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1 Mercurial Frequently Asked Questions
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2
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3 Section 1: General Usage
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4 ------------------------
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5
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6 Q. I did an 'hg pull' and my working directory is empty!
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7
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8 There are two parts to Mercurial: the repository and the working
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9 directory. 'hg pull' pulls all new changes from a remote repository
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10 into the local one but doesn't alter the working directory.
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11
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12 This keeps you from upsetting your work in progress, which may not be
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13 ready to merge with the new changes you've pulled and also allows you
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14 to manage merging more easily (see below about best practices).
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15
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16 To update your working directory, run 'hg update'. If you're sure you
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17 want to update your working directory on a pull, you can also use 'hg
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18 pull -u'. This will refuse to merge or overwrite local changes.
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19
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20
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21 Q. What is the difference between revision numbers, changeset IDs,
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22 and tags?
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23
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24 Mercurial will generally allow you to refer to a revision in three
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25 ways: by revision number, by changeset ID, and by tag.
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26
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27 A revision number is a simple decimal number that corresponds with the
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28 ordering of commits in the local repository. It is important to
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29 understand that this ordering can change from machine to machine due
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30 to Mercurial's distributed, decentralized architecture.
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31
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32 This is where changeset IDs come in. A changeset ID is a 160-bit
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33 identifier that uniquely describes a changeset and its position in the
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34 change history, regardless of which machine it's on. This is
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35 represented to the user as a 40 digit hexadecimal number. As that
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36 tends to be unwieldy, Mercurial will accept any unambiguous substring
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37 of that number when specifying versions. It will also generally print
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38 these numbers in "short form", which is the first 12 digits.
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39
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40 You should always use some form of changeset ID rather than the local
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41 revision number when discussing revisions with other Mercurial users
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42 as they may have different revision numbering on their system.
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43
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44 Finally, a tag is an arbitrary string that has been assigned a
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45 correspondence to a changeset ID. This lets you refer to revisions
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46 symbolically.
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47
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48
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49 Q. What are branches, heads, and the tip?
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50
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51 The central concept of Mercurial is branching. A 'branch' is simply
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52 an independent line of development. In most other version control
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53 systems, all users generally commit to the same line of development
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54 called 'the trunk' or 'the main branch'. In Mercurial, every developer
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55 effectively works on a private branch and there is no internal concept
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56 of 'the main branch'.
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57
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58 Thus Mercurial works hard to make repeated merging between branches
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59 easy. Simply run 'hg pull' and 'hg update -m' and commit the result.
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60
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61 'Heads' are simply the most recent commits on a branch. Technically,
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62 they are changesets which have no children. Merging is the process of
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63 joining points on two branches into one, usually at their current
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64 heads. Use 'hg heads' to find the heads in the current repository.
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65
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66 The 'tip' is the most recently changed head, and also the highest
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67 numbered revision. If you have just made a commit, that commit will be
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68 the head. Alternately, if you have just pulled from another
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69 repository, the tip of that repository becomes the current tip.
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70
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71 The 'tip' is the default revision for many commands such as update,
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72 and also functions as a special symbolic tag.
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73
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74
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75 Q. How does merging work?
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76
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77 The merge process is simple. Usually you will want to merge the tip
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78 into your working directory. Thus you run 'hg update -m' and Mercurial
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79 will incorporate the changes from tip into your local changes.
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80
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81 The first step of this process is tracing back through the history of
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82 changesets and finding the 'common ancestor' of the two versions that
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83 are being merged. This is done on a project-wide and a file by file
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84 basis.
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85
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86 For files that have been changed in both projects, a three-way merge
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87 is attempted to add the changes made remotely into the changes made
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88 locally. If there are conflicts between these changes, the user is
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89 prompted to interactively resolve them.
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90
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91 Mercurial uses a helper tool for this, which is usually found by the
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92 hgmerge script. Example tools include tkdiff, kdiff3, and the classic
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93 RCS merge.
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94
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95 After you've completed the merge and you're satisfied that the results
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96 are correct, it's a good idea to commit your changes. Mercurial won't
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97 allow you to perform another merge until you've done this commit as
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98 that would lose important history that will be needed for future
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99 merges.
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100
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101
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102 Q. How do tags work in Mercurial?
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103
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104 Tags work slightly differently in Mercurial than most revision
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105 systems. The design attempts to meet the following requirements:
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106
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107 - be version controlled and mergeable just like any other file
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108 - allow signing of tags
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109 - allow adding a tag to an already committed changeset
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110 - allow changing tags in the future
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111
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112 Thus Mercurial stores tags as a file in the working dir. This file is
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113 called .hgtags and consists of a list of changeset IDs and their
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114 corresponding tags. To add a tag to the system, simply add a line to
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115 this file and then commit it for it to take effect. The 'hg tag'
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116 command will do this for you and 'hg tags' will show the currently
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117 effective tags.
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118
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119 Note that because tags refer to changeset IDs and the changeset ID is
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120 effectively the sum of all the contents of the repository for that
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121 change, it is impossible in Mercurial to simultaneously commit and add
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122 a tag. Thus tagging a revision must be done as a second step.
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123
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124 Q. How do tags work with multiple heads?
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125
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126 The tags that are in effect at any given time are the tags specified
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127 in each head, with heads closer to the tip taking precedence.
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128
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129
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130 Q. What are some best practices for distributed development with Mercurial?
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131
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132 First, merge often! This makes merging easier for everyone and you
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133 find out about conflicts (which are often rooted in incompatible
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134 design decisions) earlier.
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135
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136 Second, don't hesitate to use multiple trees locally. Mercurial makes
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137 this fast and light-weight. Typical usage is to have an incoming tree,
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138 an outgoing tree, and a separate tree for each area being worked on.
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139
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140 The incoming tree is best maintained as a pristine copy of the
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141 upstream repository. This works as a cache so that you don't have to
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142 pull multiple copies over the network. No need to check files out here
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143 as you won't be changing them.
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144
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145 The outgoing tree contains all the changes you intend for merger into
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146 upsteam. Publish this tree with 'hg serve' or hgweb.cgi or use 'hg
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147 push' to push it to another publicly availabe repository.
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148
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149 Then, for each feature you work on, create a new tree. Commit early
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150 and commit often, merge with incoming regularly, and once you're
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151 satisfied with your feature, pull the changes into your outgoing tree.
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152
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153
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154 Q. How do I import from a repository created in a different SCM?
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155
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156 Take a look at contrib/convert-repo. This is an extensible
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157 framework for converting between repository types.
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158
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159
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160 Q. What about Windows support?
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161
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162 Patches to support Windows are being actively integrated, a fully
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163 working Windows version is probably not far off
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164
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165
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166 Section 2: Technical
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167 --------------------
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168
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169 Q. What limits does Mercurial have?
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170
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171 Mercurial currently assumes that single files, indices, and manifests
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172 can fit in memory for efficiency.
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173
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174 Offsets in revlogs are currently tracked with 32 bits, so a revlog for
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175 a single file can currently not grow beyond 4G.
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176
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177 There should otherwise be no limits on file name length, file size,
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178 file contents, number of files, or number of revisions.
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179
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180 The network protocol is big-endian.
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181
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182 File names cannot contain the null character. Committer addresses
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183 cannot contain newlines.
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184
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185 Mercurial is primarily developed for UNIX systems, so some UNIXisms
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186 may be present in ports.
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187
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188
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189 Q. How does signing work?
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190
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191 Take a look at the hgeditor script for an example. The basic idea
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192 is to sign the manifest ID inside that changelog entry. The manifest
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193 ID is a recursive hash of all of the files in the system and their
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194 complete history, and thus signing the manifest hash signs the entire
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195 project to that point.
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196
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197 More precisely: each file hash is an SHA1 hash of the contents of that
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198 file and the hashes of its parent revisions. The manifest contains a
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199 list of each file in the project along with its current file hash.
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200 This manifest is hashed similarly to the file hashes, incorporating
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201 the hashes of the parent revisions.
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202
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203
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204 Q. What about hash collisions? What about weaknesses in SHA1?
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205
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206 The SHA1 hashes are large enough that the odds of accidental hash collision
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207 are negligible for projects that could be handled by the human race.
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208 The known weaknesses in SHA1 are currently still not practical to
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209 attack, and Mercurial will switch to SHA256 hashing before that
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210 becomes a realistic concern.
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211
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212 Collisions with the "short hashes" are not a concern as they're always
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213 checked for ambiguity and are still long enough that they're not
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214 likely to happen for reasonably-sized projects (< 1M changes).
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