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1 | 1 | Mercurial Frequently Asked Questions |
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2 | 2 | ==================================== |
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3 | 3 | |
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4 | 4 | Section 1: General Usage |
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5 | 5 | ------------------------ |
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6 | 6 | |
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7 | 7 | .Q. I did an "hg pull" and my working directory is empty! |
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8 | 8 | |
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9 | 9 | There are two parts to Mercurial: the repository and the working |
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10 | 10 | directory. "hg pull" pulls all new changes from a remote repository |
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11 | 11 | into the local one but doesn't alter the working directory. |
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12 | 12 | |
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13 | 13 | This keeps you from upsetting your work in progress, which may not be |
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14 | 14 | ready to merge with the new changes you've pulled and also allows you |
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15 | 15 | to manage merging more easily (see below about best practices). |
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16 | 16 | |
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17 | 17 | To update your working directory, run "hg update". If you're sure you |
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18 | 18 | want to update your working directory on a pull, you can also use "hg |
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19 | 19 | pull -u". This will refuse to merge or overwrite local changes. |
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20 | 20 | |
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21 | 21 | |
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22 | 22 | .Q. What are revision numbers, changeset IDs, and tags? |
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23 | 23 | |
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24 | 24 | Mercurial will generally allow you to refer to a revision in three |
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25 | 25 | ways: by revision number, by changeset ID, and by tag. |
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26 | 26 | |
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27 | 27 | A revision number is a simple decimal number that corresponds with the |
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28 | 28 | ordering of commits in the local repository. It is important to |
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29 | 29 | understand that this ordering can change from machine to machine due |
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30 | 30 | to Mercurial's distributed, decentralized architecture. |
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31 | 31 | |
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32 | 32 | This is where changeset IDs come in. A changeset ID is a 160-bit |
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33 | 33 | identifier that uniquely describes a changeset and its position in the |
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34 | 34 | change history, regardless of which machine it's on. This is |
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35 | 35 | represented to the user as a 40 digit hexadecimal number. As that |
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36 | 36 | tends to be unwieldy, Mercurial will accept any unambiguous substring |
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37 | 37 | of that number when specifying versions. It will also generally print |
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38 | 38 | these numbers in "short form", which is the first 12 digits. |
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39 | 39 | |
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40 | 40 | You should always use some form of changeset ID rather than the local |
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41 | 41 | revision number when discussing revisions with other Mercurial users |
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42 | 42 | as they may have different revision numbering on their system. |
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43 | 43 | |
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44 | 44 | Finally, a tag is an arbitrary string that has been assigned a |
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45 | 45 | correspondence to a changeset ID. This lets you refer to revisions |
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46 | 46 | symbolically. |
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47 | 47 | |
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48 | 48 | |
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49 | 49 | .Q. What are branches, heads, and the tip? |
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50 | 50 | |
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51 | 51 | The central concept of Mercurial is branching. A 'branch' is simply |
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52 | 52 | an independent line of development. In most other version control |
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53 | 53 | systems, all users generally commit to the same line of development |
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54 | 54 | called 'the trunk' or 'the main branch'. In Mercurial, every developer |
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55 | 55 | effectively works on a private branch and there is no internal concept |
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56 | 56 | of 'the main branch'. |
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57 | 57 | |
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58 | 58 | Thus Mercurial works hard to make repeated merging between branches |
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59 | 59 | easy. Simply run "hg pull" and "hg update -m" and commit the result. |
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60 | 60 | |
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61 | 61 | 'Heads' are simply the most recent commits on a branch. Technically, |
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62 | 62 | they are changesets which have no children. Merging is the process of |
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63 | 63 | joining points on two branches into one, usually at their current |
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64 | 64 | heads. Use "hg heads" to find the heads in the current repository. |
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65 | 65 | |
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66 | 66 | The 'tip' is the most recently changed head, and also the highest |
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67 | 67 | numbered revision. If you have just made a commit, that commit will be |
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68 |
the |
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68 | the tip. Alternately, if you have just pulled from another | |
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69 | 69 | repository, the tip of that repository becomes the current tip. |
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70 | 70 | |
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71 | 71 | The 'tip' is the default revision for many commands such as update, |
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72 | 72 | and also functions as a special symbolic tag. |
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73 | 73 | |
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74 | 74 | |
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75 | 75 | .Q. How does merging work? |
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76 | 76 | |
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77 | 77 | The merge process is simple. Usually you will want to merge the tip |
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78 | 78 | into your working directory. Thus you run "hg update -m" and Mercurial |
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79 | 79 | will incorporate the changes from tip into your local changes. |
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80 | 80 | |
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81 | 81 | The first step of this process is tracing back through the history of |
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82 | 82 | changesets and finding the 'common ancestor' of the two versions that |
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83 | 83 | are being merged. This is done on a project-wide and a file by file |
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84 | 84 | basis. |
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85 | 85 | |
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86 | 86 | For files that have been changed in both projects, a three-way merge |
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87 | 87 | is attempted to add the changes made remotely into the changes made |
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88 | 88 | locally. If there are conflicts between these changes, the user is |
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89 | 89 | prompted to interactively resolve them. |
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90 | 90 | |
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91 | 91 | Mercurial uses a helper tool for this, which is usually found by the |
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92 | 92 | hgmerge script. Example tools include tkdiff, kdiff3, and the classic |
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93 | 93 | RCS merge. |
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94 | 94 | |
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95 | 95 | After you've completed the merge and you're satisfied that the results |
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96 | 96 | are correct, it's a good idea to commit your changes. Mercurial won't |
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97 | 97 | allow you to perform another merge until you've done this commit as |
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98 | 98 | that would lose important history that will be needed for future |
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99 | 99 | merges. |
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100 | 100 | |
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101 | 101 | |
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102 | 102 | .Q. How do tags work in Mercurial? |
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103 | 103 | |
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104 | 104 | Tags work slightly differently in Mercurial than most revision |
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105 | 105 | systems. The design attempts to meet the following requirements: |
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106 | 106 | |
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107 | 107 | - be version controlled and mergeable just like any other file |
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108 | 108 | - allow signing of tags |
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109 | 109 | - allow adding a tag to an already committed changeset |
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110 | 110 | - allow changing tags in the future |
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111 | 111 | |
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112 | 112 | Thus Mercurial stores tags as a file in the working dir. This file is |
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113 | 113 | called .hgtags and consists of a list of changeset IDs and their |
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114 | 114 | corresponding tags. To add a tag to the system, simply add a line to |
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115 | 115 | this file and then commit it for it to take effect. The "hg tag" |
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116 | 116 | command will do this for you and "hg tags" will show the currently |
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117 | 117 | effective tags. |
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118 | 118 | |
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119 | 119 | Note that because tags refer to changeset IDs and the changeset ID is |
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120 | 120 | effectively the sum of all the contents of the repository for that |
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121 | 121 | change, it is impossible in Mercurial to simultaneously commit and add |
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122 | 122 | a tag. Thus tagging a revision must be done as a second step. |
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123 | 123 | |
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124 | 124 | |
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125 | 125 | .Q. What if I want to just keep local tags? |
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126 | 126 | |
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127 | 127 | You can use "hg tag" command with an option "-l" or "--local". This |
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128 | 128 | will store the tag in the file .hg/localtags, which will not be |
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129 | 129 | distributed or versioned. The format of this file is identical to the |
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130 | 130 | one of .hgtags and the tags stored there are handled the same. |
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131 | 131 | |
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132 | 132 | |
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133 | 133 | .Q. How do tags work with multiple heads? |
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134 | 134 | |
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135 | 135 | The tags that are in effect at any given time are the tags specified |
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136 | 136 | in each head, with heads closer to the tip taking precedence. Local |
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137 | 137 | tags override all other tags. |
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138 | 138 | |
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139 | 139 | |
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140 | 140 | .Q. What are some best practices for distributed development with Mercurial? |
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141 | 141 | |
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142 | 142 | First, merge often! This makes merging easier for everyone and you |
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143 | 143 | find out about conflicts (which are often rooted in incompatible |
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144 | 144 | design decisions) earlier. |
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145 | 145 | |
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146 | 146 | Second, don't hesitate to use multiple trees locally. Mercurial makes |
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147 | 147 | this fast and light-weight. Typical usage is to have an incoming tree, |
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148 | 148 | an outgoing tree, and a separate tree for each area being worked on. |
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149 | 149 | |
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150 | 150 | The incoming tree is best maintained as a pristine copy of the |
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151 | 151 | upstream repository. This works as a cache so that you don't have to |
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152 | 152 | pull multiple copies over the network. No need to check files out here |
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153 | 153 | as you won't be changing them. |
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154 | 154 | |
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155 |
The outgoing tree contains all the changes you intend for merge |
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155 | The outgoing tree contains all the changes you intend for merge into | |
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156 | 156 | upsteam. Publish this tree with 'hg serve" or hgweb.cgi or use 'hg |
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157 | 157 | push" to push it to another publicly availabe repository. |
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158 | 158 | |
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159 | 159 | Then, for each feature you work on, create a new tree. Commit early |
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160 | 160 | and commit often, merge with incoming regularly, and once you're |
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161 | 161 | satisfied with your feature, pull the changes into your outgoing tree. |
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162 | 162 | |
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163 | 163 | |
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164 | 164 | .Q. How do I import from a repository created in a different SCM? |
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165 | 165 | |
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166 | 166 | Take a look at contrib/convert-repo. This is an extensible |
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167 | 167 | framework for converting between repository types. |
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168 | 168 | |
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169 | 169 | |
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170 | 170 | .Q. What about Windows support? |
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171 | 171 | |
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172 | 172 | Patches to support Windows are being actively integrated, a fully |
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173 | 173 | working Windows version is probably not far off |
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174 | 174 | |
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175 | 175 | |
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176 | 176 | Section 2: Bugs and Features |
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177 | 177 | ---------------------------- |
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178 | 178 | |
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179 | 179 | .Q. I found a bug, what do I do? |
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180 | 180 | |
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181 | 181 | Report it to the mercurial mailing list, mercurial@selenic.com. |
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182 | 182 | |
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183 | 183 | |
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184 | 184 | .Q. What should I include in my bug report? |
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185 | 185 | |
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186 | 186 | Enough information to reproduce or diagnose the bug. If you can, try |
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187 | 187 | using the hg -v and hg -d switches to figure out exactly what |
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188 | 188 | Mercurial is doing. |
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189 | 189 | |
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190 | 190 | If you can reproduce the bug in a simple repository, that is very |
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191 | 191 | helpful. The best is to create a simple shell script to automate this |
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192 | 192 | process, which can then be added to our test suite. |
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193 | 193 | |
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194 | 194 | |
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195 | 195 | .Q. Can Mercurial do <x>? |
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196 | 196 | |
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197 | 197 | If you'd like to request a feature, send your request to |
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198 | 198 | mercurial@selenic.com. As Mercurial is still very new, there are |
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199 |
certainly features it is missing and you can give u |
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199 | certainly features it is missing and you can give us feedback on how | |
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200 | 200 | best to implement them. |
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201 | 201 | |
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202 | 202 | |
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203 | 203 | Section 3: Technical |
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204 | 204 | -------------------- |
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205 | 205 | |
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206 | 206 | .Q. What limits does Mercurial have? |
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207 | 207 | |
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208 | 208 | Mercurial currently assumes that single files, indices, and manifests |
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209 | 209 | can fit in memory for efficiency. |
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210 | 210 | |
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211 | 211 | Offsets in revlogs are currently tracked with 32 bits, so a revlog for |
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212 | 212 | a single file can currently not grow beyond 4G. |
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213 | 213 | |
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214 | 214 | There should otherwise be no limits on file name length, file size, |
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215 | 215 | file contents, number of files, or number of revisions. |
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216 | 216 | |
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217 | 217 | The network protocol is big-endian. |
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218 | 218 | |
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219 | 219 | File names cannot contain the null character. Committer addresses |
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220 | 220 | cannot contain newlines. |
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221 | 221 | |
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222 | 222 | Mercurial is primarily developed for UNIX systems, so some UNIXisms |
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223 | 223 | may be present in ports. |
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224 | 224 | |
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225 | 225 | |
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226 | 226 | .Q. How does Mercurial store its data? |
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227 | 227 | |
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228 | 228 | The fundamental storage type in Mercurial is a "revlog". A revlog is |
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229 | 229 | the set of all revisions of a named object. Each revision is either |
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230 | 230 | stored compressed in its entirety or as a compressed binary delta |
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231 | 231 | against the previous version. The decision of when to store a full |
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232 | 232 | version is made based on how much data would be needed to reconstruct |
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233 | 233 | the file. This lets us ensure that we never need to read huge amounts |
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234 | 234 | of data to reconstruct a object, regardless of how many revisions of it |
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235 | 235 | we store. |
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236 | 236 | |
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237 | 237 | In fact, we should always be able to do it with a single read, |
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238 | 238 | provided we know when and where to read. This is where the index comes |
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239 | 239 | in. Each revlog has an index containing a special hash (nodeid) of the |
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240 | 240 | text, hashes for its parents, and where and how much of the revlog |
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241 | 241 | data we need to read to reconstruct it. Thus, with one read of the |
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242 | 242 | index and one read of the data, we can reconstruct any version in time |
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243 | 243 | proportional to the object size. |
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244 | 244 | |
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245 | 245 | Similarly, revlogs and their indices are append-only. This means that |
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246 | 246 | adding a new version is also O(1) seeks. |
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247 | 247 | |
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248 | 248 | Revlogs are used to represent all revisions of files, manifests, and |
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249 | 249 | changesets. Compression for typical objects with lots of revisions can |
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250 | 250 | range from 100 to 1 for things like project makefiles to over 2000 to |
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251 | 251 | 1 for objects like the manifest. |
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252 | 252 | |
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253 | 253 | |
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254 | 254 | .Q. How are manifests and changesets stored? |
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255 | 255 | |
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256 | 256 | A manifest is simply a list of all files in a given revision of a |
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257 | 257 | project along with the nodeids of the corresponding file revisions. So |
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258 | 258 | grabbing a given version of the project means simply looking up its |
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259 |
manifest and reconstructi |
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259 | manifest and reconstructing all the file revisions pointed to by it. | |
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260 | 260 | |
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261 | 261 | A changeset is a list of all files changed in a check-in along with a |
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262 | 262 | change description and some metadata like user and date. It also |
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263 | 263 | contains a nodeid to the relevent revision of the manifest. |
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264 | 264 | |
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265 | 265 | |
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266 | 266 | .Q. How do Mercurial hashes get calculated? |
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267 | 267 | |
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268 | 268 | Mercurial hashes both the contents of an object and the hash of its |
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269 | 269 | parents to create an identifier that uniquely identifies an object's |
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270 | 270 | contents and history. This greatly simplifies merging of histories |
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271 | 271 | because it avoid graph cycles that can occur when a object is reverted |
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272 | 272 | to an earlier state. |
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273 | 273 | |
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274 | 274 | All file revisions have an associated hash value. These are listed in |
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275 | 275 | the manifest of a given project revision, and the manifest hash is |
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276 | 276 | listed in the changeset. The changeset hash is again a hash of the |
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277 | 277 | changeset contents and its parents, so it uniquely identifies the |
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278 | 278 | entire history of the project to that point. |
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279 | 279 | |
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280 | 280 | |
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281 | 281 | .Q. What checks are there on repository integrity? |
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282 | 282 | |
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283 | 283 | Every time a revlog object is retrieved, it is checked against its |
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284 | 284 | hash for integrity. It is also incidentally doublechecked by the |
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285 | 285 | Adler32 checksum used by the underlying zlib compression. |
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286 | 286 | |
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287 | 287 | Running 'hg verify' decompresses and reconstitutes each revision of |
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288 | 288 | each object in the repository and cross-checks all of the index |
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289 | 289 | metadata with those contents. |
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290 | 290 | |
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291 | 291 | But this alone is not enough to ensure that someone hasn't tampered |
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292 | 292 | with a repository. For that, you need cryptographic signing. |
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293 | 293 | |
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294 | 294 | |
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295 | 295 | .Q. How does signing work with Mercurial? |
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296 | 296 | |
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297 | 297 | Take a look at the hgeditor script for an example. The basic idea is |
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298 | 298 | to use GPG to sign the manifest ID inside that changelog entry. The |
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299 | 299 | manifest ID is a recursive hash of all of the files in the system and |
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300 | 300 | their complete history, and thus signing the manifest hash signs the |
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301 | 301 | entire project contents. |
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302 | 302 | |
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303 | 303 | |
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304 | 304 | .Q. What about hash collisions? What about weaknesses in SHA1? |
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305 | 305 | |
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306 | 306 | The SHA1 hashes are large enough that the odds of accidental hash collision |
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307 | 307 | are negligible for projects that could be handled by the human race. |
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308 | 308 | The known weaknesses in SHA1 are currently still not practical to |
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309 | 309 | attack, and Mercurial will switch to SHA256 hashing before that |
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310 | 310 | becomes a realistic concern. |
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311 | 311 | |
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312 | 312 | Collisions with the "short hashes" are not a concern as they're always |
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313 | 313 | checked for ambiguity and are still long enough that they're not |
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314 | 314 | likely to happen for reasonably-sized projects (< 1M changes). |
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315 | 315 | |
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316 | 316 | |
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317 | 317 |
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