 bisect</command> chooses a changeset. A
|
|
bos@559
|
741 <emphasis>probe</emphasis> is what <command role="hg-cmd">hg
|
|
bos@559
|
742 bisect</command> runs to tell whether a revision is good.
|
|
bos@559
|
743 Finally, we'll use the word <quote>bisect</quote>, as both a
|
|
bos@559
|
744 noun and a verb, to stand in for the phrase <quote>search using
|
|
bos@559
|
745 the <command role="hg-cmd">hg bisect</command>
|
|
bos@559
|
746 command</quote>.</para>
|
|
bos@559
|
747
|
|
bos@559
|
748 <para>One simple way to automate the searching process would be
|
|
bos@559
|
749 simply to probe every changeset. However, this scales poorly.
|
|
bos@559
|
750 If it took ten minutes to test a single changeset, and you had
|
|
bos@559
|
751 10,000 changesets in your repository, the exhaustive approach
|
|
bos@559
|
752 would take on average 35 <emphasis>days</emphasis> to find the
|
|
bos@559
|
753 changeset that introduced a bug. Even if you knew that the bug
|
|
bos@559
|
754 was introduced by one of the last 500 changesets, and limited
|
|
bos@559
|
755 your search to those, you'd still be looking at over 40 hours to
|
|
bos@559
|
756 find the changeset that introduced your bug.</para>
|
|
bos@559
|
757
|
|
bos@559
|
758 <para>What the <command role="hg-cmd">hg bisect</command> command
|
|
bos@559
|
759 does is use its knowledge of the <quote>shape</quote> of your
|
|
bos@559
|
760 project's revision history to perform a search in time
|
|
bos@559
|
761 proportional to the <emphasis>logarithm</emphasis> of the number
|
|
bos@559
|
762 of changesets to check (the kind of search it performs is called
|
|
bos@559
|
763 a dichotomic search). With this approach, searching through
|
|
bos@559
|
764 10,000 changesets will take less than three hours, even at ten
|
|
bos@559
|
765 minutes per test (the search will require about 14 tests).
|
|
bos@559
|
766 Limit your search to the last hundred changesets, and it will
|
|
bos@559
|
767 take only about an hour (roughly seven tests).</para>
|
|
bos@559
|
768
|
|
bos@559
|
769 <para>The <command role="hg-cmd">hg bisect</command> command is
|
|
bos@559
|
770 aware of the <quote>branchy</quote> nature of a Mercurial
|
|
bos@559
|
771 project's revision history, so it has no problems dealing with
|
|
bos@559
|
772 branches, merges, or multiple heads in a repository. It can
|
|
bos@559
|
773 prune entire branches of history with a single probe, which is
|
|
bos@559
|
774 how it operates so efficiently.</para>
|
|
bos@559
|
775
|
|
bos@559
|
776 <sect2>
|
|
bos@559
|
777 <title>Using the <command role="hg-cmd">hg bisect</command>
|
|
bos@559
|
778 command</title>
|
|
bos@559
|
779
|
|
bos@559
|
780 <para>Here's an example of <command role="hg-cmd">hg
|
|
bos@559
|
781 bisect</command> in action.</para>
|
|
bos@559
|
782
|
|
bos@559
|
783 <note>
|
|
bos@559
|
784 <para> In versions 0.9.5 and earlier of Mercurial, <command
|
|
bos@559
|
785 role="hg-cmd">hg bisect</command> was not a core command:
|
|
bos@559
|
786 it was distributed with Mercurial as an extension. This
|
|
bos@559
|
787 section describes the built-in command, not the old
|
|
bos@559
|
788 extension.</para>
|
|
bos@559
|
789 </note>
|
|
bos@559
|
790
|
|
bos@559
|
791 <para>Now let's create a repository, so that we can try out the
|
|
bos@559
|
792 <command role="hg-cmd">hg bisect</command> command in
|
|
bos@567
|
793 isolation.</para>
|
|
bos@567
|
794
|
|
bos@567
|
795 &interaction.bisect.init;
|
|
bos@567
|
796
|
|
bos@567
|
797 <para>We'll simulate a project that has a bug in it in a
|
|
bos@567
|
798 simple-minded way: create trivial changes in a loop, and
|
|
bos@567
|
799 nominate one specific change that will have the
|
|
bos@567
|
800 <quote>bug</quote>. This loop creates 35 changesets, each
|
|
bos@567
|
801 adding a single file to the repository. We'll represent our
|
|
bos@567
|
802 <quote>bug</quote> with a file that contains the text <quote>i
|
|
bos@567
|
803 have a gub</quote>.</para>
|
|
bos@567
|
804
|
|
bos@567
|
805 &interaction.bisect.commits;
|
|
bos@559
|
806
|
|
bos@559
|
807 <para>The next thing that we'd like to do is figure out how to
|
|
bos@559
|
808 use the <command role="hg-cmd">hg bisect</command> command.
|
|
bos@559
|
809 We can use Mercurial's normal built-in help mechanism for
|
|
bos@567
|
810 this.</para>
|
|
bos@567
|
811
|
|
bos@567
|
812 &interaction.bisect.help;
|
|
bos@559
|
813
|
|
bos@559
|
814 <para>The <command role="hg-cmd">hg bisect</command> command
|
|
bos@559
|
815 works in steps. Each step proceeds as follows.</para>
|
|
bos@559
|
816 <orderedlist>
|
|
bos@559
|
817 <listitem><para>You run your binary test.</para>
|
|
bos@559
|
818 <itemizedlist>
|
|
bos@559
|
819 <listitem><para>If the test succeeded, you tell <command
|
|
bos@559
|
820 role="hg-cmd">hg bisect</command> by running the
|
|
bos@559
|
821 <command role="hg-cmd">hg bisect good</command>
|
|
bos@559
|
822 command.</para>
|
|
bos@559
|
823 </listitem>
|
|
bos@559
|
824 <listitem><para>If it failed, run the <command
|
|
bos@559
|
825 role="hg-cmd">hg bisect bad</command>
|
|
bos@559
|
826 command.</para></listitem></itemizedlist>
|
|
bos@559
|
827 </listitem>
|
|
bos@559
|
828 <listitem><para>The command uses your information to decide
|
|
bos@559
|
829 which changeset to test next.</para>
|
|
bos@559
|
830 </listitem>
|
|
bos@559
|
831 <listitem><para>It updates the working directory to that
|
|
bos@559
|
832 changeset, and the process begins again.</para>
|
|
bos@559
|
833 </listitem></orderedlist>
|
|
bos@559
|
834 <para>The process ends when <command role="hg-cmd">hg
|
|
bos@559
|
835 bisect</command> identifies a unique changeset that marks
|
|
bos@559
|
836 the point where your test transitioned from
|
|
bos@559
|
837 <quote>succeeding</quote> to <quote>failing</quote>.</para>
|
|
bos@559
|
838
|
|
bos@559
|
839 <para>To start the search, we must run the <command
|
|
bos@567
|
840 role="hg-cmd">hg bisect --reset</command> command.</para>
|
|
bos@567
|
841
|
|
bos@567
|
842 &interaction.bisect.search.init;
|
|
bos@559
|
843
|
|
bos@559
|
844 <para>In our case, the binary test we use is simple: we check to
|
|
bos@559
|
845 see if any file in the repository contains the string <quote>i
|
|
bos@559
|
846 have a gub</quote>. If it does, this changeset contains the
|
|
bos@559
|
847 change that <quote>caused the bug</quote>. By convention, a
|
|
bos@559
|
848 changeset that has the property we're searching for is
|
|
bos@559
|
849 <quote>bad</quote>, while one that doesn't is
|
|
bos@559
|
850 <quote>good</quote>.</para>
|
|
bos@559
|
851
|
|
bos@559
|
852 <para>Most of the time, the revision to which the working
|
|
bos@559
|
853 directory is synced (usually the tip) already exhibits the
|
|
bos@559
|
854 problem introduced by the buggy change, so we'll mark it as
|
|
bos@567
|
855 <quote>bad</quote>.</para>
|
|
bos@567
|
856
|
|
bos@567
|
857 &interaction.bisect.search.bad-init;
|
|
bos@559
|
858
|
|
bos@559
|
859 <para>Our next task is to nominate a changeset that we know
|
|
bos@559
|
860 <emphasis>doesn't</emphasis> have the bug; the <command
|
|
bos@559
|
861 role="hg-cmd">hg bisect</command> command will
|
|
bos@559
|
862 <quote>bracket</quote> its search between the first pair of
|
|
bos@559
|
863 good and bad changesets. In our case, we know that revision
|
|
bos@559
|
864 10 didn't have the bug. (I'll have more words about choosing
|
|
bos@567
|
865 the first <quote>good</quote> changeset later.)</para>
|
|
bos@567
|
866
|
|
bos@567
|
867 &interaction.bisect.search.good-init;
|
|
bos@559
|
868
|
|
bos@559
|
869 <para>Notice that this command printed some output.</para>
|
|
bos@559
|
870 <itemizedlist>
|
|
bos@559
|
871 <listitem><para>It told us how many changesets it must
|
|
bos@559
|
872 consider before it can identify the one that introduced
|
|
bos@559
|
873 the bug, and how many tests that will require.</para>
|
|
bos@559
|
874 </listitem>
|
|
bos@559
|
875 <listitem><para>It updated the working directory to the next
|
|
bos@559
|
876 changeset to test, and told us which changeset it's
|
|
bos@559
|
877 testing.</para>
|
|
bos@559
|
878 </listitem></itemizedlist>
|
|
bos@559
|
879
|
|
bos@559
|
880 <para>We now run our test in the working directory. We use the
|
|
bos@559
|
881 <command>grep</command> command to see if our
|
|
bos@559
|
882 <quote>bad</quote> file is present in the working directory.
|
|
bos@559
|
883 If it is, this revision is bad; if not, this revision is good.
|
|
bos@567
|
884 &interaction.bisect.search.step1;</para>
|
|
bos@559
|
885
|
|
bos@559
|
886 <para>This test looks like a perfect candidate for automation,
|
|
bos@567
|
887 so let's turn it into a shell function.</para>
|
|
bos@567
|
888 &interaction.bisect.search.mytest;
|
|
bos@567
|
889
|
|
bos@567
|
890 <para>We can now run an entire test step with a single command,
|
|
bos@567
|
891 <literal>mytest</literal>.</para>
|
|
bos@567
|
892
|
|
bos@567
|
893 &interaction.bisect.search.step2;
|
|
bos@567
|
894
|
|
bos@567
|
895 <para>A few more invocations of our canned test step command,
|
|
bos@567
|
896 and we're done.</para>
|
|
bos@567
|
897
|
|
bos@567
|
898 &interaction.bisect.search.rest;
|
|
bos@559
|
899
|
|
bos@559
|
900 <para>Even though we had 40 changesets to search through, the
|
|
bos@559
|
901 <command role="hg-cmd">hg bisect</command> command let us find
|
|
bos@559
|
902 the changeset that introduced our <quote>bug</quote> with only
|
|
bos@559
|
903 five tests. Because the number of tests that the <command
|
|
bos@559
|
904 role="hg-cmd">hg bisect</command> command performs grows
|
|
bos@559
|
905 logarithmically with the number of changesets to search, the
|
|
bos@559
|
906 advantage that it has over the <quote>brute force</quote>
|
|
bos@559
|
907 search approach increases with every changeset you add.</para>
|
|
bos@559
|
908
|
|
bos@559
|
909 </sect2>
|
|
bos@559
|
910 <sect2>
|
|
bos@559
|
911 <title>Cleaning up after your search</title>
|
|
bos@559
|
912
|
|
bos@559
|
913 <para>When you're finished using the <command role="hg-cmd">hg
|
|
bos@559
|
914 bisect</command> command in a repository, you can use the
|
|
bos@559
|
915 <command role="hg-cmd">hg bisect reset</command> command to
|
|
bos@559
|
916 drop the information it was using to drive your search. The
|
|
bos@559
|
917 command doesn't use much space, so it doesn't matter if you
|
|
bos@559
|
918 forget to run this command. However, <command
|
|
bos@559
|
919 role="hg-cmd">hg bisect</command> won't let you start a new
|
|
bos@559
|
920 search in that repository until you do a <command
|
|
bos@567
|
921 role="hg-cmd">hg bisect reset</command>.</para>
|
|
bos@567
|
922
|
|
bos@567
|
923 &interaction.bisect.search.reset;
|
|
bos@559
|
924
|
|
bos@559
|
925 </sect2>
|
|
bos@559
|
926 </sect1>
|
|
bos@559
|
927 <sect1>
|
|
bos@559
|
928 <title>Tips for finding bugs effectively</title>
|
|
bos@559
|
929
|
|
bos@559
|
930 <sect2>
|
|
bos@559
|
931 <title>Give consistent input</title>
|
|
bos@559
|
932
|
|
bos@559
|
933 <para>The <command role="hg-cmd">hg bisect</command> command
|
|
bos@559
|
934 requires that you correctly report the result of every test
|
|
bos@559
|
935 you perform. If you tell it that a test failed when it really
|
|
bos@559
|
936 succeeded, it <emphasis>might</emphasis> be able to detect the
|
|
bos@559
|
937 inconsistency. If it can identify an inconsistency in your
|
|
bos@559
|
938 reports, it will tell you that a particular changeset is both
|
|
bos@559
|
939 good and bad. However, it can't do this perfectly; it's about
|
|
bos@559
|
940 as likely to report the wrong changeset as the source of the
|
|
bos@559
|
941 bug.</para>
|
|
bos@559
|
942
|
|
bos@559
|
943 </sect2>
|
|
bos@559
|
944 <sect2>
|
|
bos@559
|
945 <title>Automate as much as possible</title>
|
|
bos@559
|
946
|
|
bos@559
|
947 <para>When I started using the <command role="hg-cmd">hg
|
|
bos@559
|
948 bisect</command> command, I tried a few times to run my
|
|
bos@559
|
949 tests by hand, on the command line. This is an approach that
|
|
bos@559
|
950 I, at least, am not suited to. After a few tries, I found
|
|
bos@559
|
951 that I was making enough mistakes that I was having to restart
|
|
bos@559
|
952 my searches several times before finally getting correct
|
|
bos@559
|
953 results.</para>
|
|
bos@559
|
954
|
|
bos@559
|
955 <para>My initial problems with driving the <command
|
|
bos@559
|
956 role="hg-cmd">hg bisect</command> command by hand occurred
|
|
bos@559
|
957 even with simple searches on small repositories; if the
|
|
bos@559
|
958 problem you're looking for is more subtle, or the number of
|
|
bos@559
|
959 tests that <command role="hg-cmd">hg bisect</command> must
|
|
bos@559
|
960 perform increases, the likelihood of operator error ruining
|
|
bos@559
|
961 the search is much higher. Once I started automating my
|
|
bos@559
|
962 tests, I had much better results.</para>
|
|
bos@559
|
963
|
|
bos@559
|
964 <para>The key to automated testing is twofold:</para>
|
|
bos@559
|
965 <itemizedlist>
|
|
bos@559
|
966 <listitem><para>always test for the same symptom, and</para>
|
|
bos@559
|
967 </listitem>
|
|
bos@559
|
968 <listitem><para>always feed consistent input to the <command
|
|
bos@559
|
969 role="hg-cmd">hg bisect</command> command.</para>
|
|
bos@559
|
970 </listitem></itemizedlist>
|
|
bos@559
|
971 <para>In my tutorial example above, the <command>grep</command>
|
|
bos@559
|
972 command tests for the symptom, and the <literal>if</literal>
|
|
bos@559
|
973 statement takes the result of this check and ensures that we
|
|
bos@559
|
974 always feed the same input to the <command role="hg-cmd">hg
|
|
bos@559
|
975 bisect</command> command. The <literal>mytest</literal>
|
|
bos@559
|
976 function marries these together in a reproducible way, so that
|
|
bos@559
|
977 every test is uniform and consistent.</para>
|
|
bos@559
|
978
|
|
bos@559
|
979 </sect2>
|
|
bos@559
|
980 <sect2>
|
|
bos@559
|
981 <title>Check your results</title>
|
|
bos@559
|
982
|
|
bos@559
|
983 <para>Because the output of a <command role="hg-cmd">hg
|
|
bos@559
|
984 bisect</command> search is only as good as the input you
|
|
bos@559
|
985 give it, don't take the changeset it reports as the absolute
|
|
bos@559
|
986 truth. A simple way to cross-check its report is to manually
|
|
bos@559
|
987 run your test at each of the following changesets:</para>
|
|
bos@559
|
988 <itemizedlist>
|
|
bos@559
|
989 <listitem><para>The changeset that it reports as the first bad
|
|
bos@559
|
990 revision. Your test should still report this as
|
|
bos@559
|
991 bad.</para>
|
|
bos@559
|
992 </listitem>
|
|
bos@559
|
993 <listitem><para>The parent of that changeset (either parent,
|
|
bos@559
|
994 if it's a merge). Your test should report this changeset
|
|
bos@559
|
995 as good.</para>
|
|
bos@559
|
996 </listitem>
|
|
bos@559
|
997 <listitem><para>A child of that changeset. Your test should
|
|
bos@559
|
998 report this changeset as bad.</para>
|
|
bos@559
|
999 </listitem></itemizedlist>
|
|
bos@559
|
1000
|
|
bos@559
|
1001 </sect2>
|
|
bos@559
|
1002 <sect2>
|
|
bos@559
|
1003 <title>Beware interference between bugs</title>
|
|
bos@559
|
1004
|
|
bos@559
|
1005 <para>It's possible that your search for one bug could be
|
|
bos@559
|
1006 disrupted by the presence of another. For example, let's say
|
|
bos@559
|
1007 your software crashes at revision 100, and worked correctly at
|
|
bos@559
|
1008 revision 50. Unknown to you, someone else introduced a
|
|
bos@559
|
1009 different crashing bug at revision 60, and fixed it at
|
|
bos@559
|
1010 revision 80. This could distort your results in one of
|
|
bos@559
|
1011 several ways.</para>
|
|
bos@559
|
1012
|
|
bos@559
|
1013 <para>It is possible that this other bug completely
|
|
bos@559
|
1014 <quote>masks</quote> yours, which is to say that it occurs
|
|
bos@559
|
1015 before your bug has a chance to manifest itself. If you can't
|
|
bos@559
|
1016 avoid that other bug (for example, it prevents your project
|
|
bos@559
|
1017 from building), and so can't tell whether your bug is present
|
|
bos@559
|
1018 in a particular changeset, the <command role="hg-cmd">hg
|
|
bos@559
|
1019 bisect</command> command cannot help you directly. Instead,
|
|
bos@559
|
1020 you can mark a changeset as untested by running <command
|
|
bos@559
|
1021 role="hg-cmd">hg bisect --skip</command>.</para>
|
|
bos@559
|
1022
|
|
bos@559
|
1023 <para>A different problem could arise if your test for a bug's
|
|
bos@559
|
1024 presence is not specific enough. If you check for <quote>my
|
|
bos@559
|
1025 program crashes</quote>, then both your crashing bug and an
|
|
bos@559
|
1026 unrelated crashing bug that masks it will look like the same
|
|
bos@559
|
1027 thing, and mislead <command role="hg-cmd">hg
|
|
bos@559
|
1028 bisect</command>.</para>
|
|
bos@559
|
1029
|
|
bos@559
|
1030 <para>Another useful situation in which to use <command
|
|
bos@559
|
1031 role="hg-cmd">hg bisect --skip</command> is if you can't
|
|
bos@559
|
1032 test a revision because your project was in a broken and hence
|
|
bos@559
|
1033 untestable state at that revision, perhaps because someone
|
|
bos@559
|
1034 checked in a change that prevented the project from
|
|
bos@559
|
1035 building.</para>
|
|
bos@559
|
1036
|
|
bos@559
|
1037 </sect2>
|
|
bos@559
|
1038 <sect2>
|
|
bos@559
|
1039 <title>Bracket your search lazily</title>
|
|
bos@559
|
1040
|
|
bos@559
|
1041 <para>Choosing the first <quote>good</quote> and
|
|
bos@559
|
1042 <quote>bad</quote> changesets that will mark the end points of
|
|
bos@559
|
1043 your search is often easy, but it bears a little discussion
|
|
bos@559
|
1044 nevertheless. From the perspective of <command
|
|
bos@559
|
1045 role="hg-cmd">hg bisect</command>, the <quote>newest</quote>
|
|
bos@559
|
1046 changeset is conventionally <quote>bad</quote>, and the older
|
|
bos@559
|
1047 changeset is <quote>good</quote>.</para>
|
|
bos@559
|
1048
|
|
bos@559
|
1049 <para>If you're having trouble remembering when a suitable
|
|
bos@559
|
1050 <quote>good</quote> change was, so that you can tell <command
|
|
bos@559
|
1051 role="hg-cmd">hg bisect</command>, you could do worse than
|
|
bos@559
|
1052 testing changesets at random. Just remember to eliminate
|
|
bos@559
|
1053 contenders that can't possibly exhibit the bug (perhaps
|
|
bos@559
|
1054 because the feature with the bug isn't present yet) and those
|
|
bos@559
|
1055 where another problem masks the bug (as I discussed
|
|
bos@559
|
1056 above).</para>
|
|
bos@559
|
1057
|
|
bos@559
|
1058 <para>Even if you end up <quote>early</quote> by thousands of
|
|
bos@559
|
1059 changesets or months of history, you will only add a handful
|
|
bos@559
|
1060 of tests to the total number that <command role="hg-cmd">hg
|
|
bos@559
|
1061 bisect</command> must perform, thanks to its logarithmic
|
|
bos@559
|
1062 behaviour.</para>
|
|
bos@559
|
1063
|
|
bos@559
|
1064 </sect2>
|
|
bos@559
|
1065 </sect1>
|
|
bos@559
|
1066 </chapter>
|
|
bos@559
|
1067
|
|
bos@559
|
1068 <!--
|
|
bos@559
|
1069 local variables:
|
|
bos@559
|
1070 sgml-parent-document: ("00book.xml" "book" "chapter")
|
|
bos@559
|
1071 end:
|
|
bos@559
|
1072 -->
|
