coder · hugodutka · Jun 18, 2026
diff --git a/coderd/x/chatd/messagepartbuffer/message_part_buffer.go b/coderd/x/chatd/messagepartbuffer/message_part_buffer.go
@@ -1,9 +1,25 @@
+// Package messagepartbuffer stores the transient message-part stream that a
+// chat worker emits before those parts are committed to durable chat history.
+//
+// Chat generation has two consumers with different timing. Stream endpoints
+// need to forward parts immediately, while interruption handling may need to
+// recover the partial assistant or tool message and commit it. Buffer groups
+// parts by an episode key that includes the chat, history version, and
+// generation attempt so stale workers and late subscribers do not mix parts
+// from different generations.
+//
+// Episodes are intentionally in-memory. They are closed when a generation
+// attempt ends, then retained briefly so stream subscribers and interruption
+// cleanup can drain the final parts. The cleanup loop removes closed episodes
+// after the retention window. Never-created placeholders are removed during
+// subscriber teardown, when the last early subscriber leaves.
 package messagepartbuffer
 
 import (
 	"container/heap"
 	"context"
 	"encoding/json"
+	"slices"
 	"sync"
 	"time"
 
@@ -18,7 +34,6 @@ const (
 	defaultMaxEpisodeBytes        = int64(1024 * 1024)
 	defaultClosedEpisodeRetention = 15 * time.Second
 	defaultSubscriberSendTimeout  = 10 * time.Second
-	defaultSubscriberChannelSize  = 16
 )
 
 var (
@@ -67,7 +82,6 @@ type Options struct {
 	MaxEpisodeBytes        int64
 	ClosedEpisodeRetention time.Duration
 	SubscriberSendTimeout  time.Duration
-	SubscriberChannelSize  int
 	Clock                  quartz.Clock
 }
 
@@ -148,47 +162,51 @@ func New(options Options) *Buffer {
 	if options.SubscriberSendTimeout <= 0 {
 		options.SubscriberSendTimeout = defaultSubscriberSendTimeout
 	}
-	if options.SubscriberChannelSize <= 0 {
-		options.SubscriberChannelSize = defaultSubscriberChannelSize
-	}
 	if options.Clock == nil {
 		options.Clock = quartz.NewReal()
 	}
 	buffer := &Buffer{
 		opts:     options,
 		episodes: make(map[Key]*episodeState),
-		done:     make(chan struct{}),
+		// done is unbuffered because it's only ever closed - never sent on.
+		done: make(chan struct{}),
 	}
 	buffer.startCleanupLoop()
 	return buffer
 }
 
 // CreateEpisode creates a new episode.
+//
+// Subscribers may attach before an episode is created. Creating the episode
+// makes it eligible to receive parts; the first AddPart wakes early subscribers.
 func (b *Buffer) CreateEpisode(key Key) error {
 	b.mu.Lock()
 	defer b.mu.Unlock()
 	if b.closed {
 		return ErrMessagePartBufferClosed
 	}
 	b.gcClosedEpisodesLocked(b.opts.Clock.Now("message-part-buffer", "create"))
-	episode := b.episodeLocked(key)
+	episode := b.getOrCreateEpisodeLocked(key)
 	if episode.created {
 		return ErrEpisodeExists
 	}
-	episode.created = true
+	episode.markCreated()
 	return nil
 }
 
 // AddPart appends a part to an existing episode.
+//
+// Parts receive contiguous sequence numbers so stream endpoints can detect
+// stale or broken episode subscriptions before forwarding data to clients.
 func (b *Buffer) AddPart(key Key, role codersdk.ChatMessageRole, part codersdk.ChatMessagePart) error {
 	b.mu.Lock()
 	defer b.mu.Unlock()
 	if b.closed {
 		return ErrMessagePartBufferClosed
 	}
-	episode := b.episodes[key]
-	if episode == nil || !episode.created {
-		return ErrEpisodeNotFound
+	episode, err := b.getEpisodeLocked(key)
+	if err != nil {
+		return err
 	}
 	if episode.closed {
 		return ErrEpisodeClosed
@@ -207,60 +225,71 @@ func (b *Buffer) AddPart(key Key, role codersdk.ChatMessageRole, part codersdk.C
 	}
 	episode.parts = append(episode.parts, buffered)
 	episode.bytes += sizeBytes
-	for subscriber := range episode.subscribers {
-		notifySubscriber(subscriber)
-	}
+	episode.notifySubscribers()
 	return nil
 }
 
-// CloseEpisode marks an episode closed and closes its subscribers.
+// CloseEpisode marks an episode closed.
+//
+// Closing creates the episode if it did not exist yet. This lets interruption
+// cleanup converge when a worker exits before it publishes any parts.
 func (b *Buffer) CloseEpisode(key Key) error {
 	b.mu.Lock()
 	defer b.mu.Unlock()
 	if b.closed {
 		return ErrMessagePartBufferClosed
 	}
-	episode := b.episodeLocked(key)
-	episode.created = true
-	if episode.closed {
+	episode := b.getOrCreateEpisodeLocked(key)
+	if !episode.close(b.opts.Clock.Now("message-part-buffer", "close")) {
 		return nil
 	}
-	episode.closed = true
-	episode.closedAt = b.opts.Clock.Now("message-part-buffer", "close")
 	b.queueClosedEpisodeLocked(key, episode)
-	for subscriber := range episode.subscribers {
-		notifySubscriber(subscriber)
-	}
+	episode.notifySubscribers()
 	return nil
 }
 
 // GetParts returns a snapshot of buffered parts for an episode.
+//
+// The returned slice is detached from the buffer so callers can process it
+// without holding the buffer lock.
 func (b *Buffer) GetParts(key Key) ([]Part, error) {
 	b.mu.Lock()
 	defer b.mu.Unlock()
 	if b.closed {
 		return nil, ErrMessagePartBufferClosed
 	}
 	b.gcClosedEpisodesLocked(b.opts.Clock.Now("message-part-buffer", "get"))
-	episode := b.episodes[key]
-	if episode == nil || !episode.created {
-		return nil, ErrEpisodeNotFound
+	episode, err := b.getEpisodeLocked(key)
+	if err != nil {
+		return nil, err
 	}
-	return append([]Part(nil), episode.parts...), nil
+	return slices.Clone(episode.parts), nil
 }
 
 // SubscribeToEpisode replays existing parts and streams new parts.
+//
+// Subscribers may attach before CreateEpisode is called. In that case the
+// subscription stays idle until the first part added, closure, cancellation,
+// or buffer shutdown. The returned cancel function is idempotent.
 func (b *Buffer) SubscribeToEpisode(ctx context.Context, key Key) (<-chan Part, func(), error) {
 	b.mu.Lock()
 	if b.closed {
 		b.mu.Unlock()
 		return nil, nil, ErrMessagePartBufferClosed
 	}
-	episode := b.episodeLocked(key)
+	episode := b.getOrCreateEpisodeLocked(key)
 	subscriber := &episodeSubscriber{
-		out:      make(chan Part),
+		// out is unbuffered so the delivery goroutine only advances once the
+		// subscriber has accepted each part. The send timeout bounds how long
+		// an unresponsive subscriber can keep its episode retained.
+		out: make(chan Part),
+		// notifyCh is a one-slot wakeup channel. Additional wakeups can be
+		// coalesced because the delivery goroutine copies all available parts
+		// each time it wakes.
 		notifyCh: make(chan struct{}, 1),
-		stopCh:   make(chan struct{}),
+		// stopCh is unbuffered because stop only closes it. Closing does not
+		// block and every select that observes it treats it as cancellation.
+		stopCh: make(chan struct{}),
 	}
 	if episode.subscribers == nil {
 		episode.subscribers = make(map[*episodeSubscriber]struct{})
@@ -358,7 +387,7 @@ func (b *Buffer) queueClosedEpisodeLocked(key Key, episode *episodeState) {
 	heap.Push(&b.closedEpisodes, item)
 }
 
-func (b *Buffer) episodeLocked(key Key) *episodeState {
+func (b *Buffer) getOrCreateEpisodeLocked(key Key) *episodeState {
 	episode := b.episodes[key]
 	if episode != nil {
 		return episode
@@ -368,6 +397,14 @@ func (b *Buffer) episodeLocked(key Key) *episodeState {
 	return episode
 }
 
+func (b *Buffer) getEpisodeLocked(key Key) (*episodeState, error) {
+	episode := b.episodes[key]
+	if episode == nil || !episode.created {
+		return nil, ErrEpisodeNotFound
+	}
+	return episode, nil
+}
+
 func (b *Buffer) subscriberParts(key Key, subscriber *episodeSubscriber) (parts []Part, closed bool, ok bool) {
 	b.mu.Lock()
 	defer b.mu.Unlock()
@@ -384,7 +421,7 @@ func (b *Buffer) subscriberParts(key Key, subscriber *episodeSubscriber) (parts
 	if subscriber.next > len(episode.parts) {
 		return nil, false, false
 	}
-	parts = append([]Part(nil), episode.parts[subscriber.next:]...)
+	parts = slices.Clone(episode.parts[subscriber.next:])
 	subscriber.next = len(episode.parts)
 	return parts, episode.closed && subscriber.next == len(episode.parts), true
 }
@@ -481,6 +518,27 @@ func notifySubscriber(subscriber *episodeSubscriber) {
 	}
 }
 
+func (e *episodeState) markCreated() {
+	e.created = true
+}
+
+// close marks the episode closed and returns false if it was already closed.
+func (e *episodeState) close(now time.Time) bool {
+	e.markCreated()
+	if e.closed {
+		return false
+	}
+	e.closed = true
+	e.closedAt = now
+	return true
+}
+
+func (e *episodeState) notifySubscribers() {
+	for subscriber := range e.subscribers {
+		notifySubscriber(subscriber)
+	}
+}
+
 func (s *episodeSubscriber) stop() {
 	s.stopOnce.Do(func() { close(s.stopCh) })
 }

diff --git a/coderd/x/chatd/messagepartbuffer/message_part_buffer_test.go b/coderd/x/chatd/messagepartbuffer/message_part_buffer_test.go
@@ -291,7 +291,7 @@ func TestBuffer_SlowSubscriberClosed(t *testing.T) {
 func TestBuffer_BurstyOutputDoesNotCloseSubscriberBeforeSendTimeout(t *testing.T) {
 	t.Parallel()
 
-	buffer := messagepartbuffer.New(messagepartbuffer.Options{SubscriberChannelSize: 1})
+	buffer := messagepartbuffer.New(messagepartbuffer.Options{})
 	defer buffer.Close()
 	key := testEpisodeKey()
 	require.NoError(t, buffer.CreateEpisode(key))