Add Discord outbound posting (#14)

Stacks on #13. Adds discord_post_links and a subscriber that posts roots into a public thread, replies into that thread, and edits the linked Discord message. Unset Discord env leaves the site unchanged.

Reviewed-on: #14
Co-authored-by: codegirl-007 <s.raide@gmail.com>

internal/discord/api.go

@@ -0,0 +1,102 @@
+package discord
+
+import (
+	"context"
+
+	"github.com/bwmarrin/discordgo"
+)
+
+// API is the Discord REST surface used by the outbound subscriber.
+type API interface {
+	SendToChannel(ctx context.Context, channelID string, msg Message) (messageID string, err error)
+	StartThread(ctx context.Context, channelID, messageID, name string) (threadID string, err error)
+	SendToThread(ctx context.Context, threadID string, msg Message) (messageID string, err error)
+	Edit(ctx context.Context, channelID, messageID string, msg Message) error
+	Close() error
+}
+
+type sessionAPI struct {
+	session *discordgo.Session
+}
+
+func (s *sessionAPI) SendToChannel(_ context.Context, channelID string, msg Message) (string, error) {
+	sent, err := s.session.ChannelMessageSendComplex(channelID, toMessageSend(msg))
+	if err != nil {
+		return "", err
+	}
+	return sent.ID, nil
+}
+
+func (s *sessionAPI) StartThread(_ context.Context, channelID, messageID, name string) (string, error) {
+	thread, err := s.session.MessageThreadStartComplex(channelID, messageID, &discordgo.ThreadStart{
+		Name:                name,
+		AutoArchiveDuration: 10080,
+	})
+	if err != nil {
+		return "", err
+	}
+	return thread.ID, nil
+}
+
+func (s *sessionAPI) SendToThread(ctx context.Context, threadID string, msg Message) (string, error) {
+	return s.SendToChannel(ctx, threadID, msg)
+}
+
+func (s *sessionAPI) Edit(_ context.Context, channelID, messageID string, msg Message) error {
+	embeds := toEmbeds(msg)
+	_, err := s.session.ChannelMessageEditComplex(&discordgo.MessageEdit{
+		ID:      messageID,
+		Channel: channelID,
+		Embeds:  &embeds,
+	})
+	return err
+}
+
+func (s *sessionAPI) Close() error {
+	if s == nil || s.session == nil {
+		return nil
+	}
+	return s.session.Close()
+}
+
+func toMessageSend(msg Message) *discordgo.MessageSend {
+	return &discordgo.MessageSend{
+		Embeds:          toEmbeds(msg),
+		AllowedMentions: &discordgo.MessageAllowedMentions{},
+	}
+}
+
+func toEmbeds(msg Message) []*discordgo.MessageEmbed {
+	main := &discordgo.MessageEmbed{
+		Title:       msg.Title,
+		URL:         msg.URL,
+		Description: msg.Description,
+		Color:       embedColor,
+	}
+	if msg.City != "" {
+		main.Fields = append(main.Fields, &discordgo.MessageEmbedField{
+			Name:   "City",
+			Value:  msg.City,
+			Inline: true,
+		})
+	}
+	if msg.Author != "" {
+		main.Fields = append(main.Fields, &discordgo.MessageEmbedField{
+			Name:   "Author",
+			Value:  msg.Author,
+			Inline: true,
+		})
+	}
+	embeds := []*discordgo.MessageEmbed{main}
+	for i, url := range msg.ImageURLs {
+		if i == 0 {
+			main.Image = &discordgo.MessageEmbedImage{URL: url}
+			continue
+		}
+		embeds = append(embeds, &discordgo.MessageEmbed{
+			Color: embedColor,
+			Image: &discordgo.MessageEmbedImage{URL: url},
+		})
+	}
+	return embeds
+}