Commit history detached from cancellation on every path, not just failure (#73)

Co-authored-by: Steve Dudenhoeffer <[email protected]>

internal/api/agent.go

@@ -8,6 +8,8 @@ import (
 	"io"
 	"log/slog"
 	"net/http"
+	"sync"
+	"time"
 
 	mdagent "gitea.stevedudenhoeffer.com/steve/majordomo/agent"
 	"gitea.stevedudenhoeffer.com/steve/majordomo/llm"
@@ -24,6 +26,11 @@ import (
 // by everything at once, which reads as a hang — and the whole design rests on
 // watching the canvas change as it happens.
 
+// keepAliveInterval is how often a quiet stream emits a comment frame. Well
+// under the 30–60s idle timeout typical of reverse proxies, which is the thing
+// it exists to stay ahead of.
+const keepAliveInterval = 20 * time.Second
+
 // chatRequest is the body of POST /agent/chat.
 type chatRequest struct {
 	GardenID int64  `json:"gardenId" binding:"required"`
@@ -62,13 +69,21 @@ func (h *handlers) agentChat(c *gin.Context) {
 		return
 	}
 
-	send := openEventStream(c)
+	stream := openEventStream(c)
+	send := stream.send
+
+	// A model thinking hard between tool calls sends nothing for a while, and an
+	// idle proxy will cut a quiet connection. Deferred so a panic in the run
+	// can't leak the ticker goroutine; stopping it twice is harmless.
+	stopBeat := stream.keepAlive(keepAliveInterval)
+	defer stopBeat()
 
 	turn, err := h.agent.Run(c.Request.Context(), actor.ID, req.GardenID, req.Message,
 		replayHistory(history),
 		func(s mdagent.Step) {
 			send(chatEvent{Step: &stepEvent{Index: s.Index, Tools: toolNames(s)}})
 		})
+	stopBeat()
 	if err != nil {
 		// The stream is already open, so an error is an event rather than a
 		// status code — the client has committed to reading a stream by now.
@@ -93,25 +108,74 @@ func (h *handlers) agentChat(c *gin.Context) {
 	send(chatEvent{Done: turn})
 }
 
-// openEventStream puts the response into SSE mode and returns a sender.
+// eventStream serializes writes to one SSE response.
+//
+// The mutex is load-bearing, not decoration: step events are sent from the
+// agent's run goroutine while the keep-alive ticker writes from its own, and two
+// goroutines writing a ResponseWriter concurrently is a data race that corrupts
+// frames long before it crashes anything.
+type eventStream struct {
+	c  *gin.Context
+	mu sync.Mutex
+}
+
+// openEventStream puts the response into SSE mode.
 //
 // Headers go out before the first write and the stream is flushed immediately,
 // so a proxy holding the response until it looks complete can't reintroduce
 // exactly the silence streaming exists to remove.
-func openEventStream(c *gin.Context) func(chatEvent) {
+func openEventStream(c *gin.Context) *eventStream {
 	c.Header("Content-Type", "text/event-stream")
 	c.Header("Cache-Control", "no-cache")
 	c.Header("X-Accel-Buffering", "no")
 	c.Writer.Flush()
+	return &eventStream{c: c}
+}
 
-	return func(ev chatEvent) {
-		b, err := json.Marshal(ev)
-		if err != nil {
-			slog.Error("api: encode chat event", "error", err)
-			return
+func (s *eventStream) send(ev chatEvent) {
+	b, err := json.Marshal(ev)
+	if err != nil {
+		slog.Error("api: encode chat event", "error", err)
+		return
+	}
+	s.write(fmt.Sprintf("data: %s\n\n", b))
+}
+
+func (s *eventStream) write(frame string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	_, _ = io.WriteString(s.c.Writer, frame)
+	s.c.Writer.Flush()
+}
+
+// keepAlive writes an SSE comment frame on an interval until the returned
+// function is called, so a long silence while the model thinks doesn't look like
+// a dead connection to whatever sits in between. SSE ignores comment frames, so
+// this costs the client nothing.
+func (s *eventStream) keepAlive(every time.Duration) func() {
+	done := make(chan struct{})
+	stopped := make(chan struct{})
+	go func() {
+		defer close(stopped)
+		t := time.NewTicker(every)
+		defer t.Stop()
+		for {
+			select {
+			case <-done:
+				return
+			case <-s.c.Request.Context().Done():
+				return
+			case <-t.C:
+				s.write(": keep-alive\n\n")
+			}
 		}
-		_, _ = fmt.Fprintf(c.Writer, "data: %s\n\n", b)
-		c.Writer.Flush()
+	}()
+	// Idempotent: the handler stops it explicitly when the run returns and again
+	// via defer, so a panic can't leak the goroutine.
+	var once sync.Once
+	return func() {
+		once.Do(func() { close(done) })
+		<-stopped
 	}
 }