b0fdd27b

Guard offscreen renderer against shared-state hazards

a73x 2026-04-17 15:20

Commit message

Guard offscreen renderer against shared-state hazards

renderToOffscreen now waits on in_flight_fence before touching the
shared instance buffer, preventing the host from overwriting bytes
still being read by an in-flight swapchain draw.

readbackOffscreen now asserts target.format is BGRA8, since the
BGRA→RGBA swizzle assumes that layout.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/renderer.zig

Old		New
@@ -1781,6 +1781,12 @@ pub const Context = struct {
1781	instance_data: []const Instance,	1781	instance_data: []const Instance,
1782	push: PushConstants,	1782	push: PushConstants,
1783	) !void {	1783	) !void {
		1784	// Wait for any in-flight swapchain frame to complete before touching the
		1785	// shared instance buffer. (drawCells and renderToOffscreen share
		1786	// self.instance_memory; without this wait the host would overwrite bytes
		1787	// the GPU is still reading.)
		1788	_ = try self.vkd.waitForFences(self.device, 1, @ptrCast(&self.in_flight_fence), .true, std.math.maxInt(u64));
		1789
1784	// 1. Upload instances (same path drawCells uses)	1790	// 1. Upload instances (same path drawCells uses)
1785	try self.uploadInstances(instance_data);	1791	try self.uploadInstances(instance_data);
1786		1792
@@ -1915,6 +1921,8 @@ pub const Context = struct {
1915	target: *const OffscreenTarget,	1921	target: *const OffscreenTarget,
1916	out_rgba: []u8,	1922	out_rgba: []u8,
1917	) !void {	1923	) !void {
		1924	std.debug.assert(target.format == .b8g8r8a8_unorm); // swizzle below assumes BGRA8
		1925	std.debug.assert(out_rgba.len == target.width * target.height * 4);
1918	if (out_rgba.len < target.readback_size) return error.BufferTooSmall;	1926	if (out_rgba.len < target.readback_size) return error.BufferTooSmall;
1919		1927
1920	const mapped = try self.vkd.mapMemory(	1928	const mapped = try self.vkd.mapMemory(