Rendering: SPIR-V reflection incorrectly determined writable state

[stuartcarnie]

Identified from testing #101696

Resolves a bug in RenderingDeviceDriver::_reflect_spirv:

godot/servers/rendering/rendering_device_driver.cpp

Lines 163 to 167 in 1af1edf

	if (may_be_writable) {
	uniform.writable = !(binding.type_description->decoration_flags & SPV_REFLECT_DECORATION_NON_WRITABLE) && !(binding.block.decoration_flags & SPV_REFLECT_DECORATION_NON_WRITABLE);
	} else {
	uniform.writable = false;
	}

When reflecting an image type, the binding.block struct is invalid, as that only applies to struct types. Here is the dumped value of the binding variable from the debugger, abbreviated:

{
  spirv_id = 65
  name = 0x000000011c28fd80 "current_image"
  binding = 0
  input_attachment_index = 0
  set = 0
  descriptor_type = SPV_REFLECT_DESCRIPTOR_TYPE_STORAGE_IMAGE
  resource_type = SPV_REFLECT_RESOURCE_FLAG_UAV
  image = (dim = SpvDim2D, depth = 0, arrayed = 0, ms = 0, sampled = 2, image_format = SpvImageFormatR32f)
  block = {
    spirv_id = 0
    name = 0x0000000000000000
    offset = 0
    absolute_offset = 0
    size = 0
    padded_size = 0
    decoration_flags = 0
    flags = 0
    member_count = 0
    members = nullptr
    type_description = nullptr
    word_offset = (offset = 0)
  }
  count = 1
  accessed = 1
  uav_counter_id = 4294967295
  uav_counter_binding = nullptr
  byte_address_buffer_offset_count = 0
  byte_address_buffer_offsets = 0x0000000000000000
  type_description = 0x000000011c295470
  word_offset = (binding = 175, set = 171)
  decoration_flags = 128
  user_type = SPV_REFLECT_USER_TYPE_INVALID
}

Note that the the entire binding.block struct is 0, which means this will evaluate to true, as the second part of the expression does not have the SPV_REFLECT_DECORATION_NON_WRITABLE bit set, as it is invalid and always 0:

uniform.writable = !(binding.type_description->decoration_flags & SPV_REFLECT_DECORATION_NON_WRITABLE) && !(binding.block.decoration_flags & SPV_REFLECT_DECORATION_NON_WRITABLE);

Metal has its own implementation to reflect the SPIR-V, which distinguishes the types:

godot/drivers/metal/rendering_device_driver_metal.mm

Lines 1787 to 1796 in d89cc01

	if (a_type.basetype == BT::Struct) {
	Bitset flags = compiler.get_buffer_block_flags(res.id);
	uniform.writable = !compiler.has_decoration(res.id, spv::DecorationNonWritable) && !flags.get(spv::DecorationNonWritable);
	} else if (a_type.basetype == BT::Image) {
	if (a_type.image.access == spv::AccessQualifierMax) {
	uniform.writable = !compiler.has_decoration(res.id, spv::DecorationNonWritable);
	} else {
	uniform.writable = a_type.image.access != spv::AccessQualifierReadOnly;
	}
	}

Note

SPIR-V cross throws an exception if you try to fetch the block metadata for an image.