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rust: preserve WASIp3 TLS for no-std task context - #1745
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alexcrichton merged 2 commits intoOct 12, 2026
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The prebuilt task-context helper was compiled with -mcpu=mvp, which lowers its _Thread_local storage to a plain global. Overlapping async exports on WASIp3 then share the same task state and panic in start_task. Keep wasip3_task_set in the MVP archive used by every wasm32 target. Move the versioned context accessors into a separate archive compiled for wasm32-wasip3 and link it only for that target. Regenerate the C, object, and archive files and check the TLS-base import during rebuilds. Validation: - Stock nightly-2026-10-12 and Wasmtime 47.0.2: the unpatched HTTP handler returns 20/20 concurrent 500s; the fix returns 20/20 200s. - Rust 1.99.0 builds binaries and cdylibs for WASIp1, WASIp2, and wasm32-unknown-unknown without introducing TLS-base imports. - WASIp1/WASIp2 binaries and the unknown-target probe run successfully; native unit tests and the generated doctest pass. - SDK 34 and wasm-tools 1.258.0 regenerate artifacts identically on macOS arm64. Reintroducing the MVP flag makes the rebuild guard fail. - cargo fmt --all --check and git diff --check pass.
alexcrichton
approved these changes
Oct 12, 2026
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Overlapping async exports on
wasm32-wasip3can panic whenwit-bindgen/stdis disabled. The existing C fallback declarestask_contextas_Thread_local, but its prebuilt object is compiled for the SDK’s default WASIp1 target with-mcpu=mvp. This produces a shared global, so a second export task sees the suspended task’s state and fails the assertion in start_task.This was found while testing a standard-library-using HTTP handler with wasip3 0.9.0, whose dependency configuration leaves wit-bindgen/std disabled. A second export starting before the first completes sees the first task’s state and fails the assertion in
start_task.This was found while testing concurrent requests to an HTTP handler built with Rust’s standard library and
wasip3 0.9.0, whose dependency configuration leaveswit-bindgen/stddisabled. The resulting panics cause the server to return HTTP 500 responses.