Add nascent idea to static + dynamic plugin system

This is on the road for v1: enabling plugins for vocabulary extensions
and either supporting them in static or dynamic scenarios.

This new as/ folder is temporary to play around with, it is expected to
fold back into the pub area once I am making more sweeping changes for v1.

as/registry.go

@@ -0,0 +1,105 @@
+package extension
+
+import (
+	"fmt"
+	"sync"
+)
+
+// The global mapping of names to specific vocabularies and the plugins
+// providing their implementations.
+var (
+	pluginRegistry = make(map[string]Plugin)
+	pluginRegistryMu sync.RWMutex
+)
+
+func RegisteredPlugins() []string {
+	pluginRegistryMu.RLock()
+	defer pluginRegistryMu.RUnlock()
+	s := make([]string, 0, len(pluginRegistry))
+	for name, _ := range pluginRegistry {
+		s = append(s, name)
+	}
+	return s
+}
+
+/* BEGIN TEST SPECIFIC STUFF */
+
+func unregisterAllExtensions() {
+	pluginRegistryMu.Lock()
+	defer pluginRegistryMu.Unlock()
+	pluginRegistry = make(map[string]Plugin)
+}
+
+/* BEGIN STATIC SPECIFIC STUFF */
+
+func RegisterExtension(name string, plugin Plugin) {
+	pluginRegistryMu.Lock()
+	defer pluginRegistryMu.Unlock()
+	if plugin == nil {
+		panic("nil plugin passed to RegisterExtension")
+	}
+	if _, ok := pluginRegistry[name]; ok {
+		panic("duplicate RegisterExtension name: " + name)
+	}
+	pluginRegistry[name] = plugin
+}
+
+/* BEGIN DYNAMIC SPECIFIC STUFF */
+
+var (
+	pluginProvider PluginProvider = nil
+	pluginProviderMu sync.RWMutex
+)
+
+type NamedPlugin struct {
+	Name string
+	Plugin Plugin
+}
+
+type PluginProvider interface {
+	Add() <-chan NamedPlugin
+	Remove() <-chan string
+	// nil errors will be sent in response to successful calls to Add or Remove
+	Err() chan<- error
+	Done() <-chan struct{}
+}
+
+func RegisterPluginProvider(p PluginProvider) {
+	pluginProviderMu.Lock()
+	defer pluginProviderMu.Unlock()
+	if pluginProvider != nil {
+		panic("RegisterPluginProvider already called")
+	}
+	pluginProvider = p
+	go func() {
+		addCh := pluginProvider.Add()
+		remCh := pluginProvider.Remove()
+		errCh := pluginProvider.Err()
+		doneCh := pluginProvider.Done()
+		for {
+			select {
+			case namedPlugin := <-addCh:
+				pluginRegistryMu.Lock() // LOCK
+				if _, ok := pluginRegistry[namedPlugin.Name]; ok {
+					errCh <- fmt.Errorf("cannot add: plugin already registered: %s", namedPlugin.Name)
+				} else {
+					pluginRegistry[namedPlugin.Name] = namedPlugin.Plugin
+					errCh <- nil
+				}
+				pluginRegistryMu.Unlock() // UNLOCK
+			case removeName := <-remCh:
+				pluginRegistryMu.Lock() // LOCK
+				if _, ok := pluginRegistry[removeName]; !ok {
+					errCh <- fmt.Errorf("cannot remove: plugin not registered: %s", removeName)
+				} else {
+					delete(pluginRegistry, removeName)
+					errCh <- nil
+				}
+				pluginRegistryMu.Unlock() // UNLOCK
+			case <-doneCh:
+				close(errCh)
+				return
+			}
+		}
+	}()
+}

as/vocabulary.go

@@ -0,0 +1,83 @@
+// Package extension outlines the interfaces required for an ActivityStreams
+// extension.
+package extension
+
+import ()
+
+// TODO: Figure out the new "Resolver" and "Property-Resolver" algorithms for
+// the plugin architecture. Two kinds: for deserialization, and for dispatching.
+
+// Plugin
+type Plugin interface {
+	NewTypeResolver()
+}
+
+// Resolver
+type Resolver interface {
+	Deserialize(map[string]interface{}) error
+}
+
+type PropertyResolver interface {
+}
+
+/*
+EXISTING USAGE:
+
+vocab.IsActivityType()
+vocab.HasTypeTombstone()
+vocab.HasTypeCreate()
+vocab.Serializer
+vocab.Typer
+vocab.ObjectType
+vocab.LinkType
+vocab.ActivityType
+vocab.IntransitiveActivityType
+vocab.CollectionType
+vocab.CollectionPageType
+vocab.OrderedCollectionType
+vocab.OrderedCollectionPageType
+vocab.FollowType
+vocab.TombstoneType
+vocab.Object
+vocab.Create
+vocab.Accept
+vocab.Reject
+vocab.OrderedCollection
+vocab.OrderedCollectionPage
+vocab.Collection
+vocab.CollectionPage
+vocab.Tombstone
+
+streams.Resolver
+streams.Create
+streams.Update
+streams.Delete
+streams.Follow
+streams.Accept
+streams.Reject
+streams.Add
+streams.Remove
+streams.Like
+streams.Undo
+streams.Block
+streams.Announce
+streams.Dislike
+streams.Flag
+streams.Ignore
+streams.Invite
+streams.Join
+streams.Leave
+streams.Listen
+streams.Move
+streams.Offer
+streams.Question
+streams.Read
+streams.TentativeAccept
+streams.TentativeReject
+streams.Travel
+streams.View
+streams.Collection
+streams.OrderedCollection
+streams.CollectionPage
+streams.OrderedCollectionPage
+*/