truss(1): detach more carefully

(cherry picked from commit 12f747e)

usr.bin/truss/setup.c

@@ -223,11 +223,24 @@ restore_proc(int signo __unused)
 static void
 detach_proc(pid_t pid)
 {
+	int sig, status;
 
-	/* stop the child so that we can detach */
+	/*
+	 * Stop the child so that we can detach.  Filter out possible
+	 * lingering SIGTRAP events buffered in the threads.
+	 */
 	kill(pid, SIGSTOP);
-	if (waitpid(pid, NULL, 0) < 0)
-		err(1, "Unexpected stop in waitpid");
+	for (;;) {
+		if (waitpid(pid, &status, 0) < 0)
+			err(1, "Unexpected error in waitpid");
+		sig = WIFSTOPPED(status) ? WSTOPSIG(status) : 0;
+		if (sig == SIGSTOP)
+			break;
+		if (sig == SIGTRAP)
+			sig = 0;
+		if (ptrace(PT_CONTINUE, pid, (caddr_t)1, sig) < 0)
+			err(1, "Can not continue for detach");
+	}
 
 	if (ptrace(PT_DETACH, pid, (caddr_t)1, 0) < 0)
 		err(1, "Can not detach the process");