lnwire: ensure we're able to decode legacy FailUnknownPaymentHash

In this commit, we modify the decoding of the FailUnknownPaymentHash
message to ensure we're able to fully decode the legacy serialization of
the onion error. We do this by catching the `io.EOF` error as it's
returned when _no_ bytes are read. If this is the case, then only the
error type was serialized and not also the optional amount.

lnwire/onion_error_test.go

@@ -33,10 +33,10 @@ var onionFailures = []FailureMessage{
 	&FailPermanentChannelFailure{},
 	&FailRequiredChannelFeatureMissing{},
 	&FailUnknownNextPeer{},
-	&FailUnknownPaymentHash{},
 	&FailIncorrectPaymentAmount{},
 	&FailFinalExpiryTooSoon{},
 
+	NewFailUnknownPaymentHash(99),
 	NewInvalidOnionVersion(testOnionHash),
 	NewInvalidOnionHmac(testOnionHash),
 	NewInvalidOnionKey(testOnionHash),
@@ -167,3 +167,30 @@ func TestWriteOnionErrorChanUpdate(t *testing.T) {
 			trueUpdateLength, encodedLen)
 	}
 }
+
+// TestFailUnknownPaymentHashOptionalAmount tests that we're able to decode an
+// UnknownPaymentHash error that doesn't have the optional amount. This ensures
+// we're able to decode FailUnknownPaymentHash messages from older nodes.
+func TestFailUnknownPaymentHashOptionalAmount(t *testing.T) {
+	t.Parallel()
+
+	// Creation an error that is a non-pointer will allow us to skip the
+	// type assertion for the Serializable interface. As a result, the
+	// amount body won't be written.
+	onionError := FailUnknownPaymentHash{}
+
+	var b bytes.Buffer
+	if err := EncodeFailure(&b, onionError, 0); err != nil {
+		t.Fatalf("unable to encode failure: %v", err)
+	}
+
+	onionError2, err := DecodeFailure(bytes.NewReader(b.Bytes()), 0)
+	if err != nil {
+		t.Fatalf("unable to decode error: %v", err)
+	}
+
+	if !reflect.DeepEqual(onionError, onionError) {
+		t.Fatalf("expected %v, got %v", spew.Sdump(onionError),
+			spew.Sdump(onionError2))
+	}
+}