wasm: functions in FunctionIndexSpace include wrong contents

[hajimehoshi]

1. Build a hello-world wasm:

package main

func main() {
        println("Hello, World!")
}

GOOS=js GOARCH=wasm go build -o test.wasm helloworld.go 

2. Run the below code:

package main

import (
        "fmt"
        "os"

        "github.com/go-interpreter/wagon/wasm"
)

func main() {
        f, err := os.Open("test.wasm")
        if err != nil {
                panic(err)
        }
        defer f.Close()

        mod, err := wasm.ReadModule(f, nil)
        if err != nil {
                panic(err)
        }

        fn := mod.FunctionIndexSpace[0]
        fmt.Printf("name: %q\n", fn.Name) // This is empty. That's fine since this is an imported function 'debug'.                                                                                                                           
        fmt.Printf("len(code): %d\n", len(fn.Body.Code)) // This is not empty. That's unexpected. This content is for the first function as 'go.buildid'.                                                                                     
}

It looks like the function's index and its instructions don't match. Some of the first functions should be imported functions, but include instructions for other functions. I think the function bodies are shifted.

[hajimehoshi]

wagon/wasm/index.go

Lines 76 to 80 in cbbdf08

	fn := Function{
	Sig: &m.Types.Entries[typeIndex],
	Body: &m.Code.Bodies[codeIndex],
	Name: names[uint32(codeIndex+numImports)], // Add the name string if we have it
	}

The names are shifted with numImports, which is correct, while Sig and Body are not shifted, maybe? I'm not confident.

[zhangzhiqiangcs]

Hello, I read in detail, found that the problem comes with numImports, when you don't provide a imported module but the wasm file need it, the numImports is not equal to len(FunctionIndexSpace).

wagon/wasm/index.go

Lines 84 to 85 in dc6758c

	// Add the functions from the wasm itself to the function list
	numImports := len(m.FunctionIndexSpace)

So I had a pr #193
I am not sure if we can do it in this way.
Thanks.