Fix type conversion panic for host-provided funcs

[ericflo]

It was panicking because it's of type goFunction, not *goFunction as the line is checking for.

[sbinet]

thanks for the PR.

could you add a test/example showing the panic you are trying to fix with this PR?

thanks again.

[sbinet]

hum...
this kind of type assertion, using the "comma-ok" variant, shouldn't panic.
so I must admit I am not completely clear on what panic this should cure.

that said, it seems indeed that we should probably type-assert for goFunction:

$> git grep goFunction
exec/func.go:38:type goFunction struct {
exec/func.go:43:func (fn goFunction) call(vm *VM, index int64) {
exec/native_compile.go:94:              if _, isGoFunc := vm.funcs[i].(*goFunction); isGoFunc {
exec/native_compile.go:181:             if _, isGoFunc := vm.funcs[i].(*goFunction); isGoFunc {
exec/vm.go:130:                 vm.funcs[i] = goFunction{

(as we only ever fill vm.funcs with goFunction{...} values...)

[ericflo]

Sorry, my fault for underspecifying - it passes this check, so in the code below it (line 98) it panics trying to access it as a compiledFunction

[twitchyliquid64]

Hmmm, I dont understand how we would hit this panic. Consider that vm.funcs is of type []function, and function is an interface type. The goFunction type only implements the necessary interface methods on itself, it does not implement it on its pointer receiver.

As such, I was thinking only goFunction{} could implement function, not a *goFunction ? Now I'm not sure ...

Either way, the current logic is negated, considering we only want to run the loop logic on the func if it is of type compiledFunction.

What do you think of a fix like this?

fn, isCompiledFn := vm.funcs[i].(compiledFunction)
if !isCompiledFn {
  continue
}

[ericflo]

I'll look into adding a new test, but one way to test it would be to run the existing host-provided func test with AOT enabled

[sbinet]

ping?

[fabian-z]

I can confirm that using host provided functions with the AOT backend enabled causes a panic for me that is fixed by this change (I tested with rebasing on top of master). If needed, I can share a minimal reproducer.

The panic happens here:

wagon/exec/native_compile.go

Line 98 in dc6758c

fn := vm.funcs[i].(compiledFunction)

Stack trace:

panic: interface conversion: exec.function is exec.goFunction, not exec.compiledFunction

goroutine 1 [running]:
github.com/go-interpreter/wagon/exec.(*VM).tryNativeCompile(0xc0007fc000, 0xc00006f770, 0x0)
	/home/fabian/go/pkg/mod/github.com/go-interpreter/wagon@v0.6.1-0.20200313144839-dc6758ca7262/exec/native_compile.go:98 +0x85d
github.com/go-interpreter/wagon/exec.NewVM(0xc00013c000, 0xc0007e7f30, 0x1, 0x1, 0x0, 0x0, 0x0)
	/home/fabian/go/pkg/mod/github.com/go-interpreter/wagon@v0.6.1-0.20200313144839-dc6758ca7262/exec/vm.go:175 +0x4cd
main.main()
	/home/fabian/playground/wasm-runtime/main.go:285 +0x1a8