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/basic/cgroup-util.c

@@ -223,7 +223,7 @@ int cg_kill(
 
         _cleanup_set_free_ Set *allocated_set = NULL;
         bool done = false;
-        int r, ret = 0;
+        int r, ret = 0, ret_log_kill = 0;
         pid_t my_pid;
 
         assert(sig >= 0);
@@ -267,7 +267,7 @@ int cg_kill(
                                 continue;
 
                         if (log_kill)
-                                log_kill(pid, sig, userdata);
+                                ret_log_kill = log_kill(pid, sig, userdata);
 
                         /* If we haven't killed this process yet, kill
                          * it */
@@ -278,8 +278,12 @@ int cg_kill(
                                 if (flags & CGROUP_SIGCONT)
                                         (void) kill(pid, SIGCONT);
 
-                                if (ret == 0)
-                                        ret = 1;
+                                if (ret == 0) {
+                                        if (log_kill)
+                                                ret = ret_log_kill;
+                                        else
+                                                ret = 1;
+                                }
                         }
 
                         done = false;

src/basic/cgroup-util.h

@@ -167,7 +167,7 @@ typedef enum CGroupFlags {
         CGROUP_REMOVE      = 1 << 2,
 } CGroupFlags;
 
-typedef void (*cg_kill_log_func_t)(pid_t pid, int sig, void *userdata);
+typedef int (*cg_kill_log_func_t)(pid_t pid, int sig, void *userdata);
 
 int cg_kill(const char *controller, const char *path, int sig, CGroupFlags flags, Set *s, cg_kill_log_func_t kill_log, void *userdata);
 int cg_kill_recursive(const char *controller, const char *path, int sig, CGroupFlags flags, Set *s, cg_kill_log_func_t kill_log, void *userdata);