service: add ability to pin fd store

Oftentimes it is useful to allow the per-service fd store to survive
longer than for a restart. This is useful in various scenarios:

1. An fd to some security relevant object needs to be stashed somewhere,
   that should not be cleaned automatically, because the security
   enforcement would be dropped then.

2. A user namespace fd should be allocated on first invocation and be
   kept around until the user logs out (i.e. systemd --user ends), á la
   #16328 (This does not implement what #16318 asks for, but should
   solve the use-case discussed there.)

3. There's interest in allow a concept of "userspace reboots" where the
   kernel stays running, and userspace is swapped out (i.e. all services
   exit, and the rootfs transitioned into a new version of it) while
   keeping some select resources pinned, very similar to how we
   implement a switch root. Thus it is useful to allow services to exit,
   while leaving their fds around till the very end.

This is exposed through a new FileDescriptorStorePreserve= setting that
is closely modelled after RuntimeDirectoryPreserve= (in fact it reused
the same internal type), since we want similar behaviour in the end, and
quite often they probably want to be used together.

1 files changed, 19 insertions, 2 deletions

diff --git a/man/systemd.service.xml b/man/systemd.service.xml
index 1b116b8372..7de1350a59 100644
--- a/man/systemd.service.xml
+++ b/man/systemd.service.xml
@@ -1066,7 +1066,7 @@
         <literal>FDSTORE=1</literal> messages. This is useful for implementing services that can restart
         after an explicit request or a crash without losing state. Any open sockets and other file
         descriptors which should not be closed during the restart may be stored this way. Application state
-        can either be serialized to a file in <filename>/run/</filename>, or better, stored in a
+        can either be serialized to a file in <varname>RuntimeDirectory=</varname>, or stored in a
         <citerefentry><refentrytitle>memfd_create</refentrytitle><manvolnum>2</manvolnum></citerefentry>
         memory file descriptor. Defaults to 0, i.e. no file descriptors may be stored in the service
         manager. All file descriptors passed to the service manager from a specific service are passed back
@@ -1075,7 +1075,8 @@
         details about the precise protocol used and the order in which the file descriptors are passed). Any
         file descriptors passed to the service manager are automatically closed when
         <constant>POLLHUP</constant> or <constant>POLLERR</constant> is seen on them, or when the service is
-        fully stopped and no job is queued or being executed for it. If this option is used,
+        fully stopped and no job is queued or being executed for it (the latter can be tweaked with
+        <varname>FileDescriptorStorePreserve=</varname>, see below). If this option is used,
         <varname>NotifyAccess=</varname> (see above) should be set to open access to the notification socket
         provided by systemd. If <varname>NotifyAccess=</varname> is not set, it will be implicitly set to
         <option>main</option>.</para>
@@ -1098,6 +1099,22 @@
       </varlistentry>
 
       <varlistentry>
+        <term><varname>FileDescriptorStorePreserve=</varname></term>
+        <listitem><para>Takes one of <constant>no</constant>, <constant>yes</constant>,
+        <constant>restart</constant> and controls when to release the service's file descriptor store
+        (i.e. when to close the contained file descriptors, if any). If set to <constant>no</constant> the
+        file descriptor store is automatically released when the service is stopped; if
+        <constant>restart</constant> (the default) it is kept around as long as the unit is neither inactive
+        nor failed, or a job is queued for the service, or the service is expected to be restarted. If
+        <constant>yes</constant> the file descriptor store is kept around until the unit is removed from
+        memory (i.e. is not referenced anymore and inactive). The latter is useful to keep entries in the
+        file descriptor store pinned until the service manage exits.</para>
+
+        <para>Use <command>systemctl clean --what=fdstore …</command> to release the file descriptor store
+        explicitly.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
         <term><varname>USBFunctionDescriptors=</varname></term>
         <listitem><para>Configure the location of a file containing
         <ulink