service: make killmode=cgroup|mixed, SendSIGKILL=no services singletons

KillMode=mixed and control group are used to indicate that all
process should be killed off. SendSIGKILL is used for services
that require a clean shutdown. These are typically database
service where a SigKilled process would result in a lengthy
recovery and who's shutdown or startup time is quite variable
(so Timeout settings aren't of use).

Here we take these two factors and refuse to start a service if
there are existing processes within a control group. Databases,
while generally having some protection against multiple instances
running, lets not stress the rigor of these. Also ExecStartPre
parts of the service aren't as rigoriously written to protect
against against multiple use.

closes #8630

src/core/unit.c

@@ -4428,7 +4428,7 @@ int unit_make_transient(Unit *u) {
         return 0;
 }
 
-static void log_kill(pid_t pid, int sig, void *userdata) {
+static int log_kill(pid_t pid, int sig, void *userdata) {
         _cleanup_free_ char *comm = NULL;
 
         (void) get_process_comm(pid, &comm);
@@ -4436,13 +4436,15 @@ static void log_kill(pid_t pid, int sig, void *userdata) {
         /* Don't log about processes marked with brackets, under the assumption that these are temporary processes
            only, like for example systemd's own PAM stub process. */
         if (comm && comm[0] == '(')
-                return;
+                return 0;
 
         log_unit_notice(userdata,
                         "Killing process " PID_FMT " (%s) with signal SIG%s.",
                         pid,
                         strna(comm),
                         signal_to_string(sig));
+
+        return 1;
 }
 
 static int operation_to_signal(KillContext *c, KillOperation k) {
@@ -5394,29 +5396,31 @@ int unit_prepare_exec(Unit *u) {
         return 0;
 }
 
-static void log_leftover(pid_t pid, int sig, void *userdata) {
+static int log_leftover(pid_t pid, int sig, void *userdata) {
         _cleanup_free_ char *comm = NULL;
 
         (void) get_process_comm(pid, &comm);
 
         if (comm && comm[0] == '(') /* Most likely our own helper process (PAM?), ignore */
-                return;
+                return 0;
 
         log_unit_warning(userdata,
                          "Found left-over process " PID_FMT " (%s) in control group while starting unit. Ignoring.\n"
                          "This usually indicates unclean termination of a previous run, or service implementation deficiencies.",
                          pid, strna(comm));
+
+        return 1;
 }
 
-void unit_warn_leftover_processes(Unit *u) {
+int unit_warn_leftover_processes(Unit *u) {
         assert(u);
 
         (void) unit_pick_cgroup_path(u);
 
         if (!u->cgroup_path)
-                return;
+                return 0;
 
-        (void) cg_kill_recursive(SYSTEMD_CGROUP_CONTROLLER, u->cgroup_path, 0, 0, NULL, log_leftover, u);
+        return cg_kill_recursive(SYSTEMD_CGROUP_CONTROLLER, u->cgroup_path, 0, 0, NULL, log_leftover, u);
 }
 
 bool unit_needs_console(Unit *u) {