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_store.go

@@ -32,6 +32,21 @@ const (
 	unknownFailureSourceIdx = -1
 )
 
+// missionControlDB is an interface that defines the database methods that a
+// single missionControlStore has access to. It allows the missionControlStore
+// to be unaware of the overall DB structure and restricts its access to the DB
+// by only providing it the bucket that it needs to care about.
+type missionControlDB interface {
+	// update can be used to perform reads and writes on the given bucket.
+	update(f func(bkt kvdb.RwBucket) error, reset func()) error
+
+	// view can be used to perform reads on the given bucket.
+	view(f func(bkt kvdb.RBucket) error, reset func()) error
+
+	// purge will delete all the contents in this store.
+	purge() error
+}
+
 // missionControlStore is a bolt db based implementation of a mission control
 // store. It stores the raw payment attempt data from which the internal mission
 // controls state can be rederived on startup. This allows the mission control
@@ -41,7 +56,7 @@ const (
 type missionControlStore struct {
 	done chan struct{}
 	wg   sync.WaitGroup
-	db   kvdb.Backend
+	db   missionControlDB
 
 	// queueCond is signalled when items are put into the queue.
 	queueCond *sync.Cond
@@ -67,7 +82,7 @@ type missionControlStore struct {
 	flushInterval time.Duration
 }
 
-func newMissionControlStore(db kvdb.Backend, maxRecords int,
+func newMissionControlStore(db missionControlDB, maxRecords int,
 	flushInterval time.Duration) (*missionControlStore, error) {
 
 	var (
@@ -76,13 +91,7 @@ func newMissionControlStore(db kvdb.Backend, maxRecords int,
 	)
 
 	// Create buckets if not yet existing.
-	err := kvdb.Update(db, func(tx kvdb.RwTx) error {
-		resultsBucket, err := tx.CreateTopLevelBucket(resultsKey)
-		if err != nil {
-			return fmt.Errorf("cannot create results bucket: %w",
-				err)
-		}
-
+	err := db.update(func(resultsBucket kvdb.RwBucket) error {
 		// Collect all keys to be able to quickly calculate the
 		// difference when updating the DB state.
 		c := resultsBucket.ReadCursor()
@@ -119,20 +128,12 @@ func (b *missionControlStore) clear() error {
 	b.queueCond.L.Lock()
 	defer b.queueCond.L.Unlock()
 
-	err := kvdb.Update(b.db, func(tx kvdb.RwTx) error {
-		if err := tx.DeleteTopLevelBucket(resultsKey); err != nil {
-			return err
-		}
-
-		_, err := tx.CreateTopLevelBucket(resultsKey)
-		return err
-	}, func() {})
-
-	if err != nil {
+	if err := b.db.purge(); err != nil {
 		return err
 	}
 
 	b.queue = list.New()
+
 	return nil
 }
 
@@ -140,8 +141,7 @@ func (b *missionControlStore) clear() error {
 func (b *missionControlStore) fetchAll() ([]*paymentResult, error) {
 	var results []*paymentResult
 
-	err := kvdb.View(b.db, func(tx kvdb.RTx) error {
-		resultBucket := tx.ReadBucket(resultsKey)
+	err := b.db.view(func(resultBucket kvdb.RBucket) error {
 		results = make([]*paymentResult, 0)
 
 		return resultBucket.ForEach(func(k, v []byte) error {
@@ -511,9 +511,7 @@ func (b *missionControlStore) storeResults() error {
 		}
 	}
 
-	err := kvdb.Update(b.db, func(tx kvdb.RwTx) error {
-		bucket := tx.ReadWriteBucket(resultsKey)
-
+	err := b.db.update(func(bucket kvdb.RwBucket) error {
 		for e := l.Front(); e != nil; e = e.Next() {
 			pr, ok := e.Value.(*paymentResult)
 			if !ok {