2 files changed, 289 insertions, 0 deletions
diff --git a/bridge/discord/transmitter/transmitter.go b/bridge/discord/transmitter/transmitter.go
new file mode 100644
index 00000000..41ed055b
--- /dev/null
+++ b/bridge/discord/transmitter/transmitter.go
@@ -0,0 +1,257 @@
+// Package transmitter provides functionality for transmitting
+// arbitrary webhook messages to Discord.
+//
+// The package provides the following functionality:
+// - Creating new webhooks, whenever necessary
+// - Loading webhooks that we have previously created
+// - Sending new messages
+// - Editing messages, via message ID
+// - Deleting messages, via message ID
+//
+// The package has been designed for matterbridge, but with other
+// Go bots in mind. The public API should be matterbridge-agnostic.
+package transmitter
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/matterbridge/discordgo"
+	log "github.com/sirupsen/logrus"
+)
+
+// A Transmitter represents a message manager for a single guild.
+type Transmitter struct {
+	session    *discordgo.Session
+	guild      string
+	title      string
+	autoCreate bool
+
+	// channelWebhooks maps from a channel ID to a webhook instance
+	channelWebhooks map[string]*discordgo.Webhook
+
+	mutex sync.RWMutex
+
+	Log *log.Entry
+}
+
+// ErrWebhookNotFound is returned when a valid webhook for this channel/message combination does not exist
+var ErrWebhookNotFound = errors.New("webhook for this channel and message does not exist")
+
+// ErrPermissionDenied is returned if the bot does not have permission to manage webhooks.
+//
+// It's important to note that:
+// - a bot can have both a guild-wide permission and a channel-specific permission to manage webhooks
+// - even if a bot has permission to manage the guild's webhooks, there could be channel specific overrides
+var ErrPermissionDenied = errors.New("missing 'Manage Webhooks' permission")
+
+// New returns a new Transmitter given a Discord session, guild ID, and title.
+func New(session *discordgo.Session, guild string, title string, autoCreate bool) *Transmitter {
+	return &Transmitter{
+		session:    session,
+		guild:      guild,
+		title:      title,
+		autoCreate: autoCreate,
+
+		channelWebhooks: make(map[string]*discordgo.Webhook),
+
+		Log: log.NewEntry(nil),
+	}
+}
+
+// Send transmits a message to the given channel with the provided webhook data.
+//
+// Note that this function will wait until Discord responds with an answer.
+func (t *Transmitter) Send(channelID string, params *discordgo.WebhookParams) (*discordgo.Message, error) {
+	wh, err := t.getOrCreateWebhook(channelID)
+	if err != nil {
+		return nil, err
+	}
+
+	msg, err := t.session.WebhookExecute(wh.ID, wh.Token, true, params)
+	if err != nil {
+		return nil, fmt.Errorf("execute failed: %w", err)
+	}
+
+	return msg, nil
+}
+
+// Edit will edit a message in a channel, if possible.
+func (t *Transmitter) Edit(channelID string, messageID string, params *discordgo.WebhookParams) error {
+	wh := t.getWebhook(channelID)
+
+	if wh == nil {
+		return ErrWebhookNotFound
+	}
+
+	uri := discordgo.EndpointWebhookToken(wh.ID, wh.Token) + "/messages/" + messageID
+	_, err := t.session.RequestWithBucketID("PATCH", uri, params, discordgo.EndpointWebhookToken("", ""))
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// HasWebhook checks whether the transmitter is using a particular webhook.
+func (t *Transmitter) HasWebhook(id string) bool {
+	t.mutex.RLock()
+	defer t.mutex.RUnlock()
+
+	for _, wh := range t.channelWebhooks {
+		if wh.ID == id {
+			return true
+		}
+	}
+
+	return false
+}
+
+// AddWebhook allows you to register a channel's webhook with the transmitter.
+func (t *Transmitter) AddWebhook(channelID string, webhook *discordgo.Webhook) (replaced bool) {
+	t.Log.Debugf("Manually added webhook %#v to channel %#v", webhook.ID, channelID)
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+
+	_, replaced := t.channelWebhooks[channelID]
+	t.channelWebhooks[channelID] = webhook
+	return replaced
+}
+
+// RefreshGuildWebhooks loads "relevant" webhooks into the transmitter, with careful permission handling.
+//
+// Notes:
+// - A webhook is "relevant" if it was created by this bot -- the ApplicationID should match the bot's ID.
+// - The term "having permission" means having the "Manage Webhooks" permission. See ErrPermissionDenied for more information.
+// - This function is additive and will not unload previously loaded webhooks.
+// - A nil channelIDs slice is treated the same as an empty one.
+//
+// If the bot has guild-wide permission:
+// 1. it will load any "relevant" webhooks from the entire guild
+// 2. the given slice is ignored
+//
+// If the bot does not have guild-wide permission:
+// 1. it will load any "relevant" webhooks in each channel
+// 2. a single error will be returned if any error occurs (incl. if there is no permission for any of these channels)
+//
+// If any channel has more than one "relevant" webhook, it will randomly pick one.
+func (t *Transmitter) RefreshGuildWebhooks(channelIDs []string) error {
+	t.Log.Debugln("Refreshing guild webhooks")
+
+	botID, err := getDiscordUserID(t.session)
+	if err != nil {
+		return fmt.Errorf("could not get current user: %w", err)
+	}
+
+	// Get all existing webhooks
+	hooks, err := t.session.GuildWebhooks(t.guild)
+	if err != nil {
+		switch {
+		case isDiscordPermissionError(err):
+			// We fallback on manually fetching hooks from individual channels
+			// if we don't have the "Manage Webhooks" permission globally.
+			// We can only do this if we were provided channelIDs, though.
+			if len(channelIDs) == 0 {
+				return ErrPermissionDenied
+			}
+			t.Log.Debugln("Missing global 'Manage Webhooks' permission, falling back on per-channel permission")
+			return t.fetchChannelsHooks(channelIDs, botID)
+		default:
+			return fmt.Errorf("could not get webhooks: %w", err)
+		}
+	}
+
+	t.Log.Debugln("Refreshing guild webhooks using global permission")
+	t.assignHooksByAppID(hooks, botID, false)
+	return nil
+}
+
+// createWebhook creates a webhook for a specific channel.
+func (t *Transmitter) createWebhook(channel string) (*discordgo.Webhook, error) {
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+
+	wh, err := t.session.WebhookCreate(channel, t.title+time.Now().Format(" 3:04:05PM"), "")
+	if err != nil {
+		return nil, err
+	}
+
+	t.channelWebhooks[channel] = wh
+	return wh, nil
+}
+
+func (t *Transmitter) getWebhook(channel string) *discordgo.Webhook {
+	t.mutex.RLock()
+	defer t.mutex.RUnlock()
+
+	return t.channelWebhooks[channel]
+}
+
+func (t *Transmitter) getOrCreateWebhook(channelID string) (*discordgo.Webhook, error) {
+	// If we have a webhook for this channel, immediately return it
+	wh := t.getWebhook(channelID)
+	if wh != nil {
+		return wh, nil
+	}
+
+	// Early exit if we don't want to automatically create one
+	if !t.autoCreate {
+		return nil, ErrWebhookNotFound
+	}
+
+	t.Log.Infof("Creating a webhook for %s\n", channelID)
+	wh, err := t.createWebhook(channelID)
+	if err != nil {
+		return nil, fmt.Errorf("could not create webhook: %w", err)
+	}
+
+	return wh, nil
+}
+
+// fetchChannelsHooks fetches hooks for the given channelIDs and calls assignHooksByAppID for each channel's hooks
+func (t *Transmitter) fetchChannelsHooks(channelIDs []string, botID string) error {
+	// For each channel, search for relevant hooks
+	var failedHooks []string
+	for _, channelID := range channelIDs {
+		hooks, err := t.session.ChannelWebhooks(channelID)
+		if err != nil {
+			failedHooks = append(failedHooks, "\n- "+channelID+": "+err.Error())
+			continue
+		}
+		t.assignHooksByAppID(hooks, botID, true)
+	}
+
+	// Compose an error if any hooks failed
+	if len(failedHooks) > 0 {
+		return errors.New("failed to fetch hooks:" + strings.Join(failedHooks, ""))
+	}
+
+	return nil
+}
+
+func (t *Transmitter) assignHooksByAppID(hooks []*discordgo.Webhook, appID string, channelTargeted bool) {
+	logLine := "Picking up webhook"
+	if channelTargeted {
+		logLine += " (channel targeted)"
+	}
+
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
+
+	for _, wh := range hooks {
+		if wh.ApplicationID != appID {
+			continue
+		}
+
+		t.channelWebhooks[wh.ChannelID] = wh
+		t.Log.WithFields(log.Fields{
+			"id":      wh.ID,
+			"name":    wh.Name,
+			"channel": wh.ChannelID,
+		}).Println(logLine)
+		break
+	}
+}
diff --git a/bridge/discord/transmitter/utils.go b/bridge/discord/transmitter/utils.go
new file mode 100644
index 00000000..f42e81eb
--- /dev/null
+++ b/bridge/discord/transmitter/utils.go
@@ -0,0 +1,32 @@
+package transmitter
+
+import (
+	"github.com/matterbridge/discordgo"
+)
+
+// isDiscordPermissionError returns false for nil, and true if a Discord RESTError with code discordgo.ErrorCodeMissionPermissions
+func isDiscordPermissionError(err error) bool {
+	if err == nil {
+		return false
+	}
+
+	restErr, ok := err.(*discordgo.RESTError)
+	if !ok {
+		return false
+	}
+
+	return restErr.Message != nil && restErr.Message.Code == discordgo.ErrCodeMissingPermissions
+}
+
+// getDiscordUserID gets own user ID from state, and fallback on API request
+func getDiscordUserID(session *discordgo.Session) (string, error) {
+	if user := session.State.User; user != nil {
+		return user.ID, nil
+	}
+
+	user, err := session.User("@me")
+	if err != nil {
+		return "", err
+	}
+	return user.ID, nil
+}