Mercurial > hg
annotate mercurial/help/internals/revlogs.txt @ 30746:9cb0bb0f29f0
revlog: REVIDX_EXTSTORED flag
This flag will be used by the lfs extension to mark the revision data as stored
externally.
author | Remi Chaintron <remi@fb.com> |
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date | Thu, 05 Jan 2017 17:16:51 +0000 |
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1 Revision logs - or *revlogs* - are an append only data structure for |
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2 storing discrete entries, or *revisions*. They are the primary storage |
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3 mechanism of repository data. |
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4 |
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5 Revlogs effectively model a directed acyclic graph (DAG). Each node |
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6 has edges to 1 or 2 *parent* nodes. Each node contains metadata and |
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7 the raw value for that node. |
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8 |
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9 Revlogs consist of entries which have metadata and revision data. |
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10 Metadata includes the hash of the revision's content, sizes, and |
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11 links to its *parent* entries. The collective metadata is referred |
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12 to as the *index* and the revision data is the *data*. |
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13 |
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14 Revision data is stored as a series of compressed deltas against previous |
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15 revisions. |
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16 |
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17 Revlogs are written in an append-only fashion. We never need to rewrite |
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18 a file to insert nor do we need to remove data. Rolling back in-progress |
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19 writes can be performed by truncating files. Read locks can be avoided |
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20 using simple techniques. This means that references to other data in |
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21 the same revlog *always* refer to a previous entry. |
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22 |
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23 Revlogs can be modeled as 0-indexed arrays. The first revision is |
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24 revision #0 and the second is revision #1. The revision -1 is typically |
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25 used to mean *does not exist* or *not defined*. |
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26 |
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27 File Format |
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28 =========== |
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29 |
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30 A revlog begins with a 32-bit big endian integer holding version info |
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31 and feature flags. This integer is shared with the first revision |
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32 entry. |
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33 |
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34 This integer is logically divided into 2 16-bit shorts. The least |
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35 significant half of the integer is the format/version short. The other |
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36 short holds feature flags that dictate behavior of the revlog. |
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37 |
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38 Only 1 bit of the format/version short is currently used. Remaining |
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39 bits are reserved for future use. |
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40 |
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41 The following values for the format/version short are defined: |
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42 |
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43 0 |
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44 The original revlog version. |
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45 1 |
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46 RevlogNG (*next generation*). It replaced version 0 when it was |
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47 implemented in 2006. |
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48 |
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49 The feature flags short consists of bit flags. Where 0 is the least |
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50 significant bit, the following bit offsets define flags: |
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51 |
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52 0 |
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53 Store revision data inline. |
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54 1 |
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55 Generaldelta encoding. |
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56 |
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57 2-15 |
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58 Reserved for future use. |
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59 |
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60 The following header values are common: |
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61 |
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62 00 00 00 01 |
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63 RevlogNG |
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64 00 01 00 01 |
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65 RevlogNG + inline |
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66 00 02 00 01 |
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67 RevlogNG + generaldelta |
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68 00 03 00 01 |
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69 RevlogNG + inline + generaldelta |
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70 |
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71 Following the 32-bit header is the remainder of the first index entry. |
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72 Following that are remaining *index* data. Inlined revision data is |
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73 possibly located between index entries. More on this layout is described |
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74 below. |
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75 |
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76 RevlogNG Format |
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77 =============== |
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78 |
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79 RevlogNG (version 1) begins with an index describing the revisions in |
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80 the revlog. If the ``inline`` flag is set, revision data is stored inline, |
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81 or between index entries (as opposed to in a separate container). |
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82 |
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83 Each index entry is 64 bytes. The byte layout of each entry is as |
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84 follows, with byte 0 being the first byte (all data stored as big endian): |
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85 |
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86 0-3 (4 bytes) (rev 0 only) |
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87 Revlog header |
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88 0-5 (6 bytes) |
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89 Absolute offset of revision data from beginning of revlog. |
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90 6-7 (2 bytes) |
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91 Bit flags impacting revision behavior. The following bit offsets define: |
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92 0: REVIDX_ISCENSORED revision has censor metadata, must be verified. |
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93 1: REVIDX_EXTSTORED revision data is stored externally. |
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94 8-11 (4 bytes) |
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95 Compressed length of revision data / chunk as stored in revlog. |
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96 12-15 (4 bytes) |
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97 Uncompressed length of revision data. This is the size of the full |
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98 revision data, not the size of the chunk post decompression. |
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99 16-19 (4 bytes) |
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100 Base or previous revision this revision's delta was produced against. |
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101 -1 means this revision holds full text (as opposed to a delta). |
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102 For generaldelta repos, this is the previous revision in the delta |
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103 chain. For non-generaldelta repos, this is the base or first |
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104 revision in the delta chain. |
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105 20-23 (4 bytes) |
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106 A revision this revision is *linked* to. This allows a revision in |
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107 one revlog to be forever associated with a revision in another |
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108 revlog. For example, a file's revlog may point to the changelog |
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109 revision that introduced it. |
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110 24-27 (4 bytes) |
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111 Revision of 1st parent. -1 indicates no parent. |
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112 28-31 (4 bytes) |
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113 Revision of 2nd parent. -1 indicates no 2nd parent. |
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114 32-63 (32 bytes) |
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115 Hash of revision's full text. Currently, SHA-1 is used and only |
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116 the first 20 bytes of this field are used. The rest of the bytes |
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117 are ignored and should be stored as \0. |
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118 |
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119 If inline revision data is being stored, the compressed revision data |
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120 (of length from bytes offset 8-11 from the index entry) immediately |
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121 follows the index entry. There is no header on the revision data. There |
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122 is no padding between it and the index entries before and after. |
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123 |
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124 If revision data is not inline, then raw revision data is stored in a |
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125 separate byte container. The offsets from bytes 0-5 and the compressed |
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126 length from bytes 8-11 define how to access this data. |
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127 |
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128 The first 4 bytes of the revlog are shared between the revlog header |
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129 and the 6 byte absolute offset field from the first revlog entry. |
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130 |
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131 Delta Chains |
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132 ============ |
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133 |
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134 Revision data is encoded as a chain of *chunks*. Each chain begins with |
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135 the compressed original full text for that revision. Each subsequent |
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136 *chunk* is a *delta* against the previous revision. We therefore call |
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137 these chains of chunks/deltas *delta chains*. |
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138 |
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139 The full text for a revision is reconstructed by loading the original |
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140 full text for the base revision of a *delta chain* and then applying |
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141 *deltas* until the target revision is reconstructed. |
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142 |
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143 *Delta chains* are limited in length so lookup time is bound. They are |
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144 limited to ~2x the length of the revision's data. The linear distance |
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145 between the base chunk and the final chunk is also limited so the |
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146 amount of read I/O to load all chunks in the delta chain is bound. |
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147 |
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148 Deltas and delta chains are either computed against the previous |
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149 revision in the revlog or another revision (almost certainly one of |
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150 the parents of the revision). Historically, deltas were computed against |
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151 the previous revision. The *generaldelta* revlog feature flag (enabled |
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152 by default in Mercurial 3.7) activates the mode where deltas are |
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153 computed against an arbitrary revision (almost certainly a parent revision). |
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154 |
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155 File Storage |
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156 ============ |
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157 |
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158 Revlogs logically consist of an index (metadata of entries) and |
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159 revision data. This data may be stored together in a single file or in |
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160 separate files. The mechanism used is indicated by the ``inline`` feature |
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161 flag on the revlog. |
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162 |
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163 Mercurial's behavior is to use inline storage until a revlog reaches a |
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164 certain size, at which point it will be converted to non-inline. The |
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165 reason there is a size limit on inline storage is to establish an upper |
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166 bound on how much data must be read to load the index. It would be a waste |
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167 to read tens or hundreds of extra megabytes of data just to access the |
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168 index data. |
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169 |
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170 The actual layout of revlog files on disk is governed by the repository's |
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171 *store format*. Typically, a ``.i`` file represents the index revlog |
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172 (possibly containing inline data) and a ``.d`` file holds the revision data. |
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173 |
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174 Revision Entries |
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175 ================ |
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176 |
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177 Revision entries consist of an optional 1 byte header followed by an |
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178 encoding of the revision data. The headers are as follows: |
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179 |
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180 \0 (0x00) |
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181 Revision data is the entirety of the entry, including this header. |
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182 u (0x75) |
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183 Raw revision data follows. |
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184 x (0x78) |
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185 zlib (RFC 1950) data. |
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186 |
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187 The 0x78 value is actually the first byte of the zlib header (CMF byte). |
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188 |
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189 Hash Computation |
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190 ================ |
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191 |
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192 The hash of the revision is stored in the index and is used both as a primary |
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193 key and for data integrity verification. |
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194 |
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195 Currently, SHA-1 is the only supported hashing algorithm. To obtain the SHA-1 |
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196 hash of a revision: |
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197 |
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198 1. Hash the parent nodes |
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199 2. Hash the fulltext of the revision |
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200 |
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201 The 20 byte node ids of the parents are fed into the hasher in ascending order. |