lnwire: explicitly handle empty list when encoding short chan IDs

Before this commit, both writing and reading an encoded empty set of
short channel IDs from the wire would fail. Prior to this commit, we
treated decoding an empty set as a caller error, and failed to write out
the zlib encoding of an empty set in a way that us and the other
implementations were able to read.

To fix this, rather than giving zlib an empty buffer to write out (which
results in an encoding with the zlib header data and the rest), we just
write a blank slice. When decoding, if we have an empty query body, then
we'll return a `nil` slice.

With the above changes, we'll now always write out an empty short
channel ID set as:
```
0001 (1 byte follows) || <encoding_type>
```

A new test has also been added to exercise this case for both known
encoding types.

lnwire/query_short_chan_ids.go

@@ -132,7 +132,7 @@ func decodeShortChanIDs(r io.Reader) (ShortChanIDEncoding, []ShortChannelID, err
 	}
 
 	if numBytesResp == 0 {
-		return 0, nil, fmt.Errorf("No encoding type specified")
+		return 0, nil, nil
 	}
 
 	queryBody := make([]byte, numBytesResp)
@@ -148,6 +148,13 @@ func decodeShortChanIDs(r io.Reader) (ShortChanIDEncoding, []ShortChannelID, err
 	// as that was just the encoding type.
 	queryBody = queryBody[1:]
 
+	// At this point, if there's no body remaining, then only the encoding
+	// type was specified, meaning that there're no further bytes to be
+	// parsed.
+	if len(queryBody) == 0 {
+		return encodingType, nil, nil
+	}
+
 	// Otherwise, depending on the encoding type, we'll decode the encode
 	// short channel ID's in a different manner.
 	switch encodingType {
@@ -338,27 +345,43 @@ func encodeShortChanIDs(w io.Writer, encodingType ShortChanIDEncoding,
 		var buf bytes.Buffer
 		zlibWriter := zlib.NewWriter(&buf)
 
-		// Next, we'll write out all the channel ID's directly into the
-		// zlib writer, which will do compressing on the fly.
-		for _, chanID := range shortChanIDs {
-			err := WriteElements(zlibWriter, chanID)
-			if err != nil {
-				return fmt.Errorf("unable to write short chan "+
-					"ID: %v", err)
+		// If we don't have anything at all to write, then we'll write
+		// an empty payload so we don't include things like the zlib
+		// header when the remote party is expecting no actual short
+		// channel IDs.
+		var compressedPayload []byte
+		if len(shortChanIDs) > 0 {
+			// Next, we'll write out all the channel ID's directly
+			// into the zlib writer, which will do compressing on
+			// the fly.
+			for _, chanID := range shortChanIDs {
+				err := WriteElements(zlibWriter, chanID)
+				if err != nil {
+					return fmt.Errorf("unable to write short chan "+
+						"ID: %v", err)
+				}
 			}
-		}
 
-		// Now that we've written all the elements, we'll ensure the
-		// compressed stream is written to the underlying buffer.
-		if err := zlibWriter.Close(); err != nil {
-			return fmt.Errorf("unable to finalize "+
-				"compression: %v", err)
+			// Now that we've written all the elements, we'll
+			// ensure the compressed stream is written to the
+			// underlying buffer.
+			if err := zlibWriter.Close(); err != nil {
+				return fmt.Errorf("unable to finalize "+
+					"compression: %v", err)
+			}
+
+			compressedPayload = buf.Bytes()
 		}
 
 		// Now that we have all the items compressed, we can compute
 		// what the total payload size will be. We add one to account
 		// for the byte to encode the type.
-		compressedPayload := buf.Bytes()
+		//
+		// If we don't have any actual bytes to write, then we'll end
+		// up emitting one byte for the length, followed by the
+		// encoding type, and nothing more. The spec isn't 100% clear
+		// in this area, but we do this as this is what most of the
+		// other implementations do.
 		numBytesBody := len(compressedPayload) + 1
 
 		// Finally, we can write out the number of bytes, the

lnwire/reply_channel_range_test.go

@@ -2,7 +2,11 @@ package lnwire
 
 import (
 	"bytes"
+	"encoding/hex"
+	"reflect"
 	"testing"
+
+	"github.com/davecgh/go-spew/spew"
 )
 
 // TestReplyChannelRangeUnsorted tests that decoding a ReplyChannelRange request
@@ -32,3 +36,72 @@ func TestReplyChannelRangeUnsorted(t *testing.T) {
 		})
 	}
 }
+
+// TestReplyChannelRangeEmpty tests encoding and decoding a ReplyChannelRange
+// that doesn't contain any channel results.
+func TestReplyChannelRangeEmpty(t *testing.T) {
+	t.Parallel()
+
+	emptyChannelsTests := []struct {
+		name       string
+		encType    ShortChanIDEncoding
+		encodedHex string
+	}{
+		{
+			name:    "empty plain encoding",
+			encType: EncodingSortedPlain,
+			encodedHex: "000000000000000000000000000000000000000" +
+				"00000000000000000000000000000000100000002" +
+				"01000100",
+		},
+		{
+			name:    "empty zlib encoding",
+			encType: EncodingSortedZlib,
+			encodedHex: "00000000000000000000000000000000000000" +
+				"0000000000000000000000000000000001000000" +
+				"0201000101",
+		},
+	}
+
+	for _, test := range emptyChannelsTests {
+		test := test
+		t.Run(test.name, func(t *testing.T) {
+			req := ReplyChannelRange{
+				QueryChannelRange: QueryChannelRange{
+					FirstBlockHeight: 1,
+					NumBlocks:        2,
+				},
+				Complete:     1,
+				EncodingType: test.encType,
+				ShortChanIDs: nil,
+			}
+
+			// First decode the hex string in the test case into a
+			// new ReplyChannelRange message. It should be
+			// identical to the one created above.
+			var req2 ReplyChannelRange
+			b, _ := hex.DecodeString(test.encodedHex)
+			err := req2.Decode(bytes.NewReader(b), 0)
+			if err != nil {
+				t.Fatalf("unable to decode req: %v", err)
+			}
+			if !reflect.DeepEqual(req, req2) {
+				t.Fatalf("requests don't match: expected %v got %v",
+					spew.Sdump(req), spew.Sdump(req2))
+			}
+
+			// Next, we go in the reverse direction: encode the
+			// request created above, and assert that it matches
+			// the raw byte encoding.
+			var b2 bytes.Buffer
+			err = req.Encode(&b2, 0)
+			if err != nil {
+				t.Fatalf("unable to encode req: %v", err)
+			}
+			if !bytes.Equal(b, b2.Bytes()) {
+				t.Fatalf("encoded requests don't match: expected %x got %x",
+					b, b2.Bytes())
+			}
+		})
+	}
+}