lnwire: ensure we precisely encode the length for onion errors w/ chan updates

In this commit we fix a compatibility issue with other implementations.
Before this commit, when writing out an onion error that includes a
`ChannelUpdate` we would use the `MaxPayloadLength` to get the length to
encode. However, a recent update has modified that to be the max
`brontide` payload length as it's possible to pad out the message with
optional fields we're unaware of. As a result, we would always write out
a length of 65KB or so. This didn't effect our parser as we ignore the
length and decode the channel update directly as we don't need the
length to do that. However, other implementations depended on the length
rather than just reading the channel update, meaning that they weren't
able to decode our onion errors that had channel updates.

In this commit we fix that by introducing a new
`writeOnionErrorChanUpdate` which will write out the precise length
instead of using the max payload size.

Fixes #2450.

lnwire/onion_error.go

@@ -665,12 +665,7 @@ func (f *FailAmountBelowMinimum) Encode(w io.Writer, pver uint32) error {
 		return err
 	}
 
-	err := WriteElement(w, uint16(f.Update.MaxPayloadLength(pver)))
-	if err != nil {
-		return err
-	}
-
-	return f.Update.Encode(w, pver)
+	return writeOnionErrorChanUpdate(w, &f.Update, pver)
 }
 
 // FailFeeInsufficient is returned if the HTLC does not pay sufficient fee, we
@@ -738,12 +733,7 @@ func (f *FailFeeInsufficient) Encode(w io.Writer, pver uint32) error {
 		return err
 	}
 
-	err := WriteElement(w, uint16(f.Update.MaxPayloadLength(pver)))
-	if err != nil {
-		return err
-	}
-
-	return f.Update.Encode(w, pver)
+	return writeOnionErrorChanUpdate(w, &f.Update, pver)
 }
 
 // FailIncorrectCltvExpiry is returned if outgoing cltv value does not match
@@ -811,12 +801,7 @@ func (f *FailIncorrectCltvExpiry) Encode(w io.Writer, pver uint32) error {
 		return err
 	}
 
-	err := WriteElement(w, uint16(f.Update.MaxPayloadLength(pver)))
-	if err != nil {
-		return err
-	}
-
-	return f.Update.Encode(w, pver)
+	return writeOnionErrorChanUpdate(w, &f.Update, pver)
 }
 
 // FailExpiryTooSoon is returned if the ctlv-expiry is too near, we tell them
@@ -869,12 +854,7 @@ func (f *FailExpiryTooSoon) Decode(r io.Reader, pver uint32) error {
 //
 // NOTE: Part of the Serializable interface.
 func (f *FailExpiryTooSoon) Encode(w io.Writer, pver uint32) error {
-	err := WriteElement(w, uint16(f.Update.MaxPayloadLength(pver)))
-	if err != nil {
-		return err
-	}
-
-	return f.Update.Encode(w, pver)
+	return writeOnionErrorChanUpdate(w, &f.Update, pver)
 }
 
 // FailChannelDisabled is returned if the channel is disabled, we tell them the
@@ -942,12 +922,7 @@ func (f *FailChannelDisabled) Encode(w io.Writer, pver uint32) error {
 		return err
 	}
 
-	err := WriteElement(w, uint16(f.Update.MaxPayloadLength(pver)))
-	if err != nil {
-		return err
-	}
-
-	return f.Update.Encode(w, pver)
+	return writeOnionErrorChanUpdate(w, &f.Update, pver)
 }
 
 // FailFinalIncorrectCltvExpiry is returned if the outgoing_cltv_value does not
@@ -1231,3 +1206,32 @@ func makeEmptyOnionError(code FailCode) (FailureMessage, error) {
 		return nil, errors.Errorf("unknown error code: %v", code)
 	}
 }
+
+// writeOnionErrorChanUpdate writes out a ChannelUpdate using the onion error
+// format. The format is that we first write out the true serialized length of
+// the channel update, followed by the serialized channel update itself.
+func writeOnionErrorChanUpdate(w io.Writer, chanUpdate *ChannelUpdate,
+	pver uint32) error {
+
+	// First, we encode the channel update in a temporary buffer in order
+	// to get the exact serialized size.
+	var b bytes.Buffer
+	if err := chanUpdate.Encode(&b, pver); err != nil {
+		return err
+	}
+
+	// Now that we know the size, we can write the length out in the main
+	// writer.
+	updateLen := b.Len()
+	if err := WriteElement(w, uint16(updateLen)); err != nil {
+		return err
+	}
+
+	// With the length written, we'll then write out the serialized channel
+	// update.
+	if _, err := w.Write(b.Bytes()); err != nil {
+		return err
+	}
+
+	return nil
+}