Stream: general

Topic: Panic in Wasmtime 41.0.3 (runtime/concurrent/component)

Boris (Feb 18 2026 at 06:51):

Hi!

After upgrading Wasmtime to v41 (from v34), I started getting:

thread 'tokio-runtime-worker' panicked at wasmtime-41.0.3/src/runtime/component/concurrent.rs:4384:9
assertion failed: self.threads.is_empty()

Worth noting that I reuse the store for a short period of time (10-20 calls) to reduce the overhead of instantiating components.

What happens:

• We create an async call future (via TypedFunc::call_async).
• The future gets dropped before it’s ever polled.
• TaskId::host_future_dropped called.
• Because the future was never polled, the call parameters were never lowered (already_lowered_parameters == false), or there is no params.
• Waitable::Guest(self.task).delete_from(...) called and it asserts that the task has no guest threads (self.threads.is_empty()), and that assertion fails, causing the panic.

Reproduce: https://github.com/bmartynov/wasmtime-threads-assert (cargo run)

Joel Dice (Feb 18 2026 at 14:14):

Thanks for reporting this! Have you tried running the same test using the main branch of Wasmtime? We've been doing a lot of work related to that code recently.

Boris (Feb 18 2026 at 14:15):

No, will try soon, thanks.

For now I just disabled component-model-async feature

Boris (Feb 18 2026 at 18:31):

It doesn’t reproduce on the main branch(bbfcd588641e7ce0f69b3627d7eed69a57af67cf)

Last updated: Feb 24 2026 at 04:36 UTC