pidref: add pidref_verify() helper

This new helper can be used after reading process info from procfs, to
verify that the data that was just read actually matches the pidfd, and
does not belong to some new process that just reused the numeric PID of
the process we originally pinned.

src/basic/pidref.c

@@ -240,6 +240,26 @@ int pidref_sigqueue(PidRef *pidref, int sig, int value) {
         return -ESRCH;
 }
 
+int pidref_verify(PidRef *pidref) {
+        int r;
+
+        /* This is a helper that is supposed to be called after reading information from procfs via a
+         * PidRef. It ensures that the PID we track still matches the PIDFD we pin. If this value differs
+         * after a procfs read, we might have read the data from a recycled PID. */
+
+        if (!pidref_is_set(pidref))
+                return -ESRCH;
+
+        if (pidref->fd < 0)
+                return 0; /* If we don't have a pidfd we cannot validate it, hence we assume it's all OK → return 0 */
+
+        r = pidfd_verify_pid(pidref->fd, pidref->pid);
+        if (r < 0)
+                return r;
+
+        return 1; /* We have a pidfd and it still points to the PID we have, hence all is *really* OK → return 1 */
+}
+
 static void pidref_hash_func(const PidRef *pidref, struct siphash *state) {
         siphash24_compress(&pidref->pid, sizeof(pidref->pid), state);
 }