go - Plugin symbol as function return

Question

I'm running into a Go behaviour which I don't understand. My idea is to import a plugin which implements an interface that is out of both packages. If a struct is returned it works fine, but to be sure it implements the interface, I want to return an interface which fails.

Interface definition:

package iface

type IPlugin interface{
   SayHello(string)
   SayGoodby(string)
   WhatsYourName() string
}

The main program looks like this:

package main

import (
    "plugin"
    "plugin_test/iface"
    "errors"
    "fmt"
)

//go:generate go build -buildmode=plugin -o ./pg/test.so ./pg/test.go

func main(){
    path := "pg/test.so"
    plug, err := plugin.Open(path)
    if err != nil {
        panic(err)
    }

    sym, err := plug.Lookup("Greeter")
    if err != nil {
        panic(err)
    }

    var pg iface.IPlugin
    pg, ok := sym.(iface.IPlugin)
    if !ok {
        panic(errors.New("error binding plugin to interface"))
    }

    fmt.Printf("You're now connected to: %s 
", pg.WhatsYourName())
    pg.SayHello("user")
    pg.SayGoodby("user")
}

The plugin (stored in pg/test.go)

package main

import (
    "fmt"
    "plugin_test/iface"
)

type testpl struct {}

func(pl testpl) SayHello(s string){
    fmt.Printf("Plugin says hello to %s 
", s)
}
func(pl testpl) SayGoodby(s string){
    fmt.Printf("Plugin says goodby to %s 
", s)
}
func(pl testpl) WhatsYourName() string{
    return "my name is Test-Plugin"
}

/* This function works */
func getPlugin() testpl{
    return testpl{}
}

/* This function doesn't work */
func getPlugin() iface.IPlugin{
    return testpl{}
}

/* This function also doesn't work */
func getPlugin() interface{}{
    return testpl{}
}

var Greeter = getPlugin()

I tried every getPlugin function on its own.

The function returning testpl prints the expected output:

You're now connected to: my name is Test-Plugin
Plugin says hello to user
Plugin says goodby to user 

The other functions end on sym.(iface.IPlugin)

panic: error binding plugin to interface

goroutine 1 [running]:
main.main()
        /home/../../../main.go:27 +0x343
exit status 2

Can someone explain why this isn't possible? Wouldn't it be easier to create a plugin if it did't let you build your plugin in such a case?

Answer 1

Waitting for answers