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release borrows before task.return - #256
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dicej merged 2 commits intoOct 30, 2026
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October 29, 2026 23:16
Add an async world export that takes a `borrow` of an imported resource, and a host test that calls it. The Canonical ABI requires every borrow lent to a task to be released before `task.return`, so this currently traps with "borrow handles still remain at the end of the call".
The Canonical ABI traps if a task returns while any borrow lent to it is still outstanding. `call_task_return` called `task.return` first and released the borrows afterwards, so every async-lifted export that took a `borrow` trapped with "borrow handles still remain at the end of the call" on Wasmtime 43 and later. Release the borrows first. Results can't hold borrows, so lowering them afterwards is unaffected. Fixes bytecodealliance#255
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Thanks, @FrancoisLDaigneault!
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Async-lifted exports that receive a
borrow<T>trap when the Python coroutine returns:async_::call_task_returncallstask.returnfirst and releases the borrows afterwards. The Canonical ABI requires the opposite order:Task.return_indesign/mvp/canonical-abi/definitions.pyrunstrap_if(self.num_borrows > 0)before it resolves the task, andConcurrency.md("Borrows") says that a callee which returns whilenum_borrowsis greater than zero traps. Wasmtime 43 and later enforce this, andmainbuilds against Wasmtime 48. The synchronous path already releases borrows before returning.This change releases the borrows first, then calls
task.return. A result type can't hold aborrow<T>, so lowering the results afterwards is unaffected.The first commit adds the regression test: an async world export that takes a
borrowof an imported resource, with its guest method and a host test. The second commit is the fix.Fork runs of the
Testworkflow:resource_borrow_import_async_exportfails with the trap above (126 passed, 1 failed).Fixes #255