Prevents direct use of host functions via ExportedFunction (#2259)

Signed-off-by: Takeshi Yoneda <t.y.mathetake@gmail.com>

builder.go

@@ -179,6 +179,9 @@ type HostFunctionBuilder interface {
 //     are deferred until Compile.
 //   - Functions are indexed in order of calls to NewFunctionBuilder as
 //     insertion ordering is needed by ABI such as Emscripten (invoke_*).
+//   - The semantics of host functions assumes the existence of an "importing module" because, for example, the host function needs access to
+//     the memory of the importing module. Therefore, direct use of ExportedFunction is forbidden for host modules.
+//     Practically speaking, it is usually meaningless to directly call a host function from Go code as it is already somewhere in Go code.
 type HostModuleBuilder interface {
 	// Note: until golang/go#5860, we can't use example tests to embed code in interface godocs.
 
@@ -341,12 +344,24 @@ func (b *hostModuleBuilder) Compile(ctx context.Context) (CompiledModule, error)
 	return c, nil
 }
 
+// hostModuleInstance is a wrapper around api.Module that prevents calling ExportedFunction.
+type hostModuleInstance struct{ api.Module }
+
+// ExportedFunction implements api.Module ExportedFunction.
+func (h hostModuleInstance) ExportedFunction(name string) api.Function {
+	panic("calling ExportedFunction is forbidden on host modules. See the note on ExportedFunction interface")
+}
+
 // Instantiate implements HostModuleBuilder.Instantiate
 func (b *hostModuleBuilder) Instantiate(ctx context.Context) (api.Module, error) {
 	if compiled, err := b.Compile(ctx); err != nil {
 		return nil, err
 	} else {
 		compiled.(*compiledModule).closeWithModule = true
-		return b.r.InstantiateModule(ctx, compiled, NewModuleConfig())
+		m, err := b.r.InstantiateModule(ctx, compiled, NewModuleConfig())
+		if err != nil {
+			return nil, err
+		}
+		return hostModuleInstance{m}, nil
 	}
 }