Humaid AlQassimi

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Reliable Plugins in Go

Jul 4, 2020 · 6 min read

When working on our university group project on a smart home system, I had an idea to support a marketplace, something similar to Firefox’s extensions. Go supports loading plugins, which are shared libraries which are dynamically loaded. At first this was fine, but we quickly faced issues with reliability, such as:

  • When a plugin panics, the host process crashes.
  • When a plugin is loaded, it cannot be unloaded/reloaded.
  • Plugins may interfere or conflict with libraries.
  • Plugins share the same memory space of the host process.

Also there is no formal interface which these plugins implement, we only rely on “looking up” if a function is implemented or not. These are dealbreakers when building a system which aims to be reliable.

HashiCorp’s Go plugin system seems to be the best solution to our current problem, it is used throughout HashiCorp’s products. It was a bit difficult for me to understand, as there is barely any documentation about usage, only example code is provided which was confusing. I hope I am able to explain how HashiCorp’s go plugin system works, which I’ll refer to from now on as go-plugin.

Overview

We’ll be implementing a go-plugin without gRPC, so we don’t have to deal with Protobuf yet. Let’s split this plugin architecture into three well-defined parts, so we know what we are talking about.

  • Host: This is the program which loads and manages the plugin.
  • Interface: This is a simple library which is imported by both the Host and Plugin, containing the interface which must be implemented by the Plugin.
  • Plugin: This is a program which implements the Interface, and is loaded by the Host.

In this guide, we are building a plugin system for a Host called Iglu. Both the Host and Plugin must have the same handshake config, so they know that they can communicate. So we can write the following in both the Host and Plugin:

var handshakeConfig = plugin.HandshakeConfig{
	ProtocolVersion:  1,
	MagicCookieKey:   "IGLU_PLUGIN",
	MagicCookieValue: "MzlK0OGpIRs",
}

Note: This is not a security measure, this is not used in encryption or cryptography in the communication between the Host and Plugin.

Interface implementation

The Interface is a the library which is used by both the host and plugin, so they can agree on the functions which the plugin implements.

Let’s implement a simple interface, with a function called GetManifest() which returns a struct, defined in this interface.

// Iglu is the interface that we're exposing as a plugin.
type Iglu interface {
	GetManifest() PluginManifest
}


// PluginManifest is used to describe the plugin's id, name, author, version, etc.
type PluginManifest struct {
	Id, Name, Author, Version string
}

And we can implement the basic go-plugin interface using this boilerplate:

// This is the implementation of plugin.Plugin.
type IgluPlugin struct {
	Impl Iglu
}

func (p *IgluPlugin) Server(*plugin.MuxBroker) (interface{}, error) {
	return &IgluRPCServer{Impl: p.Impl}, nil
}

func (IgluPlugin) Client(b *plugin.MuxBroker, c *rpc.Client) (interface{}, error) {
	return &IgluRPC{client: c}, nil
}

We will first implement the basic structures which are used before implementing the GetManifest() function.

// IgluRPC is what the Host uses to communicate to the Plugin.
type IgluRPC struct {
	client *rpc.Client
}

// IgluRPCServer is the implementation of the interface, running on the Plugin.
type IgluRPCServer struct {
	Impl Iglu
}

We have to now implement this GetManifest() function in both the Plugin and the Host side.

The first is the Plugin implementation, which simply passes the response of the plugin back to the Host:

func (s *IgluRPCServer) GetManifest(args interface{}, resp *GetManifestReply) error {
	resp.Manifest = s.Impl.GetManifest()
	return nil
}

And the second is the RPC implementation for the Host, which allows us to easily call these functions:

func (i *IgluRPC) GetManifest() PluginManifest {
	rep := GetManifestReply{}
	err := i.client.Call("Plugin.GetManifest", new(interface{}), &rep)
	if err != nil {
		panic(err)
	}
	return rep.Manifest
}

So for every function we want our plugin to implement, we need to make these two implementations. When creating a plugin API with a large set of functions, it may be a good idea to use Go templates to generate these, which is Mattermost’s solution.

Host implementation

In the Host, we imported the interface library as sdk. We can first start by implementing a function which simply loads the plugin and prints the manifest, given a plugin filename.

import (
	hclog "github.com/hashicorp/go-hclog"
	"github.com/hashicorp/go-plugin"
	// import your interface implementation
)

// pluginMap is a map of supported plugin interfaces.
var pluginMap = map[string]plugin.Plugin{
	"iglu_plugin": &sdk.IgluPlugin{},
}

func loadPlugin(f string) {
	// 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,
		Managed:         true,
		Cmd:             exec.Command(fmt.Sprintf("./plugins/%s", f)),
		Logger:          logger,
	})

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

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

	plugin := raw.(sdk.Iglu)
	fmt.Println(plugin.GetManifest())
}

Now we can run the plugin’s functions directly. To manage the state of these plugins, we’ll create a struct:

type PluginState int

const (
	Stopped = iota
	Running
	Crashed
)

// IgluPlugin represents a loaded Iglu plugin.
type IgluPlugin struct {
	Plugin   sdk.Iglu             // This is the interface we'll be calling
	client   *plugin.Client       // This is the go-plugin interface
	State    PluginState          // This is our enum to keep track of the state
	Filename string               // In case we need to reload the plugin.
	Manifest sdk.PluginManifest   // We'll cache the manifest, so we can get it if it crashes.
}

So now we have a way to represent our plugin’s state, and cache the manifest (or other static fields). Let’s amend our loadPlugin() function:

func loadPlugin(f string) IgluPlugin {
	[ ... ]
	
	return IgluPlugin {
		Plugin: plugin,
		client: client,
		Filename: f,
		Manifest: plugin.GetManifest(),
	}
}

Plugin implementation

Now the plugin implementation should contain a main() function and the handshake configuration. We’ll call this implementation AmazingPlugin.

var handshakeConfig = plugin.HandshakeConfig{
	ProtocolVersion:  1,
	MagicCookieKey:   "IGLU_PLUGIN",
	MagicCookieValue: "MzlK0OGpIRs",
}


// AmazingPlugin is an implementation of IgluPlugin
type AmazingPlugin struct {
	logger hclog.Logger
}

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

	amazing := &AmazingPlugin{
		logger: logger,
	}

	// pluginMap is the map of plugins we can dispense.
	var pluginMap = map[string]plugin.Plugin{
		"iglu_plugin": &sdk.IgluPlugin{Impl: amazing},
	}

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

Note: We should import and implement the same version of the Interface, otherwise you might get errors if the interfaces don’t match.

Now we can implement our GetManifest() method, remember that sdk is the Interface library in this case.

func (g *AmazingPlugin) GetManifest() sdk.PluginManifest {
	return sdk.PluginManifest{
		Id:      "amazing",
		Name:    "Amazing Plugin",
		Author:  "Someone Amazing",
		Version: "v0.1.0",
	}
}

Just remember that we must implement all the interface functions, otherwise you won’t be able to compile the plugin.

Loading plugins in a directory

We can define a folder, let’s say ./plugins, where all the plugins would be located, and load the plugins from there.

var loadedPlugins = make(map[string]IgluPlugin)

func LoadPlugins() {
	files, err := ioutil.ReadDir("./plugins")
	if err == nil {
		for _, f := range files {
			if !f.IsDir() {
				pl := loadPlugin(f.Name())
				loadedPlugins[pl.Manifest.Id] = pl
			}
		}
	}
}

Handling crashes gracefully

What if a plugin crashes? We could check the state of the plugin, if go-plugin states that the client exited while it is supposed to be running, we can mark that as crashed.

func markCrashedPlugins() {
	for i, plugin := range loadedPlugins {
		if plugin.client.Exited() && plugin.State == Running {
			loadedPlugins[i].State = Crashed
		}
	}
}

You can see further implementations as references, such as my iglü project, Mattermost’s implementation, or go-plugin’s example implementations.

This is my fifth post in the #100DaysToOffload challenge.


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