service: allow services of Type=oneshot that specify no ExecStart= commands

This is useful for services that simply want to run something on
shutdown, but not at bootup. They should only set ExecStop= but leave
ExecStart= unset.

2 files changed, 56 insertions, 27 deletions

diff --git a/man/systemd.service.xml b/man/systemd.service.xml
index 5c4bd6569..e584a1f00 100644
--- a/man/systemd.service.xml
+++ b/man/systemd.service.xml
@@ -139,9 +139,10 @@
 
                                 <para>If set to
                                 <option>simple</option> (the default
-                                value if neither
+                                if neither
                                 <varname>Type=</varname> nor
-                                <varname>BusName=</varname> are
+                                <varname>BusName=</varname>, but
+                                <varname>ExecStart=</varname> are
                                 specified), it is expected that the
                                 process configured with
                                 <varname>ExecStart=</varname> is the
@@ -177,13 +178,17 @@
                                 exits.</para>
 
                                 <para>Behavior of
-                                <option>oneshot</option> is similar
-                                to <option>simple</option>; however,
-                                it is expected that the process has to
+                                <option>oneshot</option> is similar to
+                                <option>simple</option>; however, it
+                                is expected that the process has to
                                 exit before systemd starts follow-up
                                 units. <varname>RemainAfterExit=</varname>
                                 is particularly useful for this type
-                                of service.</para>
+                                of service. This is the implied
+                                default if neither
+                                <varname>Type=</varname> or
+                                <varname>ExecStart=</varname> are
+                                specified.</para>
 
                                 <para>Behavior of
                                 <option>dbus</option> is similar to
@@ -313,22 +318,27 @@
 
                                 <para>When <varname>Type</varname> is
                                 not <option>oneshot</option>, only one
-                                command may be given. When
+                                command may and must be given. When
                                 <varname>Type=oneshot</varname> is
-                                used, more than one command may be
-                                specified. Multiple command lines may
-                                be concatenated in a single directive
-                                by separating them with semicolons
-                                (these semicolons must be passed as
-                                separate words). Alternatively, this
-                                directive may be specified more than
-                                once with the same effect.
-                                Lone semicolons may be escaped as
+                                used, none or more than one command
+                                may be specified. Multiple command
+                                lines may be concatenated in a single
+                                directive by separating them with
+                                semicolons (these semicolons must be
+                                passed as separate
+                                words). Alternatively, this directive
+                                may be specified more than once with
+                                the same effect.  Lone semicolons may
+                                be escaped as
                                 <literal>\;</literal>. If the empty
                                 string is assigned to this option, the
                                 list of commands to start is reset,
                                 prior assignments of this option will
-                                have no effect.</para>
+                                have no effect. If no
+                                <varname>ExecStart=</varname> is
+                                specified, then the service must have
+                                <varname>RemainAfterExit=yes</varname>
+                                set.</para>
 
                                 <para>Each command line is split on
                                 whitespace, with the first item being
diff --git a/src/core/service.c b/src/core/service.c
index 7d6ea73e0..1b864c4c8 100644
--- a/src/core/service.c
+++ b/src/core/service.c
@@ -313,14 +313,23 @@ static int service_verify(Service *s) {
         if (UNIT(s)->load_state != UNIT_LOADED)
                 return 0;
 
-        if (!s->exec_command[SERVICE_EXEC_START]) {
-                log_error_unit(UNIT(s)->id, "%s lacks ExecStart setting. Refusing.", UNIT(s)->id);
+        if (!s->exec_command[SERVICE_EXEC_START] && !s->exec_command[SERVICE_EXEC_STOP]) {
+                log_error_unit(UNIT(s)->id, "%s lacks both ExecStart= and ExecStop= setting. Refusing.", UNIT(s)->id);
                 return -EINVAL;
         }
 
-        if (s->type != SERVICE_ONESHOT &&
-            s->exec_command[SERVICE_EXEC_START]->command_next) {
-                log_error_unit(UNIT(s)->id, "%s has more than one ExecStart setting, which is only allowed for Type=oneshot services. Refusing.", UNIT(s)->id);
+        if (s->type != SERVICE_ONESHOT && !s->exec_command[SERVICE_EXEC_START]) {
+                log_error_unit(UNIT(s)->id, "%s has no ExecStart= setting, which is only allowed for Type=oneshot services. Refusing.", UNIT(s)->id);
+                return -EINVAL;
+        }
+
+        if (!s->remain_after_exit && !s->exec_command[SERVICE_EXEC_START]) {
+                log_error_unit(UNIT(s)->id, "%s has no ExecStart= setting, which is only allowed for RemainAfterExit=yes services. Refusing.", UNIT(s)->id);
+                return -EINVAL;
+        }
+
+        if (s->type != SERVICE_ONESHOT && s->exec_command[SERVICE_EXEC_START]->command_next) {
+                log_error_unit(UNIT(s)->id, "%s has more than one ExecStart= setting, which is only allowed for Type=oneshot services. Refusing.", UNIT(s)->id);
                 return -EINVAL;
         }
 
@@ -410,8 +419,15 @@ static int service_load(Unit *u) {
                 if (r < 0)
                         return r;
 
-                if (s->type == _SERVICE_TYPE_INVALID)
-                        s->type = s->bus_name ? SERVICE_DBUS : SERVICE_SIMPLE;
+                if (s->type == _SERVICE_TYPE_INVALID) {
+                        /* Figure out a type automatically */
+                        if (s->bus_name)
+                                s->type = SERVICE_DBUS;
+                        else if (s->exec_command[SERVICE_EXEC_START])
+                                s->type = SERVICE_SIMPLE;
+                        else
+                                s->type = SERVICE_ONESHOT;
+                }
 
                 /* Oneshot services have disabled start timeout by default */
                 if (s->type == SERVICE_ONESHOT && !s->start_timeout_defined)
@@ -1309,9 +1325,6 @@ static void service_enter_start(Service *s) {
 
         assert(s);
 
-        assert(s->exec_command[SERVICE_EXEC_START]);
-        assert(!s->exec_command[SERVICE_EXEC_START]->command_next || s->type == SERVICE_ONESHOT);
-
         service_unwatch_control_pid(s);
         service_unwatch_main_pid(s);
 
@@ -1332,6 +1345,12 @@ static void service_enter_start(Service *s) {
                 c = s->main_command = s->exec_command[SERVICE_EXEC_START];
         }
 
+        if (!c) {
+                assert(s->type == SERVICE_ONESHOT);
+                service_enter_start_post(s);
+                return;
+        }
+
         r = service_spawn(s,
                           c,
                           IN_SET(s->type, SERVICE_FORKING, SERVICE_DBUS, SERVICE_NOTIFY, SERVICE_ONESHOT) ? s->timeout_start_usec : 0,