Support deinstantiation (mod.Close)

[codefromthecrypt]

We need to eventually handle long lived processes, and graceful shutdown. If we had a way to deinstantiate a module, I think we can also handle a WASI quit signal scoped to module instead of the host process (#290)

I think we should make a new type that implements ModuleExports called wazero.InstantiatedModule (we maybe need this for host also, but let's start with normal modules). The only additional function there would be .Close and this would have the following effects.

• set a goroutine visible status on the internal moduleInstance closeCalled
  • this may need to be an array of length one to ensure visibility across goroutines, and 1 means closeCalled
• make shared implementations err if they see this status which will cause existing functions to exit
  • make engine.call implementations check this status prior to invoking new functions and return an error (I think this handles table also as table is only call_indirect and not exposed otherwise)
  • make memory, global implementations check this status before read/write and return an error
• ensure calls begin with a CancelContext added to the callEngine, and invoke that hook so that any host functions doing I/O finish.
  • this could possibly be limited to wrapping the context in wazero.StartWASICommand as maybe non-wasi shouldn't be doing I/O anyway
• remove all the references for this module in Store added by instantiate
  • ensure nice errors when calling an imported function, memory, table or global that was removed

[codefromthecrypt]

This would also be needed for wapc-go (cc @pkedy) because it is otherwise a memory leak. For example, modules are instantiated with instance counters, which I assume would eventually run out of memory depending on the scope and frequency of creation (remember memory is required by WASI and minimum memory is 64KB).

// Instantiate creates a single instance of the module with its own memory.
func (m *Module) Instantiate() (wapc.Instance, error) {
	moduleName := fmt.Sprintf("%d", atomic.AddUint64(&m.instanceCounter, 1))

[codefromthecrypt]

There's a not really documented thing in AssemblyScript that would also needs this for abort handling. If someone can find docs on it please do!

// envAbort is the AssemblyScript abort handler.
// Ex in WebAssembly 1.0 (MVP) Text Format:
//	(import "env" "abort" (func $~lib/builtins/abort (param i32 i32 i32 i32)))
func (t *thing) envAbort(ctx wasm.ModuleContext, messageOffset, fileOffset, line, col uint32) {
	t.module.Close()
}

Note: AssemblyScript maybe maps this function to WASI sigabrt ...

[mathetake]

yeah we also tweak Engine interface so that we can unmap the MMapped memory regions

[mathetake]

so the preliminary work #294 has just landed, so the next is to make store goroutine-safe