backend-headless: Don't try to finish frame if it was cancelled

When multiple outputs are present, a failure in one's repaint will
cause any repaints of all other outputs to be cancelled. If this
happens during the headless "frame" time, the assertion on
repaint_status in weston_output_finish_frame fails.

Do the same as the DRM backend does in its equivalent callback
drm_output_update_complete and don't finish the frame.

Signed-off-by: Ray Smith <rsmith@brightsign.biz>

libweston/backend-headless/headless.c

@@ -140,6 +140,14 @@ finish_frame_handler(void *data)
 	struct timespec ts;
 
 	weston_compositor_read_presentation_clock(output->base.compositor, &ts);
+
+	/* If another output's repaint failed before this repaint finished,
+	 * this repaint will have been cancelled by now. In this case we cannot
+	 * go through finish_frame, because the repaint machinery does not
+	 * expect this. */
+	if (output->base.repaint_status != REPAINT_AWAITING_COMPLETION)
+		return -1;
+
 	weston_output_finish_frame(&output->base, &ts, 0);
 
 	return 1;