process-util: introduce pid_is_my_child() helper

No functional changes.

src/basic/process-util.c

@@ -934,6 +934,20 @@ int getenv_for_pid(pid_t pid, const char *field, char **ret) {
         return 0;
 }
 
+int pid_is_my_child(pid_t pid) {
+        pid_t ppid;
+        int r;
+
+        if (pid <= 1)
+                return false;
+
+        r = get_process_ppid(pid, &ppid);
+        if (r < 0)
+                return r;
+
+        return ppid == getpid_cached();
+}
+
 bool pid_is_unwaited(pid_t pid) {
         /* Checks whether a PID is still valid at all, including a zombie */
 

src/basic/process-util.h

@@ -69,6 +69,7 @@ int getenv_for_pid(pid_t pid, const char *field, char **_value);
 
 bool pid_is_alive(pid_t pid);
 bool pid_is_unwaited(pid_t pid);
+int pid_is_my_child(pid_t pid);
 int pid_from_same_root_fs(pid_t pid);
 
 bool is_main_thread(void);

src/core/cgroup.c

@@ -2269,7 +2269,7 @@ void unit_prune_cgroup(Unit *u) {
 
 int unit_search_main_pid(Unit *u, pid_t *ret) {
         _cleanup_fclose_ FILE *f = NULL;
-        pid_t pid = 0, npid, mypid;
+        pid_t pid = 0, npid;
         int r;
 
         assert(u);
@@ -2282,15 +2282,12 @@ int unit_search_main_pid(Unit *u, pid_t *ret) {
         if (r < 0)
                 return r;
 
-        mypid = getpid_cached();
         while (cg_read_pid(f, &npid) > 0)  {
-                pid_t ppid;
 
                 if (npid == pid)
                         continue;
 
-                /* Ignore processes that aren't our kids */
-                if (get_process_ppid(npid, &ppid) >= 0 && ppid != mypid)
+                if (pid_is_my_child(npid) == 0)
                         continue;
 
                 if (pid != 0)

src/core/service.c

@@ -143,8 +143,6 @@ static void service_unwatch_pid_file(Service *s) {
 }
 
 static int service_set_main_pid(Service *s, pid_t pid) {
-        pid_t ppid;
-
         assert(s);
 
         if (pid <= 1)
@@ -163,12 +161,10 @@ static int service_set_main_pid(Service *s, pid_t pid) {
 
         s->main_pid = pid;
         s->main_pid_known = true;
+        s->main_pid_alien = pid_is_my_child(pid) == 0;
 
-        if (get_process_ppid(pid, &ppid) >= 0 && ppid != getpid_cached()) {
+        if (s->main_pid_alien)
                 log_unit_warning(UNIT(s), "Supervising process "PID_FMT" which is not our child. We'll most likely not notice when it exits.", pid);
-                s->main_pid_alien = true;
-        } else
-                s->main_pid_alien = false;
 
         return 0;
 }