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, 2 insertions, 0 deletions

diff --git a/src/core/dbus-execute.h b/src/core/dbus-execute.h
index c53834140e..5926bdb4b1 100644
--- a/src/core/dbus-execute.h
+++ b/src/core/dbus-execute.h
@@ -28,6 +28,8 @@ int bus_property_get_exec_output(sd_bus *bus, const char *path, const char *inte
 int bus_property_get_exec_command(sd_bus *bus, const char *path, const char *interface, const char *property, sd_bus_message *reply, void *userdata, sd_bus_error *ret_error);
 int bus_property_get_exec_command_list(sd_bus *bus, const char *path, const char *interface, const char *property, sd_bus_message *reply, void *userdata, sd_bus_error *ret_error);
 int bus_property_get_exec_ex_command_list(sd_bus *bus, const char *path, const char *interface, const char *property, sd_bus_message *reply, void *userdata, sd_bus_error *ret_error);
+int bus_property_get_exec_preserve_mode(sd_bus *bus, const char *path, const char *interface, const char *property, sd_bus_message *reply, void *userdata, sd_bus_error *ret_error);
 
 int bus_exec_context_set_transient_property(Unit *u, ExecContext *c, const char *name, sd_bus_message *message, UnitWriteFlags flags, sd_bus_error *error);
 int bus_set_transient_exec_command(Unit *u, const char *name, ExecCommand **exec_command, sd_bus_message *message, UnitWriteFlags flags, sd_bus_error *error);
+int bus_set_transient_exec_preserve_mode(Unit *u, const char *name, ExecPreserveMode *p, sd_bus_message *message, UnitWriteFlags flags, sd_bus_error *error);