1# -*- coding: utf-8 -*- 2 3# pidlockfile.py 4# 5# Copyright © 2008–2009 Ben Finney <ben+python@benfinney.id.au> 6# 7# This is free software: you may copy, modify, and/or distribute this work 8# under the terms of the Python Software Foundation License, version 2 or 9# later as published by the Python Software Foundation. 10# No warranty expressed or implied. See the file LICENSE.PSF-2 for details. 11 12""" Lockfile behaviour implemented via Unix PID files. 13 """ 14 15from __future__ import absolute_import 16 17import errno 18import os 19import time 20 21from . import (LockBase, AlreadyLocked, LockFailed, NotLocked, NotMyLock, 22 LockTimeout) 23 24 25class PIDLockFile(LockBase): 26 """ Lockfile implemented as a Unix PID file. 27 28 The lock file is a normal file named by the attribute `path`. 29 A lock's PID file contains a single line of text, containing 30 the process ID (PID) of the process that acquired the lock. 31 32 >>> lock = PIDLockFile('somefile') 33 >>> lock = PIDLockFile('somefile') 34 """ 35 36 def __init__(self, path, threaded=False, timeout=None): 37 # pid lockfiles don't support threaded operation, so always force 38 # False as the threaded arg. 39 LockBase.__init__(self, path, False, timeout) 40 self.unique_name = self.path 41 42 def read_pid(self): 43 """ Get the PID from the lock file. 44 """ 45 return read_pid_from_pidfile(self.path) 46 47 def is_locked(self): 48 """ Test if the lock is currently held. 49 50 The lock is held if the PID file for this lock exists. 51 52 """ 53 return os.path.exists(self.path) 54 55 def i_am_locking(self): 56 """ Test if the lock is held by the current process. 57 58 Returns ``True`` if the current process ID matches the 59 number stored in the PID file. 60 """ 61 return self.is_locked() and os.getpid() == self.read_pid() 62 63 def acquire(self, timeout=None): 64 """ Acquire the lock. 65 66 Creates the PID file for this lock, or raises an error if 67 the lock could not be acquired. 68 """ 69 70 timeout = timeout if timeout is not None else self.timeout 71 end_time = time.time() 72 if timeout is not None and timeout > 0: 73 end_time += timeout 74 75 while True: 76 try: 77 write_pid_to_pidfile(self.path) 78 except OSError as exc: 79 if exc.errno == errno.EEXIST: 80 # The lock creation failed. Maybe sleep a bit. 81 if time.time() > end_time: 82 if timeout is not None and timeout > 0: 83 raise LockTimeout("Timeout waiting to acquire" 84 " lock for %s" % 85 self.path) 86 else: 87 raise AlreadyLocked("%s is already locked" % 88 self.path) 89 time.sleep(timeout is not None and timeout / 10 or 0.1) 90 else: 91 raise LockFailed("failed to create %s" % self.path) 92 else: 93 return 94 95 def release(self): 96 """ Release the lock. 97 98 Removes the PID file to release the lock, or raises an 99 error if the current process does not hold the lock. 100 101 """ 102 if not self.is_locked(): 103 raise NotLocked("%s is not locked" % self.path) 104 if not self.i_am_locking(): 105 raise NotMyLock("%s is locked, but not by me" % self.path) 106 remove_existing_pidfile(self.path) 107 108 def break_lock(self): 109 """ Break an existing lock. 110 111 Removes the PID file if it already exists, otherwise does 112 nothing. 113 114 """ 115 remove_existing_pidfile(self.path) 116 117 118def read_pid_from_pidfile(pidfile_path): 119 """ Read the PID recorded in the named PID file. 120 121 Read and return the numeric PID recorded as text in the named 122 PID file. If the PID file cannot be read, or if the content is 123 not a valid PID, return ``None``. 124 125 """ 126 pid = None 127 try: 128 pidfile = open(pidfile_path, 'r') 129 except IOError: 130 pass 131 else: 132 # According to the FHS 2.3 section on PID files in /var/run: 133 # 134 # The file must consist of the process identifier in 135 # ASCII-encoded decimal, followed by a newline character. 136 # 137 # Programs that read PID files should be somewhat flexible 138 # in what they accept; i.e., they should ignore extra 139 # whitespace, leading zeroes, absence of the trailing 140 # newline, or additional lines in the PID file. 141 142 line = pidfile.readline().strip() 143 try: 144 pid = int(line) 145 except ValueError: 146 pass 147 pidfile.close() 148 149 return pid 150 151 152def write_pid_to_pidfile(pidfile_path): 153 """ Write the PID in the named PID file. 154 155 Get the numeric process ID (“PID”) of the current process 156 and write it to the named file as a line of text. 157 158 """ 159 open_flags = (os.O_CREAT | os.O_EXCL | os.O_WRONLY) 160 open_mode = 0o644 161 pidfile_fd = os.open(pidfile_path, open_flags, open_mode) 162 pidfile = os.fdopen(pidfile_fd, 'w') 163 164 # According to the FHS 2.3 section on PID files in /var/run: 165 # 166 # The file must consist of the process identifier in 167 # ASCII-encoded decimal, followed by a newline character. For 168 # example, if crond was process number 25, /var/run/crond.pid 169 # would contain three characters: two, five, and newline. 170 171 pid = os.getpid() 172 pidfile.write("%s\n" % pid) 173 pidfile.close() 174 175 176def remove_existing_pidfile(pidfile_path): 177 """ Remove the named PID file if it exists. 178 179 Removing a PID file that doesn't already exist puts us in the 180 desired state, so we ignore the condition if the file does not 181 exist. 182 183 """ 184 try: 185 os.remove(pidfile_path) 186 except OSError as exc: 187 if exc.errno == errno.ENOENT: 188 pass 189 else: 190 raise 191