Let go of the other queues while an ask runs, and read the let-through race right

A message that reaches a builder's subscription outside a fetch was held unheartbeated for
the whole run of another ask, behind which a delivery waited (issue 1747). The queues the
running ask did not come from are let go of and bound again after it.

internal/link/stray_test.go

@@ -0,0 +1,36 @@
+package link
+
+import (
+	"testing"
+
+	"github.com/nats-io/nats.go"
+)
+
+// Issue 1747: a builder never holds a message it does not start. A message the server sends after a fetch gave up lands in that
+// queue's subscription; while an ask runs, the queues it does not come from are let go of, so such a message is not held
+// unheartbeated for the whole run, and the queues are bound again before the next fetch.
+
+func TestARunningAskLetsGoOfTheQueuesItDidNotComeFrom(t *testing.T) {
+	m, builds, _, other, _ := aMachine()
+	// A delivery build reached the builds queue's subscription after its fetch gave up: it sits in the buffer.
+	builds.waiting = []*nats.Msg{buildAsk()}
+	took := m.subs[1] // the running ask came from the ready queue
+
+	m.quiesce(took)
+
+	if !took.bound || took.pull == nil {
+		t.Error("the queue the running ask came from was let go of; its acknowledgements need it")
+	}
+	for _, q := range []*takeQueue{m.subs[0], m.subs[2]} {
+		if q.bound || q.pull != nil || q.sub != nil {
+			t.Errorf("the %s queue is still held while an ask runs", q.kind)
+		}
+	}
+	// Nothing is fetched from a queue that was let go of: a message in it cannot be taken unheartbeated.
+	if msg, _, _ := m.next(t.Context()); msg != nil {
+		t.Fatalf("a message was taken from a queue that was let go of: %s", msg.Data)
+	}
+	if builds.fetches() != 0 || other.fetches() != 0 {
+		t.Errorf("fetched from a queue that was let go of: builds %d, other %d", builds.fetches(), other.fetches())
+	}
+}