lncfg: add configuration for user specified db backend

This commit extends lncfg to support user specified database backend.
This supports configuration for both bolt and etcd (while only allowing
one or the other).

lncfg/db.go

@@ -0,0 +1,85 @@
+package lncfg
+
+import (
+	"fmt"
+
+	"github.com/lightningnetwork/lnd/channeldb/kvdb"
+	"github.com/lightningnetwork/lnd/channeldb/kvdb/etcd"
+)
+
+const (
+	dbName      = "channel.db"
+	boltBackend = "bolt"
+	etcdBackend = "etcd"
+)
+
+// BoltDB holds bolt configuration.
+type BoltDB struct {
+	NoFreeListSync bool `long:"nofreelistsync" description:"If true, prevents the database from syncing its freelist to disk"`
+}
+
+// EtcdDB hold etcd configuration.
+type EtcdDB struct {
+	Host string `long:"host" description:"Etcd database host."`
+
+	User string `long:"user" description:"Etcd database user."`
+
+	Pass string `long:"pass" description:"Password for the database user."`
+
+	CollectStats bool `long:"collect_stats" description:"Wheter to collect etcd commit stats."`
+}
+
+// DB holds database configuration for LND.
+type DB struct {
+	Backend string `long:"backend" description:"The selected database backend."`
+
+	Etcd *EtcdDB `group:"etcd" namespace:"etcd" description:"Etcd settings."`
+
+	Bolt *BoltDB `group:"bolt" namespace:"bolt" description:"Bolt settings."`
+}
+
+// NewDB creates and returns a new default DB config.
+func DefaultDB() *DB {
+	return &DB{
+		Backend: boltBackend,
+		Bolt: &BoltDB{
+			NoFreeListSync: true,
+		},
+	}
+}
+
+// Validate validates the DB config.
+func (db *DB) Validate() error {
+	switch db.Backend {
+	case boltBackend:
+
+	case etcdBackend:
+		if db.Etcd.Host == "" {
+			return fmt.Errorf("etcd host must be set")
+		}
+
+	default:
+		return fmt.Errorf("unknown backend, must be either \"%v\" or \"%v\"",
+			boltBackend, etcdBackend)
+	}
+
+	return nil
+}
+
+// GetBackend returns a kvdb.Backend as set in the DB config.
+func (db *DB) GetBackend(path string) (kvdb.Backend, error) {
+	if db.Backend == etcdBackend {
+		backendConfig := etcd.BackendConfig{
+			Host:               db.Etcd.Host,
+			User:               db.Etcd.User,
+			Pass:               db.Etcd.Pass,
+			CollectCommitStats: db.Etcd.CollectStats,
+		}
+		return kvdb.Open(kvdb.EtcdBackendName, backendConfig)
+	}
+
+	return kvdb.GetBoltBackend(path, dbName, db.Bolt.NoFreeListSync)
+}
+
+// Compile-time constraint to ensure Workers implements the Validator interface.
+var _ Validator = (*DB)(nil)