delivery: a turn never holds the check of heads that wait in it, so two approved merges in one turn never hold each other's carry (ADR 0548 §3; issue 568, review round 1)

go/delivery/gate_test.go

@@ -202,9 +202,9 @@ func TestGateValuesRoundTrip(t *testing.T) { // 1201/AC4
 			t.Errorf("the turn lacks %s: %s", field, data)
 		}
 	}
-	// ADR 0548 §3: a turn holds off every check but the one of a waiting delivery's own heads.
-	if !TurnHolds(tu, "fedcba987654") || TurnHolds(Turn{Waiting: tu.Waiting[:1]}, tu.Waiting[0].Heads) || !TurnHolds(tu, tu.Waiting[0].Heads) {
-		t.Error("TurnHolds: a turn holds other heads' checks and never its only delivery's own")
+	// ADR 0548 §3: a turn holds off every check but those of the heads that wait in it, whoever else waits there.
+	if !TurnHolds(tu, "fedcba987654") || TurnHolds(tu, tu.Waiting[0].Heads) || TurnHolds(tu, tu.Waiting[1].Heads) || TurnHolds(Turn{}, "fedcba987654") {
+		t.Error("TurnHolds: a turn holds other heads' checks and never the check of heads waiting in it")
 	}
 	for name, dec := range map[string]func([]byte) error{
 		"turn":   func(b []byte) error { _, err := DecodeTurn(b); return err },

go/delivery/turns.go

@@ -117,15 +117,19 @@ func (t Turn) check() error {
 	return nil
 }
 
-// TurnHolds says whether a turn holds off the check of the heads: it names a delivery of other heads. The check of a waiting
-// delivery's own heads (the carry its merge waits for) is never held by its own turn.
+// TurnHolds says whether a turn holds off the check of the heads: it names deliveries, and none of them of these heads. A
+// turn never holds the check of heads that wait in it (the carry a waiting merge needs): two approved merges in one turn whose
+// trunk moved would otherwise each hold the other's carry, and neither would ever merge (review of ADR 0548's build, round 1).
 func TurnHolds(t Turn, heads string) bool {
+	if len(t.Waiting) == 0 {
+		return false
+	}
 	for _, w := range t.Waiting {
-		if w.Heads != heads {
-			return true
+		if w.Heads == heads {
+			return false
 		}
 	}
-	return false
+	return true
 }
 
 // EncodeTurn is t as JSON at Version; it refuses a turn with no delivery, a repository that is not lowercase <owner>/<name>,