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man: document that daemons can close fds they receive via sd_listen_fds() if they like

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Fixes: #18872
This commit is contained in:
Lennart Poettering 2021-10-28 11:05:22 +02:00
parent 1d69754988
commit c896eb7ad6
1 changed files with 18 additions and 8 deletions
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26
man/sd_listen_fds.xml
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@ -45,14 +45,24 @@
<refsect1>
<title>Description</title>
<para><function>sd_listen_fds()</function> may be invoked by a
daemon to check for file descriptors passed by the service manager as
part of the socket-based activation logic. It returns the number
of received file descriptors. If no file descriptors have been
received, zero is returned. The first file descriptor may be found
at file descriptor number 3
(i.e. <constant>SD_LISTEN_FDS_START</constant>), the remaining
descriptors follow at 4, 5, 6, …, if any.</para>
<para><function>sd_listen_fds()</function> may be invoked by a daemon to check for file descriptors
passed by the service manager as part of the socket-based activation logic. It returns the number of
received file descriptors. If no file descriptors have been received, zero is returned. The first file
descriptor may be found at file descriptor number 3 (i.e. <constant>SD_LISTEN_FDS_START</constant>), the
remaining descriptors follow at 4, 5, 6, …, if any.</para>
<para>The file descriptors passed this way may be closed at will by the processes receiving them: it's up
to the processes themselves to close them after use or whether to leave them open until the process exits
(in which case the kernel closes them automatically). Note that the file descriptors received by daemons
are duplicates of the file descriptors the service manager originally allocated and bound and of which it
continously keeps a copy (except if <varname>Accept=yes</varname> is used). This means any socket option
changes and other changes made to the sockets will visible to the service manager too. Most importanly
this means it's generally not a good idea to invoke <citerefentry
project='man-pages'><refentrytitle>shutdown</refentrytitle><manvolnum>2</manvolnum></citerefentry> on
such sockets, since it will shut down communication on the file descriptor the service manager holds for
the same socket, too. Also note that if a daemon is restarted (and its associated sockets are not) it
will receive file descriptors to the very same sockets as the earlier invocations, thus all socket
options applied then will still apply.</para>
<para>If a daemon receives more than one file descriptor, they will be passed in the same order as
configured in the systemd socket unit file (see