Use wiggle "trappable error" to implement wasi-common #5279
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alexcrichton
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Nov 16, 2022
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November 16, 2022 13:28
fun fact! the Send and Recv errors here that just had a `.context` on them were previously not being captured in the downcasting either. They need to be traps, and would have ended up that way by ommission, but you'd never actually know that by reading the code!
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Based on #5276
This PR replaces the use of
anyhow::Errorwith a wiggle-generatedsnapshots::preview_1::types::Errorthroughout the wasi-common (and child) crates.This change was ultimately for better type safety. The old conversion code https://github.com/bytecodealliance/wasmtime/pull/5279/files#diff-40a165c1f24fb0428d93215436730254f0379f22e816a70e0e29a44ba99c8be4L45-L66 from anyhow::Error to the concrete Errno type missed a downcast of
cap_rand::Error, which gets convereted to ananyhow::Errorhere https://github.com/bytecodealliance/wasmtime/blob/91d1d20cb628bf7875199fe5f4a4b232933bdbcb/crates/wasi-common/src/snapshots/preview_1.rs#LL1036-L1036C5 since 2019, and I probably never would have discovered it if I hadn't tried refactoring the error representation to be more typesafe.We're preparing wasi-common for the coming component-based preview 2, which will have (among other improvements) a wider range of errno types. With the increase in complexity, I was more worried about correctness than ergonomics. I also got reports from other crate authors implementing the
WasiFileandWasiDirtrait that it was a burden to ensure they only converted errors toanyhow::Errorwhich the corewasi-commonknew how to downcast into anErrno.