parse-util: introduce pid_is_valid()

Checking for validity of a PID is relatively easy, but let's add a
helper cal for this too, in order to make things more readable and more
similar to uid_is_valid(), gid_is_valid() and friends.

src/basic/parse-util.c

@@ -59,7 +59,7 @@ int parse_pid(const char *s, pid_t* ret_pid) {
         if ((unsigned long) pid != ul)
                 return -ERANGE;
 
-        if (pid <= 0)
+        if (!pid_is_valid(pid))
                 return -ERANGE;
 
         *ret_pid = pid;

src/basic/process-util.h

@@ -118,6 +118,10 @@ static inline bool ioprio_priority_is_valid(int i) {
         return i >= 0 && i < IOPRIO_BE_NR;
 }
 
+static inline bool pid_is_valid(pid_t p) {
+        return p > 0;
+}
+
 int ioprio_parse_priority(const char *s, int *ret);
 
 pid_t getpid_cached(void);

src/login/logind-session.c

@@ -45,6 +45,7 @@
 #include "terminal-util.h"
 #include "user-util.h"
 #include "util.h"
+#include "process-util.h"
 
 #define RELEASE_USEC (20*USEC_PER_SEC)
 
@@ -281,7 +282,7 @@ int session_save(Session *s) {
         if (!s->vtnr)
                 fprintf(f, "POSITION=%u\n", s->position);
 
-        if (s->leader > 0)
+        if (pid_is_valid(s->leader))
                 fprintf(f, "LEADER="PID_FMT"\n", s->leader);
 
         if (audit_session_is_valid(s->audit_id))