core: add new logic for services to store file descriptors in PID 1

With this change it is possible to send file descriptors to PID 1, via
sd_pid_notify_with_fds() which PID 1 will store individually for each
service, and pass via the usual fd passing logic on next invocation.
This is useful for enable daemon reload schemes where daemons serialize
their state to /run, push their fds into PID 1 and terminate, restoring
their state on next start from the data in /run and passed in from PID
1.

The fds are kept by PID 1 as long as no POLLHUP or POLLERR is seen on
them, and the service they belong to are either not dead or failed, or
have a job queued.

1 files changed, 16 insertions, 5 deletions

diff --git a/man/sd_listen_fds.xml b/man/sd_listen_fds.xml
index 6999db9804..4377745634 100644
--- a/man/sd_listen_fds.xml
+++ b/man/sd_listen_fds.xml
@@ -73,7 +73,7 @@
                 <para>If the <parameter>unset_environment</parameter>
                 parameter is non-zero,
                 <function>sd_listen_fds()</function> will unset the
-                <varname>$LISTEN_FDS</varname>/<varname>$LISTEN_PID</varname>
+                <varname>$LISTEN_FDS</varname> and <varname>$LISTEN_PID</varname>
                 environment variables before returning (regardless of
                 whether the function call itself succeeded or
                 not). Further calls to
@@ -83,10 +83,11 @@
 
                 <para>If a daemon receives more than one file
                 descriptor, they will be passed in the same order as
-                configured in the systemd socket definition
-                file. Nonetheless, it is recommended to verify the
-                correct socket types before using them. To simplify
-                this checking, the functions
+                configured in the systemd socket unit file (see
+                <citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+                for details). Nonetheless, it is recommended to verify
+                the correct socket types before using them. To
+                simplify this checking, the functions
                 <citerefentry><refentrytitle>sd_is_fifo</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
                 <citerefentry><refentrytitle>sd_is_socket</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
                 <citerefentry><refentrytitle>sd_is_socket_inet</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
@@ -103,6 +104,16 @@
                 <para>This function call will set the FD_CLOEXEC flag
                 for all passed file descriptors to avoid further
                 inheritance to children of the calling process.</para>
+
+                <para>If multiple socket units activate the same
+                service the order of the file descriptors passed to
+                its main process is undefined. If additional file
+                descriptors have been passed to the service manager
+                using
+                <citerefentry><refentrytitle>sd_pid_notify_with_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>'s
+                <literal>FDSTORE=1</literal> messages, these file
+                descriptors are passed last, in arbitrary order, and
+                with duplicates removed.</para>
         </refsect1>
 
         <refsect1>