Mercurial > hg
annotate hgext/clonebundles.py @ 36821:0c7350656f93
xdiff: add comments for fields in xdfile_t
This makes the related code easier to understand.
Differential Revision: https://phab.mercurial-scm.org/D2685
author | Jun Wu <quark@fb.com> |
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date | Sun, 04 Mar 2018 11:30:16 -0800 |
parents | d25802b0eef5 |
children | aacfca6f9767 |
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1 # This software may be used and distributed according to the terms of the |
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2 # GNU General Public License version 2 or any later version. |
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3 |
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4 """advertise pre-generated bundles to seed clones |
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5 |
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6 "clonebundles" is a server-side extension used to advertise the existence |
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7 of pre-generated, externally hosted bundle files to clients that are |
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8 cloning so that cloning can be faster, more reliable, and require less |
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9 resources on the server. |
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10 |
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11 Cloning can be a CPU and I/O intensive operation on servers. Traditionally, |
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12 the server, in response to a client's request to clone, dynamically generates |
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13 a bundle containing the entire repository content and sends it to the client. |
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14 There is no caching on the server and the server will have to redundantly |
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15 generate the same outgoing bundle in response to each clone request. For |
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16 servers with large repositories or with high clone volume, the load from |
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17 clones can make scaling the server challenging and costly. |
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18 |
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19 This extension provides server operators the ability to offload potentially |
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20 expensive clone load to an external service. Here's how it works. |
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21 |
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22 1. A server operator establishes a mechanism for making bundle files available |
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23 on a hosting service where Mercurial clients can fetch them. |
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24 2. A manifest file listing available bundle URLs and some optional metadata |
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25 is added to the Mercurial repository on the server. |
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26 3. A client initiates a clone against a clone bundles aware server. |
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27 4. The client sees the server is advertising clone bundles and fetches the |
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28 manifest listing available bundles. |
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29 5. The client filters and sorts the available bundles based on what it |
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30 supports and prefers. |
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31 6. The client downloads and applies an available bundle from the |
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32 server-specified URL. |
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33 7. The client reconnects to the original server and performs the equivalent |
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34 of :hg:`pull` to retrieve all repository data not in the bundle. (The |
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35 repository could have been updated between when the bundle was created |
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36 and when the client started the clone.) |
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37 |
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38 Instead of the server generating full repository bundles for every clone |
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39 request, it generates full bundles once and they are subsequently reused to |
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40 bootstrap new clones. The server may still transfer data at clone time. |
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41 However, this is only data that has been added/changed since the bundle was |
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42 created. For large, established repositories, this can reduce server load for |
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43 clones to less than 1% of original. |
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44 |
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45 To work, this extension requires the following of server operators: |
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46 |
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47 * Generating bundle files of repository content (typically periodically, |
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48 such as once per day). |
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49 * A file server that clients have network access to and that Python knows |
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50 how to talk to through its normal URL handling facility (typically an |
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51 HTTP server). |
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52 * A process for keeping the bundles manifest in sync with available bundle |
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53 files. |
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54 |
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55 Strictly speaking, using a static file hosting server isn't required: a server |
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56 operator could use a dynamic service for retrieving bundle data. However, |
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57 static file hosting services are simple and scalable and should be sufficient |
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58 for most needs. |
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59 |
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60 Bundle files can be generated with the :hg:`bundle` command. Typically |
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61 :hg:`bundle --all` is used to produce a bundle of the entire repository. |
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62 |
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63 :hg:`debugcreatestreamclonebundle` can be used to produce a special |
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64 *streaming clone bundle*. These are bundle files that are extremely efficient |
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65 to produce and consume (read: fast). However, they are larger than |
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66 traditional bundle formats and require that clients support the exact set |
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67 of repository data store formats in use by the repository that created them. |
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68 Typically, a newer server can serve data that is compatible with older clients. |
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69 However, *streaming clone bundles* don't have this guarantee. **Server |
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70 operators need to be aware that newer versions of Mercurial may produce |
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71 streaming clone bundles incompatible with older Mercurial versions.** |
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72 |
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73 A server operator is responsible for creating a ``.hg/clonebundles.manifest`` |
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74 file containing the list of available bundle files suitable for seeding |
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75 clones. If this file does not exist, the repository will not advertise the |
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76 existence of clone bundles when clients connect. |
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77 |
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78 The manifest file contains a newline (\\n) delimited list of entries. |
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79 |
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80 Each line in this file defines an available bundle. Lines have the format: |
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81 |
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82 <URL> [<key>=<value>[ <key>=<value>]] |
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83 |
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84 That is, a URL followed by an optional, space-delimited list of key=value |
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85 pairs describing additional properties of this bundle. Both keys and values |
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86 are URI encoded. |
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87 |
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88 Keys in UPPERCASE are reserved for use by Mercurial and are defined below. |
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89 All non-uppercase keys can be used by site installations. An example use |
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90 for custom properties is to use the *datacenter* attribute to define which |
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91 data center a file is hosted in. Clients could then prefer a server in the |
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92 data center closest to them. |
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93 |
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94 The following reserved keys are currently defined: |
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95 |
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96 BUNDLESPEC |
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97 A "bundle specification" string that describes the type of the bundle. |
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98 |
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99 These are string values that are accepted by the "--type" argument of |
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100 :hg:`bundle`. |
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101 |
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102 The values are parsed in strict mode, which means they must be of the |
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103 "<compression>-<type>" form. See |
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104 mercurial.exchange.parsebundlespec() for more details. |
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105 |
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106 :hg:`debugbundle --spec` can be used to print the bundle specification |
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107 string for a bundle file. The output of this command can be used verbatim |
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108 for the value of ``BUNDLESPEC`` (it is already escaped). |
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109 |
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110 Clients will automatically filter out specifications that are unknown or |
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111 unsupported so they won't attempt to download something that likely won't |
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112 apply. |
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113 |
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114 The actual value doesn't impact client behavior beyond filtering: |
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115 clients will still sniff the bundle type from the header of downloaded |
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116 files. |
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117 |
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118 **Use of this key is highly recommended**, as it allows clients to |
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119 easily skip unsupported bundles. If this key is not defined, an old |
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120 client may attempt to apply a bundle that it is incapable of reading. |
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121 |
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122 REQUIRESNI |
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123 Whether Server Name Indication (SNI) is required to connect to the URL. |
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124 SNI allows servers to use multiple certificates on the same IP. It is |
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125 somewhat common in CDNs and other hosting providers. Older Python |
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126 versions do not support SNI. Defining this attribute enables clients |
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127 with older Python versions to filter this entry without experiencing |
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128 an opaque SSL failure at connection time. |
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129 |
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130 If this is defined, it is important to advertise a non-SNI fallback |
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131 URL or clients running old Python releases may not be able to clone |
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132 with the clonebundles facility. |
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133 |
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134 Value should be "true". |
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135 |
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136 Manifests can contain multiple entries. Assuming metadata is defined, clients |
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137 will filter entries from the manifest that they don't support. The remaining |
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138 entries are optionally sorted by client preferences |
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139 (``ui.clonebundleprefers`` config option). The client then attempts |
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140 to fetch the bundle at the first URL in the remaining list. |
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141 |
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142 **Errors when downloading a bundle will fail the entire clone operation: |
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143 clients do not automatically fall back to a traditional clone.** The reason |
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144 for this is that if a server is using clone bundles, it is probably doing so |
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145 because the feature is necessary to help it scale. In other words, there |
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146 is an assumption that clone load will be offloaded to another service and |
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147 that the Mercurial server isn't responsible for serving this clone load. |
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148 If that other service experiences issues and clients start mass falling back to |
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149 the original Mercurial server, the added clone load could overwhelm the server |
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150 due to unexpected load and effectively take it offline. Not having clients |
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151 automatically fall back to cloning from the original server mitigates this |
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152 scenario. |
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153 |
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154 Because there is no automatic Mercurial server fallback on failure of the |
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155 bundle hosting service, it is important for server operators to view the bundle |
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156 hosting service as an extension of the Mercurial server in terms of |
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157 availability and service level agreements: if the bundle hosting service goes |
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158 down, so does the ability for clients to clone. Note: clients will see a |
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159 message informing them how to bypass the clone bundles facility when a failure |
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160 occurs. So server operators should prepare for some people to follow these |
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161 instructions when a failure occurs, thus driving more load to the original |
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162 Mercurial server when the bundle hosting service fails. |
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163 """ |
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164 |
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165 from __future__ import absolute_import |
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166 |
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167 from mercurial import ( |
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168 extensions, |
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169 wireproto, |
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170 ) |
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171 |
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172 testedwith = 'ships-with-hg-core' |
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173 |
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174 def capabilities(orig, repo, proto): |
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175 caps = orig(repo, proto) |
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176 |
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177 # Only advertise if a manifest exists. This does add some I/O to requests. |
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178 # But this should be cheaper than a wasted network round trip due to |
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179 # missing file. |
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180 if repo.vfs.exists('clonebundles.manifest'): |
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181 caps.append('clonebundles') |
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182 |
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183 return caps |
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184 |
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185 def extsetup(ui): |
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186 extensions.wrapfunction(wireproto, '_capabilities', capabilities) |