Call waitpid() after ptrace(PTRACE_ATTACH)

man ptrace says:

    PTRACE_ATTACH
        Attach to the process specified in pid, making it a tracee
        of the calling process. The tracee is sent a SIGSTOP, but
        will not necessarily have stopped by the completion of this
        call; use waitpid(2) to wait for the tracee to stop.

This means there is a race condition: for a short time after
ptrace(PTRACE_ATTACH) we cannot access memory of traced process.

collector/dump.c

@@ -98,11 +98,22 @@ open_pid (int pid)
 {
 	char name[1024];
 	DumpState *s;
+	int status;
 
 	if (ptrace (PTRACE_ATTACH, pid, 0, 0)) {
 		log ("cannot ptrace %d: %s\n", pid, strerror (errno));
 		return NULL;
 	}
+	if (waitpid (pid, &status, 0) < 0) {
+		log ("waitpid(%d) failed: %s\n", pid, strerror (errno));
+		ptrace (PTRACE_DETACH, pid, 0, 0);
+		return NULL;
+	}
+	if (!WIFSTOPPED(status)) {
+		log ("waitpid(%d) returned unexpected status %d\n", pid, status);
+		ptrace (PTRACE_DETACH, pid, 0, 0);
+		return NULL;
+	}
 
 	snprintf (name, 1024, "/proc/%d/mem", pid);
 	s = calloc (sizeof (DumpState), 1);