multi: update channel db HTLC OnionBlob to array

We know that onion blobs in lightning are _exactly_ 1366 bytes in
lightning, but they are currently expressed as a byte slice in
channeldb's HTLC struct. Blobs are currently serialized as var bytes,
so we can take advantage of this known length and variable length
to add additional data to the inline serialization of our HTLCs, which
are otherwise not easily extensible (without creating a new bucket).

channeldb/channel.go

@@ -2001,7 +2001,7 @@ func (c *OpenChannel) ActiveHtlcs() []HTLC {
 	// which ones are present on their commitment.
 	remoteHtlcs := make(map[[32]byte]struct{})
 	for _, htlc := range c.RemoteCommitment.Htlcs {
-		onionHash := sha256.Sum256(htlc.OnionBlob)
+		onionHash := sha256.Sum256(htlc.OnionBlob[:])
 		remoteHtlcs[onionHash] = struct{}{}
 	}
 
@@ -2009,7 +2009,7 @@ func (c *OpenChannel) ActiveHtlcs() []HTLC {
 	// as active if *we* know them as well.
 	activeHtlcs := make([]HTLC, 0, len(remoteHtlcs))
 	for _, htlc := range c.LocalCommitment.Htlcs {
-		onionHash := sha256.Sum256(htlc.OnionBlob)
+		onionHash := sha256.Sum256(htlc.OnionBlob[:])
 		if _, ok := remoteHtlcs[onionHash]; !ok {
 			continue
 		}
@@ -2056,7 +2056,7 @@ type HTLC struct {
 
 	// OnionBlob is an opaque blob which is used to complete multi-hop
 	// routing.
-	OnionBlob []byte
+	OnionBlob [lnwire.OnionPacketSize]byte
 
 	// HtlcIndex is the HTLC counter index of this active, outstanding
 	// HTLC. This differs from the LogIndex, as the HtlcIndex is only
@@ -2113,14 +2113,17 @@ func DeserializeHtlcs(r io.Reader) ([]HTLC, error) {
 
 	htlcs = make([]HTLC, numHtlcs)
 	for i := uint16(0); i < numHtlcs; i++ {
+		var onionBlob []byte
 		if err := ReadElements(r,
 			&htlcs[i].Signature, &htlcs[i].RHash, &htlcs[i].Amt,
 			&htlcs[i].RefundTimeout, &htlcs[i].OutputIndex,
-			&htlcs[i].Incoming, &htlcs[i].OnionBlob,
+			&htlcs[i].Incoming, &onionBlob,
 			&htlcs[i].HtlcIndex, &htlcs[i].LogIndex,
 		); err != nil {
 			return htlcs, err
 		}
+
+		copy(htlcs[i].OnionBlob[:], onionBlob)
 	}
 
 	return htlcs, nil