Leave a delivery cut off by a stop for the next start (closes #527)

Stopping the engine cancels the context every http and slack request is
sent with. An attempt cut off that way was recorded as failed, which for
a target with no retries, or on a last retry, ended the delivery failed
and nothing sent it again.

The shared retry core now records nothing for a failed attempt when the
engine's context is cancelled: no result, no circuit breaker outcome, no
status change. The delivery stays pending or retrying and startup
recovery sends it again. A target's own timeout is still a failed
attempt. The README's Graceful shutdown bullet, Shutdown section and the
at-least-once note say so.

Model: opus-5-5

README.md

@@ -1325,9 +1325,11 @@ webhooker solves this by acting as a durable intermediary:
     deliberately left in a recoverable state rather than marked done — losing a
     delivery is the worse failure — so the pending sweep picks it up about
     fifteen minutes later, or the next restart does, and the target receives a
-    payload it already got. webhooker adds no delivery identifier of its own to
-    an outbound request, so **make your receiver idempotent** against whatever
-    the payload itself carries.
+    payload it already got. A send still in flight when the service stops is
+    left the same way: its attempt is not recorded, and the next start sends it
+    again (see Shutdown). webhooker adds no delivery identifier of its own to an
+    outbound request, so **make your receiver idempotent** against whatever the
+    payload itself carries.
 
 3. **Observability** — Full request/response logging for every webhook received
    and every delivery attempted. Prometheus metrics expose volume, latency, and
@@ -1768,7 +1770,7 @@ is what bounds it.
 #### DeliveryResult
 
 The result of a single delivery attempt. Every attempt (including retries) is
-individually logged for full observability.
+individually logged, except one cut off by a stop (see Shutdown).
 
 | Field           | Type    | Description                               |
 | --------------- | ------- | ----------------------------------------- |
@@ -2206,12 +2208,14 @@ This means:
   buffer rather than spawning more goroutines.
 - **Independent results** — each worker records its own delivery result in the
   per-webhook database without coordination.
-- **Graceful shutdown** — cancel the context, workers finish their current task
-  and exit. The stop hook waits for the pool via `lifecycle.WaitForShutdown`,
-  which bounds that wait by fx's stop timeout rather than blocking forever on a
-  wedged worker. On timeout it logs at `ERROR` and returns an error, and the
-  goroutines that did not finish are still running — an unclean shutdown is
-  reported rather than hidden.
+- **Graceful shutdown** — the stop hook cancels the workers' context, and each
+  worker exits once its current task returns. Cancelling cuts off an `http` or
+  `slack` request still in flight; that attempt is not recorded, and the next
+  start sends the delivery again (see Shutdown). The stop hook waits for the
+  pool via `lifecycle.WaitForShutdown`, which bounds that wait by fx's stop
+  timeout rather than blocking forever on a wedged worker. On timeout it logs at
+  `ERROR` and returns an error, and the goroutines that did not finish are still
+  running — an unclean shutdown is reported rather than hidden.
 
 **Recovery paths:**
 
@@ -3284,7 +3288,8 @@ stop-hook log:
 3. `server` — the HTTP drain, bounded by `server.ShutdownTimeout` (**3
    seconds**) and by what the hooks before it left, then a Sentry flush if
    `SENTRY_DSN` is set
-4. `delivery.Engine` — waits for its workers, then closes the archive databases
+4. `delivery.Engine` — cuts off any `http` or `slack` request in flight, waits
+   for its workers, then closes the archive databases
 5. `healthcheck`
 6. `WebhookDBManager`
 7. the database close
@@ -3295,6 +3300,12 @@ by the stop context, so a wedge there consumes the 5 seconds before the HTTP
 server hook is ever entered. The hooks after the server are microsecond-scale in
 normal operation.
 
+The delivery engine does not wait for a delivery request in flight: it cancels
+it. That attempt is not recorded and the delivery keeps its `pending` or
+`retrying` status, so startup recovery sends it again on the next start (see
+Recovery paths under Bounded Worker Pool). If the receiver already had it, it
+receives it twice: the at-least-once duplicate described under Rationale.
+
 The HTTP drain budget is deliberately **shorter** than the sequence budget. Were
 the two equal, a drain that used its whole budget would exhaust the sequence
 budget at the instant it finished, and every later hook — the delivery engine,

internal/delivery/engine_lifecycle_test.go

@@ -2,11 +2,15 @@ package delivery_test
 
 import (
 	"context"
+	"net/http"
+	"net/http/httptest"
 	"path/filepath"
+	"sync"
 	"testing"
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/fx"
 	"gorm.io/gorm/clause"
@@ -272,6 +276,147 @@ func TestEngine_StopHookHonoursStopTimeout(t *testing.T) {
 	requireStopHookExpires(t, lc.hooks[0], "delivery engine")
 }
 
+// seedHeldDelivery gives the setup's webhook an http target with
+// maxRetries, pointed at a receiver that keeps every request open until
+// the client gives up on it, and seeds one delivery to that target at
+// status. It returns the task that sends attempt attemptNum of the
+// delivery, and a channel that is closed when a request reaches the
+// receiver.
+func seedHeldDelivery(
+	t *testing.T,
+	s iSetup,
+	maxRetries int,
+	status database.DeliveryStatus,
+	attemptNum int,
+) (delivery.Task, <-chan struct{}) {
+	t.Helper()
+
+	arrived := make(chan struct{})
+
+	var once sync.Once
+
+	ts := httptest.NewServer(http.HandlerFunc(
+		func(_ http.ResponseWriter, r *http.Request) {
+			once.Do(func() { close(arrived) })
+
+			<-r.Context().Done()
+		},
+	))
+	t.Cleanup(ts.Close)
+
+	cfg := iHTTPConfig(ts.URL)
+	targetID := uuid.New().String()
+
+	iCreateWebhook(t, s.MainDB, s.WebhookID, "hook")
+	iCreateTarget(
+		t, s.MainDB, targetID, s.WebhookID, "held",
+		database.TargetTypeHTTP, cfg, maxRetries,
+	)
+
+	event := iSeedEvent(t, s.WebhookDB, s.WebhookID, `{"held":true}`)
+	d := iSeedDelivery(t, s.WebhookDB, event.ID, targetID, status)
+
+	bodyStr := event.Body
+
+	return iTask(
+		d, event, s.WebhookID, targetID, "held", cfg,
+		maxRetries, attemptNum, &bodyStr,
+	), arrived
+}
+
+// stopDuringAttempt waits until the receiver holds an attempt open, then
+// stops the engine through its real stop hook.
+func stopDuringAttempt(
+	t *testing.T, lc *recordingLifecycle, arrived <-chan struct{},
+) {
+	t.Helper()
+
+	select {
+	case <-arrived:
+	case <-time.After(5 * time.Second):
+		t.Fatal("the delivery attempt never reached the receiver")
+	}
+
+	require.NoError(t, lc.hooks[0].OnStop(context.Background()))
+}
+
+// requireRequeued runs restart recovery on the stopped engine and
+// requires it to queue task's attempt again on ch.
+func requireRequeued(
+	t *testing.T, s iSetup, ch <-chan delivery.Task, task delivery.Task,
+) {
+	t.Helper()
+
+	s.Engine.ExportRecoverWebhookDeliveries(
+		context.Background(), s.WebhookID,
+	)
+
+	select {
+	case got := <-ch:
+		assert.Equal(t, task.DeliveryID, got.DeliveryID)
+		assert.Equal(t, task.AttemptNum, got.AttemptNum)
+	case <-time.After(5 * time.Second):
+		t.Fatal("restart recovery did not queue the delivery again")
+	}
+}
+
+// TestEngine_StopLeavesFireAndForgetAttemptPending is the regression
+// test for a stop that turned an attempt in flight into a terminal
+// failure. Stopping the engine cancels the request; for a target with
+// no retries that was the delivery's only attempt, recorded as failed
+// with "context canceled", and nothing ever sent it again. The
+// delivery must instead stay pending, with no result, for restart
+// recovery to send.
+func TestEngine_StopLeavesFireAndForgetAttemptPending(t *testing.T) {
+	t.Parallel()
+
+	s := newISetup(t)
+	lc := startEngineViaHook(t, s.Engine)
+
+	task, arrived := seedHeldDelivery(
+		t, s, 0, database.DeliveryStatusPending, 1,
+	)
+
+	s.Engine.Notify([]delivery.Task{task})
+	stopDuringAttempt(t, lc, arrived)
+
+	iAssertStatus(
+		t, s.WebhookDB, task.DeliveryID, database.DeliveryStatusPending,
+	)
+	assert.Empty(t, iResults(t, s.WebhookDB, task.DeliveryID))
+
+	requireRequeued(t, s, s.Engine.ExportDeliveryCh(), task)
+}
+
+// TestEngine_StopLeavesLastRetryAttemptRetrying is the same regression
+// for a target with retries, on its last attempt: the cancelled attempt
+// used to exhaust the retries and end the delivery failed. It must stay
+// retrying with only its earlier attempt recorded, and restart recovery
+// must send that last attempt again.
+func TestEngine_StopLeavesLastRetryAttemptRetrying(t *testing.T) {
+	t.Parallel()
+
+	const maxRetries = 2
+
+	s := newISetup(t)
+	lc := startEngineViaHook(t, s.Engine)
+
+	task, arrived := seedHeldDelivery(
+		t, s, maxRetries, database.DeliveryStatusRetrying, maxRetries,
+	)
+	iSeedFailedResult(t, s.WebhookDB, task.DeliveryID)
+
+	s.Engine.ExportScheduleRetry(task, 0)
+	stopDuringAttempt(t, lc, arrived)
+
+	iAssertStatus(
+		t, s.WebhookDB, task.DeliveryID, database.DeliveryStatusRetrying,
+	)
+	assert.Len(t, iResults(t, s.WebhookDB, task.DeliveryID), 1)
+
+	requireRequeued(t, s, s.Engine.ExportRetryCh(), task)
+}
+
 // deliverToArchive gives the setup's webhook a database target,
 // runs one delivery to it through the running engine, and returns
 // the target's ID and archive file path. The archive writer holds

internal/delivery/target_http.go

@@ -57,7 +57,11 @@ type httpCore struct {
 // recorded and no circuit breaker is consulted. A positive
 // maxRetries gates the attempt on the circuit breaker and
 // schedules a backed-off retry on failure.
+//
+// ctx is the context attempt sends its request with. Stopping the
+// engine cancels it; see interruptedByStop.
 func (c *httpCore) deliver(
+	ctx context.Context,
 	webhookDB *gorm.DB,
 	d *database.Delivery,
 	task *Task,
@@ -66,23 +70,56 @@ func (c *httpCore) deliver(
 	attempt func() attemptResult,
 ) {
 	if maxRetries == 0 {
-		c.fireAndForget(webhookDB, d, attempt())
+		c.fireAndForget(ctx, webhookDB, d, attempt())
 
 		return
 	}
 
 	c.withRetry(
-		webhookDB, d, task, sched, maxRetries, attempt,
+		ctx, webhookDB, d, task, sched, maxRetries, attempt,
 	)
 }
 
+// interruptedByStop reports whether an attempt failed because the
+// engine is stopping, which cancels the context its request was sent
+// with. Such an attempt is not recorded: no DeliveryResult, no circuit
+// breaker outcome, no status change. The delivery stays pending or
+// retrying, and restart recovery sends it again on the next start.
+//
+// A target's own timeout leaves that context alone, so its attempt is
+// still recorded as the failure it is.
+func (c *httpCore) interruptedByStop(
+	ctx context.Context,
+	d *database.Delivery,
+	res attemptResult,
+) bool {
+	if res.success || ctx.Err() == nil {
+		return false
+	}
+
+	c.eng.log.Warn(
+		"delivery attempt interrupted by shutdown; not recorded, "+
+			"it is sent again on the next start",
+		"delivery_id", d.ID,
+		"event_id", d.EventID,
+		"target_id", d.TargetID,
+	)
+
+	return true
+}
+
 func (c *httpCore) fireAndForget(
+	ctx context.Context,
 	webhookDB *gorm.DB,
 	d *database.Delivery,
 	res attemptResult,
 ) {
 	c.eng.observeAttempt(d.Target.Type, res.elapsed())
 
+	if c.interruptedByStop(ctx, d, res) {
+		return
+	}
+
 	err := c.eng.recordResult(
 		webhookDB, d, 1, res.success,
 		res.statusCode, res.respBody, res.errMsg,
@@ -110,6 +147,7 @@ func (c *httpCore) fireAndForget(
 }
 
 func (c *httpCore) withRetry(
+	ctx context.Context,
 	webhookDB *gorm.DB,
 	d *database.Delivery,
 	task *Task,
@@ -133,6 +171,10 @@ func (c *httpCore) withRetry(
 
 	c.eng.observeAttempt(d.Target.Type, res.elapsed())
 
+	if c.interruptedByStop(ctx, d, res) {
+		return
+	}
+
 	err := c.eng.recordResult(
 		webhookDB, d, attemptNum, res.success,
 		res.statusCode, res.respBody, res.errMsg,
@@ -395,7 +437,7 @@ func (t *httpTarget) Deliver(
 	}
 
 	t.deliver(
-		webhookDB, d, task, sched,
+		ctx, webhookDB, d, task, sched,
 		d.Target.MaxRetries, attempt,
 	)
 }

internal/delivery/target_slack.go

@@ -82,7 +82,7 @@ func (t *slackTarget) Deliver(
 	}
 
 	t.deliver(
-		webhookDB, d, task, sched,
+		ctx, webhookDB, d, task, sched,
 		d.Target.MaxRetries, attempt,
 	)
 }