htlcswitch+routing: implement expiry_too_far failure

In this commit we add a check to HtlcSatifiesPolicy to verify that the
time lock for the outgoing htlc that is requested in the onion packet
isn't too far in the future.

Without this check, anyone could force an unreasonably long time lock on
the forwarding node.

htlcswitch/link_test.go

@@ -5119,3 +5119,77 @@ func TestForwardingAsymmetricTimeLockPolicies(t *testing.T) {
 		t.Fatalf("unable to send payment: %v", err)
 	}
 }
+
+// TestHtlcSatisfyPolicy tests that a link is properly enforcing the HTLC
+// forwarding policy.
+func TestHtlcSatisfyPolicy(t *testing.T) {
+
+	fetchLastChannelUpdate := func(lnwire.ShortChannelID) (
+		*lnwire.ChannelUpdate, error) {
+
+		return &lnwire.ChannelUpdate{}, nil
+	}
+
+	link := channelLink{
+		cfg: ChannelLinkConfig{
+			FwrdingPolicy: ForwardingPolicy{
+				TimeLockDelta: 20,
+				MinHTLC:       500,
+				BaseFee:       10,
+			},
+			FetchLastChannelUpdate: fetchLastChannelUpdate,
+		},
+	}
+
+	var hash [32]byte
+
+	t.Run("satisfied", func(t *testing.T) {
+		result := link.HtlcSatifiesPolicy(hash, 1500, 1000,
+			200, 150, 0)
+		if result != nil {
+			t.Fatalf("expected policy to be satisfied")
+		}
+	})
+
+	t.Run("below minhtlc", func(t *testing.T) {
+		result := link.HtlcSatifiesPolicy(hash, 100, 50,
+			200, 150, 0)
+		if _, ok := result.(*lnwire.FailAmountBelowMinimum); !ok {
+			t.Fatalf("expected FailAmountBelowMinimum failure code")
+		}
+	})
+
+	t.Run("insufficient fee", func(t *testing.T) {
+		result := link.HtlcSatifiesPolicy(hash, 1005, 1000,
+			200, 150, 0)
+		if _, ok := result.(*lnwire.FailFeeInsufficient); !ok {
+			t.Fatalf("expected FailFeeInsufficient failure code")
+		}
+	})
+
+	t.Run("expiry too soon", func(t *testing.T) {
+		result := link.HtlcSatifiesPolicy(hash, 1500, 1000,
+			200, 150, 190)
+		if _, ok := result.(*lnwire.FailExpiryTooSoon); !ok {
+			t.Fatalf("expected FailExpiryTooSoon failure code")
+		}
+	})
+
+	t.Run("incorrect cltv expiry", func(t *testing.T) {
+		result := link.HtlcSatifiesPolicy(hash, 1500, 1000,
+			200, 190, 0)
+		if _, ok := result.(*lnwire.FailIncorrectCltvExpiry); !ok {
+			t.Fatalf("expected FailIncorrectCltvExpiry failure code")
+		}
+
+	})
+
+	t.Run("cltv expiry too far in the future", func(t *testing.T) {
+		// Check that expiry isn't too far in the future.
+		result := link.HtlcSatifiesPolicy(hash, 1500, 1000,
+			10200, 10100, 0)
+		if _, ok := result.(*lnwire.FailExpiryTooFar); !ok {
+			t.Fatalf("expected FailExpiryTooFar failure code")
+		}
+	})
+}