fix(webgpu): reading a mapped buffer throws on a detached ArrayBuffer - #4
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Cloning a buffer duplicated its cached mapped ArrayBuffer rather than sharing it, so unmapping through one clone left every other clone holding an ArrayBuffer that Javascript had already detached. The next read built a Uint8Array view over that buffer and threw "Cannot perform Construct on a detached ArrayBuffer". The same applies to the mapped range recorded by map_async, which was invisible to any other clone. Upstream carries this same change from 29.0.1 onward; 27.0.4 is still affected.
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Connections
None. Found while debugging the SpatialEngine WASM viewer after fsl_libs moved from wgpu 26 to 27.
Description
Reading a mapped buffer on the WebGPU backend can throw "Cannot perform Construct on a detached ArrayBuffer".
WebBuffer caches the array buffer from getMappedRange so several Rust-side reads share one JS call. Since 27, DispatchBuffer holds WebBuffer by value rather than behind an Arc, so cloning a buffer copies that cache instead of sharing it. Unmapping through one clone clears only its own copy while Javascript detaches the array buffer for every clone, and the next read builds a view over the detached buffer. The range recorded by map_async diverges the same way.
An Rc restores shared ownership, matching upstream from 29.0.1 onward. The patch-27.0.4 branch is still affected and wants the same change.
Testing
Reproduced in the SpatialEngine WASM viewer, where volumesight's streaming readback clones a buffer into its recycle pool before unmapping it. The error appears on every page load with 27.0.1, and bisecting showed it absent on 26.0.1. Check, clippy and fmt for wgpu on wasm32-unknown-unknown all pass.
Not run against wgpu's own test suite and not exercised on a native backend, though only the WebGPU path changes. A reviewer with a WebGPU mapping test should confirm nothing relies on clones having independent caches.
Squash or Rebase?
Single commit, ready to rebase.