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fix(compositor): copy the decoder surface before sampling it - #895
EtienneLescot merged 2 commits into
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ChangesNV12 shader-resource views
Priority: ⬆️ High Estimated code review effort: 2 (Simple) | ~12 minutes Change: Bug fix Merge Risk: ⚪ Minimal · up to The private-copy path addresses decoder-surface sampling failures, and cache validation prevents incompatible storage reuse. No actionable merge-blocking risk remains in the supplied evidence; Windows runtime tests were not independently executed. Security Architecture ReviewSecurity architecture risk: 🔵 Low · up to The change improves separation between decoded video and displayed frames. No introduced security issue was established in the inspected playback path, but complete caller and resource-lifetime coverage remains unavailable. Retained concerns Security review detailsSecurity Blast Radius
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Review comments at @crates/compositor/src/compositor_windows.rs:
- Around line 853-892: Update the srv_cache entries used by the key-based lookup
to retain the source ID3D11Texture2D alongside the destination and
shader-resource views. In the cached lookup, reuse those resources only when the
retained source matches src; otherwise create and cache a fresh destination and
views for the current source.
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crates/compositor/src/compositor_windows.rs
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`nv12_srvs` built its Y/UV views straight on the ffmpeg D3D11VA output — a texture array slice, addressed through `FirstArraySlice`. Two documented rules say that is not a supported way to read decoded video. `D3D11_BIND_FLAG` is explicit about the first: "you cannot use texture arrays that are created with this flag in calls to `ID3D11Device::CreateShaderResourceView`". Our pool is exactly such an array — `get_hw_format` asks for `initial_pool_size = 32` with `D3D11_BIND_DECODER | D3D11_BIND_SHADER_RESOURCE`. Drivers are free to let the view creation succeed anyway, and most do, which is why this held up everywhere else. The second rule is the one that actually bit. The decoded surface stays the decoder's reference frame, and between the video engine and the 3D pipeline "there is no automatic hazard tracking" — so the shader may sample a surface the decoder is concurrently rewriting. ffmpeg's `ID3D11VideoContext` is the very same object as our immediate context (it comes out of a `QueryInterface` on it), and `SetMultithreadProtected(TRUE)` only makes an individual call atomic, never a sequence. Nothing ordered the decode against our draws. On a Snapdragon X Elite (Adreno X1-85) the editor preview drew opaque black for most of playback. Measured on 1.13, same build, same recording, the two paths selected by an environment variable: 768 black frames out of 792 sampling the decoder surface, 2 out of 545 through the copy — and those two are the transparent frames before the first compose, not black ones. The black was opaque and total, so not "one frame late" but no frame at all. What hid the cause for so long: exporting the same project never produced a single black frame (28 342 verified), because the export drains the GPU every frame through the encoder, which lets the decode finish before the read. Everything that slowed the live loop — one more readback, a lock, a sleep — cut the black proportionally without ever removing it. Four mitigations along those lines were tried and discarded: a `Flush` and then a `D3D11_QUERY_EVENT` wait around the readback, holding `ID3D11Multithread::Enter` across compose and readback (90 % black down to 57 %), and capping the loop period (69 % at 0 ms, 38 % at 33 ms). They were all treating the symptom. The fix copies the slice into a private texture — `ArraySize = 1`, `BIND_SHADER_RESOURCE` only — with `CopySubresourceRegion` on the immediate context, which IS ordered against the draws that follow, and samples that copy. One allocation per decoder texture, then one GPU→GPU copy per frame and per source; nothing goes back to system memory. `clear_srv_cache` keeps its meaning and now also drops the copies, so a new decoder texture landing on a recycled address cannot inherit one sized for the old one. Export output is unchanged: the same project re-exported frame for frame identical, 326 frames, mean luminance 168.0, min 84.3. This also supersedes the diagnosis in `feat/compositor-force-cpu-backend`, which read the same symptom as a broken D3D11 hardware path on that adapter and added `OPENSCREEN_FORCE_CPU_BACKEND` to work around it. The compose path was never at fault — the export proves it on the same GPU — so that override is no longer the answer here. Not covered: no regression test. The failure only appears where the video engine and the 3D pipeline actually race, so it does not reproduce on the CI adapters, and a test that passes everywhere would prove nothing. The change is exercised by the existing compose and export tests; the evidence above is the A/B on the affected hardware. Refs: - https://learn.microsoft.com/windows/win32/api/d3d11/ne-d3d11-d3d11_bind_flag - https://learn.microsoft.com/windows/win32/direct3d11/overviews-direct3d-11-devices-intro#threading-considerations - https://learn.microsoft.com/windows/win32/api/d3d11_4/nn-d3d11_4-id3d11multithread - https://learn.microsoft.com/windows/win32/api/dxgiformat/ne-dxgiformat-dxgi_format
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The copy cache is keyed by the source texture's address and no longer holds a reference to it. A decoder replaced without clear_srv_cache (mid-stream resolution change, CpuFrames::ensure_tex) can hand a new texture the address of an old one; a larger source then met a smaller destination and CopySubresourceRegion dropped the copy silently, freezing the picture. Check the destination's size and format against the source on every hit and reallocate on mismatch. Adds a regression test that re-keys a small entry under a larger source's address.
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Pushed one commit on top (e11e9ab): Why: the copy cache is keyed by the source texture's address and, unlike the old SRVs, no longer keeps the source alive. If a decoder is replaced without What changed:
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2 participants
The symptom
On a Snapdragon X Elite (Adreno X1-85), the editor preview draws opaque black for most of playback. Pausing brings the picture back for a few seconds; playing turns it black again. Exporting the very same project is flawless.
The cause
Compositor::nv12_srvscreated its Y/UV shader resource views directly on the ffmpeg D3D11VA output surface — a texture array slice, addressed throughFirstArraySlice. Two documented rules say that is not a supported way to read decoded video.1. You may not create an SRV on a
BIND_DECODERtexture array.D3D11_BIND_FLAGis explicit:Our pool is exactly such an array —
get_hw_formatrequestsinitial_pool_size = 32withD3D11_BIND_DECODER | D3D11_BIND_SHADER_RESOURCE. Drivers may let the view creation succeed regardless, and most do, which is why this held up on every other adapter.2. Nothing orders the decode against our draws. This is the rule that actually bit. The decoded surface remains the decoder's reference frame, and between the video engine and the 3D pipeline there is no automatic hazard tracking — the shader can sample a surface the decoder is concurrently rewriting. ffmpeg's
ID3D11VideoContextis the same object as our immediate context (it comes out of aQueryInterfaceon it, see Threading considerations), andID3D11Multithreadspells out the limit of what we had:SetMultithreadProtected(TRUE)makes an individual call atomic. It never made our compose sequence atomic against ffmpeg's decode submissions.The fix
Copy the slice into a private texture —
ArraySize = 1,BIND_SHADER_RESOURCEonly — withCopySubresourceRegionon the immediate context, which is ordered against the draws that follow, and sample that copy. One allocation per decoder texture, then one GPU→GPU copy per frame and per source; nothing goes back to system memory.clear_srv_cachekeeps its meaning and now also drops the copies, so a new decoder texture landing on a recycled address cannot inherit one sized for the old one.The NV12 two-plane view split is unchanged and still follows the
DXGI_FORMATremarks: luma asR8_UNORM, chroma asR8G8_UNORM.Evidence
Same build, same recording, the two paths selected by an environment variable during bring-up:
The two remaining ones are the transparent frames before the first compose (alpha 0), not black ones — a separate, known preview-warmup defect.
Export is unchanged. The same project re-exported frame for frame identical: 326 frames, mean luminance 168.0, min 84.3,
blackdetectsilent.Why this took a while to find, and what was ruled out
Exporting the same project never produced a single black frame — 28 342 frames verified — because the export drains the GPU every frame through the encoder, which lets the decode finish before the read. Export and preview share
compose_frame, so compose and the hardware rasteriser were never at fault.Everything that slowed the live loop cut the black proportionally without ever removing it. Four mitigations along those lines were tried and discarded, all of them treating the symptom:
Flush()before the readbackMapD3D11_QUERY_EVENTwait before theMapID3D11Multithread::Enteracross compose + readbackDecoupling the loop the other way — skipping the readback while the previous frame sits unconsumed — made it worse (96/102), which is what finally pointed at the real mechanism: the readback was never the load, it was an accidental synchronisation that rescued the frames after it.
Ruled out by measurement: the renderer and canvas (main-process packet bytes and canvas pixels match frame for frame), the geometry plan (every
plan_framefield byte-identical for black and bright frames), a resize loop, slice selection, readback ordering, and segmentation.Supersedes
feat/compositor-force-cpu-backendread the same symptom as a broken D3D11 hardware path on that adapter and addedOPENSCREEN_FORCE_CPU_BACKENDto sidestep it. That diagnosis does not survive the export evidence — the compose path is sound on this GPU — so the override is no longer the answer here.Not covered
No regression test. The failure only appears where the video engine and the 3D pipeline actually race, so it does not reproduce on the CI adapters, and a test that passes everywhere would prove nothing. The change is exercised by the existing compose and export tests; the A/B above is the evidence. Verified on Windows on ARM only — macOS and Linux have their own
nv12_srvsand are untouched by this change.Summary by CodeRabbit