Keep the store's one connection, delete the seam it cannot survive (V-642)

`SetMaxOpenConns(1)` under WAL gives up concurrent reads, and the task
asked whether that costs anything. Measured over a fixed two-second
window, a paced writer against a read loop, three runs per cap:
reads do not queue. Four connections buy 70µs at p50 on a turn that
spends 1.19s in the resident model, and write throughput more than
halves. A 19ms worst case also cannot be the source of the 2.7s router
figure, so that line of enquiry is closed.

What the cap cannot survive is a long-lived transaction. It holds the
only connection, so a second read never completes: two seconds and
`context deadline exceeded`, against 1ms at a cap of four.

`Store.DB` handed out exactly that transaction. It had been there since
the initial commit with no production caller, and its comment described
a loop that never materialised. Its one user was a test helper reading
`delivery_attempts` by raw SQL, which `ListDeliveryAttempts` has covered
since V-390. So the cap stays and the seam goes, and the hazard is gone
by construction rather than by documentation.

`internal/store/conncap_test.go` stays as the standing measurement,
skipped under -short. The comment at the cap and the one in
`internal/ipc/server.go` that leans on it now state the invariant and
cite the numbers.

Measurement: docs/evals/2026-08-07-store-connection-cap.md

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/delivery/durability_test.go

@@ -94,20 +94,22 @@ func openTestStore(t *testing.T) *store.Store {
 
 // attemptStatus reads one attempt row back. Returns ok=false when the row is
 // gone, which would itself be a broken promise (a dropped attempt).
+//
+// It goes through ListDeliveryAttempts rather than raw SQL. This helper used to
+// reach past the store into store.DB, which was the tell that the outbox was
+// write-only; the reader landed in V-390 and this caller was not moved over.
 func attemptStatus(t *testing.T, st *store.Store, id int64) (status string, completed bool, ok bool) {
 	t.Helper()
-	tx, err := st.DB(context.Background())
+	attempts, err := st.ListDeliveryAttempts(context.Background(), "", 200)
 	if err != nil {
-		t.Fatalf("read tx: %v", err)
+		t.Fatalf("ListDeliveryAttempts: %v", err)
 	}
-	defer func() { _ = tx.Rollback() }()
-	var completedTS *int64
-	err = tx.QueryRowContext(context.Background(),
-		`SELECT status, completed_ts FROM delivery_attempts WHERE id = ?`, id).Scan(&status, &completedTS)
-	if err != nil {
-		return "", false, false
+	for _, a := range attempts {
+		if a.ID == id {
+			return a.Status, a.HasComplete, true
+		}
 	}
-	return status, completedTS != nil, true
+	return "", false, false
 }
 
 // TestCrashBetweenBeginAndCompleteBecomesUnknown — simulate the crash window:

internal/ipc/server.go

@@ -17,9 +17,11 @@ import (
 // Server — the core side of the boundary. Listens on a unix domain socket,
 // accepts module connections, frames requests to a CoreAPI and responses back.
 // One Server per daemon process; concurrent connections are handled in their
-// own goroutine but share the single CoreAPI (and therefore the single store
-// writer — store is single-connection, SetMaxOpenConns(1), so serialization is
-// already guaranteed at the db; the Server adds no locking of its own).
+// own goroutine but share the single CoreAPI, and so the single store writer.
+// The store opens at SetMaxOpenConns(1), so serialisation is already guaranteed
+// at the database and the Server adds no locking of its own. That cap is an
+// invariant this comment depends on, measured and kept on 07-08-2026 (V-642,
+// docs/evals/2026-08-07-store-connection-cap.md).
 type Server struct {
 	api  atomic.Value // stores CoreAPI
 	path string

internal/store/conncap_test.go

@@ -0,0 +1,154 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"path/filepath"
+	"sort"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+// conncap_test.go measures whether a read queues behind a write at
+// SetMaxOpenConns(1), which is what openAt sets (V-642). It is a measurement
+// harness, not an assertion: the numbers it prints are the evidence, and the
+// decision to move the cap or leave it belongs in docs/evals.
+//
+// Run it with -v, and note that it is skipped under -short because it spends
+// seconds on purpose.
+
+// openCapped opens a plaintext store at the given connection cap. In-package,
+// so it can reach the handle openAt caps at 1.
+func openCapped(t *testing.T, cap int) *Store {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), "cap.db")
+	db, err := openAt(context.Background(), path)
+	if err != nil {
+		t.Fatalf("openAt: %v", err)
+	}
+	db.SetMaxOpenConns(cap)
+	s := &Store{db: db}
+	t.Cleanup(func() { _ = s.Close() })
+	return s
+}
+
+func percentile(d []time.Duration, p float64) time.Duration {
+	if len(d) == 0 {
+		return 0
+	}
+	i := int(float64(len(d)-1) * p)
+	return d[i]
+}
+
+// seedFacts writes n facts so a read has rows to decode.
+func seedFacts(t *testing.T, s *Store, n int) {
+	t.Helper()
+	ctx := context.Background()
+	now := time.Now().UTC()
+	for i := 0; i < n; i++ {
+		key := fmt.Sprintf("seed_%d", i)
+		if _, err := s.SetValue(ctx, KindSelf, key, "tap:test",
+			map[string]int{"ml": i}, now.Add(time.Duration(i)*time.Millisecond)); err != nil {
+			t.Fatalf("seed %d: %v", i, err)
+		}
+	}
+}
+
+// measureReadsUnderWrites reports read latency percentiles while a writer
+// writes at a fixed pace. The pace matters: an unpaced writer completes a
+// different number of writes at each cap, because at a higher cap it competes
+// with the readers for the write lock instead of taking turns on one
+// connection. Two runs that did different work cannot be compared.
+// It runs for a fixed wall-clock window rather than a fixed read count, so the
+// paced writer does the same work at every cap. Tying the window to a read
+// count made the faster configuration receive fewer writes.
+func measureReadsUnderWrites(t *testing.T, s *Store, window, pace time.Duration) []time.Duration {
+	t.Helper()
+	ctx := context.Background()
+
+	var stop atomic.Bool
+	var writes atomic.Int64
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		now := time.Now().UTC()
+		for i := 0; !stop.Load(); i++ {
+			key := fmt.Sprintf("hot_%d", i%16)
+			if _, err := s.SetValue(ctx, KindSelf, key, "tap:test",
+				map[string]int{"n": i}, now.Add(time.Duration(i)*time.Millisecond)); err != nil {
+				t.Errorf("write: %v", err)
+				return
+			}
+			writes.Add(1)
+			time.Sleep(pace)
+		}
+	}()
+
+	var lat []time.Duration
+	deadline := time.Now().Add(window)
+	for time.Now().Before(deadline) {
+		start := time.Now()
+		if _, err := s.RecentFacts(ctx, 50); err != nil {
+			t.Fatalf("RecentFacts: %v", err)
+		}
+		lat = append(lat, time.Since(start))
+	}
+	stop.Store(true)
+	wg.Wait()
+	t.Logf("in %v: %d reads, %d writes", window, len(lat), writes.Load())
+
+	sort.Slice(lat, func(i, j int) bool { return lat[i] < lat[j] })
+	return lat
+}
+
+// TestConnCap_ReadLatencyUnderWrites is the V-642 measurement: read latency at
+// cap 1 against cap 4, same workload, same schema, same driver.
+func TestConnCap_ReadLatencyUnderWrites(t *testing.T) {
+	if testing.Short() {
+		t.Skip("measurement harness; runs for seconds")
+	}
+	for _, cap := range []int{1, 4} {
+		t.Run(fmt.Sprintf("cap=%d", cap), func(t *testing.T) {
+			s := openCapped(t, cap)
+			seedFacts(t, s, 500)
+			lat := measureReadsUnderWrites(t, s, 2*time.Second, 2*time.Millisecond)
+			t.Logf("cap=%d reads=%d p50=%v p95=%v max=%v",
+				cap, len(lat), percentile(lat, 0.50), percentile(lat, 0.95), lat[len(lat)-1])
+		})
+	}
+}
+
+// TestConnCap_ReadBlocksBehindOpenSnapshot is the sharper claim: at cap 1 an
+// open read-only transaction holds the only connection, so an unrelated read
+// cannot proceed until it commits. This is why the store exposes no way to
+// begin one — `Store.DB` used to, and was deleted in V-642 with no caller. The
+// test stays as the reason, so re-adding that seam fails a measurement rather
+// than shipping a stall.
+func TestConnCap_ReadBlocksBehindOpenSnapshot(t *testing.T) {
+	if testing.Short() {
+		t.Skip("measurement harness; waits on a timeout")
+	}
+	for _, cap := range []int{1, 4} {
+		t.Run(fmt.Sprintf("cap=%d", cap), func(t *testing.T) {
+			s := openCapped(t, cap)
+			seedFacts(t, s, 50)
+
+			tx, err := s.db.BeginTx(context.Background(), &sql.TxOptions{ReadOnly: true})
+			if err != nil {
+				t.Fatalf("BeginTx: %v", err)
+			}
+			defer func() { _ = tx.Rollback() }()
+
+			ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+			defer cancel()
+			start := time.Now()
+			_, err = s.RecentFacts(ctx, 10)
+			t.Logf("cap=%d read alongside an open snapshot: waited %v, err=%v",
+				cap, time.Since(start).Round(time.Millisecond), err)
+		})
+	}
+}

internal/store/store.go

@@ -95,7 +95,20 @@ func openAt(ctx context.Context, path string) (*sql.DB, error) {
 	if err != nil {
 		return nil, fmt.Errorf("open %s: %w", path, err)
 	}
-	// single writer expected; the daemon is the only process touching the db.
+	// One connection, so every statement is serialised at the database and no
+	// caller above needs a lock of its own. internal/ipc's Server relies on
+	// exactly this, which is why the cap is an invariant rather than a tuning
+	// knob: raising it moves the serialisation guarantee somewhere it is not
+	// written down.
+	//
+	// Measured on 07-08-2026 (V-642, docs/evals/2026-08-07-store-connection-cap.md).
+	// WAL exists to let readers run beside one writer, and the cap gives that
+	// up, but reads do not queue: p50 594µs against 525µs at a cap of four,
+	// while write throughput more than halves. The one thing the cap cannot
+	// survive is a long-lived transaction, which holds the only connection and
+	// stalls every read for its lifetime. So the store begins none, and
+	// TestConnCap_ReadBlocksBehindOpenSnapshot is the standing measurement of
+	// what re-adding one would cost.
 	db.SetMaxOpenConns(1)
 	if _, err := db.ExecContext(ctx, schemaSQL); err != nil {
 		if closeErr := db.Close(); closeErr != nil {
@@ -133,13 +146,6 @@ func (s *Store) Close() error {
 	return s.enc.closeAndSeal(s.db)
 }
 
-// DB exposes the underlying handle for internal read-only snapshots.
-// Used by the loop to take a consistent read under a single transaction.
-// Modules never receive this handle — core mediates.
-func (s *Store) DB(ctx context.Context) (*sql.Tx, error) {
-	return s.db.BeginTx(ctx, &sql.TxOptions{ReadOnly: true})
-}
-
 var (
 	// ErrNoFact — no non-voided row exists for this key.
 	ErrNoFact = errors.New("store: no fact for key")