annotate mercurial/revlogutils/randomaccessfile.py @ 50896:b2b8c25f9462

hgwebmod: use sysstr to check for attribute presence We do not need bytes here.
author Pierre-Yves David <pierre-yves.david@octobus.net>
date Wed, 30 Aug 2023 13:28:09 +0200
parents 642e31cb55f0
children 3314c41c3759
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1 # Copyright Mercurial Contributors
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2 #
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3 # This software may be used and distributed according to the terms of the
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4 # GNU General Public License version 2 or any later version.
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5
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6 import contextlib
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7
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8 from ..i18n import _
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9 from .. import (
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10 error,
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11 util,
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12 )
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13
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14
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15 _MAX_CACHED_CHUNK_SIZE = 1048576 # 1 MiB
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16
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17 PARTIAL_READ_MSG = _(
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18 b'partial read of revlog %s; expected %d bytes from offset %d, got %d'
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19 )
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20
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21
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22 def _is_power_of_two(n):
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23 return (n & (n - 1) == 0) and n != 0
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24
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25
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26 class randomaccessfile:
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27 """Accessing arbitrary chuncks of data within a file, with some caching"""
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28
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29 def __init__(
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30 self,
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31 opener,
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32 filename,
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33 default_cached_chunk_size,
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34 initial_cache=None,
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35 ):
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36 # Required by bitwise manipulation below
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37 assert _is_power_of_two(default_cached_chunk_size)
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38
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39 self.opener = opener
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40 self.filename = filename
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41 self.default_cached_chunk_size = default_cached_chunk_size
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42 self.writing_handle = None # This is set from revlog.py
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43 self.reading_handle = None
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44 self._cached_chunk = b''
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45 self._cached_chunk_position = 0 # Offset from the start of the file
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46 if initial_cache:
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47 self._cached_chunk_position, self._cached_chunk = initial_cache
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48
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49 def clear_cache(self):
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50 self._cached_chunk = b''
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51 self._cached_chunk_position = 0
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52
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53 def _open(self, mode=b'r'):
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54 """Return a file object"""
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55 return self.opener(self.filename, mode=mode)
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56
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57 @contextlib.contextmanager
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58 def _open_read(self, existing_file_obj=None):
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59 """File object suitable for reading data"""
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60 # Use explicit file handle, if given.
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61 if existing_file_obj is not None:
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62 yield existing_file_obj
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63
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64 # Use a file handle being actively used for writes, if available.
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65 # There is some danger to doing this because reads will seek the
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66 # file. However, revlog._writeentry performs a SEEK_END before all
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67 # writes, so we should be safe.
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68 elif self.writing_handle:
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69 yield self.writing_handle
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70
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71 elif self.reading_handle:
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72 yield self.reading_handle
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73
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74 # Otherwise open a new file handle.
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75 else:
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76 with self._open() as fp:
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77 yield fp
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78
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79 @contextlib.contextmanager
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80 def reading(self):
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81 """Context manager that keeps the file open for reading"""
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82 if (
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83 self.reading_handle is None
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84 and self.writing_handle is None
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85 and self.filename is not None
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86 ):
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87 with self._open() as fp:
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88 self.reading_handle = fp
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89 try:
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90 yield
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91 finally:
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92 self.reading_handle = None
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93 else:
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94 yield
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95
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96 def read_chunk(self, offset, length, existing_file_obj=None):
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97 """Read a chunk of bytes from the file.
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98
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99 Accepts an absolute offset, length to read, and an optional existing
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100 file handle to read from.
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101
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102 If an existing file handle is passed, it will be seeked and the
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103 original seek position will NOT be restored.
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104
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105 Returns a str or buffer of raw byte data.
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106
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107 Raises if the requested number of bytes could not be read.
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108 """
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109 end = offset + length
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110 cache_start = self._cached_chunk_position
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111 cache_end = cache_start + len(self._cached_chunk)
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112 # Is the requested chunk within the cache?
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113 if cache_start <= offset and end <= cache_end:
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114 if cache_start == offset and end == cache_end:
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115 return self._cached_chunk # avoid a copy
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116 relative_start = offset - cache_start
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117 return util.buffer(self._cached_chunk, relative_start, length)
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118
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119 return self._read_and_update_cache(offset, length, existing_file_obj)
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120
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121 def _read_and_update_cache(self, offset, length, existing_file_obj=None):
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122 # Cache data both forward and backward around the requested
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123 # data, in a fixed size window. This helps speed up operations
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124 # involving reading the revlog backwards.
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125 real_offset = offset & ~(self.default_cached_chunk_size - 1)
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126 real_length = (
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127 (offset + length + self.default_cached_chunk_size)
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128 & ~(self.default_cached_chunk_size - 1)
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129 ) - real_offset
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130 with self._open_read(existing_file_obj) as file_obj:
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131 file_obj.seek(real_offset)
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132 data = file_obj.read(real_length)
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133
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134 self._add_cached_chunk(real_offset, data)
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135
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136 relative_offset = offset - real_offset
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137 got = len(data) - relative_offset
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138 if got < length:
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139 message = PARTIAL_READ_MSG % (self.filename, length, offset, got)
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140 raise error.RevlogError(message)
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141
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142 if offset != real_offset or real_length != length:
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143 return util.buffer(data, relative_offset, length)
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144 return data
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145
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146 def _add_cached_chunk(self, offset, data):
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147 """Add to or replace the cached data chunk.
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148
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149 Accepts an absolute offset and the data that is at that location.
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150 """
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151 if (
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152 self._cached_chunk_position + len(self._cached_chunk) == offset
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153 and len(self._cached_chunk) + len(data) < _MAX_CACHED_CHUNK_SIZE
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154 ):
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155 # add to existing cache
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156 self._cached_chunk += data
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157 else:
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158 self._cached_chunk = data
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159 self._cached_chunk_position = offset