delivery log: measure the sweep before deciding its fate (audit H3, 0007)

Migration 0007 (DDL only): delivery_log_days, a row per UTC day, kept a
year, shown nowhere - read off a copy of the database after a month by the
decision rule written in the audit plan.

- reconcile_closed: the sweep adds the rows it actually closed.
- reconcile_contradicted: store.UpdateStatus, in one transaction, counts the
  rows with reason "reconcile" a delivery line now speaks of, writes the
  log's status and clears the reason - so a re-read line counts once.
- tailer_blind_seconds: follow() tracks stretches in which the file is gone
  or reopening fails (logtail.blindness); stored when a stretch ends and
  every minute while it lasts; under 3 s not counted (the rotation's tick
  between rename and create loses nothing).
- queued_over_lifetime: every sweep overwrites the day's level with the rows
  queued past LongestLifetime + MaximalBackoff (a message Postfix still
  works on stays queued until its first retry after the lifetime).
- retentionLoop prunes days older than a year.

Tests: store (sums per day, the level overwritten, under a second nothing,
prune; a contradicted sweep counted once and its reason cleared; count of
queued rows); logtail (the sweep counts what it closed; countStuck records
the level; blindness - a one-tick gap not counted, 75 s stored as 60 + 15
from where the last part ended). Schema tests at user_version 7.

schema-migrations.md: chain head 7 (it still said 5 since 2.4.0), row 7.
CHANGELOG [Unreleased], audit plan line ticked, roadmap 10/61.
Evidence: go vet ./... clean; go test ./internal/store ok, ./internal/logtail
ok but for TestFollowTailsAndRotates (Windows); the server run of the stage
follows before the push.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/logtail/logtail.go

@@ -32,6 +32,12 @@ type StatusStore interface {
 	UpdateStatus(queueID, recipient, status string) (int64, error)
 	ListQueuedOlderThan(cutoff time.Time) ([]store.QueuedDelivery, error)
 	CloseUnreported(queueID, recipient string) (int64, error)
+	// The counters of delivery_log_days (store/deliverylog.go).
+	AddReconcileClosed(at time.Time, n int64) error
+	AddTailerBlind(from time.Time, d time.Duration) error
+	CountQueuedOlderThan(cutoff time.Time) (int64, error)
+	SetQueuedOverLifetime(at time.Time, n int64) error
+	PruneDeliveryLogDays(now time.Time) error
 	DeleteSendLogBefore(cutoff time.Time) (int64, error)
 	InsertSendLogLines(lines []store.SendLogLine, once bool) (int, error)
 	PruneSendLogLines(cutoff time.Time) (int64, error)
@@ -56,6 +62,10 @@ func RetentionInterval() time.Duration {
 // It is a var so tests can answer without a running Postfix.
 var queueIDs = postfix.QueueIDs
 
+// retryPolicy reads Postfix's queue lifetimes, for counting the rows left
+// queued past them. A var for the same reason.
+var retryPolicy = postfix.LoadRetryPolicy
+
 const (
 	// reconcileInterval is how often the sweep compares stuck rows against the
 	// Postfix queue, and reconcileGrace how long a row is left alone first.
@@ -179,6 +189,11 @@ func reconcileLoop(ctx context.Context, st StatusStore, caughtUp <-chan struct{}
 	case <-caughtUp:
 	}
 
+	// A message Postfix still works on stays queued until its first retry after
+	// the queue lifetime, which can be a whole backoff later.
+	policy := retryPolicy()
+	overLifetime := policy.LongestLifetime() + policy.MaximalBackoff
+
 	t := time.NewTicker(reconcileInterval)
 	defer t.Stop()
 	for {
@@ -186,11 +201,26 @@ func reconcileLoop(ctx context.Context, st StatusStore, caughtUp <-chan struct{}
 		case <-ctx.Done():
 			return
 		case <-t.C:
-			reconcile(st, time.Now().UTC().Add(-reconcileGrace))
+			now := time.Now().UTC()
+			reconcile(st, now.Add(-reconcileGrace))
+			countStuck(st, now, overLifetime)
 		}
 	}
 }
 
+// countStuck records how many rows are still queued past Postfix's queue
+// lifetime - what the log would leave unresolved if there were no sweep. Each
+// sweep overwrites the day's value, so a past day keeps its last sweep's.
+func countStuck(st StatusStore, now time.Time, overLifetime time.Duration) {
+	n, err := st.CountQueuedOlderThan(now.Add(-overLifetime))
+	if err == nil {
+		err = st.SetQueuedOverLifetime(now, n)
+	}
+	if err != nil {
+		log.Printf("log-tailer: count rows queued past the queue lifetime: %v", err)
+	}
+}
+
 // reconcile marks as bounced every row still "queued" from before cutoff whose
 // message Postfix no longer holds, with reason "reconcile". The close itself
 // is conditional on the row still being queued, so a delivery line read
@@ -236,6 +266,9 @@ func reconcile(st StatusStore, cutoff time.Time) {
 	}
 	if closed > 0 {
 		log.Printf("log-tailer: reconcile: closed %d row(s) Postfix no longer holds and never reported", closed)
+		if err := st.AddReconcileClosed(time.Now(), closed); err != nil {
+			log.Printf("log-tailer: reconcile: count: %v", err)
+		}
 	}
 }
 
@@ -260,8 +293,15 @@ func retentionLoop(ctx context.Context, st StatusStore, retention RetentionDays)
 		}
 	}
 
+	pruneDays := func() {
+		if err := st.PruneDeliveryLogDays(time.Now()); err != nil {
+			log.Printf("log-tailer: %v", err)
+		}
+	}
+
 	prune()
 	pruneLines(st)
+	pruneDays()
 	t := time.NewTicker(retentionInterval)
 	defer t.Stop()
 	for {
@@ -271,6 +311,7 @@ func retentionLoop(ctx context.Context, st StatusStore, retention RetentionDays)
 		case <-t.C:
 			prune()
 			pruneLines(st)
+			pruneDays()
 		}
 	}
 }
@@ -552,12 +593,14 @@ func follow(ctx context.Context, path string, tr *tracker, caughtUp chan struct{
 	}
 
 	var once sync.Once
+	blind := &blindness{add: tr.st.AddTailerBlind}
 
 	ticker := time.NewTicker(pollInterval)
 	defer ticker.Stop()
 	for {
 		select {
 		case <-ctx.Done():
+			blind.see(time.Now(), false)
 			tr.record(f, read(), true) // shutdown: the next start resumes here
 			return nil
 		case <-ticker.C:
@@ -565,6 +608,7 @@ func follow(ctx context.Context, path string, tr *tracker, caughtUp chan struct{
 			once.Do(func() { close(caughtUp) })
 			ni, err := os.Stat(path)
 			if err != nil {
+				blind.see(time.Now(), true)
 				continue // file briefly gone mid-rotation; try again next tick
 			}
 			pos, _ := f.Seek(0, io.SeekCurrent)
@@ -576,12 +620,57 @@ func follow(ctx context.Context, path string, tr *tracker, caughtUp chan struct{
 				drain()
 				if err := openAt(0, io.SeekStart); err != nil {
 					log.Printf("log-tailer: reopen %s: %v", path, err)
+					blind.see(time.Now(), true)
 					continue
 				}
+				blind.see(time.Now(), false)
 				tr.adopt(f)
 				continue
 			}
+			blind.see(time.Now(), false)
 			tr.record(f, read(), false)
 		}
 	}
 }
+
+// blindness measures the stretches in which follow cannot read mail.log - the
+// file gone, reopening it failing - for delivery_log_days. A stretch is
+// stored when it ends, and every minute while it lasts, so a long one is not
+// lost to a restart and lands on the days it covered. A stretch shorter than
+// blindMin is not counted: a rotation leaves the name missing for a tick
+// between rename and create, and that loses nothing - the new file is read
+// from its start.
+type blindness struct {
+	add   func(from time.Time, d time.Duration) error
+	since time.Time // start of the current stretch; zero when reading
+	long  bool      // the current stretch has passed blindMin
+}
+
+const blindMin = 3 * time.Second
+
+func (b *blindness) see(now time.Time, blind bool) {
+	if blind && b.since.IsZero() {
+		b.since = now
+		return
+	}
+	if b.since.IsZero() {
+		return
+	}
+	d := now.Sub(b.since)
+	if d >= blindMin {
+		b.long = true
+	}
+	if blind && d < time.Minute {
+		return
+	}
+	if b.long {
+		if err := b.add(b.since, d); err != nil {
+			log.Printf("log-tailer: count the time mail.log could not be read: %v", err)
+		}
+	}
+	if blind {
+		b.since = now // the stretch goes on; the next part starts here
+		return
+	}
+	b.since, b.long = time.Time{}, false
+}