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1 ‘python-daemon’ Frequently Asked Questions
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2 ##########################################
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3
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4 :Author: Ben Finney <ben+python@benfinney.id.au>
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5 :Updated: 2015-01-10
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6
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7 .. contents::
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8 ..
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9 1 General
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10 1.1 What is the purpose of the ‘python-daemon’ library?
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11 1.2 How can I run a service communicating with a separate daemon process?
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12 2 Security
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13 2.1 Why is the umask set to 0 by default?
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14 3 File descriptors
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15 3.1 Why does the output stop after opening the daemon context?
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16 3.2 How can I preserve a ‘logging’ handler's file descriptor?
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17
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18 General
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19 =======
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20
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21 What is the purpose of the ‘python-daemon’ library?
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22 ---------------------------------------------------
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23
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24 The ‘python-daemon’ library has a deliberately narrow focus: that of
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25 being a reference implementation for `PEP 3143`_, “Standard daemon
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26 process library”.
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27
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28 .. _`PEP 3143`: http://www.python.org/dev/peps/pep-3143
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29
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30 How can I run a service communicating with a separate daemon process?
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31 ---------------------------------------------------------------------
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32
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33 As specified in `PEP 3143`_, the ‘python-daemon’ library is
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34 specifically focussed on the goal of having the *current running
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35 program* become a well-behaved Unix daemon process. This leaves open
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36 the question of how this program is started, or about multiple
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37 programs interacting. As detailed in PEP 3143:
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38
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39 A daemon is not a service
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40
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41 There is a related concept in many systems, called a “service”. A
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42 service differs from the model in this PEP, in that rather than
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43 having the *current* program continue to run as a daemon process,
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44 a service starts an *additional* process to run in the background,
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45 and the current process communicates with that additional process
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46 via some defined channels.
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47
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48 The Unix-style daemon model in this PEP can be used, among other
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49 things, to implement the background-process part of a service; but
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50 this PEP does not address the other aspects of setting up and
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51 managing a service.
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52
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53 A possible starting point for such a “service” model of execution is
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54 in a `message from 2009-01-30`_ to the ``python-ideas`` forum.
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55
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56 .. _`message from 2009-01-30`: http://mail.python.org/pipermail/python-ideas/2009-January/002606.html
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57
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58
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59 Security
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60 ========
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61
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62 Why is the umask set to 0 by default?
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63 -------------------------------------
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64
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65 A daemon should not rely on the parent process's umask value, which is
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66 beyond its control and may prevent creating a file with the required
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67 access mode. So when the daemon context opens, the umask is set to an
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68 explicit known value.
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69
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70 If the conventional value of 0 is too open, consider setting a value
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71 such as 0o022, 0o027, 0o077, or another specific value. Otherwise,
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72 ensure the daemon creates every file with an explicit access mode for
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73 the purpose.
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74
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75
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76 File descriptors
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77 ================
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78
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79 Why does the output stop after opening the daemon context?
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80 ----------------------------------------------------------
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81
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82 The specified behaviour in `PEP 3143`_ includes the requirement to
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83 detach the process from the controlling terminal (to allow the process
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84 to continue to run as a daemon), and to close all file descriptors not
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85 known to be safe once detached (to ensure any files that continue to
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86 be used are under the control of the daemon process).
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87
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88 If you want the process to generate output via the system streams
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89 ‘sys.stdout’ and ‘sys.stderr’, set the ‘DaemonContext’'s ‘stdout’
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90 and/or ‘stderr’ options to a file-like object (e.g. the ‘stream’
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91 attribute of a ‘logging.Handler’ instance). If these objects have file
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92 descriptors, they will be preserved when the daemon context opens.
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93
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94 How can I preserve a ‘logging’ handler's file descriptor?
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95 ---------------------------------------------------------
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96
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97 The ‘DaemonContext.open’ method conforms to `PEP 3143`_ by closing all
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98 open file descriptors, but excluding those files specified in the
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99 ‘files_preserve’ option. This option is a list of files or file
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100 descriptors.
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101
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102 The Python standard library ‘logging’ module provides log handlers
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103 that write to streams, including to files via the ‘StreamHandler’
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104 class and its sub-classes. The documentation (both the online `logging
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105 module documentation`_ and the docstrings for the code) makes no
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106 mention of a way to get at the stream associated with a handler
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107 object.
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108
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109 However, looking at the source code for ‘StreamHandler’, in Python 2.5
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110 as ``/usr/lib/python2.5/logging/__init__.py``, shows a ‘stream’
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111 attribute that is bound to the stream object. The attribute is not
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112 marked private (i.e. it is not named with a leading underscore), so we
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113 can presume it is part of the public API.
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114
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115 That attribute can then be used to specify that a logging handler's
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116 file descriptor should, when the ‘DaemonContext’ opens, be excluded
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117 from closure::
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118
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119 import logging
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120 import daemon
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121
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122 # any subclass of StreamHandler should provide the ‘stream’ attribute.
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123 lh = logging.handlers.TimedRotatingFileHandler(
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124 "/var/log/foo.log",
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125 # …
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126 )
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127
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128 # … do some logging and other activity …
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129
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130 daemon_context = daemon.DaemonContext()
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131 daemon_context.files_preserve = [lh.stream]
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132
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133 daemon_context.open()
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134
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135 # … continue as a daemon process …
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136
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137 .. _`logging module documentation`: http://docs.python.org/library/logging
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138
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139
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140 ..
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141 This is free software: you may copy, modify, and/or distribute this work
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142 under the terms of the Apache License version 2.0 as published by the
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143 Apache Software Foundation.
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144 No warranty expressed or implied. See the file ‘LICENSE.ASF-2’ for details.
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145
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146 ..
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147 Local variables:
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148 coding: utf-8
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149 mode: text
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150 mode: rst
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151 time-stamp-format: "%:y-%02m-%02d"
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152 time-stamp-start: "^:Updated:[ ]+"
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153 time-stamp-end: "$"
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154 time-stamp-line-limit: 20
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155 End:
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156 vim: fileencoding=utf-8 filetype=rst :
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