tests: Ensure that overflow test always overflows

While the default Unix socket buffer size on Linux is relatively
small, on some computers the default size may be configured to
be huge, making the overflow test never actually overflow the
Wayland display socket.

The changed code now explicitly sets the display socket send buffer
size to be small enough to guarantee an overflow.

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>

tests/display-test.c

@@ -1430,13 +1430,19 @@ send_overflow_client(void *data)
 	int i, err = 0;
 	int *pipes = data;
 	char tmp = '\0';
+	int sock, optval = 16384;
+
+	/* Limit the send buffer size for the display socket to guarantee
+	 * that the test will cause an overflow. */
+	sock = wl_display_get_fd(c->wl_display);
+	assert(setsockopt(sock, SOL_SOCKET, SO_SNDBUF, &optval, sizeof(optval)) == 0);
 
 	/* Request to break out of 'display_run' in the main process */
 	assert(stop_display(c, 1) >= 0);
 
-	/* On Linux, the Unix socket default buffer size is <=256KB, and
+	/* On Linux, the actual socket data + metadata space is twice `optval`;
-	 * each noop request requires 8 bytes; the buffer should thus
+	 * since each noop request requires 8 bytes, the buffer should overflow
-	 * overflow within about 32K /unhandled/ iterations */
+	 * within <=4096 iterations. */
 	for (i = 0; i < 1000000; i++) {
 		noop_request(c);
 		err = wl_display_get_error(c->wl_display);