Port to wasmtime

[autodidaddict]

Current version of waPC utilizes wasmer, wasmer-runtime, and wasmer-wasi for its webassembly interpreting and JIT compiling. See if it's possible to port this underlying support to the bytecode alliance's wasmtime library.

NOTES - It looks like function calling is pretty straightforward and should be easily translatable. The one embedding sample I've seen doesn't show how to construct the externs you pass to the module instance, nor does it show you how to manipulate module memory from a guest invocation, so those feel like the biggest unknowns.

Ideally I'd also like to know how well wasmtime performs when making simple invocations. The main things we need to be able to continue to do in the wasmtime implementation:

• Uniquely identify module instances (using a monotonically increasing number right now)
• Support both "pure isolated" and WASI loading

[autodidaddict]

For reference, the following functions will need to be ported to working within wasmtime's constraints:

externs, exported by host, imported by guest (wapc namespace):

• __console_log
• __host_call
• __guest_request
• __host_response
• __host_response_len
• __guest_response
• __guest_error
• __host_error
• __host_error_len

Functions called by host, exported by guest:

• __guest_call

[autodidaddict]

Note to self: One other thing that concerns me is that wasmer gives us a Ctx inside the signature of the calls made out of the guest module into the host (which is 99% of the activity of the waPC contract). It looks like wasmtime's Callable is in a trait, so it should theoretically be possible to maintain a reference to the wasmtime Instance inside some struct so that in each of these functions we can manipulate the module memory.

[autodidaddict]

More research. It looks like there's a way for the externs to have a VMContext parameter, which should let me access the module's memory and functions. Deep down in the bowels of the cmdline binary, there's a signature translator:

https://github.com/bytecodealliance/wasmtime/blob/39e57e3e9ac9c15bef45eb77a2544a7c0b76501a/crates/environ/src/module_environ.rs#L363

Still trying to decipher how, or if, this is applicable.