Engine deadlocks when calling from host function to WASM.

[Berrysoft]

The Engine locks when calling a WASM function. If the WASM function calls a host function, and the host function calls another WASM function, it deadlocks.

         lock                                deadlock
host ------------> WASM ------------> host ------------> WASM

Unfortunately, my use case heavily depends on this calling routine. The host function calls a WASM function to make an ABI-compatible allocation. The same code passes on Wasmtime. It seems not a simple problem to solve, but I think it is better to point it out.

[Robbepop]

I am aware of this problem. I was not aware that Wasmtime allows this behavior though. However, it is actually simpler with a JIT compiler compared to an interpreter since there is much more state in an interpreter that needs to be carried over to new function executions.
As you pointed out this won't be simple to implement on the wasmi side. Can you tell me more about your use case in particular? I thought it was not very important so far which is why we never focused on fixing this issue.

It is not impossible to fix. We could probably solve this by allowing multiple executors per engine and create new executors on the fly and reuse old ones to reduce frequent heap allocations. Currently we only have a single executor per engine.

[Berrysoft]

Well, if it is difficult to solve, it's OK. I'll try to remove these allocations to make sure no deadlocks :)

[Robbepop]

@Berrysoft btw thank you a lot for writing all those issues! That really help us improving wasmi. 😊

[Robbepop]

Closed in favor of #572.