Clear the SSE write deadline so an agent turn can outlive WriteTimeout (#87)

Closes #78. The 30s server WriteTimeout is an absolute deadline that was cutting
every agent turn over 30s mid-stream — and the keep-alive from #73 could never
work because it ticked into a connection destroyed at 30s. Refresh a per-write
deadline instead: unbounded stream, bounded writes, so a stuck reader still can't
pin the run goroutine. Two client-side regression tests, one per failure mode.

Gadfly: no material issues on final review.

internal/api/agent.go

@@ -108,6 +108,21 @@ func (h *handlers) agentChat(c *gin.Context) {
 	send(chatEvent{Done: turn})
 }
 
+// sseWriteTimeout bounds ONE write to the stream, not the stream itself.
+//
+// It is refreshed per frame, which is the only shape that satisfies both ends:
+// the server's absolute WriteTimeout would cut a long turn (#78), while removing
+// the deadline entirely would let a client that stops reading block a write
+// forever once the socket buffer fills — pinning the run goroutine and this
+// stream's mutex with it, and taking the keep-alive down too since it needs the
+// same lock. Generous, because it is a backstop against a stuck peer and not a
+// pacing mechanism.
+//
+// A var, not a const, ONLY so the test can shrink it to prove the deadline is
+// refreshed per frame rather than set once — a set-once 30s deadline would pass
+// a test whose whole run is under a second. Production never reassigns it.
+var sseWriteTimeout = 30 * time.Second
+
 // eventStream serializes writes to one SSE response.
 //
 // The mutex is load-bearing, not decoration: step events are sent from the
@@ -116,6 +131,7 @@ func (h *handlers) agentChat(c *gin.Context) {
 // frames long before it crashes anything.
 type eventStream struct {
 	c  *gin.Context
+	rc *http.ResponseController
 	mu sync.Mutex
 }
 
@@ -124,12 +140,34 @@ type eventStream struct {
 // 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.
+//
+// Taking the write deadline off the server's absolute WriteTimeout and onto a
+// per-write one is what makes a turn longer than 30s possible at all (#78).
+// WriteTimeout is an ABSOLUTE deadline from when the request header was read,
+// not an idle timeout, so a streaming response is cut mid-turn however recently
+// it wrote. Without this the 4-minute runTimeout is unreachable and the
+// keep-alive below tops out at one tick — pacing a connection that is destroyed
+// underneath it.
+//
+// That failure is INVISIBLE from in here: writes past the deadline return
+// err == nil and their bytes are dropped, so there is nothing to detect on the
+// write path. Only the client sees it, as a truncated stream it reports as a
+// dropped connection. Hence a deadline set up front and refreshed per frame,
+// rather than anything checked after the fact.
 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")
+	s := &eventStream{c: c, rc: http.NewResponseController(c.Writer)}
+	// Probe once here rather than reporting per frame: a writer that can't take
+	// deadlines will fail identically on every write, and the operator needs to
+	// hear it once. If this fails the stream still works — it is just back to
+	// being cut at WriteTimeout, which is worth saying out loud.
+	if err := s.rc.SetWriteDeadline(time.Now().Add(sseWriteTimeout)); err != nil {
+		slog.Error("api: SSE write deadlines unavailable; long turns will be truncated at the server WriteTimeout", "error", err)
+	}
 	c.Writer.Flush()
-	return &eventStream{c: c}
+	return s
 }
 
 func (s *eventStream) send(ev chatEvent) {
@@ -144,6 +182,11 @@ func (s *eventStream) send(ev chatEvent) {
 func (s *eventStream) write(frame string) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
+	// Refresh for THIS write, so the stream as a whole is unbounded but no single
+	// write is. Error deliberately unchecked: openEventStream already reported
+	// whether deadlines work at all, and this call can only fail the same way, so
+	// checking here would log once per frame to say the same thing.
+	_ = s.rc.SetWriteDeadline(time.Now().Add(sseWriteTimeout))
 	_, _ = io.WriteString(s.c.Writer, frame)
 	s.c.Writer.Flush()
 }

internal/api/sse_deadline_test.go

@@ -0,0 +1,101 @@
+package api
+
+import (
+	"bufio"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/gin-gonic/gin"
+)
+
+// streamFrames spins up a real http.Server with the given WriteTimeout and an
+// SSE handler that emits `frames` data frames, one every `tick`, then returns.
+// It reports how many frames the client actually received and any read error —
+// the only vantage point from which the deadline failures in #78/#87 are
+// visible, since the writes themselves return nil when the bytes are dropped.
+func streamFrames(t *testing.T, serverWriteTimeout, tick time.Duration, frames int) (int, error) {
+	t.Helper()
+	gin.SetMode(gin.TestMode)
+	r := gin.New()
+	r.GET("/stream", func(c *gin.Context) {
+		s := openEventStream(c)
+		for i := 0; i < frames; i++ {
+			time.Sleep(tick)
+			s.send(chatEvent{Error: "frame"})
+		}
+	})
+
+	srv := httptest.NewUnstartedServer(r)
+	srv.Config.WriteTimeout = serverWriteTimeout
+	srv.Start()
+	defer srv.Close()
+
+	resp, err := srv.Client().Get(srv.URL + "/stream")
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	defer resp.Body.Close()
+
+	got := 0
+	sc := bufio.NewScanner(resp.Body)
+	for sc.Scan() {
+		if strings.HasPrefix(sc.Text(), "data: ") {
+			got++
+		}
+	}
+	return got, sc.Err()
+}
+
+// TestEventStreamOutlivesServerWriteTimeout is the regression test for #78.
+//
+// http.Server.WriteTimeout is an ABSOLUTE deadline measured from when the
+// request header was read — not an idle timeout — so a streaming response is cut
+// once it passes, however recently the handler wrote. pansy sets it to 30s while
+// an agent turn may run for minutes. openEventStream must override it.
+//
+// This has to be asserted from the CLIENT side because the failure cannot be
+// observed from the handler: writes made after the deadline return err == nil
+// and their bytes are silently discarded. A test that checked the return of
+// io.WriteString would pass against the bug.
+func TestEventStreamOutlivesServerWriteTimeout(t *testing.T) {
+	// sseWriteTimeout stays at its 30s default here, so the per-frame refresh
+	// keeps the stream alive with a huge margin — CI slowness only ever makes
+	// this pass more surely. The server's 300ms WriteTimeout is the thing being
+	// overridden; frames straddle it (300ms/600ms/900ms).
+	got, err := streamFrames(t, 300*time.Millisecond, 300*time.Millisecond, 3)
+	if err != nil {
+		t.Errorf("client read error after %d/3 frames: %v", got, err)
+	}
+	if got != 3 {
+		t.Errorf("client received %d frames, want 3 — the stream was cut at the server WriteTimeout", got)
+	}
+}
+
+// TestEventStreamRefreshesDeadlinePerFrame guards the #87 fix specifically: the
+// write deadline is refreshed on EVERY frame, not set once.
+//
+// A set-once deadline is a plausible "simplification" and it reintroduces the
+// unbounded-block risk the per-frame refresh exists to prevent — yet it would
+// sail through the test above, whose whole run is far under sseWriteTimeout. So
+// shrink sseWriteTimeout below the stream's total duration and send frames whose
+// gap stays comfortably under it: per-frame refresh delivers them all, while a
+// deadline set once at open would expire mid-stream and cut it short.
+func TestEventStreamRefreshesDeadlinePerFrame(t *testing.T) {
+	orig := sseWriteTimeout
+	sseWriteTimeout = 400 * time.Millisecond
+	t.Cleanup(func() { sseWriteTimeout = orig })
+
+	// The server WriteTimeout is generous (5s), so it isn't the limiter — the
+	// per-frame sseWriteTimeout is. 8 frames at a 100ms tick span 800ms, well past
+	// the 400ms deadline, but each 100ms gap is a 4× margin under it.
+	got, err := streamFrames(t, 5*time.Second, 100*time.Millisecond, 8)
+	if err != nil {
+		t.Errorf("client read error after %d/8 frames: %v", got, err)
+	}
+	if got != 8 {
+		t.Errorf("client received %d frames, want 8 — a set-once deadline would cut the stream at ~%v; the refresh must be per-frame",
+			got, sseWriteTimeout)
+	}
+}