`export_falls_back_to_the_cpu_when_gpu_work_is_lost` asserted on
`lightcraft_gpu::last_fallback()`, one value for the whole process that
every other engine test's GPU render overwrites between the export and
the assertion (out of device memory under parallel load, once "a GPU
render panicked"). It also switched the GPU off for the whole process
through the injected fault and cleared everyone's state with
`reset_failures()`.
A render now reports its own fallback: `lightcraft_gpu::try_render`
returns `Result<Rendered, Fallback>` (`NotTried` when the GPU was not
used, `Failed(reason)` when it was tried and the CPU must render), and
`render` is a thin `.ok()` over it. `media::develop_reporting` passes
the reason on as `RenderResult::gpu_fallback`, an export carries it as
`Exported::gpu_fallback`, and each file in an `app.export` batch result
has `gpuFallback` when its render fell back. `last_fallback()` stays as
the process-wide diagnostic for `ui.inspect` → `perf.gpuFallback`.
A simulated failure (`inject_fault`) is thread-local, and so are its
consequences now: a fatal injected fault stops GPU use on the injecting
thread only (`available()` false, `unavailable_reason()` set there);
`reset_failures()` clears that and nothing else, so neither reaches the
other tests in the process. Real failures still stop the whole process.
The engine test asserts on its own export's reason and checks that
another thread still has the GPU; `crates/gpu/tests/fallback.rs` uses
`try_render` and checks the same; a new batch test checks `gpuFallback`
lands on the file whose render fell back. Docs: gpu-pipeline.md,
control-protocol.md (app.export result), parity.md.
A GPU render no longer hands back an image it cannot vouch for:
- set the wgpu device-lost callback (wgpu never reports a lost device as an
error, so a driver reset went unnoticed); readback waits with a timeout and
checks the poll result and map callback instead of blocking forever
- each render runs in OutOfMemory/Validation/Internal error scopes; errors
fail that render (OOM and limit failures for that render only, device
errors for the process) and it is redone on the CPU
- every buffer is checked against the device limit when created (no
validation error, no process-wide disable); LIGHTCRAFT_GPU_LIMITS=webgpu|<MiB>
creates the device with lower limits to reproduce small adapters
- the output buffer is cleared and the per-pixel kernel writes alpha 255:
any unwritten pixel (work silently dropped, the issue's all-black export)
fails the render; an all-black result is re-rendered on the CPU
- renders submit per stage and every 16 M invocations, the per-pixel stage
runs in ~4 MP bands: no long submission for a slow iGPU's watchdog to reset
- ui.inspect perf.gpuReason / perf.gpuFallback, app.gpu reason/lastFallback,
System Info and Settings say why the GPU is off (e.g. llvmpipe skipped) and
why the last render fell back
Tests: crates/gpu/tests/fallback.rs (limit, mid-render limit, validation
error, dropped work, black photo), engine export falls back to the CPU image,
banded 3000x2000 GPU/CPU equivalence.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Jason Baldwin
Brandon Thomas