util: generalize code that checks whether PIDs are alive or unwaited for

src/core/service.c

@@ -1368,25 +1368,10 @@ static int service_load_pid_file(Service *s, bool may_warn) {
                 return r;
         }
 
-        if (kill(pid, 0) < 0 && errno != EPERM) {
+        if (!pid_is_alive(pid)) {
                 if (may_warn)
-                        log_info_unit(UNIT(s)->id,
-                                      "PID "PID_FMT" read from file %s does not exist.",
-                                      pid, s->pid_file);
-                return -ESRCH;
-        }
+                        log_info_unit(UNIT(s)->id, "PID "PID_FMT" read from file %s does not exist or is a zombie.", pid, s->pid_file);
 
-        r = get_process_state(pid);
-        if (r < 0) {
-                if (may_warn)
-                        log_info_unit(UNIT(s)->id, "Failed to read /proc/%d/stat: %s",
-                                      pid, strerror(-r));
-                return r;
-        } else if (r == 'Z') {
-                if (may_warn)
-                        log_info_unit(UNIT(s)->id,
-                                      "PID "PID_FMT" read from file %s is a zombie.",
-                                      pid, s->pid_file);
                 return -ESRCH;
         }
 
@@ -1579,7 +1564,7 @@ static int service_coldplug(Unit *u) {
                                 return r;
                 }
 
-                if (pid_valid(s->main_pid) &&
+                if (pid_is_unwaited(s->main_pid) &&
                     ((s->deserialized_state == SERVICE_START && IN_SET(s->type, SERVICE_FORKING, SERVICE_DBUS, SERVICE_ONESHOT, SERVICE_NOTIFY)) ||
                      IN_SET(s->deserialized_state,
                             SERVICE_START, SERVICE_START_POST,
@@ -1592,7 +1577,7 @@ static int service_coldplug(Unit *u) {
                                 return r;
                 }
 
-                if (pid_valid(s->control_pid) &&
+                if (pid_is_unwaited(s->control_pid) &&
                     IN_SET(s->deserialized_state,
                            SERVICE_START_PRE, SERVICE_START, SERVICE_START_POST,
                            SERVICE_RELOAD,
@@ -1824,7 +1809,7 @@ static int main_pid_good(Service *s) {
                 /* If it's an alien child let's check if it is still
                  * alive ... */
                 if (s->main_pid_alien && s->main_pid > 0)
-                        return kill(s->main_pid, 0) >= 0 || errno != ESRCH;
+                        return pid_is_alive(s->main_pid);
 
                 /* .. otherwise assume we'll get a SIGCHLD for it,
                  * which we really should wait for to collect exit

src/core/unit.c

@@ -1833,7 +1833,7 @@ void unit_tidy_watch_pids(Unit *u, pid_t except1, pid_t except2) {
                 if (pid == except1 || pid == except2)
                         continue;
 
-                if (kill(pid, 0) < 0 && errno == ESRCH)
+                if (!pid_is_unwaited(pid))
                         set_remove(u->pids, e);
         }
 }

src/libsystemd/sd-bus/bus-creds.c

@@ -148,7 +148,7 @@ _public_ int sd_bus_creds_new_from_pid(pid_t pid, uint64_t mask, sd_bus_creds **
 
         /* Check if the process existed at all, in case we haven't
          * figured that out already */
-        if (kill(pid, 0) < 0 && errno == ESRCH) {
+        if (!pid_is_alive(pid)) {
                 sd_bus_creds_unref(c);
                 return -ESRCH;
         }

src/shared/util.c

@@ -6045,7 +6045,9 @@ int namespace_enter(int pidns_fd, int mntns_fd, int root_fd) {
         return 0;
 }
 
-bool pid_valid(pid_t pid) {
+bool pid_is_unwaited(pid_t pid) {
+        /* Checks whether a PID is still valid at all, including a zombie */
+
         if (pid <= 0)
                 return false;
 
@@ -6055,6 +6057,21 @@ bool pid_valid(pid_t pid) {
         return errno != ESRCH;
 }
 
+bool pid_is_alive(pid_t pid) {
+        int r;
+
+        /* Checks whether a PID is still valid and not a zombie */
+
+        if (pid <= 0)
+                return false;
+
+        r = get_process_state(pid);
+        if (r == -ENOENT || r == 'Z')
+                return false;
+
+        return true;
+}
+
 int getpeercred(int fd, struct ucred *ucred) {
         socklen_t n = sizeof(struct ucred);
         struct ucred u;

src/shared/util.h

@@ -858,7 +858,8 @@ int container_get_leader(const char *machine, pid_t *pid);
 int namespace_open(pid_t pid, int *pidns_fd, int *mntns_fd, int *root_fd);
 int namespace_enter(int pidns_fd, int mntns_fd, int root_fd);
 
-bool pid_valid(pid_t pid);
+bool pid_is_alive(pid_t pid);
+bool pid_is_unwaited(pid_t pid);
 
 int getpeercred(int fd, struct ucred *ucred);
 int getpeersec(int fd, char **ret);

src/tty-ask-password-agent/tty-ask-password-agent.c

@@ -294,9 +294,7 @@ static int parse_password(const char *filename, char **wall) {
                 }
         }
 
-        if (pid > 0 &&
-            kill(pid, 0) < 0 &&
-            errno == ESRCH) {
+        if (pid > 0 && !pid_is_alive(pid)) {
                 r = 0;
                 goto finish;
         }