一个golang项目笔记 （二） 动态加载库

这个项目需要用到动态链接库技术， 主程序动态加载一些功能模块，这样在扩充功能时，无须修改主程序，只需要新增功能模块动态调用就可以了。 研究了一下golang官方支持的plugin功能，发现有几点不足。 

     1.官方plugin功能本质上是用cgo实现的，编译一个so文件，然后再调用

     2. 只支持linux， 不支持windows

     3.  plugin模块panic时， 主程序也会panic, 无法做到隔离。

   基于上述原因，我开始另外寻找合适的第三方支持。后来发现这样一个开源库，https://github.com/hashicorp/go-plugin , 感觉符合我的需求。它基于net/rpc ，grpc实现，主程序和plugin程序是两个qtj独立进程，可以通过主程序调用plugin进程启动，也可以附加进程的方式。通过本地网络通讯，达到类似动态链接库调用的效果。 

官方示例如下：

plugin:  

package main

import (
	"os"

	"github.com/hashicorp/go-hclog"
	"github.com/hashicorp/go-plugin"
	"github.com/hashicorp/go-plugin/examples/basic/commons"
)

// Here is a real implementation of Greeter
type GreeterHello struct {
	logger hclog.Logger
}

func (g *GreeterHello) Greet() string {
	g.logger.Debug("message from GreeterHello.Greet")
	return "Hello!"
}

// handshakeConfigs are used to just do a basic handshake between
// a plugin and host. If the handshake fails, a user friendly error is shown.
// This prevents users from executing bad plugins or executing a plugin
// directory. It is a UX feature, not a security feature.
var handshakeConfig = plugin.HandshakeConfig{
	ProtocolVersion:  1,
	MagicCookieKey:   "BASIC_PLUGIN",
	MagicCookieValue: "hello",
}

func main() {
	logger := hclog.New(&hclog.LoggerOptions{
		Level:      hclog.Trace,
		Output:     os.Stderr,
		JSONFormat: true,
	})

	greeter := &GreeterHello{
		logger: logger,
	}
	// pluginMap is the map of plugins we can dispense.
	var pluginMap = map[string]plugin.Plugin{
		"greeter": &example.GreeterPlugin{Impl: greeter},
	}

	logger.Debug("message from plugin", "foo", "bar")

	plugin.Serve(&plugin.ServeConfig{
		HandshakeConfig: handshakeConfig,
		Plugins:         pluginMap,
	})
}

　　主程序 （调用方）：

package main

import (
	"fmt"
	"log"
	"os"
	"os/exec"

	hclog "github.com/hashicorp/go-hclog"
	"github.com/hashicorp/go-plugin"
	"github.com/hashicorp/go-plugin/examples/basic/commons"
)

func main() {
	// Create an hclog.Logger
	logger := hclog.New(&hclog.LoggerOptions{
		Name:   "plugin",
		Output: os.Stdout,
		Level:  hclog.Debug,
	})

	// We're a host! Start by launching the plugin process.
	client := plugin.NewClient(&plugin.ClientConfig{
		HandshakeConfig: handshakeConfig,
		Plugins:         pluginMap,
		Cmd:             exec.Command("./plugin/greeter"),
		Logger:          logger,
	})
	defer client.Kill()

	// Connect via RPC
	rpcClient, err := client.Client()
	if err != nil {
		log.Fatal(err)
	}

	// Request the plugin
	raw, err := rpcClient.Dispense("greeter")
	if err != nil {
		log.Fatal(err)
	}

	// We should have a Greeter now! This feels like a normal interface
	// implementation but is in fact over an RPC connection.
	greeter := raw.(example.Greeter)
	fmt.Println(greeter.Greet())
}

// handshakeConfigs are used to just do a basic handshake between
// a plugin and host. If the handshake fails, a user friendly error is shown.
// This prevents users from executing bad plugins or executing a plugin
// directory. It is a UX feature, not a security feature.
var handshakeConfig = plugin.HandshakeConfig{
	ProtocolVersion:  1,
	MagicCookieKey:   "BASIC_PLUGIN",
	MagicCookieValue: "hello",
}

// pluginMap is the map of plugins we can dispense.
var pluginMap = map[string]plugin.Plugin{
	"greeter": &example.GreeterPlugin{},
}