channeldb/forwarding_package: loosen Add/SettleFailRef acking

This commit loosens the fwdpkg reference acking to be more tolerant
of prior deletions. Specifically, we won't fail if certain channels
are not found or fwdpkgs do not exist. This will make us more
tolerant to future changes where we:
 - remove fwdpkgs on channel close
 - defensively cleanup stray responses

channeldb/forwarding_package.go

@@ -3,11 +3,11 @@ package channeldb
 import (
 	"bytes"
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"io"
 
 	"github.com/coreos/bbolt"
-	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
@@ -759,7 +759,11 @@ func ackAddHtlcsAtHeight(sourceBkt *bolt.Bucket, height uint64,
 	heightKey := makeLogKey(height)
 	heightBkt := sourceBkt.Bucket(heightKey[:])
 	if heightBkt == nil {
-		return ErrCorruptedFwdPkg
+		// If the height bucket isn't found, this could be because the
+		// forwarding package was already removed. We'll return nil to
+		// signal that the operation is successful, as there is nothing
+		// to ack.
+		return nil
 	}
 
 	// Load ack filter from disk.
@@ -824,12 +828,17 @@ func ackSettleFails(tx *bolt.Tx, settleFailRefs []SettleFailRef) error {
 	}
 
 	// With the references organized by destination and height, we now load
-	// each remote bucket, and update the settle fail filter for  any
+	// each remote bucket, and update the settle fail filter for any
 	// settle/fail htlcs.
 	for dest, destHeights := range destHeightDiffs {
 		destKey := makeLogKey(dest.ToUint64())
 		destBkt := fwdPkgBkt.Bucket(destKey[:])
 		if destBkt == nil {
+			// If the destination bucket is not found, this is
+			// likely the result of the destination channel being
+			// closed and having it's forwarding packages wiped. We
+			// won't treat this as an error, because the response
+			// will no longer be retransmitted internally.
 			continue
 		}
 
@@ -852,6 +861,9 @@ func ackSettleFailsAtHeight(destBkt *bolt.Bucket, height uint64,
 	heightKey := makeLogKey(height)
 	heightBkt := destBkt.Bucket(heightKey[:])
 	if heightBkt == nil {
+		// If the height bucket isn't found, this could be because the
+		// forwarding package was already removed. We'll return nil to
+		// signal that the operation is as there is nothing to ack.
 		return nil
 	}