Make the periodic reconciliation test wait for the startup pass (closes #189)

TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup slept for
three eviction intervals before writing its file, so on a slow start the
startup pass could still be running and adopt the file itself, and the
test passed without a periodic pass. It now holds the test database's
only connection until the startup pass waits for it, writes the file and
lets the connection go, as TestEvictionRunsOnPeriodicSchedule does, so
only a periodic reconciliation pass can adopt the file. Test only.

Model: opus-5-5

internal/imgcache/eviction_internal_test.go

@@ -847,15 +847,28 @@ func TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup(t *testing.T) {
 
 	cache, _ := newEvictionTestCache(t, 1<<30)
 
-	const interval = 100 * time.Millisecond
+	// Hold the test database's only connection, so the startup pass
+	// waits for it after walking the still empty variant directory: the
+	// file written while it waits is first seen by a periodic pass.
+	conn, err := cache.db.Conn(t.Context())
+	if err != nil {
+		t.Fatalf("failed to take the database connection: %v", err)
+	}
 
-	cache.StartEviction(interval)
+	defer func() { _ = conn.Close() }()
+
+	cache.StartEviction(100 * time.Millisecond)
 	defer func() { _ = cache.StopEviction(t.Context()) }()
 
-	// Let startup reconciliation run and settle on an empty cache
-	// before introducing the untracked file, so the adoption we assert
-	// below can only be the work of a later, periodic pass.
-	time.Sleep(3 * interval)
+	deadline := time.Now().Add(5 * time.Second)
+
+	for cache.db.Stats().WaitCount == 0 {
+		if time.Now().After(deadline) {
+			t.Fatal("the startup pass never waited for the database")
+		}
+
+		time.Sleep(10 * time.Millisecond)
+	}
 
 	// Simulate a variant whose accounting insert failed after the
 	// process was already running and serving requests: the content
@@ -864,14 +877,16 @@ func TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup(t *testing.T) {
 	// insert had failed and only the file write had succeeded.
 	untracked := bytes.Repeat([]byte{0x41}, 900)
 
-	_, err := cache.variants.Store(
+	_, err = cache.variants.Store(
 		"aabbccdd0099", bytes.NewReader(untracked), "image/webp",
 	)
 	if err != nil {
 		t.Fatalf("failed to store untracked variant file: %v", err)
 	}
 
-	deadline := time.Now().Add(5 * time.Second)
+	_ = conn.Close()
+
+	deadline = time.Now().Add(5 * time.Second)
 
 	var usage int64