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annotate rust/hg-core/src/lock.rs @ 52167:7be39c5110c9

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hg-core: add a complete VFS This will be used from Python in a later change. More changes are needed in hg-core and rhg to properly clean up the APIs of the old VFS implementation but it can be done when the dust settles and we start adding more functionality to the pure Rust VFS.
author Rapha?l Gom?s <rgomes@octobus.net>
date Mon, 29 Jul 2024 20:47:43 +0200
parents db7dbe6f7bb2
children
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::vfs::Vfs;
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6 use crate::vfs::VfsImpl;
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7 use std::io;
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8 use std::io::ErrorKind;
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9 use std::path::Path;
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10
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11 #[derive(derive_more::From)]
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12 pub enum LockError {
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13 AlreadyHeld,
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14 #[from]
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15 Other(HgError),
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16 }
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17
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18 /// Try to call `f` with the lock acquired, without waiting.
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19 ///
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20 /// If the lock is aready held, `f` is not called and `LockError::AlreadyHeld`
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21 /// is returned. `LockError::Io` is returned for any unexpected I/O error
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22 /// accessing the lock file, including for removing it after `f` was called.
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23 /// The return value of `f` is dropped in that case. If all is successful, the
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24 /// return value of `f` is forwarded.
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25 pub fn try_with_lock_no_wait<R>(
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26 hg_vfs: &VfsImpl,
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27 lock_filename: &str,
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28 f: impl FnOnce() -> R,
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29 ) -> Result<R, LockError> {
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30 let our_lock_data = &*OUR_LOCK_DATA;
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31 for _retry in 0..5 {
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32 match make_lock(hg_vfs, lock_filename, our_lock_data) {
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33 Ok(()) => {
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34 let result = f();
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35 unlock(hg_vfs, lock_filename)?;
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36 return Ok(result);
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37 }
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38 Err(HgError::IoError { error, .. })
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39 if error.kind() == ErrorKind::AlreadyExists =>
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40 {
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41 let lock_data = read_lock(hg_vfs, lock_filename)?;
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42 if lock_data.is_none() {
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43 // Lock was apparently just released, retry acquiring it
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44 continue;
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45 }
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46 if !lock_should_be_broken(&lock_data) {
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47 return Err(LockError::AlreadyHeld);
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48 }
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49 // The lock file is left over from a process not running
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50 // anymore. Break it, but with another lock to
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51 // avoid a race.
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52 break_lock(hg_vfs, lock_filename)?;
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53
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54 // Retry acquiring
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55 }
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56 Err(error) => Err(error)?,
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57 }
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58 }
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59 Err(LockError::AlreadyHeld)
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60 }
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61
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62 fn break_lock(hg_vfs: &VfsImpl, lock_filename: &str) -> Result<(), LockError> {
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63 try_with_lock_no_wait(hg_vfs, &format!("{}.break", lock_filename), || {
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64 // Check again in case some other process broke and
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65 // acquired the lock in the meantime
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66 let lock_data = read_lock(hg_vfs, lock_filename)?;
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67 if !lock_should_be_broken(&lock_data) {
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68 return Err(LockError::AlreadyHeld);
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69 }
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70 Ok(hg_vfs.unlink(Path::new(lock_filename))?)
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71 })?
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72 }
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73
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74 #[cfg(unix)]
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75 fn make_lock(
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76 hg_vfs: &VfsImpl,
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77 lock_filename: &str,
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78 data: &str,
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79 ) -> Result<(), HgError> {
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80 // Use a symbolic link because creating it is atomic.
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81 // The link’s "target" contains data not representing any path.
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82 let fake_symlink_target = data;
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83 hg_vfs.create_symlink(lock_filename, fake_symlink_target)
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84 }
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85
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86 fn read_lock(
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87 hg_vfs: &VfsImpl,
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88 lock_filename: &str,
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89 ) -> Result<Option<String>, HgError> {
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90 let link_target =
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91 hg_vfs.read_link(lock_filename).io_not_found_as_none()?;
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92 if let Some(target) = link_target {
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93 let data = target
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94 .into_os_string()
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95 .into_string()
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96 .map_err(|_| HgError::corrupted("non-UTF-8 lock data"))?;
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97 Ok(Some(data))
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98 } else {
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99 Ok(None)
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100 }
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101 }
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102
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103 fn unlock(hg_vfs: &VfsImpl, lock_filename: &str) -> Result<(), HgError> {
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104 hg_vfs.unlink(Path::new(lock_filename))
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105 }
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106
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107 /// Return whether the process that is/was holding the lock is known not to be
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108 /// running anymore.
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109 fn lock_should_be_broken(data: &Option<String>) -> bool {
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110 (|| -> Option<bool> {
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111 let (prefix, pid) = data.as_ref()?.split_once(':')?;
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112 if prefix != *LOCK_PREFIX {
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113 return Some(false);
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114 }
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115 let process_is_running;
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116
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117 #[cfg(unix)]
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118 {
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119 let pid: libc::pid_t = pid.parse().ok()?;
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120 unsafe {
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121 let signal = 0; // Test if we could send a signal, without sending
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122 let result = libc::kill(pid, signal);
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123 if result == 0 {
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124 process_is_running = true
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125 } else {
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126 let errno =
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127 io::Error::last_os_error().raw_os_error().unwrap();
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128 process_is_running = errno != libc::ESRCH
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129 }
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130 }
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131 }
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132
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133 Some(!process_is_running)
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134 })()
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135 .unwrap_or(false)
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136 }
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137
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138 lazy_static::lazy_static! {
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139 /// A string which is used to differentiate pid namespaces
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140 ///
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141 /// It's useful to detect "dead" processes and remove stale locks with
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142 /// confidence. Typically it's just hostname. On modern linux, we include an
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143 /// extra Linux-specific pid namespace identifier.
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144 static ref LOCK_PREFIX: String = {
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145 // Note: this must match the behavior of `_getlockprefix` in `mercurial/lock.py`
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146
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147 /// Same as https://github.com/python/cpython/blob/v3.10.0/Modules/socketmodule.c#L5414
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148 const BUFFER_SIZE: usize = 1024;
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149 // This cast is *needed* for platforms with signed chars
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150 #[allow(clippy::unnecessary_cast)]
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151 let mut buffer = [0 as libc::c_char; BUFFER_SIZE];
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152 let hostname_bytes = unsafe {
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153 let result = libc::gethostname(buffer.as_mut_ptr(), BUFFER_SIZE);
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154 if result != 0 {
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155 panic!("gethostname: {}", io::Error::last_os_error())
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156 }
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157 std::ffi::CStr::from_ptr(buffer.as_mut_ptr()).to_bytes()
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158 };
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159 let hostname =
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160 std::str::from_utf8(hostname_bytes).expect("non-UTF-8 hostname");
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161
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162 #[cfg(target_os = "linux")]
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163 {
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164 use std::os::linux::fs::MetadataExt;
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165 match std::fs::metadata("/proc/self/ns/pid") {
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166 Ok(meta) => {
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167 return format!("{}/{:x}", hostname, meta.st_ino())
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168 }
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169 Err(error) => {
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170 // TODO: match on `error.kind()` when `NotADirectory`
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171 // is available on all supported Rust versions:
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172 // https://github.com/rust-lang/rust/issues/86442
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173 use libc::{
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174 ENOENT, // ErrorKind::NotFound
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175 ENOTDIR, // ErrorKind::NotADirectory
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176 EACCES, // ErrorKind::PermissionDenied
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177 };
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178 match error.raw_os_error() {
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179 Some(ENOENT) | Some(ENOTDIR) | Some(EACCES) => {}
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180 _ => panic!("stat /proc/self/ns/pid: {}", error),
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181 }
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182 }
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183 }
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184 }
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185
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186 hostname.to_owned()
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187 };
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188
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189 static ref OUR_LOCK_DATA: String = format!("{}:{}", &*LOCK_PREFIX, std::process::id());
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190 }