channeldb/db: define FetchClosedChannelForID

FetchClosedChannelForID is used to find the channel close summary given
only a channel ID.

channeldb/db.go

@@ -12,6 +12,7 @@ import (
 	"github.com/btcsuite/btcd/wire"
 	"github.com/coreos/bbolt"
 	"github.com/go-errors/errors"
+	"github.com/lightningnetwork/lnd/lnwire"
 )
 
 const (
@@ -616,6 +617,54 @@ func (d *DB) FetchClosedChannel(chanID *wire.OutPoint) (*ChannelCloseSummary, er
 	return chanSummary, nil
 }
 
+// FetchClosedChannelForID queries for a channel close summary using the
+// channel ID of the channel in question.
+func (d *DB) FetchClosedChannelForID(cid lnwire.ChannelID) (
+	*ChannelCloseSummary, error) {
+
+	var chanSummary *ChannelCloseSummary
+	if err := d.View(func(tx *bolt.Tx) error {
+		closeBucket := tx.Bucket(closedChannelBucket)
+		if closeBucket == nil {
+			return ErrClosedChannelNotFound
+		}
+
+		// The first 30 bytes of the channel ID and outpoint will be
+		// equal.
+		cursor := closeBucket.Cursor()
+		op, c := cursor.Seek(cid[:30])
+
+		// We scan over all possible candidates for this channel ID.
+		for ; op != nil && bytes.Compare(cid[:30], op[:30]) <= 0; op, c = cursor.Next() {
+			var outPoint wire.OutPoint
+			err := readOutpoint(bytes.NewReader(op), &outPoint)
+			if err != nil {
+				return err
+			}
+
+			// If the found outpoint does not correspond to this
+			// channel ID, we continue.
+			if !cid.IsChanPoint(&outPoint) {
+				continue
+			}
+
+			// Deserialize the close summary and return.
+			r := bytes.NewReader(c)
+			chanSummary, err = deserializeCloseChannelSummary(r)
+			if err != nil {
+				return err
+			}
+
+			return nil
+		}
+		return ErrClosedChannelNotFound
+	}); err != nil {
+		return nil, err
+	}
+
+	return chanSummary, nil
+}
+
 // MarkChanFullyClosed marks a channel as fully closed within the database. A
 // channel should be marked as fully closed if the channel was initially
 // cooperatively closed and it's reached a single confirmation, or after all