去插件变量初始化

Question

This is a general question about Go Plugin initialization.

I want to make a certain package in a Go program to a Go Plugin.

The package (say mypackage) has a variable which is initialized with a function call from a certain package in the executable. (E.g. a variable of type interface Logger which is to be initialized by a logger.CreateLogger function.)

In order to make mypackage a plugin, I need to create a main package, embed mypackage inside main, and export an api Init in package main which accepts a function which can get me a logger.

I want to do this so as to reduce the dependencies of my plugin. The mypackage plugin should depend on the interface Logger rather than Logger's implementation.

Now the problem is the initialization, in the case of executable, the initialization could have happened as below:

package mypackage

var dlog = logger.CreateLogger("mypackage")

And the logger can be used in any func init() function of mypackage.

After converting to a plugin, it can't be initialized like this. It has to be initialized at a later point after the main.Init is invoked.

The func init is invoked when the plugin is Opened, so it cannot be used to initialize variables.

Only solution seems to be a creating a function per package, and invoke it from an exported function in main package.

package mypackage

var dlog Logger

func Init(f createLoggerType){
   dlog = f()
   yetanotherpackage.Init(f)
}

package main

func Init(f createLoogerType) {
    mypackage.Init(f)
    anotherpackage.Init(f)        
}

Is there a better way to initialize? I tried checking github.com/facebookgo/inject but couldn't figure out how it can be used in case of plugins.

Answer 1

There is nothing wrong with your solution, but if you want to be able to use it in variable declarations and package init() functions, this is how you can do it. I also think it's easier to use the plugin this way, but this requires a common, shared package between your main app and the plugin.

One option would be to create a "store" package, which could store factory functions or pre-created loggers.

Let's say the Logger interface definition is in mylogger:

package mylogger

type Logger interface { Log(string) }

The store for example:

package store

import "mylogger"

var LoggerFactory func(string) mylogger.Logger

And in your main app initialize it before you load / open the mypackage plugin:

package main

import "store"

func main() {
    store.LoggerFactory = logger.CreateLogger

    // Now proceed to load / open mypackage plugin
}

Then the mypackage plugin may look like this:

package mypackage

import "store"

var dlog = store.LoggerFactory("mypackage")