WAMR Exception Handling

[csegarragonz]

Faasm relies on exceptions being propagated from native function calls (i.e. host interface functions called from WASM) to the runtime code. For example, WASM code may call __faasm_migrate_point which, in turn, will throw a faabric::util::FunctionMigratedException. A very simplfied call stack would look something like the following:

<__faasm_migrate_point> -> throws exception
<functionDef2> -> WASM function
<functionDef1> -> WASM function
<WASM runtime call to invoke WASM> -> PROBLEM: may be written in C
<Faasm's runtime call to WASM runtime> -> try/catch exception

The previous works fine when the WASM runtime is implemented in C++ (e..g WAVM), as C++ exceptions are correctly propagated. Unfortunately, WAMR is written in C, so exceptions are not correctly propagated, and they are impossible to catch.

To work around this issue, we make use of setjmp/longjmp, inspired from here. This may feel like a big hack, but I don't think there's another way around it. This also means that we need to add a layer of indirection when throwing exceptions from native function calls (and potentially a number of dynamic_casts), but exception throwing and catching is not really performance critical (for now) so we can bear the cost.

In the process I homogeneise the interface to execute WASM functions with WAMR (i.e. making call by name and call by pointer use the same low-level functions).