message: Ensure color placements are possible
diff: @@ -1491,6 +1491,11 @@ boost::optional<PlacementPlan> Scheduler::TryMakeFallbackPlan(
  case proto::SchedulePass::kColorIoUnique: {
  // Assign all inputs, rotating through the memory units, so that
  // we guarantee they have unique colors.
  + if (placements.size() > units_.size()) {
  + // This can't be scheduled with IO coloring -- there are more
  + // placements than units.
  + return boost::none;
  + }
  std::set<stripe::Affine> used_output_units;
  auto unit_it = units_.begin();
  for (const auto& pkey_placement : placements) {
  