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annotate mercurial/dirstateutils/timestamp.py @ 52055:b332ae615714

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merge: improve working-copy mtime race handling Explanations inline. This also makes use of `make_mtime_reliable`, which unifies our mtime raciness logic from the status. On top of this, this fixes the handling of the pure dirstate status to better catch racy status, as we've been doing in Rust for a long time now.
author Rapha?l Gom?s <rgomes@octobus.net>
date Wed, 16 Oct 2024 19:14:30 +0200
parents 572d80e51094
children 5cc8deb96b48
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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 from __future__ import annotations
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7
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8 import functools
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9 import os
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10 import stat
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11 import time
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12 from typing import Optional, Tuple
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13
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14 from .. import error
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15
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16
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17 rangemask = 0x7FFFFFFF
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18
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19
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20 @functools.total_ordering
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21 class timestamp(tuple):
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22 """
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23 A Unix timestamp with optional nanoseconds precision,
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24 modulo 2**31 seconds.
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25
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26 A 3-tuple containing:
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27
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28 `truncated_seconds`: seconds since the Unix epoch,
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29 truncated to its lower 31 bits
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30
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31 `subsecond_nanoseconds`: number of nanoseconds since `truncated_seconds`.
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32 When this is zero, the sub-second precision is considered unknown.
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33
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34 `second_ambiguous`: whether this timestamp is still "reliable"
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35 (see `reliable_mtime_of`) if we drop its sub-second component.
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36 """
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37
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38 def __new__(cls, value):
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39 truncated_seconds, subsec_nanos, second_ambiguous = value
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40 value = (truncated_seconds & rangemask, subsec_nanos, second_ambiguous)
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41 return super(timestamp, cls).__new__(cls, value)
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42
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43 def __eq__(self, other):
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44 raise error.ProgrammingError(
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45 'timestamp should never be compared directly'
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46 )
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47
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48 def __gt__(self, other):
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49 raise error.ProgrammingError(
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50 'timestamp should never be compared directly'
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51 )
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52
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53
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54 def get_fs_now(vfs) -> Optional[timestamp]:
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55 """return a timestamp for "now" in the current vfs
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56
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57 This will raise an exception if no temporary files could be created.
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58 """
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59 tmpfd, tmpname = vfs.mkstemp()
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60 try:
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61 return mtime_of(os.fstat(tmpfd))
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62 finally:
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63 os.close(tmpfd)
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64 vfs.unlink(tmpname)
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65
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66
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67 def zero() -> timestamp:
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68 """
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69 Returns the `timestamp` at the Unix epoch.
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70 """
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71 return tuple.__new__(timestamp, (0, 0))
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72
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73
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74 def mtime_of(stat_result: os.stat_result) -> timestamp:
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75 """
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76 Takes an `os.stat_result`-like object and returns a `timestamp` object
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77 for its modification time.
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78 """
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79 try:
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80 # TODO: add this attribute to `osutil.stat` objects,
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81 # see `mercurial/cext/osutil.c`.
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82 #
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83 # This attribute is also not available on Python 2.
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84 nanos = stat_result.st_mtime_ns
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85 except AttributeError:
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86 # https://docs.python.org/2/library/os.html#os.stat_float_times
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87 # "For compatibility with older Python versions,
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88 # accessing stat_result as a tuple always returns integers."
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89 secs = stat_result[stat.ST_MTIME]
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90
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91 subsec_nanos = 0
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92 else:
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93 billion = int(1e9)
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94 secs = nanos // billion
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95 subsec_nanos = nanos % billion
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96
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97 return timestamp((secs, subsec_nanos, False))
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98
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99
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100 def reliable_mtime_of(
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101 stat_result: os.stat_result, present_mtime: timestamp
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102 ) -> Optional[timestamp]:
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103 """Wrapper for `make_mtime_reliable` for stat objects"""
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104 file_mtime = mtime_of(stat_result)
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105 return make_mtime_reliable(file_mtime, present_mtime)
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106
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107
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108 def make_mtime_reliable(
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109 file_timestamp: timestamp, present_mtime: timestamp
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110 ) -> Optional[timestamp]:
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111 """Same as `mtime_of`, but return `None` or a `Timestamp` with
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112 `second_ambiguous` set if the date might be ambiguous.
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113
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114 A modification time is reliable if it is older than "present_time" (or
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115 sufficiently in the future).
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116
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117 Otherwise a concurrent modification might happens with the same mtime.
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118 """
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119 file_second = file_timestamp[0]
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120 file_ns = file_timestamp[1]
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121 boundary_second = present_mtime[0]
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122 boundary_ns = present_mtime[1]
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123 # If the mtime of the ambiguous file is younger (or equal) to the starting
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124 # point of the `status` walk, we cannot garantee that another, racy, write
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125 # will not happen right after with the same mtime and we cannot cache the
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126 # information.
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127 #
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128 # However if the mtime is far away in the future, this is likely some
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129 # mismatch between the current clock and previous file system operation. So
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130 # mtime more than one days in the future are considered fine.
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131 if boundary_second == file_second:
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132 if file_ns and boundary_ns:
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133 if file_ns < boundary_ns:
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134 return timestamp((file_second, file_ns, True))
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135 return None
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136 elif boundary_second < file_second < (3600 * 24 + boundary_second):
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137 return None
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138 else:
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139 return file_timestamp
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140
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141
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142 FS_TICK_WAIT_TIMEOUT = 0.1 # 100 milliseconds
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143
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144
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145 def wait_until_fs_tick(vfs) -> Optional[Tuple[timestamp, bool]]:
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146 """Wait until the next update from the filesystem time by writing in a loop
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147 a new temporary file inside the working directory and checking if its time
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148 differs from the first one observed.
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149
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150 Returns `None` if we are unable to get the filesystem time,
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151 `(timestamp, True)` if we've timed out waiting for the filesystem clock
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152 to tick, and `(timestamp, False)` if we've waited successfully.
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153
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154 On Linux, your average tick is going to be a "jiffy", or 1/HZ.
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155 HZ is your kernel's tick rate (if it has one configured) and the value
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156 is the one returned by `grep 'CONFIG_HZ=' /boot/config-$(uname -r)`,
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157 again assuming a normal setup.
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158
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159 In my case (Alphare) at the time of writing, I get `CONFIG_HZ=250`,
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160 which equates to 4ms.
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161 This might change with a series that could make it to Linux 6.12:
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162 https://lore.kernel.org/all/20241002-mgtime-v10-8-d1c4717f5284@kernel.org
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163 """
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164 start = time.monotonic()
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165
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166 try:
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167 old_fs_time = get_fs_now(vfs)
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168 new_fs_time = get_fs_now(vfs)
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169
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170 while (
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171 new_fs_time[0] == old_fs_time[0]
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172 and new_fs_time[1] == old_fs_time[1]
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173 ):
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174 if time.monotonic() - start > FS_TICK_WAIT_TIMEOUT:
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175 return (old_fs_time, True)
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176 new_fs_time = get_fs_now(vfs)
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177 except OSError:
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178 return None
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179 else:
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180 return (new_fs_time, False)