Disallow direct call of host functions (#723)

Signed-off-by: Takeshi Yoneda <takeshi@tetrate.io>

examples/basic/add.go

@@ -9,7 +9,6 @@ import (
 	"strconv"
 
 	"github.com/tetratelabs/wazero"
-	"github.com/tetratelabs/wazero/api"
 )
 
 // addWasm was generated by the following:
@@ -34,28 +33,17 @@ func main() {
 		log.Panicln(err)
 	}
 
-	// Add a module to the runtime named "host/math" which exports one function "add", implemented in Go.
-	host, err := r.NewModuleBuilder("host/math").
-		ExportFunction("add", func(v1, v2 uint32) uint32 {
-			return v1 + v2
-		}).Instantiate(ctx, r)
+	// Read two args to add.
+	x, y := readTwoArgs()
+
+	// Call the `add` function in the module and print the results to the console.
+	add := wasm.ExportedFunction("add")
+	results, err := add.Call(ctx, x, y)
 	if err != nil {
 		log.Panicln(err)
 	}
 
-	// Read two args to add.
-	x, y := readTwoArgs()
-
-	// Call the same function in both modules and print the results to the console.
-	for _, mod := range []api.Module{wasm, host} {
-		add := mod.ExportedFunction("add")
-		results, err := add.Call(ctx, x, y)
-		if err != nil {
-			log.Panicln(err)
-		}
-
-		fmt.Printf("%s: %d + %d = %d\n", mod.Name(), x, y, results[0])
-	}
+	fmt.Printf("%s: %d + %d = %d\n", wasm.Name(), x, y, results[0])
 }
 
 func readTwoArgs() (uint64, uint64) {

examples/basic/add_test.go

@@ -13,6 +13,5 @@ import (
 func Test_main(t *testing.T) {
 	stdout, _ := maintester.TestMain(t, main, "add", "7", "9")
 	require.Equal(t, `wasm/math: 7 + 9 = 16
-host/math: 7 + 9 = 16
 `, stdout)
 }