Update plugin load and snapshot service

Allow plugins to be mapped and returned by their ID.
Add skip plugin to allow plugins to decide whether they should
be loaded.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

plugin/context.go

@@ -9,7 +9,7 @@ import (
 	"github.com/containerd/containerd/log"
 )
 
-func NewContext(ctx context.Context, plugins map[PluginType][]interface{}, root, id string) *InitContext {
+func NewContext(ctx context.Context, plugins map[PluginType]map[string]interface{}, root, id string) *InitContext {
 	return &InitContext{
 		plugins: plugins,
 		Root:    filepath.Join(root, id),
@@ -23,18 +23,17 @@ type InitContext struct {
 	Config  interface{}
 	Emitter *events.Emitter
 
-	plugins map[PluginType][]interface{}
+	plugins map[PluginType]map[string]interface{}
 }
 
 func (i *InitContext) Get(t PluginType) (interface{}, error) {
-	p := i.plugins[t]
-	if len(p) == 0 {
-		return nil, fmt.Errorf("no plugins registered for %s", t)
+	for _, v := range i.plugins[t] {
+		return v, nil
 	}
-	return p[0], nil
+	return nil, fmt.Errorf("no plugins registered for %s", t)
 }
 
-func (i *InitContext) GetAll(t PluginType) ([]interface{}, error) {
+func (i *InitContext) GetAll(t PluginType) (map[string]interface{}, error) {
 	p, ok := i.plugins[t]
 	if !ok {
 		return nil, fmt.Errorf("no plugins registered for %s", t)

plugin/plugin.go

@@ -1,18 +1,31 @@
 package plugin
 
 import (
-	"errors"
 	"fmt"
 	"sync"
 
+	"github.com/pkg/errors"
 	"google.golang.org/grpc"
 )
 
 var (
 	ErrNoPluginType = errors.New("plugin: no type")
 	ErrNoPluginID   = errors.New("plugin: no id")
+
+	// SkipPlugin is used when a plugin is not initialized and should not be loaded,
+	// this allows the plugin loader differentiate between a plugin which is configured
+	// not to load and one that fails to load.
+	SkipPlugin = errors.New("skip plugin")
 )
 
+// IsSkipPlugin returns true if the error is skipping the plugin
+func IsSkipPlugin(err error) bool {
+	if errors.Cause(err) == SkipPlugin {
+		return true
+	}
+	return false
+}
+
 type PluginType string
 
 const (