Publish Discord inbound replies on the post event bus.

Reviewed-on: #18

cmd/server/main.go

@@ -37,14 +37,15 @@ func main() {
 		log.Fatalf("mail: %v", err)
 	}
 	bus := events.New()
-	bot, err := discord.FromEnv(store.NewDiscordLinks(db), bus, store.NewPostgres(db), notifier)
+	mail.Subscribe(bus, store.NewPostgres(db), notifier)
+	bot, err := discord.FromEnv(store.NewDiscordLinks(db), bus, store.NewPostgres(db))
 	if err != nil {
 		log.Fatalf("discord: %v", err)
 	}
 	if bot != nil {
 		defer bot.Close()
 	}
-	handler := newHandler(db, sessions, uploader, notifier, bus)
+	handler := newHandler(db, sessions, uploader, bus)
 	run(&http.Server{
 		Addr:              listenAddr(),
 		Handler:           handler,
@@ -68,13 +69,12 @@ func openDB() (*sql.DB, *store.SessionStore) {
 	return db, sessions
 }
 
-func newHandler(db *sql.DB, sessions *store.SessionStore, uploader blob.Uploader, notifier mail.Notifier, bus events.Publisher) http.Handler {
+func newHandler(db *sql.DB, sessions *store.SessionStore, uploader blob.Uploader, bus events.Publisher) http.Handler {
 	srv, err := web.New(store.NewPostgres(db), sessions.Store(), plumber.TemplateFS, plumber.StaticFS, web.Config{
 		AdminSetupSecret: strings.TrimSpace(os.Getenv("ADMIN_SETUP_SECRET")),
 		SecureCookie:     secureCookieFromEnv(),
 		TrustedProxies:   parseTrustedProxies(os.Getenv("TRUSTED_PROXY_CIDRS")),
 		Blob:             uploader,
-		Mail:             notifier,
 		Events:           bus,
 		BaseURL:          strings.TrimRight(strings.TrimSpace(os.Getenv("APP_BASE_URL")), "/"),
 	})

internal/discord/api.go

@@ -9,7 +9,7 @@ import (
 // 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)
+	StartThread(ctx context.Context, channelID, 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
@@ -27,9 +27,10 @@ func (s *sessionAPI) SendToChannel(_ context.Context, channelID string, msg Mess
 	return sent.ID, nil
 }
 
-func (s *sessionAPI) StartThread(_ context.Context, channelID, messageID, name string) (string, error) {
+func (s *sessionAPI) StartThread(_ context.Context, channelID, name string) (string, error) {
-	thread, err := s.session.MessageThreadStartComplex(channelID, messageID, &discordgo.ThreadStart{
+	thread, err := s.session.ThreadStartComplex(channelID, &discordgo.ThreadStart{
 		Name:                name,
+		Type:                discordgo.ChannelTypeGuildPublicThread,
 		AutoArchiveDuration: 10080,
 	})
 	if err != nil {
@@ -43,10 +44,12 @@ func (s *sessionAPI) SendToThread(ctx context.Context, threadID string, msg Mess
 }
 
 func (s *sessionAPI) Edit(_ context.Context, channelID, messageID string, msg Message) error {
+	content := messageContent(msg)
 	embeds := toEmbeds(msg)
 	_, err := s.session.ChannelMessageEditComplex(&discordgo.MessageEdit{
 		ID:      messageID,
 		Channel: channelID,
+		Content: &content,
 		Embeds:  &embeds,
 	})
 	return err
@@ -61,6 +64,7 @@ func (s *sessionAPI) Close() error {
 
 func toMessageSend(msg Message) *discordgo.MessageSend {
 	return &discordgo.MessageSend{
+		Content:         messageContent(msg),
 		Embeds:          toEmbeds(msg),
 		AllowedMentions: &discordgo.MessageAllowedMentions{},
 	}
@@ -69,7 +73,7 @@ func toMessageSend(msg Message) *discordgo.MessageSend {
 func toEmbeds(msg Message) []*discordgo.MessageEmbed {
 	main := &discordgo.MessageEmbed{
 		Title:       msg.Title,
-		URL:         msg.URL,
+		URL:         publicURL(msg.URL),
 		Description: msg.Description,
 		Color:       embedColor,
 	}

internal/discord/bot.go

@@ -13,7 +13,6 @@ import (
 	"github.com/bwmarrin/discordgo"
 
 	"plumber/internal/events"
-	"plumber/internal/mail"
 	"plumber/internal/store"
 )
 
@@ -25,7 +24,7 @@ type Bot struct {
 	links     store.DiscordLinkStore
 	api       API
 	store     store.Store
-	mail      mail.Notifier
+	bus       events.Publisher
 	admins    map[string]string
 	botUserID string
 }
@@ -36,7 +35,7 @@ func New(channelID string, links store.DiscordLinkStore, api API) *Bot {
 }
 
 // FromEnv builds a bot when Discord env is set. Missing config is a no-op.
-func FromEnv(links store.DiscordLinkStore, bus *events.Bus, st store.Store, mailer mail.Notifier) (*Bot, error) {
+func FromEnv(links store.DiscordLinkStore, bus *events.Bus, st store.Store) (*Bot, error) {
 	token := strings.TrimSpace(os.Getenv("DISCORD_BOT_TOKEN"))
 	channelID := strings.TrimSpace(os.Getenv("DISCORD_CHANNEL_ID"))
 	if token == "" && channelID == "" {
@@ -56,12 +55,9 @@ func FromEnv(links store.DiscordLinkStore, bus *events.Bus, st store.Store, mail
 		return nil, err
 	}
 	session.Identify.Intents = discordgo.IntentsGuilds | discordgo.IntentsGuildMessages | discordgo.IntentsMessageContent
-	if mailer == nil {
-		mailer = mail.Nop{}
-	}
 	bot := New(channelID, links, &sessionAPI{session: session})
 	bot.store = st
-	bot.mail = mailer
+	bot.bus = bus
 	bot.admins = parseAdminMap(os.Getenv("DISCORD_ADMIN_MAP"))
 	session.AddHandler(bot.onMessageCreate)
 	if bus != nil {
@@ -102,6 +98,14 @@ func (b *Bot) Handle(_ context.Context, ev any) {
 }
 
 func (b *Bot) onCreated(ctx context.Context, ev events.PostEvent) {
+	_, err := b.links.GetByPostID(ctx, ev.PostID)
+	if err == nil {
+		return
+	}
+	if !errors.Is(err, sql.ErrNoRows) {
+		log.Printf("discord: load link %s: %v", ev.PostID, err)
+		return
+	}
 	if isRoot(ev) {
 		b.createRoot(ctx, ev)
 		return
@@ -133,16 +137,16 @@ func (b *Bot) onUpdated(ctx context.Context, ev events.PostEvent) {
 
 func (b *Bot) createRoot(ctx context.Context, ev events.PostEvent) {
 	msg := formatMessage(ev)
-	messageID, err := b.api.SendToChannel(ctx, b.channelID, msg)
+	threadID, err := b.api.StartThread(ctx, b.channelID, msg.ThreadName)
-	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
 	}
+	messageID, err := b.api.SendToThread(ctx, threadID, msg)
+	if err != nil {
+		log.Printf("discord: send root %s: %v", ev.PostID, err)
+		return
+	}
 	if err := b.links.Upsert(ctx, store.DiscordLink{
 		PostID:    ev.PostID,
 		MessageID: messageID,
@@ -185,7 +189,7 @@ func (b *Bot) createReply(ctx context.Context, ev events.PostEvent) {
 
 func (b *Bot) editChannel(ctx context.Context, ev events.PostEvent, link *store.DiscordLink) (string, error) {
 	if strings.TrimSpace(link.ThreadID) != "" {
-		return b.channelID, nil
+		return link.ThreadID, nil
 	}
 	root, err := b.links.GetByPostID(ctx, ev.RootID)
 	if err != nil {

internal/discord/bot_test.go

@@ -2,11 +2,11 @@ package discord
 
 import (
 	"context"
+	"strconv"
+	"strings"
 	"sync"
 	"testing"
 
-	"strconv"
-
 	"plumber/internal/events"
 	"plumber/internal/store"
 )
@@ -30,17 +30,18 @@ func (f *fakeAPI) SendToChannel(_ context.Context, channelID string, msg Message
 	return f.record("channel", channelID, "", msg)
 }
 
-func (f *fakeAPI) StartThread(_ context.Context, channelID, messageID, name string) (string, error) {
+func (f *fakeAPI) StartThread(_ context.Context, channelID, name string) (string, error) {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 	f.next++
+	id := "thread-" + strconv.Itoa(f.next)
 	f.sends = append(f.sends, recordedSend{
 		Kind:      "thread",
 		ChannelID: channelID,
 		Name:      name,
-		Msg:       Message{ThreadName: name, URL: messageID},
+		Msg:       Message{ThreadName: name},
 	})
-	return "thread-" + messageID, nil
+	return id, nil
 }
 
 func (f *fakeAPI) SendToThread(_ context.Context, threadID string, msg Message) (string, error) {
@@ -93,17 +94,21 @@ func TestOutboundRootReplyAndEdit(t *testing.T) {
 	}
 	bot.Handle(ctx, events.PostCreated{PostEvent: root})
 
-	if len(api.sends) != 2 || api.sends[0].Kind != "channel" || api.sends[1].Kind != "thread" {
+	if len(api.sends) != 2 ||
+		api.sends[0].Kind != "thread" ||
+		api.sends[1].Kind != "thread-msg" {
 		t.Fatalf("root sends = %+v", api.sends)
 	}
-	if api.sends[0].ChannelID != "channel-1" || api.sends[1].Name != "Leaky sink" {
+	if api.sends[0].ChannelID != "channel-1" ||
+		api.sends[0].Name != "sam asks: Leaky sink" ||
+		api.sends[1].ChannelID != "thread-1" {
 		t.Fatalf("root routing = %+v", api.sends)
 	}
-	if got := api.sends[0].Msg.ImageURLs; len(got) != 2 || got[0] != "https://cdn.example/a.jpg" {
+	if got := api.sends[1].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" {
+	if err != nil || link.MessageID != "msg-2" || link.ThreadID != "thread-1" {
 		t.Fatalf("root link = %+v, %v", link, err)
 	}
 
@@ -116,7 +121,7 @@ func TestOutboundRootReplyAndEdit(t *testing.T) {
 		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" {
+	if len(api.sends) != 3 || api.sends[2].Kind != "thread-msg" || api.sends[2].ChannelID != "thread-1" {
 		t.Fatalf("reply sends = %+v", api.sends)
 	}
 	replyLink, err := links.GetByPostID(ctx, "reply-1")
@@ -126,7 +131,7 @@ func TestOutboundRootReplyAndEdit(t *testing.T) {
 
 	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" {
+	if len(api.edits) != 1 || api.edits[0].ChannelID != "thread-1" || api.edits[0].Name != "msg-2" {
 		t.Fatalf("root edit = %+v", api.edits)
 	}
 	if api.edits[0].Msg.Description != "Updated leak." {
@@ -135,7 +140,7 @@ func TestOutboundRootReplyAndEdit(t *testing.T) {
 
 	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" {
+	if len(api.edits) != 2 || api.edits[1].ChannelID != "thread-1" || api.edits[1].Name != "msg-3" {
 		t.Fatalf("reply edit = %+v", api.edits)
 	}
 }
@@ -156,6 +161,36 @@ func TestOutboundSkipsReplyWithoutRootLink(t *testing.T) {
 	}
 }
 
+func TestOutboundSkipsAlreadyLinkedPost(t *testing.T) {
+	t.Parallel()
+
+	links := newMemoryLinks()
+	api := &fakeAPI{}
+	bot := New("channel-1", links, api)
+	if err := links.Upsert(context.Background(), store.DiscordLink{
+		PostID:    "reply-1",
+		MessageID: "d-reply-1",
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if err := links.Upsert(context.Background(), store.DiscordLink{
+		PostID:    "root-1",
+		MessageID: "d-root",
+		ThreadID:  "thread-1",
+	}); err != nil {
+		t.Fatal(err)
+	}
+	bot.Handle(context.Background(), events.PostCreated{PostEvent: events.PostEvent{
+		PostID:   "reply-1",
+		RootID:   "root-1",
+		ParentID: "root-1",
+		Body:     "Already on Discord.",
+	}})
+	if len(api.sends) != 0 {
+		t.Fatalf("echoed already-linked reply: %+v", api.sends)
+	}
+}
+
 func TestOutboundUpdateWithoutLinkCreates(t *testing.T) {
 	t.Parallel()
 
@@ -190,21 +225,30 @@ func TestFormatMessage(t *testing.T) {
 		got.City != "Oakland" ||
 		got.Author != "sam" ||
 		got.URL != "https://example.com/q" ||
-		got.ThreadName != "Leaky sink" ||
+		got.ThreadName != "sam asks: 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" {
+	if reply.Title != "Reply" || reply.Author != "Someone" || reply.ThreadName != "Someone asks: Question" {
 		t.Fatalf("reply format = %+v", reply)
 	}
+	content := messageContent(got)
+	if strings.Contains(content, "Leaky sink") ||
+		!strings.Contains(content, "It drips.") ||
+		!strings.Contains(content, "Oakland") {
+		t.Fatalf("content = %q", content)
+	}
+	if publicURL("/questions/x") != "" || publicURL("http://localhost:8080/q") != "" {
+		t.Fatal("localhost or relative permalink should not be an embed URL")
+	}
 }
 
 func TestFromEnvDisabled(t *testing.T) {
 	t.Setenv("DISCORD_BOT_TOKEN", "")
 	t.Setenv("DISCORD_CHANNEL_ID", "")
-	bot, err := FromEnv(newMemoryLinks(), nil, nil, nil)
+	bot, err := FromEnv(newMemoryLinks(), nil, nil)
 	if err != nil || bot != nil {
 		t.Fatalf("disabled FromEnv = (%v, %v)", bot, err)
 	}
@@ -213,12 +257,12 @@ func TestFromEnvDisabled(t *testing.T) {
 func TestFromEnvRequiresBoth(t *testing.T) {
 	t.Setenv("DISCORD_BOT_TOKEN", "token")
 	t.Setenv("DISCORD_CHANNEL_ID", "")
-	if _, err := FromEnv(newMemoryLinks(), nil, nil, nil); err == nil {
+	if _, err := FromEnv(newMemoryLinks(), nil, 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, nil, nil); err == nil {
+	if _, err := FromEnv(newMemoryLinks(), nil, nil); err == nil {
 		t.Fatal("expected error when token is missing")
 	}
 }

internal/discord/format.go

@@ -39,7 +39,7 @@ func formatMessage(ev events.PostEvent) Message {
 		Description: truncateRunes(strings.TrimSpace(ev.Body), embedDescriptionLimit),
 		City:        strings.TrimSpace(ev.City),
 		Author:      author,
-		ThreadName:  threadName(ev.Title),
+		ThreadName:  threadName(author, ev.Title),
 	}
 	for _, img := range ev.Images {
 		url := strings.TrimSpace(img.URL)
@@ -51,12 +51,16 @@ func formatMessage(ev events.PostEvent) Message {
 	return msg
 }
 
-func threadName(title string) string {
+func threadName(author, title string) string {
+	author = strings.TrimSpace(author)
+	if author == "" {
+		author = "Someone"
+	}
 	title = strings.TrimSpace(title)
 	if title == "" {
-		return "Question"
+		title = "Question"
 	}
-	return truncateRunes(title, threadNameLimit)
+	return truncateRunes(author+" asks: "+title, threadNameLimit)
 }
 
 func truncateRunes(s string, max int) string {
@@ -73,3 +77,35 @@ func truncateRunes(s string, max int) string {
 func isRoot(ev events.PostEvent) bool {
 	return strings.TrimSpace(ev.ParentID) == ""
 }
+
+func messageContent(msg Message) string {
+	var parts []string
+	if body := strings.TrimSpace(msg.Description); body != "" {
+		parts = append(parts, body)
+	}
+	var meta []string
+	if msg.City != "" {
+		meta = append(meta, msg.City)
+	}
+	if msg.Author != "" {
+		meta = append(meta, msg.Author)
+	}
+	if len(meta) > 0 {
+		parts = append(parts, strings.Join(meta, " · "))
+	}
+	if u := publicURL(msg.URL); u != "" {
+		parts = append(parts, u)
+	}
+	return truncateRunes(strings.Join(parts, "\n"), 2000)
+}
+
+func publicURL(raw string) string {
+	raw = strings.TrimSpace(raw)
+	if !strings.HasPrefix(raw, "https://") {
+		return ""
+	}
+	if strings.Contains(raw, "localhost") || strings.Contains(raw, "127.0.0.1") {
+		return ""
+	}
+	return raw
+}

internal/discord/inbound.go

@@ -7,11 +7,10 @@ import (
 	"fmt"
 	"log"
 	"strings"
-	"time"
 
 	"github.com/bwmarrin/discordgo"
 
-	"plumber/internal/mail"
+	"plumber/internal/events"
 	"plumber/internal/store"
 )
 
@@ -127,8 +126,9 @@ func (b *Bot) handleInbound(in inboundMessage) {
 		MessageID: in.ID,
 	}); err != nil {
 		log.Printf("discord: save inbound link %s: %v", reply.ID, err)
+		return
 	}
-	b.notifyInboundReply(parent, root, reply, author)
+	b.publishInbound(reply, root, author)
 	log.Printf("discord: inbound reply %s -> post %s", in.ID, reply.ID)
 }
 
@@ -184,44 +184,20 @@ func (b *Bot) postAndRoot(ctx context.Context, postID string) (*store.Post, *sto
 	return post, current, nil
 }
 
-func (b *Bot) notifyInboundReply(parent, root, reply *store.Post, author *store.User) {
+func (b *Bot) publishInbound(reply, root *store.Post, author *store.User) {
-	if parent == nil || root == nil || reply == nil || author == nil || b.mail == nil {
+	if b == nil || b.bus == nil || reply == nil || root == nil || author == nil {
 		return
 	}
-	if _, disabled := b.mail.(mail.Nop); disabled {
+	ev := events.PostEvent{
-		return
+		PostID:     reply.ID,
-	}
-	recipientID := parent.AuthorID
-	if author.Admin() {
-		recipientID = root.AuthorID
-	}
-	if recipientID == author.ID {
-		return
-	}
-	msg := mail.PostReply{
 		RootID:     root.ID,
-		RootTitle:       root.Title,
+		Body:       reply.Body,
-		ReplyID:         reply.ID,
+		AuthorID:   author.ID,
-		ReplyBody:       reply.Body,
+		AuthorName: author.Name,
-		ReplyAuthorName: author.Name,
+		AuthorRole: string(author.Role),
 	}
-	go func() {
+	if reply.ParentID != nil {
-		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		ev.ParentID = *reply.ParentID
-		defer cancel()
-		recipient, err := b.store.UserByID(ctx, recipientID)
-		if err != nil {
-			log.Printf("notify reply %s: load recipient: %v", msg.ReplyID, err)
-			return
 	}
-		if recipient == nil || strings.TrimSpace(recipient.Email) == "" {
+	b.bus.Publish(context.Background(), events.PostCreated{PostEvent: ev})
-			return
-		}
-		msg.ToEmail = recipient.Email
-		msg.ToName = recipient.Name
-		if err := b.mail.NotifyPostReply(ctx, msg); err != nil {
-			log.Printf("notify reply %s: %v", msg.ReplyID, err)
-			return
-		}
-		log.Printf("notify reply %s: accepted", msg.ReplyID)
-	}()
 }

internal/discord/inbound_test.go

@@ -27,9 +27,11 @@ func TestInboundCreatesSiteReply(t *testing.T) {
 	links := newMemoryLinks()
 	api := &fakeAPI{}
 	mailer := &mail.Recording{}
-	bot := inboundTestBot(mem, links, api, mailer, admin.Username)
+	bot := inboundTestBot(mem, links, api, admin.Username)
 	bus := events.New()
 	defer bus.Close()
+	bot.bus = bus
+	mail.Subscribe(bus, mem, mailer)
 	bus.Subscribe(bot.Handle)
 	root := seedLinkedRoot(t, mem, links, homeowner.ID, "thread-1")
 
@@ -68,7 +70,7 @@ func TestInboundParentsFromReference(t *testing.T) {
 
 	mem, homeowner, admin := seedInboundUsers(t)
 	links := newMemoryLinks()
-	bot := inboundTestBot(mem, links, &fakeAPI{}, &mail.Recording{}, admin.Username)
+	bot := inboundTestBot(mem, links, &fakeAPI{}, admin.Username)
 	root := seedLinkedRoot(t, mem, links, homeowner.ID, "thread-1")
 	plumberReply := &store.Post{ParentID: &root.ID, AuthorID: admin.ID, Body: "First look."}
 	if err := mem.CreatePost(context.Background(), plumberReply); err != nil {
@@ -106,7 +108,7 @@ func TestInboundIgnoresAllowlistHiddenAndEchoSources(t *testing.T) {
 	mem, homeowner, admin := seedInboundUsers(t)
 	links := newMemoryLinks()
 	api := &fakeAPI{}
-	bot := inboundTestBot(mem, links, api, mail.Nop{}, admin.Username)
+	bot := inboundTestBot(mem, links, api, admin.Username)
 	root := seedLinkedRoot(t, mem, links, homeowner.ID, "thread-1")
 	hidden := &store.Post{
 		AuthorID:  homeowner.ID,
@@ -148,10 +150,9 @@ func TestInboundIgnoresAllowlistHiddenAndEchoSources(t *testing.T) {
 	}
 }
 
-func inboundTestBot(mem *store.Memory, links *memoryLinks, api *fakeAPI, mailer mail.Notifier, adminUsername string) *Bot {
+func inboundTestBot(mem *store.Memory, links *memoryLinks, api *fakeAPI, adminUsername string) *Bot {
 	bot := New("channel-1", links, api)
 	bot.store = mem
-	bot.mail = mailer
 	bot.admins = map[string]string{"snow-admin": adminUsername}
 	bot.botUserID = "bot-1"
 	return bot

internal/mail/subscriber.go

@@ -0,0 +1,89 @@
+package mail
+
+import (
+	"context"
+	"log"
+	"strings"
+	"time"
+
+	"plumber/internal/events"
+	"plumber/internal/store"
+)
+
+// Subscribe sends reply emails from PostCreated events. Nop or nil is a no-op.
+func Subscribe(bus *events.Bus, st store.Store, n Notifier) {
+	if bus == nil || st == nil || n == nil {
+		return
+	}
+	if _, disabled := n.(Nop); disabled {
+		return
+	}
+	s := subscriber{store: st, mail: n}
+	bus.Subscribe(s.handle)
+}
+
+type subscriber struct {
+	store store.Store
+	mail  Notifier
+}
+
+func (s subscriber) handle(_ context.Context, ev any) {
+	created, ok := ev.(events.PostCreated)
+	if !ok {
+		return
+	}
+	if strings.TrimSpace(created.ParentID) == "" {
+		return
+	}
+	go s.notifyReply(created.PostEvent)
+}
+
+func (s subscriber) notifyReply(ev events.PostEvent) {
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	parent, err := s.store.GetPost(ctx, ev.ParentID)
+	if err != nil {
+		log.Printf("notify reply %s: load parent: %v", ev.PostID, err)
+		return
+	}
+	root, err := s.store.GetPost(ctx, ev.RootID)
+	if err != nil {
+		log.Printf("notify reply %s: load root: %v", ev.PostID, err)
+		return
+	}
+	author, err := s.store.UserByID(ctx, ev.AuthorID)
+	if err != nil {
+		log.Printf("notify reply %s: load author: %v", ev.PostID, err)
+		return
+	}
+	recipientID := parent.AuthorID
+	if author.Admin() {
+		recipientID = root.AuthorID
+	}
+	if recipientID == author.ID {
+		return
+	}
+	msg := PostReply{
+		RootID:          root.ID,
+		RootTitle:       root.Title,
+		ReplyID:         ev.PostID,
+		ReplyBody:       ev.Body,
+		ReplyAuthorName: author.Name,
+	}
+	recipient, err := s.store.UserByID(ctx, recipientID)
+	if err != nil {
+		log.Printf("notify reply %s: load recipient: %v", msg.ReplyID, err)
+		return
+	}
+	if recipient == nil || strings.TrimSpace(recipient.Email) == "" {
+		return
+	}
+	msg.ToEmail = recipient.Email
+	msg.ToName = recipient.Name
+	if err := s.mail.NotifyPostReply(ctx, msg); err != nil {
+		log.Printf("notify reply %s: %v", msg.ReplyID, err)
+		return
+	}
+	log.Printf("notify reply %s: accepted", msg.ReplyID)
+}

internal/mail/subscriber_test.go

@@ -0,0 +1,200 @@
+package mail
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"plumber/internal/events"
+	"plumber/internal/pacific"
+	"plumber/internal/store"
+)
+
+func TestSubscribeReplyNotifications(t *testing.T) {
+	t.Parallel()
+
+	mem := store.NewMemory()
+	homeowner := seedMailUser(t, mem, "homeowner", store.RoleUser, "sam@example.com")
+	admin := seedMailUser(t, mem, "plumber", store.RoleAdmin, "pat@example.com")
+	root := seedMailRoot(t, mem, homeowner.ID, "Leaky sink", "It drips.")
+	adminReply := seedMailReply(t, mem, admin.ID, root.ID)
+	homeownerReply := seedMailReply(t, mem, homeowner.ID, adminReply.ID)
+
+	bus := events.New()
+	defer bus.Close()
+	recording := &Recording{}
+	Subscribe(bus, mem, recording)
+
+	ctx := context.Background()
+	bus.Publish(ctx, events.PostCreated{PostEvent: events.PostEvent{
+		PostID:   root.ID,
+		RootID:   root.ID,
+		Title:    root.Title,
+		Body:     root.Body,
+		AuthorID: homeowner.ID,
+	}})
+	bus.Publish(ctx, events.PostUpdated{PostEvent: events.PostEvent{
+		PostID:   adminReply.ID,
+		RootID:   root.ID,
+		ParentID: root.ID,
+		Body:     "Edited",
+		AuthorID: admin.ID,
+	}})
+
+	bus.Publish(ctx, events.PostCreated{PostEvent: events.PostEvent{
+		PostID:   adminReply.ID,
+		RootID:   root.ID,
+		ParentID: root.ID,
+		Body:     adminReply.Body,
+		AuthorID: admin.ID,
+	}})
+	msgs := waitForMail(t, recording, 1)
+	if msg := msgs[0]; msg.ToEmail != homeowner.Email ||
+		msg.RootID != root.ID ||
+		msg.RootTitle != root.Title ||
+		msg.ReplyID != adminReply.ID ||
+		msg.ReplyBody != adminReply.Body ||
+		msg.ReplyAuthorName != admin.Name {
+		t.Fatalf("admin reply = %+v", msg)
+	}
+
+	bus.Publish(ctx, events.PostCreated{PostEvent: events.PostEvent{
+		PostID:   homeownerReply.ID,
+		RootID:   root.ID,
+		ParentID: adminReply.ID,
+		Body:     homeownerReply.Body,
+		AuthorID: homeowner.ID,
+	}})
+	msgs = waitForMail(t, recording, 2)
+	if msg := msgs[1]; msg.ToEmail != admin.Email ||
+		msg.RootID != root.ID ||
+		msg.ReplyID != homeownerReply.ID ||
+		msg.ReplyAuthorName != homeowner.Name {
+		t.Fatalf("homeowner reply = %+v", msg)
+	}
+
+	nestedAdmin := seedMailReply(t, mem, admin.ID, adminReply.ID)
+	bus.Publish(ctx, events.PostCreated{PostEvent: events.PostEvent{
+		PostID:   nestedAdmin.ID,
+		RootID:   root.ID,
+		ParentID: adminReply.ID,
+		Body:     nestedAdmin.Body,
+		AuthorID: admin.ID,
+	}})
+	msgs = waitForMail(t, recording, 3)
+	if msg := msgs[2]; msg.ToEmail != homeowner.Email ||
+		msg.RootID != root.ID ||
+		msg.ReplyBody != nestedAdmin.Body ||
+		msg.ReplyAuthorName != admin.Name {
+		t.Fatalf("nested admin reply = %+v", msg)
+	}
+
+	self := seedMailReply(t, mem, homeowner.ID, root.ID)
+	bus.Publish(ctx, events.PostCreated{PostEvent: events.PostEvent{
+		PostID:   self.ID,
+		RootID:   root.ID,
+		ParentID: root.ID,
+		Body:     self.Body,
+		AuthorID: homeowner.ID,
+	}})
+
+	noEmail := seedMailUser(t, mem, "quiet", store.RoleUser, "")
+	quietRoot := seedMailRoot(t, mem, noEmail.ID, "Quiet thread", "No email.")
+	quietReply := seedMailReply(t, mem, admin.ID, quietRoot.ID)
+	bus.Publish(ctx, events.PostCreated{PostEvent: events.PostEvent{
+		PostID:   quietReply.ID,
+		RootID:   quietRoot.ID,
+		ParentID: quietRoot.ID,
+		Body:     quietReply.Body,
+		AuthorID: admin.ID,
+	}})
+
+	time.Sleep(50 * time.Millisecond)
+	if recording.Len() != 3 {
+		t.Fatalf("self, root, edit, or no-email sent mail: %+v", recording.Snapshot())
+	}
+}
+
+func TestSubscribeNopIgnoresReplies(t *testing.T) {
+	t.Parallel()
+
+	mem := store.NewMemory()
+	homeowner := seedMailUser(t, mem, "homeowner", store.RoleUser, "sam@example.com")
+	admin := seedMailUser(t, mem, "plumber", store.RoleAdmin, "pat@example.com")
+	root := seedMailRoot(t, mem, homeowner.ID, "Leaky sink", "It drips.")
+	reply := seedMailReply(t, mem, admin.ID, root.ID)
+
+	bus := events.New()
+	defer bus.Close()
+	recording := &Recording{}
+	Subscribe(bus, mem, Nop{})
+	Subscribe(nil, mem, recording)
+	Subscribe(bus, mem, nil)
+
+	bus.Publish(context.Background(), events.PostCreated{PostEvent: events.PostEvent{
+		PostID:   reply.ID,
+		RootID:   root.ID,
+		ParentID: root.ID,
+		Body:     reply.Body,
+		AuthorID: admin.ID,
+	}})
+	time.Sleep(50 * time.Millisecond)
+	if recording.Len() != 0 {
+		t.Fatalf("Nop or nil subscribe sent mail: %+v", recording.Snapshot())
+	}
+}
+
+func seedMailUser(t *testing.T, mem *store.Memory, username string, role store.Role, email string) *store.User {
+	t.Helper()
+	u := &store.User{
+		Username:     username,
+		Name:         username,
+		Email:        email,
+		PasswordHash: "x",
+		Role:         role,
+	}
+	if err := mem.CreateUser(context.Background(), u); err != nil {
+		t.Fatal(err)
+	}
+	return u
+}
+
+func seedMailRoot(t *testing.T, mem *store.Memory, authorID, title, body string) *store.Post {
+	t.Helper()
+	root := &store.Post{
+		AuthorID: authorID,
+		Title:    title,
+		Body:     body,
+		PostDate: pacific.Today(),
+	}
+	if err := mem.CreatePost(context.Background(), root); err != nil {
+		t.Fatal(err)
+	}
+	return root
+}
+
+func seedMailReply(t *testing.T, mem *store.Memory, authorID, parentID string) *store.Post {
+	t.Helper()
+	reply := &store.Post{
+		ParentID: &parentID,
+		AuthorID: authorID,
+		Body:     "Reply from " + authorID,
+	}
+	if err := mem.CreatePost(context.Background(), reply); err != nil {
+		t.Fatal(err)
+	}
+	return reply
+}
+
+func waitForMail(t *testing.T, recording *Recording, want int) []PostReply {
+	t.Helper()
+	deadline := time.Now().Add(2 * time.Second)
+	for time.Now().Before(deadline) {
+		if recording.Len() >= want {
+			return recording.Snapshot()
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	t.Fatalf("recorded %d notifications, want %d", recording.Len(), want)
+	return nil
+}

internal/web/posts.go

@@ -5,15 +5,12 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
-	"log"
 	"net/http"
 	"net/url"
 	"strings"
-	"time"
 
 	"github.com/go-chi/chi/v5"
 
-	"plumber/internal/mail"
 	"plumber/internal/store"
 )
 
@@ -82,9 +79,6 @@ func (s *Server) handleCreatePost(w http.ResponseWriter, r *http.Request) {
 	if root == nil {
 		root = post
 	}
-	if parent != nil {
-		s.notifyPostReply(parent, root, post, user)
-	}
 	s.publishPostCreated(post, root, user)
 	http.Redirect(
 		w,
@@ -94,59 +88,6 @@ func (s *Server) handleCreatePost(w http.ResponseWriter, r *http.Request) {
 	)
 }
 
-// notifyPostReply emails the root homeowner for admin replies and the direct
-// parent author for homeowner replies.
-func (s *Server) notifyPostReply(
-	parent *store.Post,
-	root *store.Post,
-	reply *store.Post,
-	replyAuthor *store.User,
-) {
-	if parent == nil ||
-		root == nil ||
-		reply == nil ||
-		replyAuthor == nil ||
-		s.cfg.Mail == nil {
-		return
-	}
-	if _, disabled := s.cfg.Mail.(mail.Nop); disabled {
-		return
-	}
-	recipientID := parent.AuthorID
-	if replyAuthor.Admin() {
-		recipientID = root.AuthorID
-	}
-	if recipientID == replyAuthor.ID {
-		return
-	}
-	msg := mail.PostReply{
-		RootID:          root.ID,
-		RootTitle:       root.Title,
-		ReplyID:         reply.ID,
-		ReplyBody:       reply.Body,
-		ReplyAuthorName: replyAuthor.Name,
-	}
-	go func() {
-		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
-		defer cancel()
-		recipient, err := s.store.UserByID(ctx, recipientID)
-		if err != nil {
-			log.Printf("notify reply %s: load recipient: %v", msg.ReplyID, err)
-			return
-		}
-		if recipient == nil || strings.TrimSpace(recipient.Email) == "" {
-			return
-		}
-		msg.ToEmail = recipient.Email
-		msg.ToName = recipient.Name
-		if err := s.cfg.Mail.NotifyPostReply(ctx, msg); err != nil {
-			log.Printf("notify reply %s: %v", msg.ReplyID, err)
-			return
-		}
-		log.Printf("notify reply %s: accepted", msg.ReplyID)
-	}()
-}
-
 // handleEditPost updates only a post's body after verifying that the current
 // homeowner owns it or that an admin is editing an admin-authored post.
 func (s *Server) handleEditPost(w http.ResponseWriter, r *http.Request) {

internal/web/posts_test.go

@@ -9,6 +9,7 @@ import (
 	"testing"
 	"time"
 
+	"plumber/internal/events"
 	"plumber/internal/mail"
 	"plumber/internal/pacific"
 	"plumber/internal/store"
@@ -263,8 +264,12 @@ func TestEditPostRoutePermissions(t *testing.T) {
 func TestPostReplyNotifications(t *testing.T) {
 	t.Parallel()
 
+	mem := store.NewMemory()
+	bus := events.New()
+	defer bus.Close()
 	recording := &mail.Recording{}
-	srv, mem := newTestServer(t, Config{Mail: recording})
+	mail.Subscribe(bus, mem, recording)
+	srv := newTestServerStore(t, mem, Config{Events: bus})
 	handler := srv.Handler()
 	homeowner := seedUser(t, mem, uniq("homeowner"), "hunter22", store.RoleUser)
 	admin := seedUser(t, mem, uniq("admin"), "hunter22", store.RoleAdmin)

internal/web/server.go

@@ -22,7 +22,6 @@ import (
 	"plumber/internal/blob"
 	"plumber/internal/events"
 	"plumber/internal/geo"
-	"plumber/internal/mail"
 	"plumber/internal/pacific"
 	"plumber/internal/store"
 )
@@ -35,7 +34,6 @@ type Config struct {
 	// TrustedProxies are CIDRs allowed to set X-Forwarded-For (direct peer).
 	TrustedProxies []*net.IPNet
 	Blob           blob.Uploader
-	Mail           mail.Notifier
 	Events         events.Publisher
 	BaseURL        string
 }
@@ -110,9 +108,6 @@ func New(st store.Store, sessionStore scs.Store, templateFS fs.FS, staticFS fs.F
 	if cfg.Blob == nil {
 		cfg.Blob = blob.Disabled{}
 	}
-	if cfg.Mail == nil {
-		cfg.Mail = mail.Nop{}
-	}
 	if cfg.Events == nil {
 		cfg.Events = events.Nop{}
 	}