Add Discord outbound posting.

.env.example

@@ -26,3 +26,7 @@ SECURE_COOKIE=0
 # SPACES_BUCKET=your-bucket
 # SPACES_ENDPOINT=https://nyc3.digitaloceanspaces.com
 # SPACES_CDN_BASE=https://your-bucket.nyc3.cdn.digitaloceanspaces.com
+# Discord bot subscriber. Leave unset to disable.
+# DISCORD_BOT_TOKEN=
+# DISCORD_CHANNEL_ID=
+# DISCORD_ADMIN_MAP=123456789012345678:plumber,234567890123456789:otheradmin

cmd/server/main.go

@@ -17,6 +17,7 @@ import (
 
 	"plumber"
 	"plumber/internal/blob"
+	"plumber/internal/discord"
 	"plumber/internal/events"
 	"plumber/internal/mail"
 	"plumber/internal/store"
@@ -35,7 +36,15 @@ func main() {
 	if err != nil {
 		log.Fatalf("mail: %v", err)
 	}
-	handler := newHandler(db, sessions, uploader, notifier, events.New())
+	bus := events.New()
+	bot, err := discord.FromEnv(store.NewDiscordLinks(db), bus)
+	if err != nil {
+		log.Fatalf("discord: %v", err)
+	}
+	if bot != nil {
+		defer bot.Close()
+	}
+	handler := newHandler(db, sessions, uploader, notifier, bus)
 	run(&http.Server{
 		Addr:              listenAddr(),
 		Handler:           handler,

db/queries/discord_links.sql

@@ -0,0 +1,32 @@
+-- name: GetDiscordPostLinkByPostID :one
+SELECT post_id, discord_message_id, discord_thread_id, created_at
+FROM discord_post_links
+WHERE post_id = sqlc.arg(post_id);
+
+-- name: GetDiscordPostLinkByMessageID :one
+SELECT post_id, discord_message_id, discord_thread_id, created_at
+FROM discord_post_links
+WHERE discord_message_id = sqlc.arg(discord_message_id);
+
+-- name: GetDiscordPostLinkByThreadID :one
+SELECT post_id, discord_message_id, discord_thread_id, created_at
+FROM discord_post_links
+WHERE discord_thread_id = sqlc.arg(discord_thread_id)
+  AND discord_thread_id <> '';
+
+-- name: UpsertDiscordPostLink :exec
+INSERT INTO discord_post_links (
+  post_id, discord_message_id, discord_thread_id, created_at
+)
+VALUES (
+  sqlc.arg(post_id),
+  sqlc.arg(discord_message_id),
+  sqlc.arg(discord_thread_id),
+  sqlc.arg(created_at)
+)
+ON CONFLICT (post_id) DO UPDATE SET
+  discord_message_id = EXCLUDED.discord_message_id,
+  discord_thread_id = CASE
+    WHEN EXCLUDED.discord_thread_id <> '' THEN EXCLUDED.discord_thread_id
+    ELSE discord_post_links.discord_thread_id
+  END;

go.mod

@@ -7,10 +7,12 @@ require (
 	github.com/aws/aws-sdk-go-v2 v1.43.7
 	github.com/aws/aws-sdk-go-v2/credentials v1.19.37
 	github.com/aws/aws-sdk-go-v2/service/s3 v1.107.3
+	github.com/bwmarrin/discordgo v0.29.0
 	github.com/go-chi/chi/v5 v5.3.1
 	github.com/google/uuid v1.6.0
 	github.com/jackc/pgx/v5 v5.10.0
 	github.com/joho/godotenv v1.5.1
+	github.com/resend/resend-go/v3 v3.16.0
 	golang.org/x/crypto v0.55.0
 	golang.org/x/image v0.45.0
 )
@@ -25,10 +27,11 @@ require (
 	github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.13.38 // indirect
 	github.com/aws/aws-sdk-go-v2/service/internal/s3shared v1.19.39 // indirect
 	github.com/aws/smithy-go v1.27.8 // indirect
+	github.com/gorilla/websocket v1.4.2 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
 	github.com/jackc/puddle/v2 v2.2.2 // indirect
-	github.com/resend/resend-go/v3 v3.16.0 // indirect
 	golang.org/x/sync v0.22.0 // indirect
+	golang.org/x/sys v0.47.0 // indirect
 	golang.org/x/text v0.41.0 // indirect
 )

go.sum

@@ -24,6 +24,8 @@ github.com/aws/aws-sdk-go-v2/service/s3 v1.107.3 h1:IKoCZqfWfZzSBi16QFQ+QcbQ3LRQ
 github.com/aws/aws-sdk-go-v2/service/s3 v1.107.3/go.mod h1:RBpRcXiM4s2pOInVs32GsBonnje+fiAj4mcrStRmlCA=
 github.com/aws/smithy-go v1.27.8 h1:FR0dxZfIlV7Z8eh2iHfIofdunw382XsDV3Mxt9nUvRY=
 github.com/aws/smithy-go v1.27.8/go.mod h1:YE2RhdIuDbA5E5bTdciG9KrW3+TiEONeUWCqxX9i1Fc=
+github.com/bwmarrin/discordgo v0.29.0 h1:FmWeXFaKUwrcL3Cx65c20bTRW+vOb6k8AnaP+EgjDno=
+github.com/bwmarrin/discordgo v0.29.0/go.mod h1:NJZpH+1AfhIcyQsPeuBKsUtYrRnjkyu0kIVMCHkZtRY=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
@@ -31,6 +33,8 @@ github.com/go-chi/chi/v5 v5.3.1 h1:3j4HZLGZQ3JpMCrPJF/Jl3mYJfWLKBfNJ6quurUGCf8=
 github.com/go-chi/chi/v5 v5.3.1/go.mod h1:R+tYY2hNuVUUjxoPtqUdgBqevM9s9njzkTLutVsOCto=
 github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
 github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/gorilla/websocket v1.4.2 h1:+/TMaTYc4QFitKJxsQ7Yye35DkWvkdLcvGKqM+x0Ufc=
+github.com/gorilla/websocket v1.4.2/go.mod h1:YR8l580nyteQvAITg2hZ9XVh4b55+EU/adAjf1fMHhE=
 github.com/jackc/pgpassfile v1.0.0 h1:/6Hmqy13Ss2zCq62VdNG8tM1wchn8zjSGOBJ6icpsIM=
 github.com/jackc/pgpassfile v1.0.0/go.mod h1:CEx0iS5ambNFdcRtxPj5JhEz+xB6uRky5eyVu/W2HEg=
 github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 h1:iCEnooe7UlwOQYpKFhBabPMi4aNAfoODPEFNiAnClxo=
@@ -50,14 +54,22 @@ github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UV
 github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
 github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
+golang.org/x/crypto v0.0.0-20210421170649-83a5a9bb288b/go.mod h1:T9bdIzuCu7OtxOm1hfPfRQxPLYneinmdGuTeoZ9dtd4=
 golang.org/x/crypto v0.55.0 h1:+KWHjbgOaAQ66dh/YlkZKHlz9ZUlq61AFirAR9ntP8M=
 golang.org/x/crypto v0.55.0/go.mod h1:uq0V9dE/fzQuJtbnL+2EhWOE63vo164FY8xqEnV9xis=
 golang.org/x/image v0.45.0 h1:FMb1nTbH5H9vF55SriQHgFw5GnNL9Jg6L25BwXKzhB0=
 golang.org/x/image v0.45.0/go.mod h1:n62x/7RqlwXDvGsSU4u6IUTUf6KghUZ9Bt7cG/T9Fx4=
+golang.org/x/net v0.0.0-20210226172049-e18ecbb05110/go.mod h1:m0MpNAwzfU5UDzcl9v0D8zg8gWTRqZa9RBIspLL5mdg=
 golang.org/x/sync v0.22.0 h1:SZjpbeLmrCk4xhRSZFNZW5gFUeCeFgjekvI/+gfScek=
 golang.org/x/sync v0.22.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=
+golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.47.0 h1:o7XGOvZQCADBQQ4Y7VNq2dRWQR7JmOUW8Kxx4ZsNgWs=
+golang.org/x/sys v0.47.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
+golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
+golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
 golang.org/x/text v0.41.0 h1:vz/seA0lnX87Othu2f/0L24RcgrXD9/YFTSuGjj3rH8=
 golang.org/x/text v0.41.0/go.mod h1:jvf1O8ajNzZqhSrQBPbutR/EB83Cc0CFrezNQIwbb5M=
+golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=

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
+}

internal/discord/bot.go

@@ -0,0 +1,178 @@
+package discord
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"fmt"
+	"log"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/bwmarrin/discordgo"
+
+	"plumber/internal/events"
+	"plumber/internal/store"
+)
+
+const discordTimeout = 15 * time.Second
+
+// Bot posts site events to a Discord channel and owns post-to-message links.
+type Bot struct {
+	channelID string
+	links     store.DiscordLinkStore
+	api       API
+}
+
+// New constructs an outbound subscriber. Tests inject a fake API.
+func New(channelID string, links store.DiscordLinkStore, api API) *Bot {
+	return &Bot{channelID: strings.TrimSpace(channelID), links: links, api: api}
+}
+
+// FromEnv builds a bot when Discord env is set. Missing config is a no-op.
+func FromEnv(links store.DiscordLinkStore, bus *events.Bus) (*Bot, error) {
+	token := strings.TrimSpace(os.Getenv("DISCORD_BOT_TOKEN"))
+	channelID := strings.TrimSpace(os.Getenv("DISCORD_CHANNEL_ID"))
+	if token == "" && channelID == "" {
+		return nil, nil
+	}
+	if token == "" {
+		return nil, fmt.Errorf("DISCORD_BOT_TOKEN is required when DISCORD_CHANNEL_ID is set")
+	}
+	if channelID == "" {
+		return nil, fmt.Errorf("DISCORD_CHANNEL_ID is required when DISCORD_BOT_TOKEN is set")
+	}
+	if links == nil {
+		return nil, fmt.Errorf("discord links store is required")
+	}
+	session, err := discordgo.New("Bot " + token)
+	if err != nil {
+		return nil, err
+	}
+	bot := New(channelID, links, &sessionAPI{session: session})
+	if bus != nil {
+		bus.Subscribe(bot.Handle)
+	}
+	log.Printf("discord: outbound subscriber enabled")
+	return bot, nil
+}
+
+// Close releases the Discord session.
+func (b *Bot) Close() error {
+	if b == nil || b.api == nil {
+		return nil
+	}
+	return b.api.Close()
+}
+
+// Handle processes one site event. Failures are logged and do not fail the request.
+func (b *Bot) Handle(_ context.Context, ev any) {
+	if b == nil {
+		return
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), discordTimeout)
+	defer cancel()
+	switch e := ev.(type) {
+	case events.PostCreated:
+		b.onCreated(ctx, e.PostEvent)
+	case events.PostUpdated:
+		b.onUpdated(ctx, e.PostEvent)
+	}
+}
+
+func (b *Bot) onCreated(ctx context.Context, ev events.PostEvent) {
+	if isRoot(ev) {
+		b.createRoot(ctx, ev)
+		return
+	}
+	b.createReply(ctx, ev)
+}
+
+func (b *Bot) onUpdated(ctx context.Context, ev events.PostEvent) {
+	link, err := b.links.GetByPostID(ctx, ev.PostID)
+	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			b.onCreated(ctx, ev)
+			return
+		}
+		log.Printf("discord: load link %s: %v", ev.PostID, err)
+		return
+	}
+	channelID, err := b.editChannel(ctx, ev, link)
+	if err != nil {
+		log.Printf("discord: edit channel %s: %v", ev.PostID, err)
+		return
+	}
+	if err := b.api.Edit(ctx, channelID, link.MessageID, formatMessage(ev)); err != nil {
+		log.Printf("discord: edit %s: %v", ev.PostID, err)
+		return
+	}
+	log.Printf("discord: edited %s", ev.PostID)
+}
+
+func (b *Bot) createRoot(ctx context.Context, ev events.PostEvent) {
+	msg := formatMessage(ev)
+	messageID, err := b.api.SendToChannel(ctx, b.channelID, msg)
+	if err != nil {
+		log.Printf("discord: send root %s: %v", ev.PostID, err)
+		return
+	}
+	threadID, err := b.api.StartThread(ctx, b.channelID, messageID, msg.ThreadName)
+	if err != nil {
+		log.Printf("discord: start thread %s: %v", ev.PostID, err)
+		return
+	}
+	if err := b.links.Upsert(ctx, store.DiscordLink{
+		PostID:    ev.PostID,
+		MessageID: messageID,
+		ThreadID:  threadID,
+	}); err != nil {
+		log.Printf("discord: save root link %s: %v", ev.PostID, err)
+		return
+	}
+	log.Printf("discord: posted root %s", ev.PostID)
+}
+
+func (b *Bot) createReply(ctx context.Context, ev events.PostEvent) {
+	root, err := b.links.GetByPostID(ctx, ev.RootID)
+	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			log.Printf("discord: skip reply %s: no root thread", ev.PostID)
+			return
+		}
+		log.Printf("discord: load root link %s: %v", ev.RootID, err)
+		return
+	}
+	if strings.TrimSpace(root.ThreadID) == "" {
+		log.Printf("discord: skip reply %s: no root thread", ev.PostID)
+		return
+	}
+	messageID, err := b.api.SendToThread(ctx, root.ThreadID, formatMessage(ev))
+	if err != nil {
+		log.Printf("discord: send reply %s: %v", ev.PostID, err)
+		return
+	}
+	if err := b.links.Upsert(ctx, store.DiscordLink{
+		PostID:    ev.PostID,
+		MessageID: messageID,
+	}); err != nil {
+		log.Printf("discord: save reply link %s: %v", ev.PostID, err)
+		return
+	}
+	log.Printf("discord: posted reply %s", ev.PostID)
+}
+
+func (b *Bot) editChannel(ctx context.Context, ev events.PostEvent, link *store.DiscordLink) (string, error) {
+	if strings.TrimSpace(link.ThreadID) != "" {
+		return b.channelID, nil
+	}
+	root, err := b.links.GetByPostID(ctx, ev.RootID)
+	if err != nil {
+		return "", err
+	}
+	if strings.TrimSpace(root.ThreadID) == "" {
+		return "", fmt.Errorf("root %s has no thread", ev.RootID)
+	}
+	return root.ThreadID, nil
+}

internal/discord/bot_test.go

@@ -0,0 +1,247 @@
+package discord
+
+import (
+	"context"
+	"sync"
+	"testing"
+
+	"strconv"
+
+	"plumber/internal/events"
+	"plumber/internal/store"
+)
+
+type recordedSend struct {
+	Kind      string
+	ChannelID string
+	Name      string
+	Msg       Message
+}
+
+type fakeAPI struct {
+	mu       sync.Mutex
+	sends    []recordedSend
+	edits    []recordedSend
+	next     int
+	failSend error
+}
+
+func (f *fakeAPI) SendToChannel(_ context.Context, channelID string, msg Message) (string, error) {
+	return f.record("channel", channelID, "", msg)
+}
+
+func (f *fakeAPI) StartThread(_ context.Context, channelID, messageID, name string) (string, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.next++
+	f.sends = append(f.sends, recordedSend{
+		Kind:      "thread",
+		ChannelID: channelID,
+		Name:      name,
+		Msg:       Message{ThreadName: name, URL: messageID},
+	})
+	return "thread-" + messageID, nil
+}
+
+func (f *fakeAPI) SendToThread(_ context.Context, threadID string, msg Message) (string, error) {
+	return f.record("thread-msg", threadID, "", msg)
+}
+
+func (f *fakeAPI) Edit(_ context.Context, channelID, messageID string, msg Message) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.edits = append(f.edits, recordedSend{
+		Kind:      "edit",
+		ChannelID: channelID,
+		Name:      messageID,
+		Msg:       msg,
+	})
+	return nil
+}
+
+func (f *fakeAPI) Close() error { return nil }
+
+func (f *fakeAPI) record(kind, channelID, name string, msg Message) (string, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	if f.failSend != nil {
+		return "", f.failSend
+	}
+	f.next++
+	id := "msg-" + strconv.Itoa(f.next)
+	f.sends = append(f.sends, recordedSend{Kind: kind, ChannelID: channelID, Name: name, Msg: msg})
+	return id, nil
+}
+
+func TestOutboundRootReplyAndEdit(t *testing.T) {
+	t.Parallel()
+
+	links := newMemoryLinks()
+	api := &fakeAPI{}
+	bot := New("channel-1", links, api)
+	ctx := context.Background()
+
+	root := events.PostEvent{
+		PostID:     "root-1",
+		RootID:     "root-1",
+		Title:      "Leaky sink",
+		Body:       "Water under the cabinet.",
+		City:       "Oakland",
+		AuthorName: "sam",
+		Permalink:  "https://www.askaplumberfirst.com/questions/root-1#post-root-1",
+		Images:     []events.Image{{URL: "https://cdn.example/a.jpg"}, {URL: "https://cdn.example/b.jpg"}},
+	}
+	bot.Handle(ctx, events.PostCreated{PostEvent: root})
+
+	if len(api.sends) != 2 || api.sends[0].Kind != "channel" || api.sends[1].Kind != "thread" {
+		t.Fatalf("root sends = %+v", api.sends)
+	}
+	if api.sends[0].ChannelID != "channel-1" || api.sends[1].Name != "Leaky sink" {
+		t.Fatalf("root routing = %+v", api.sends)
+	}
+	if got := api.sends[0].Msg.ImageURLs; len(got) != 2 || got[0] != "https://cdn.example/a.jpg" {
+		t.Fatalf("root images = %v", got)
+	}
+	link, err := links.GetByPostID(ctx, "root-1")
+	if err != nil || link.MessageID != "msg-1" || link.ThreadID != "thread-msg-1" {
+		t.Fatalf("root link = %+v, %v", link, err)
+	}
+
+	reply := events.PostEvent{
+		PostID:     "reply-1",
+		RootID:     "root-1",
+		ParentID:   "root-1",
+		Body:       "Replace the cartridge.",
+		AuthorName: "plumber",
+		Permalink:  "https://www.askaplumberfirst.com/questions/root-1#post-reply-1",
+	}
+	bot.Handle(ctx, events.PostCreated{PostEvent: reply})
+	if len(api.sends) != 3 || api.sends[2].Kind != "thread-msg" || api.sends[2].ChannelID != "thread-msg-1" {
+		t.Fatalf("reply sends = %+v", api.sends)
+	}
+	replyLink, err := links.GetByPostID(ctx, "reply-1")
+	if err != nil || replyLink.MessageID != "msg-3" || replyLink.ThreadID != "" {
+		t.Fatalf("reply link = %+v, %v", replyLink, err)
+	}
+
+	root.Body = "Updated leak."
+	bot.Handle(ctx, events.PostUpdated{PostEvent: root})
+	if len(api.edits) != 1 || api.edits[0].ChannelID != "channel-1" || api.edits[0].Name != "msg-1" {
+		t.Fatalf("root edit = %+v", api.edits)
+	}
+	if api.edits[0].Msg.Description != "Updated leak." {
+		t.Fatalf("root edit body = %+v", api.edits[0].Msg)
+	}
+
+	reply.Body = "Use a ceramic cartridge."
+	bot.Handle(ctx, events.PostUpdated{PostEvent: reply})
+	if len(api.edits) != 2 || api.edits[1].ChannelID != "thread-msg-1" || api.edits[1].Name != "msg-3" {
+		t.Fatalf("reply edit = %+v", api.edits)
+	}
+}
+
+func TestOutboundSkipsReplyWithoutRootLink(t *testing.T) {
+	t.Parallel()
+
+	api := &fakeAPI{}
+	bot := New("channel-1", newMemoryLinks(), api)
+	bot.Handle(context.Background(), events.PostCreated{PostEvent: events.PostEvent{
+		PostID:   "reply-1",
+		RootID:   "missing",
+		ParentID: "missing",
+		Body:     "Orphan reply",
+	}})
+	if len(api.sends) != 0 {
+		t.Fatalf("unexpected sends %+v", api.sends)
+	}
+}
+
+func TestOutboundUpdateWithoutLinkCreates(t *testing.T) {
+	t.Parallel()
+
+	links := newMemoryLinks()
+	api := &fakeAPI{}
+	bot := New("channel-1", links, api)
+	bot.Handle(context.Background(), events.PostUpdated{PostEvent: events.PostEvent{
+		PostID: "root-2",
+		RootID: "root-2",
+		Title:  "Late question",
+		Body:   "Created while Discord was down.",
+	}})
+	link, err := links.GetByPostID(context.Background(), "root-2")
+	if err != nil || link.ThreadID == "" || len(api.sends) != 2 {
+		t.Fatalf("late create link=%+v sends=%+v err=%v", link, api.sends, err)
+	}
+}
+
+func TestFormatMessage(t *testing.T) {
+	t.Parallel()
+
+	got := formatMessage(events.PostEvent{
+		Title:      "Leaky sink",
+		Body:       "It drips.",
+		City:       "Oakland",
+		AuthorName: "sam",
+		Permalink:  "https://example.com/q",
+		Images:     []events.Image{{URL: "https://cdn.example/a.jpg", Description: "ignored"}},
+	})
+	if got.Title != "Leaky sink" ||
+		got.Description != "It drips." ||
+		got.City != "Oakland" ||
+		got.Author != "sam" ||
+		got.URL != "https://example.com/q" ||
+		got.ThreadName != "Leaky sink" ||
+		len(got.ImageURLs) != 1 {
+		t.Fatalf("format = %+v", got)
+	}
+
+	reply := formatMessage(events.PostEvent{Body: "Thanks", AuthorName: ""})
+	if reply.Title != "Reply" || reply.Author != "Someone" || reply.ThreadName != "Question" {
+		t.Fatalf("reply format = %+v", reply)
+	}
+}
+
+func TestFromEnvDisabled(t *testing.T) {
+	t.Setenv("DISCORD_BOT_TOKEN", "")
+	t.Setenv("DISCORD_CHANNEL_ID", "")
+	bot, err := FromEnv(newMemoryLinks(), nil)
+	if err != nil || bot != nil {
+		t.Fatalf("disabled FromEnv = (%v, %v)", bot, err)
+	}
+}
+
+func TestFromEnvRequiresBoth(t *testing.T) {
+	t.Setenv("DISCORD_BOT_TOKEN", "token")
+	t.Setenv("DISCORD_CHANNEL_ID", "")
+	if _, err := FromEnv(newMemoryLinks(), nil); err == nil {
+		t.Fatal("expected error when channel is missing")
+	}
+	t.Setenv("DISCORD_BOT_TOKEN", "")
+	t.Setenv("DISCORD_CHANNEL_ID", "channel")
+	if _, err := FromEnv(newMemoryLinks(), nil); err == nil {
+		t.Fatal("expected error when token is missing")
+	}
+}
+
+func TestMemoryLinkUpsertKeepsThread(t *testing.T) {
+	t.Parallel()
+
+	links := newMemoryLinks()
+	ctx := context.Background()
+	if err := links.Upsert(ctx, store.DiscordLink{PostID: "p", MessageID: "m1", ThreadID: "t1"}); err != nil {
+		t.Fatal(err)
+	}
+	if err := links.Upsert(ctx, store.DiscordLink{PostID: "p", MessageID: "m2"}); err != nil {
+		t.Fatal(err)
+	}
+	got, err := links.GetByPostID(ctx, "p")
+	if err != nil || got.MessageID != "m2" || got.ThreadID != "t1" {
+		t.Fatalf("upsert keep thread = %+v, %v", got, err)
+	}
+	if _, err := links.GetByMessageID(ctx, "m2"); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := links.GetRootByThreadID(ctx, "t1"); err != nil {
+		t.Fatal(err)
+	}
+}

internal/discord/format.go

@@ -0,0 +1,75 @@
+package discord
+
+import (
+	"strings"
+
+	"plumber/internal/events"
+)
+
+const (
+	embedTitleLimit       = 256
+	embedDescriptionLimit = 4096
+	threadNameLimit       = 100
+	embedColor            = 0xe96a26
+)
+
+// Message is a Discord-ready snapshot of a site post event.
+type Message struct {
+	Title       string
+	URL         string
+	Description string
+	City        string
+	Author      string
+	ImageURLs   []string
+	ThreadName  string
+}
+
+func formatMessage(ev events.PostEvent) Message {
+	title := strings.TrimSpace(ev.Title)
+	if title == "" {
+		title = "Reply"
+	}
+	author := strings.TrimSpace(ev.AuthorName)
+	if author == "" {
+		author = "Someone"
+	}
+	msg := Message{
+		Title:       truncateRunes(title, embedTitleLimit),
+		URL:         strings.TrimSpace(ev.Permalink),
+		Description: truncateRunes(strings.TrimSpace(ev.Body), embedDescriptionLimit),
+		City:        strings.TrimSpace(ev.City),
+		Author:      author,
+		ThreadName:  threadName(ev.Title),
+	}
+	for _, img := range ev.Images {
+		url := strings.TrimSpace(img.URL)
+		if url == "" {
+			continue
+		}
+		msg.ImageURLs = append(msg.ImageURLs, url)
+	}
+	return msg
+}
+
+func threadName(title string) string {
+	title = strings.TrimSpace(title)
+	if title == "" {
+		return "Question"
+	}
+	return truncateRunes(title, threadNameLimit)
+}
+
+func truncateRunes(s string, max int) string {
+	if max <= 0 {
+		return ""
+	}
+	runes := []rune(s)
+	if len(runes) <= max {
+		return s
+	}
+	return string(runes[:max])
+}
+
+func isRoot(ev events.PostEvent) bool {
+	return strings.TrimSpace(ev.ParentID) == ""
+}

internal/discord/memory_links.go

@@ -0,0 +1,88 @@
+package discord
+
+import (
+	"context"
+	"database/sql"
+	"strings"
+	"sync"
+	"time"
+
+	"plumber/internal/store"
+)
+
+// memoryLinks is an in-process DiscordLinkStore for tests.
+type memoryLinks struct {
+	mu        sync.Mutex
+	byPost    map[string]store.DiscordLink
+	byMessage map[string]string
+	byThread  map[string]string
+}
+
+func newMemoryLinks() *memoryLinks {
+	return &memoryLinks{
+		byPost:    map[string]store.DiscordLink{},
+		byMessage: map[string]string{},
+		byThread:  map[string]string{},
+	}
+}
+
+func (m *memoryLinks) GetByPostID(_ context.Context, postID string) (*store.DiscordLink, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	link, ok := m.byPost[strings.TrimSpace(postID)]
+	if !ok {
+		return nil, sql.ErrNoRows
+	}
+	cp := link
+	return &cp, nil
+}
+
+func (m *memoryLinks) GetByMessageID(_ context.Context, messageID string) (*store.DiscordLink, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	postID, ok := m.byMessage[strings.TrimSpace(messageID)]
+	if !ok {
+		return nil, sql.ErrNoRows
+	}
+	link := m.byPost[postID]
+	cp := link
+	return &cp, nil
+}
+
+func (m *memoryLinks) GetRootByThreadID(_ context.Context, threadID string) (*store.DiscordLink, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	postID, ok := m.byThread[strings.TrimSpace(threadID)]
+	if !ok {
+		return nil, sql.ErrNoRows
+	}
+	link := m.byPost[postID]
+	cp := link
+	return &cp, nil
+}
+
+func (m *memoryLinks) Upsert(_ context.Context, link store.DiscordLink) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	link.PostID = strings.TrimSpace(link.PostID)
+	link.MessageID = strings.TrimSpace(link.MessageID)
+	link.ThreadID = strings.TrimSpace(link.ThreadID)
+	if link.CreatedAt == "" {
+		link.CreatedAt = time.Now().UTC().Format(time.RFC3339Nano)
+	}
+	if prev, ok := m.byPost[link.PostID]; ok {
+		delete(m.byMessage, prev.MessageID)
+		if prev.ThreadID != "" {
+			delete(m.byThread, prev.ThreadID)
+		}
+		if link.ThreadID == "" {
+			link.ThreadID = prev.ThreadID
+		}
+	}
+	m.byPost[link.PostID] = link
+	m.byMessage[link.MessageID] = link.PostID
+	if link.ThreadID != "" {
+		m.byThread[link.ThreadID] = link.PostID
+	}
+	return nil
+}

internal/store/discord_link.go

@@ -0,0 +1,105 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"strings"
+	"time"
+
+	"plumber/internal/store/sqlc"
+)
+
+// DiscordLink is the bot-owned mapping from a site post to a Discord message.
+type DiscordLink struct {
+	PostID    string
+	MessageID string
+	ThreadID  string
+	CreatedAt string
+}
+
+// DiscordLinkStore is the mapping table used only by the Discord subscriber.
+// It is not part of Store.
+type DiscordLinkStore interface {
+	GetByPostID(ctx context.Context, postID string) (*DiscordLink, error)
+	GetByMessageID(ctx context.Context, messageID string) (*DiscordLink, error)
+	GetRootByThreadID(ctx context.Context, threadID string) (*DiscordLink, error)
+	Upsert(ctx context.Context, link DiscordLink) error
+}
+
+// DiscordLinks implements DiscordLinkStore against Postgres.
+type DiscordLinks struct {
+	db *sql.DB
+}
+
+// NewDiscordLinks wraps db. It is independent of Store.
+func NewDiscordLinks(db *sql.DB) *DiscordLinks {
+	return &DiscordLinks{db: db}
+}
+
+// GetByPostID returns the link for a site post.
+func (d *DiscordLinks) GetByPostID(ctx context.Context, postID string) (*DiscordLink, error) {
+	if d == nil || d.db == nil {
+		return nil, fmt.Errorf("discord links: no database")
+	}
+	row, err := sqlc.New(d.db).GetDiscordPostLinkByPostID(ctx, strings.TrimSpace(postID))
+	if err != nil {
+		return nil, err
+	}
+	return discordLinkFromRow(row), nil
+}
+
+// GetByMessageID returns the link for a Discord message.
+func (d *DiscordLinks) GetByMessageID(ctx context.Context, messageID string) (*DiscordLink, error) {
+	if d == nil || d.db == nil {
+		return nil, fmt.Errorf("discord links: no database")
+	}
+	row, err := sqlc.New(d.db).GetDiscordPostLinkByMessageID(ctx, strings.TrimSpace(messageID))
+	if err != nil {
+		return nil, err
+	}
+	return discordLinkFromRow(row), nil
+}
+
+// GetRootByThreadID returns the root link for a Discord thread.
+func (d *DiscordLinks) GetRootByThreadID(ctx context.Context, threadID string) (*DiscordLink, error) {
+	if d == nil || d.db == nil {
+		return nil, fmt.Errorf("discord links: no database")
+	}
+	row, err := sqlc.New(d.db).GetDiscordPostLinkByThreadID(ctx, strings.TrimSpace(threadID))
+	if err != nil {
+		return nil, err
+	}
+	return discordLinkFromRow(row), nil
+}
+
+// Upsert inserts or replaces the Discord IDs for a post.
+func (d *DiscordLinks) Upsert(ctx context.Context, link DiscordLink) error {
+	if d == nil || d.db == nil {
+		return fmt.Errorf("discord links: no database")
+	}
+	link.PostID = strings.TrimSpace(link.PostID)
+	link.MessageID = strings.TrimSpace(link.MessageID)
+	link.ThreadID = strings.TrimSpace(link.ThreadID)
+	if link.PostID == "" || link.MessageID == "" {
+		return fmt.Errorf("discord links: post and message ids are required")
+	}
+	if link.CreatedAt == "" {
+		link.CreatedAt = time.Now().UTC().Format(time.RFC3339Nano)
+	}
+	return sqlc.New(d.db).UpsertDiscordPostLink(ctx, sqlc.UpsertDiscordPostLinkParams{
+		PostID:           link.PostID,
+		DiscordMessageID: link.MessageID,
+		DiscordThreadID:  link.ThreadID,
+		CreatedAt:        link.CreatedAt,
+	})
+}
+
+func discordLinkFromRow(row sqlc.DiscordPostLink) *DiscordLink {
+	return &DiscordLink{
+		PostID:    row.PostID,
+		MessageID: row.DiscordMessageID,
+		ThreadID:  row.DiscordThreadID,
+		CreatedAt: row.CreatedAt,
+	}
+}

internal/store/migrate.go

@@ -139,6 +139,25 @@ CREATE TABLE IF NOT EXISTS post_images (
 	return nil
 }
 
+func migrateDiscordPostLinks(ctx context.Context, exec execContext) error {
+	if _, err := exec.ExecContext(ctx, `
+CREATE TABLE IF NOT EXISTS discord_post_links (
+  post_id TEXT PRIMARY KEY REFERENCES posts(id) ON DELETE CASCADE,
+  discord_message_id TEXT NOT NULL UNIQUE,
+  discord_thread_id TEXT NOT NULL DEFAULT '',
+  created_at TEXT NOT NULL
+)`); err != nil {
+		return fmt.Errorf("create discord_post_links: %w", err)
+	}
+	if _, err := exec.ExecContext(ctx, `
+CREATE UNIQUE INDEX IF NOT EXISTS discord_post_links_thread_uidx
+  ON discord_post_links (discord_thread_id)
+  WHERE discord_thread_id <> ''`); err != nil {
+		return fmt.Errorf("discord_post_links_thread_uidx: %w", err)
+	}
+	return nil
+}
+
 func migratePostDate(ctx context.Context, exec execContext) error {
 	steps := []struct {
 		name string
@@ -331,6 +350,7 @@ CREATE TABLE IF NOT EXISTS schema_migrations (
 		{"008_post_author_index", migratePostAuthorIndex},
 		{"009_drop_legacy_post_tables", migrateDropLegacyPostTables},
 		{"010_post_images", migratePostImages},
+		{"011_discord_post_links", migrateDiscordPostLinks},
 	}
 	for _, m := range migrations {
 		if applied[m.version] {

internal/store/migrate_posts_test.go

@@ -81,6 +81,12 @@ CREATE TABLE users (
 	if err := migratePostImages(ctx, conn); err != nil {
 		t.Fatalf("post images migration is not idempotent: %v", err)
 	}
+	if err := migrateDiscordPostLinks(ctx, conn); err != nil {
+		t.Fatal(err)
+	}
+	if err := migrateDiscordPostLinks(ctx, conn); err != nil {
+		t.Fatalf("discord post links migration is not idempotent: %v", err)
+	}
 	if _, err := conn.ExecContext(ctx, `
 INSERT INTO users (id, name, role)
 VALUES ('homeowner', 'Home Owner', 'user'), ('plumber', 'The Plumber', 'admin');
@@ -143,6 +149,42 @@ VALUES ('homeowner', 'root-1', 1);`); err != nil {
 	}); err == nil {
 		t.Fatal("fifth image position unexpectedly succeeded")
 	}
+	if err := imageQueries.UpsertDiscordPostLink(ctx, sqlc.UpsertDiscordPostLinkParams{
+		PostID:           "root-1",
+		DiscordMessageID: "msg-root",
+		DiscordThreadID:  "thread-root",
+		CreatedAt:        "2026-08-26T08:00:00Z",
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if err := imageQueries.UpsertDiscordPostLink(ctx, sqlc.UpsertDiscordPostLinkParams{
+		PostID:           "reply-1",
+		DiscordMessageID: "msg-reply",
+		DiscordThreadID:  "",
+		CreatedAt:        "2026-08-26T09:00:00Z",
+	}); err != nil {
+		t.Fatal(err)
+	}
+	rootLink, err := imageQueries.GetDiscordPostLinkByPostID(ctx, "root-1")
+	if err != nil || rootLink.DiscordMessageID != "msg-root" || rootLink.DiscordThreadID != "thread-root" {
+		t.Fatalf("root discord link = %+v, %v", rootLink, err)
+	}
+	threadLink, err := imageQueries.GetDiscordPostLinkByThreadID(ctx, "thread-root")
+	if err != nil || threadLink.PostID != "root-1" {
+		t.Fatalf("thread discord link = %+v, %v", threadLink, err)
+	}
+	if err := imageQueries.UpsertDiscordPostLink(ctx, sqlc.UpsertDiscordPostLinkParams{
+		PostID:           "reply-1",
+		DiscordMessageID: "msg-reply-2",
+		DiscordThreadID:  "",
+		CreatedAt:        "2026-08-26T09:01:00Z",
+	}); err != nil {
+		t.Fatal(err)
+	}
+	replyLink, err := imageQueries.GetDiscordPostLinkByPostID(ctx, "reply-1")
+	if err != nil || replyLink.DiscordMessageID != "msg-reply-2" || replyLink.DiscordThreadID != "" {
+		t.Fatalf("reply upsert = %+v, %v", replyLink, err)
+	}
 	var postVoteIndexCount int
 	if err := conn.QueryRowContext(ctx, `
 SELECT count(*)

internal/store/sqlc/discord_links.sql.go

@@ -0,0 +1,100 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.31.1
+// source: discord_links.sql
+
+package sqlc
+
+import (
+	"context"
+)
+
+const getDiscordPostLinkByMessageID = `-- name: GetDiscordPostLinkByMessageID :one
+SELECT post_id, discord_message_id, discord_thread_id, created_at
+FROM discord_post_links
+WHERE discord_message_id = $1
+`
+
+func (q *Queries) GetDiscordPostLinkByMessageID(ctx context.Context, discordMessageID string) (DiscordPostLink, error) {
+	row := q.db.QueryRowContext(ctx, getDiscordPostLinkByMessageID, discordMessageID)
+	var i DiscordPostLink
+	err := row.Scan(
+		&i.PostID,
+		&i.DiscordMessageID,
+		&i.DiscordThreadID,
+		&i.CreatedAt,
+	)
+	return i, err
+}
+
+const getDiscordPostLinkByPostID = `-- name: GetDiscordPostLinkByPostID :one
+SELECT post_id, discord_message_id, discord_thread_id, created_at
+FROM discord_post_links
+WHERE post_id = $1
+`
+
+func (q *Queries) GetDiscordPostLinkByPostID(ctx context.Context, postID string) (DiscordPostLink, error) {
+	row := q.db.QueryRowContext(ctx, getDiscordPostLinkByPostID, postID)
+	var i DiscordPostLink
+	err := row.Scan(
+		&i.PostID,
+		&i.DiscordMessageID,
+		&i.DiscordThreadID,
+		&i.CreatedAt,
+	)
+	return i, err
+}
+
+const getDiscordPostLinkByThreadID = `-- name: GetDiscordPostLinkByThreadID :one
+SELECT post_id, discord_message_id, discord_thread_id, created_at
+FROM discord_post_links
+WHERE discord_thread_id = $1
+  AND discord_thread_id <> ''
+`
+
+func (q *Queries) GetDiscordPostLinkByThreadID(ctx context.Context, discordThreadID string) (DiscordPostLink, error) {
+	row := q.db.QueryRowContext(ctx, getDiscordPostLinkByThreadID, discordThreadID)
+	var i DiscordPostLink
+	err := row.Scan(
+		&i.PostID,
+		&i.DiscordMessageID,
+		&i.DiscordThreadID,
+		&i.CreatedAt,
+	)
+	return i, err
+}
+
+const upsertDiscordPostLink = `-- name: UpsertDiscordPostLink :exec
+INSERT INTO discord_post_links (
+  post_id, discord_message_id, discord_thread_id, created_at
+)
+VALUES (
+  $1,
+  $2,
+  $3,
+  $4
+)
+ON CONFLICT (post_id) DO UPDATE SET
+  discord_message_id = EXCLUDED.discord_message_id,
+  discord_thread_id = CASE
+    WHEN EXCLUDED.discord_thread_id <> '' THEN EXCLUDED.discord_thread_id
+    ELSE discord_post_links.discord_thread_id
+  END
+`
+
+type UpsertDiscordPostLinkParams struct {
+	PostID           string
+	DiscordMessageID string
+	DiscordThreadID  string
+	CreatedAt        string
+}
+
+func (q *Queries) UpsertDiscordPostLink(ctx context.Context, arg UpsertDiscordPostLinkParams) error {
+	_, err := q.db.ExecContext(ctx, upsertDiscordPostLink,
+		arg.PostID,
+		arg.DiscordMessageID,
+		arg.DiscordThreadID,
+		arg.CreatedAt,
+	)
+	return err
+}

internal/store/sqlc/models.go

@@ -9,6 +9,13 @@ import (
 	"time"
 )
 
+type DiscordPostLink struct {
+	PostID           string
+	DiscordMessageID string
+	DiscordThreadID  string
+	CreatedAt        string
+}
+
 type Post struct {
 	ID        string
 	ParentID  sql.NullString

schema.sql

@@ -65,6 +65,17 @@ CREATE TABLE IF NOT EXISTS post_votes (
 CREATE INDEX IF NOT EXISTS idx_post_votes_post_id
   ON post_votes(post_id);
 
+CREATE TABLE IF NOT EXISTS discord_post_links (
+  post_id TEXT PRIMARY KEY REFERENCES posts(id) ON DELETE CASCADE,
+  discord_message_id TEXT NOT NULL UNIQUE,
+  discord_thread_id TEXT NOT NULL DEFAULT '',
+  created_at TEXT NOT NULL
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS discord_post_links_thread_uidx
+  ON discord_post_links (discord_thread_id)
+  WHERE discord_thread_id <> '';
+
 CREATE TABLE IF NOT EXISTS sessions (
   token TEXT PRIMARY KEY,
   data BYTEA NOT NULL,