routing: introduce missionControlDB abstraction

So that `missionControlStore` can be unaware of the backing DB structure
it is writing to. In an upcoming commit when we change mission control
to write to namespaced buckets instead, we then only need to update the
`namespacedDB` implementation.

routing/missioncontrol.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcwallet/walletdb"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/kvdb"
@@ -226,7 +227,8 @@ func NewMissionControl(db kvdb.Backend, self route.Vertex,
 	}
 
 	store, err := newMissionControlStore(
-		db, cfg.MaxMcHistory, cfg.McFlushInterval,
+		newNamespacedDB(db), cfg.MaxMcHistory,
+		cfg.McFlushInterval,
 	)
 	if err != nil {
 		return nil, err
@@ -545,3 +547,74 @@ func (m *MissionControl) applyPaymentResult(
 
 	return i.finalFailureReason
 }
+
+// namespacedDB is an implementation of the missionControlDB that gives a user
+// of the interface access to the top level mission control bucket. In a
+// follow-up commit (accompanied by a migration), this will change to giving
+// the user of the interface access to a namespaced sub-bucket instead.
+type namespacedDB struct {
+	topLevelBucketKey []byte
+	db                kvdb.Backend
+}
+
+// A compile-time check to ensure that namespacedDB implements missionControlDB.
+var _ missionControlDB = (*namespacedDB)(nil)
+
+// newNamespacedDB creates a new instance of missionControlDB where the DB will
+// have access to the top level bucket.
+func newNamespacedDB(db kvdb.Backend) missionControlDB {
+	return &namespacedDB{
+		db:                db,
+		topLevelBucketKey: resultsKey,
+	}
+}
+
+// update can be used to perform reads and writes on the given bucket.
+//
+// NOTE: this is part of the missionControlDB interface.
+func (n *namespacedDB) update(f func(bkt walletdb.ReadWriteBucket) error,
+	reset func()) error {
+
+	return n.db.Update(func(tx kvdb.RwTx) error {
+		mcStoreBkt, err := tx.CreateTopLevelBucket(n.topLevelBucketKey)
+		if err != nil {
+			return fmt.Errorf("cannot create top level mission "+
+				"control bucket: %w", err)
+		}
+
+		return f(mcStoreBkt)
+	}, reset)
+}
+
+// view can be used to perform reads on the given bucket.
+//
+// NOTE: this is part of the missionControlDB interface.
+func (n *namespacedDB) view(f func(bkt walletdb.ReadBucket) error,
+	reset func()) error {
+
+	return n.db.View(func(tx kvdb.RTx) error {
+		mcStoreBkt := tx.ReadBucket(n.topLevelBucketKey)
+		if mcStoreBkt == nil {
+			return fmt.Errorf("top level mission control bucket " +
+				"not found")
+		}
+
+		return f(mcStoreBkt)
+	}, reset)
+}
+
+// purge will delete all the contents in the namespace.
+//
+// NOTE: this is part of the missionControlDB interface.
+func (n *namespacedDB) purge() error {
+	return n.db.Update(func(tx kvdb.RwTx) error {
+		err := tx.DeleteTopLevelBucket(n.topLevelBucketKey)
+		if err != nil {
+			return err
+		}
+
+		_, err = tx.CreateTopLevelBucket(n.topLevelBucketKey)
+
+		return err
+	}, func() {})
+}