lnd: add ExtractChannel function

lnd/chanbackup.go

@@ -4,6 +4,9 @@ import (
 	"bytes"
 	"fmt"
 
+	"github.com/btcsuite/btcd/btcutil/hdkeychain"
+	"github.com/btcsuite/btcd/chaincfg"
+	"github.com/lightninglabs/chantools/dump"
 	"github.com/lightningnetwork/lnd/chanbackup"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/keychain"
@@ -35,3 +38,32 @@ func CreateChannelBackup(db *channeldb.DB, multiFile *chanbackup.MultiFile,
 	}
 	return nil
 }
+
+// ExtractChannel extracts a single channel from the given backup file and
+// returns it as a dump.BackupSingle struct.
+func ExtractChannel(extendedKey *hdkeychain.ExtendedKey,
+	chainParams *chaincfg.Params, multiFilePath,
+	channelPoint string) (*dump.BackupSingle, error) {
+
+	multiFile := chanbackup.NewMultiFile(multiFilePath)
+	keyRing := &HDKeyRing{
+		ExtendedKey: extendedKey,
+		ChainParams: chainParams,
+	}
+
+	multi, err := multiFile.ExtractMulti(keyRing)
+	if err != nil {
+		return nil, fmt.Errorf("could not extract multi file: %w", err)
+	}
+
+	channels := dump.BackupDump(multi, chainParams)
+	for _, channel := range channels {
+		channel := channel
+
+		if channel.FundingOutpoint == channelPoint {
+			return &channel, nil
+		}
+	}
+
+	return nil, fmt.Errorf("channel %s not found in backup", channelPoint)
+}