Testing golang plugins with package init()

cmd-main-main.go

package main

import (
	"fmt"
	"github.com/dnephin/ppp/foo"
	"plugin"
)

func main() {
	fmt.Println("MAIN")
	plug, err := plugin.Open("plugin.so")
	if err != nil {
		panic(err)
	}
	sym, err := plug.Lookup("Plugin")
	if err != nil {
		panic(err)
	}
	foo.Foo()

	sym.(func())()
}

cmd-plugin-main.go

package main

import (
	"fmt"
	"github.com/dnephin/ppp/foo"
)

func main() {
	fmt.Println("PLUGIN MAIN")
}

func Plugin() {
	fmt.Println("PLUGIN")
	foo.Foo()
}

foo-pkg.go

package foo

import "fmt"

func init() {
	fmt.Println("RUNNING")
}

func Foo() {
	fmt.Println("FOO")
}

output

# go build --buildmode=plugin ./cmd/plugin/
# go run ./cmd/main/main.go
RUNNING
MAIN
FOO
PLUGIN
FOO

So in summary, the plugin init() will only be called when any symbol from the plugin is loaded. Whereas a regular package's init will occur before any execution has occured at all. Thanks for the testing, this should have been included in the docs.