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annotate rust/hg-core/src/lock.rs @ 50222:ecd28d89c29e stable

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dirstate-v2: add devel config option to control write behavior This will help us to write predictable tests checking behavior in each case.
author Rapha?l Gom?s <rgomes@octobus.net>
date Tue, 28 Feb 2023 15:49:53 +0100
parents 59be65b7cdfd
children 29cf3167e459
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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1 //! Filesystem-based locks for local repositories
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2
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3 use crate::errors::HgError;
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4 use crate::errors::HgResultExt;
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5 use crate::utils::StrExt;
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6 use crate::vfs::Vfs;
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7 use std::io;
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8 use std::io::ErrorKind;
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9
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10 #[derive(derive_more::From)]
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11 pub enum LockError {
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12 AlreadyHeld,
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13 #[from]
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14 Other(HgError),
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15 }
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16
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17 /// Try to call `f` with the lock acquired, without waiting.
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18 ///
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19 /// If the lock is aready held, `f` is not called and `LockError::AlreadyHeld`
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20 /// is returned. `LockError::Io` is returned for any unexpected I/O error
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21 /// accessing the lock file, including for removing it after `f` was called.
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22 /// The return value of `f` is dropped in that case. If all is successful, the
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23 /// return value of `f` is forwarded.
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24 pub fn try_with_lock_no_wait<R>(
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25 hg_vfs: Vfs,
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26 lock_filename: &str,
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27 f: impl FnOnce() -> R,
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28 ) -> Result<R, LockError> {
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29 let our_lock_data = &*OUR_LOCK_DATA;
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30 for _retry in 0..5 {
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31 match make_lock(hg_vfs, lock_filename, our_lock_data) {
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32 Ok(()) => {
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33 let result = f();
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34 unlock(hg_vfs, lock_filename)?;
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35 return Ok(result);
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36 }
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37 Err(HgError::IoError { error, .. })
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38 if error.kind() == ErrorKind::AlreadyExists =>
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39 {
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40 let lock_data = read_lock(hg_vfs, lock_filename)?;
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41 if lock_data.is_none() {
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42 // Lock was apparently just released, retry acquiring it
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43 continue;
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44 }
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45 if !lock_should_be_broken(&lock_data) {
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46 return Err(LockError::AlreadyHeld);
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47 }
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48 // The lock file is left over from a process not running
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49 // anymore. Break it, but with another lock to
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50 // avoid a race.
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51 break_lock(hg_vfs, lock_filename)?;
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52
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53 // Retry acquiring
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54 }
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55 Err(error) => Err(error)?,
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56 }
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57 }
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58 Err(LockError::AlreadyHeld)
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59 }
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60
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61 fn break_lock(hg_vfs: Vfs, lock_filename: &str) -> Result<(), LockError> {
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62 try_with_lock_no_wait(hg_vfs, &format!("{}.break", lock_filename), || {
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63 // Check again in case some other process broke and
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64 // acquired the lock in the meantime
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65 let lock_data = read_lock(hg_vfs, lock_filename)?;
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66 if !lock_should_be_broken(&lock_data) {
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67 return Err(LockError::AlreadyHeld);
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68 }
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69 Ok(hg_vfs.remove_file(lock_filename)?)
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70 })?
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71 }
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72
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73 #[cfg(unix)]
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74 fn make_lock(
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75 hg_vfs: Vfs,
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76 lock_filename: &str,
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77 data: &str,
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78 ) -> Result<(), HgError> {
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79 // Use a symbolic link because creating it is atomic.
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80 // The link’s "target" contains data not representing any path.
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81 let fake_symlink_target = data;
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82 hg_vfs.create_symlink(lock_filename, fake_symlink_target)
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83 }
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84
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85 fn read_lock(
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86 hg_vfs: Vfs,
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87 lock_filename: &str,
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88 ) -> Result<Option<String>, HgError> {
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89 let link_target =
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90 hg_vfs.read_link(lock_filename).io_not_found_as_none()?;
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91 if let Some(target) = link_target {
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92 let data = target
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93 .into_os_string()
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94 .into_string()
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95 .map_err(|_| HgError::corrupted("non-UTF-8 lock data"))?;
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96 Ok(Some(data))
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97 } else {
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98 Ok(None)
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99 }
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100 }
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101
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102 fn unlock(hg_vfs: Vfs, lock_filename: &str) -> Result<(), HgError> {
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103 hg_vfs.remove_file(lock_filename)
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104 }
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105
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106 /// Return whether the process that is/was holding the lock is known not to be
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107 /// running anymore.
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108 fn lock_should_be_broken(data: &Option<String>) -> bool {
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109 (|| -> Option<bool> {
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110 let (prefix, pid) = data.as_ref()?.split_2(':')?;
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111 if prefix != &*LOCK_PREFIX {
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112 return Some(false);
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113 }
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114 let process_is_running;
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115
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116 #[cfg(unix)]
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117 {
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118 let pid: libc::pid_t = pid.parse().ok()?;
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119 unsafe {
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120 let signal = 0; // Test if we could send a signal, without sending
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121 let result = libc::kill(pid, signal);
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122 if result == 0 {
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123 process_is_running = true
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124 } else {
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125 let errno =
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126 io::Error::last_os_error().raw_os_error().unwrap();
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127 process_is_running = errno != libc::ESRCH
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128 }
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129 }
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130 }
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131
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132 Some(!process_is_running)
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133 })()
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134 .unwrap_or(false)
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135 }
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136
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137 lazy_static::lazy_static! {
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138 /// A string which is used to differentiate pid namespaces
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139 ///
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140 /// It's useful to detect "dead" processes and remove stale locks with
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141 /// confidence. Typically it's just hostname. On modern linux, we include an
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142 /// extra Linux-specific pid namespace identifier.
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143 static ref LOCK_PREFIX: String = {
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144 // Note: this must match the behavior of `_getlockprefix` in `mercurial/lock.py`
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145
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146 /// Same as https://github.com/python/cpython/blob/v3.10.0/Modules/socketmodule.c#L5414
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147 const BUFFER_SIZE: usize = 1024;
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148 let mut buffer = [0 as libc::c_char; BUFFER_SIZE];
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149 let hostname_bytes = unsafe {
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150 let result = libc::gethostname(buffer.as_mut_ptr(), BUFFER_SIZE);
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151 if result != 0 {
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152 panic!("gethostname: {}", io::Error::last_os_error())
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153 }
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154 std::ffi::CStr::from_ptr(buffer.as_mut_ptr()).to_bytes()
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155 };
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156 let hostname =
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157 std::str::from_utf8(hostname_bytes).expect("non-UTF-8 hostname");
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158
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159 #[cfg(target_os = "linux")]
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160 {
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161 use std::os::linux::fs::MetadataExt;
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162 match std::fs::metadata("/proc/self/ns/pid") {
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163 Ok(meta) => {
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164 return format!("{}/{:x}", hostname, meta.st_ino())
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165 }
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166 Err(error) => {
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167 // TODO: match on `error.kind()` when `NotADirectory`
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168 // is available on all supported Rust versions:
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169 // https://github.com/rust-lang/rust/issues/86442
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170 use libc::{
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171 ENOENT, // ErrorKind::NotFound
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172 ENOTDIR, // ErrorKind::NotADirectory
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173 EACCES, // ErrorKind::PermissionDenied
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174 };
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175 match error.raw_os_error() {
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176 Some(ENOENT) | Some(ENOTDIR) | Some(EACCES) => {}
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177 _ => panic!("stat /proc/self/ns/pid: {}", error),
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178 }
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179 }
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180 }
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181 }
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182
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183 hostname.to_owned()
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184 };
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185
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186 static ref OUR_LOCK_DATA: String = format!("{}:{}", &*LOCK_PREFIX, std::process::id());
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187 }