Imported upstream version 0.59.3
[hcoop/debian/courier-authlib.git] / liblock / lockmail.html
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4 Copyright 1998 - 2007 Double Precision, Inc. See COPYING for distribution
5 information.
7 --></head><body><div class="refentry" lang="en" xml:lang="en"><a id="lockmail" shape="rect"> </a><div class="titlepage"/><div class="refnamediv"><h2>Name</h2><p>lockmail — create mail lock files</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><div class="cmdsynopsis"><p><code class="command">lockmail</code> [-r] [-t <em class="replaceable"><code>timeout</code></em>] {<em class="replaceable"><code>lockfile</code></em>} {<em class="replaceable"><code>program</code></em>} [argument...]</p></div></div><div class="refsect1" lang="en" xml:lang="en"><a id="id282326" shape="rect"> </a><h2>DESCRIPTION</h2><p>
8 <span><strong class="command">lockmail</strong></span> is a helper utility for working with mailbox
9 files.
10 Mailbox files must be locked to prevent other applications from modifying the
11 mailbox at the same time.
12 Different system use different locking conventions.
13 <span><strong class="command">lockmail</strong></span> uses two of the most common locking mechanisms
14 in use, which should work reliably on most systems.</p><p>
15 <em class="replaceable"><code>lockfile</code></em> is the pathname to an existing mailbox
16 file.
17 By default, <span><strong class="command">lockmail</strong></span> tries to lock the mailbox every
18 five seconds (if the mailbox is already locked), and will give up after
19 three minutes.
20 After the mailbox is succesfully locked, <span><strong class="command">lockmail</strong></span> runs
21 <em class="replaceable"><code>program</code></em> as a child process, with any optional
22 <em class="replaceable"><code>argument</code></em>s.
23 When <em class="replaceable"><code>program</code></em> terminates, <span><strong class="command">lockmail</strong></span>
24 removes the mailbox lock, and terminates itself.</p></div><div class="refsect1" lang="en" xml:lang="en"><a id="id282387" shape="rect"> </a><h2>OPTIONS</h2><div class="variablelist"><dl><dt><span class="term">-r</span></dt><dd><p>
25 If a regular lock fails, try a read-only lock.
26 Use this option to lock mailbox files in a read-only directory.</p></dd><dt><span class="term">-t <em class="replaceable"><code>timeout</code></em></span></dt><dd><p>
27 If the lock attempt fails, try again for up to
28 <em class="replaceable"><code>timeout</code></em> seconds.
29 The actual timeout is rounded up to the next five second interval
30 (a lock attempt is tried every five seconds).</p></dd></dl></div></div><div class="refsect1" lang="en" xml:lang="en"><a id="id282626" shape="rect"> </a><h2>DESCRIPTION</h2><p>
31 This section briefly describes the locking mechanism used by
32 <span><strong class="command">lockmail</strong></span>.
33 <span><strong class="command">lockmail</strong></span> uses three different locking conventions in
34 order to maximize compatibility with other mail software:
35 C-Client folder locks, dot-locks, and file locks.
36 </p><div class="refsect2" lang="en" xml:lang="en"><a id="id282648" shape="rect"> </a><h3>C-Client folder locks</h3><p>
37 Mail software based on the <code class="literal">C-Client</code> library creates
38 lock files named
39 <code class="filename">/tmp/.<em class="replaceable"><code>dddddd</code></em>.<em class="replaceable"><code>iiiiii</code></em></code>.
40 Here, <em class="replaceable"><code>dddddd</code></em> and <em class="replaceable"><code>iiiiii</code></em>
41 are the device number and the inode number of the mailbox file
42 (the <em class="structfield"><code>st_dev</code></em> and <em class="structfield"><code>st_ino</code></em>
43 fields in the inode), in hexadecimal.
44 If the process ID saved in the C-Client folder lock file is not valid,
45 <span><strong class="command">lockmail</strong></span> concludes that it's a stale lock file, and
46 will remove it.</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
47 A race condition exists where a <code class="literal">C-Client</code> process is
48 killed after it creates a lock file, but before saving its process ID in the
49 lock file.
50 The race window is very small, but it exists.
51 The <code class="literal">C-Client</code> library does not appear to ever clear out
52 the lock file.</p><p>
53 <span><strong class="command">lockmail</strong></span>
54 attempts to resolve this race condition by deleting zero-length lock files
55 that are at least five minutes old.</p></div></div><div class="refsect2" lang="en" xml:lang="en"><a id="id281884" shape="rect"> </a><h3>dot-locks</h3><p>
56 <span><strong class="command">lockmail</strong></span>
57 also creates, and honors dot-lock files.
58 Dot-lock files are first created as temporary files, then linked to
59 <code class="filename"><em class="replaceable"><code>lockfile</code></em>.lock</code>.
60 The link operation fails if the dot-lock file already exists.
61 <span><strong class="command">lockmail</strong></span>
62 uses an enhanced method of dot-locking, where its process ID, and the name
63 of the server where <span><strong class="command">lockmail</strong></span> is running is also saved
64 in its dot-lock file.
65 If the operation fails due to an existing dot-lock file that was created
66 by another <span><strong class="command">lockmail</strong></span> process on the same server, and the
67 process ID no longer exists, this stale dot-lock file is removed immediately.
68 In all other situations a dot-lock file older than five minutes is considered
69 stale, and removed.
70 </p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
71 A failure to create a dot-lock file is silently ignored if the reason for
72 the failure is because
73 <span><strong class="command">lockmail</strong></span>
74 does not have the write permission in the dot-lock file's directory.
75 The incoming mail spool directory (usually
76 <code class="filename">/var/mail</code>)
77 typically does not have global write permissions, so the attempt to
78 create the dot-lock file in the spool directory will fail, and
79 <span><strong class="command">lockmail</strong></span>
80 will be content with using file-locking only.
81 </p></div></div><div class="refsect2" lang="en" xml:lang="en"><a id="id281958" shape="rect"> </a><h3>File locks</h3><p>
82 The final locking mechanism
83 <span><strong class="command">lockmail</strong></span>
84 uses is the operating system's file locking facility.
85 If
86 <span><strong class="command">lockmail</strong></span>
87 fails to obtain all three locks,
88 <span><strong class="command">lockmail</strong></span>
89 will sleep for five seconds and try again.
90 The only exception is a failure to create a dot-lock because of no write
91 access to the dot-lock file's directory, which is ignored.
92 If
93 <span><strong class="command">lockmail</strong></span>
94 still fails to obtain all required locks in the amount of time specified
95 by the <code class="option">-t</code> option (or its default value),
96 <span><strong class="command">lockmail</strong></span> will terminate with the
97 <code class="literal">EX_TEMPFAIL</code> exit code.
98 </p><p>
99 <span><strong class="command">lockmail</strong></span>
100 runs <em class="replaceable"><code>program</code></em> after obtaining the last file
101 lock, waits until <em class="replaceable"><code>program</code></em> terminates, and
102 releases all locks.
103 <em class="replaceable"><code>program</code></em> must terminate before any of the locks
104 obtained by <span><strong class="command">lockmail</strong></span> expire, and are considered stale.
105 <span><strong class="command">lockmail</strong></span> will then terminate with the same exit code
106 as <em class="replaceable"><code>program</code></em>.</p></div></div><div class="refsect1" lang="en" xml:lang="en"><a id="id325274" shape="rect"> </a><h2>EXIT STATUS</h2><p>
107 <span><strong class="command">lockmail</strong></span> terminates with the same exit status as
108 <em class="replaceable"><code>program</code></em>
109 <span><strong class="command">lockmail</strong></span> terminates with the <code class="literal">EX_TEMPFAIL</code>
110 exit status if it was unable to obtain a lock, or if
111 <em class="replaceable"><code>program</code></em>
112 was killed by a signal.</p></div><div class="refsect1" lang="en" xml:lang="en"><a id="id325310" shape="rect"> </a><h2>SEE ALSO</h2><p>
113 <a href="maildrop.html" target="_top" shape="rect"><span class="citerefentry"><span class="refentrytitle">maildrop</span>(1)</span></a>,
114 <span class="citerefentry"><span class="refentrytitle">sendmail</span>(8)</span>.
115 </p></div></div></body></html>