multi: update internals to support updating max htlc

In this commit, we update the router and link to support users
updating the max HTLC policy for their channels. By updating these internal
systems before updating the RPC server and lncli, we protect users from
being shown an option that doesn't actually work.

htlcswitch/link_test.go

@@ -971,6 +971,47 @@ func TestUpdateForwardingPolicy(t *testing.T) {
 	default:
 		t.Fatalf("expected FailFeeInsufficient instead got: %v", err)
 	}
+
+	// Reset the policy so we can then test updating the max HTLC policy.
+	n.secondBobChannelLink.UpdateForwardingPolicy(n.globalPolicy)
+
+	// As a sanity check, ensure the original payment now succeeds again.
+	_, err = makePayment(
+		n.aliceServer, n.carolServer, firstHop, hops, amountNoFee,
+		htlcAmt, htlcExpiry,
+	).Wait(30 * time.Second)
+	if err != nil {
+		t.Fatalf("unable to send payment: %v", err)
+	}
+
+	// Now we'll update Bob's policy to lower his max HTLC to an extent
+	// that'll cause him to reject the same HTLC that we just sent.
+	newPolicy = n.globalPolicy
+	newPolicy.MaxHTLC = amountNoFee - 1
+	n.secondBobChannelLink.UpdateForwardingPolicy(newPolicy)
+
+	// Next, we'll send the payment again, using the exact same per-hop
+	// payload for each node. This payment should fail as it won't factor
+	// in Bob's new max HTLC policy.
+	_, err = makePayment(
+		n.aliceServer, n.carolServer, firstHop, hops, amountNoFee,
+		htlcAmt, htlcExpiry,
+	).Wait(30 * time.Second)
+	if err == nil {
+		t.Fatalf("payment should've been rejected")
+	}
+
+	ferr, ok = err.(*ForwardingError)
+	if !ok {
+		t.Fatalf("expected a ForwardingError, instead got (%T): %v",
+			err, err)
+	}
+	switch ferr.FailureMessage.(type) {
+	case *lnwire.FailTemporaryChannelFailure:
+	default:
+		t.Fatalf("expected TemporaryChannelFailure, instead got: %v",
+			err)
+	}
 }
 
 // TestChannelLinkMultiHopInsufficientPayment checks that we receive error if