channeldb: add migration34

Migration34 garbage collects the decayed log database. This commit
only adds the migration code and does not use it.

channeldb/migration34/log.go

@@ -0,0 +1,14 @@
+package migration34
+
+import (
+	"github.com/btcsuite/btclog/v2"
+)
+
+// log is a logger that is initialized as disabled.  This means the package will
+// not perform any logging by default until a logger is set.
+var log = btclog.Disabled
+
+// UseLogger uses a specified Logger to output package logging info.
+func UseLogger(logger btclog.Logger) {
+	log = logger
+}

channeldb/migration34/migration.go

@@ -0,0 +1,76 @@
+package migration34
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/lightningnetwork/lnd/kvdb"
+)
+
+// Migration34 is an optional migration that garbage collects the decayed log
+// in particular the `batch-replay` bucket. However we did choose to use an
+// optional migration which defaults to true because the decayed log db is
+// separate from the channeldb and if we would have implemented it as a
+// required migration, then it would have required a bigger change to the
+// codebase.
+//
+// Most of the decayed log db will shrink significantly after this migration
+// because the other bucket called `shared-secrets` is garbage collected
+// continuously and the `batch-replay` bucket will be deleted.
+
+var (
+	// batchReplayBucket is a bucket that maps batch identifiers to
+	// serialized ReplaySets. This is used to give idempotency in the event
+	// that a batch is processed more than once.
+	batchReplayBucket = []byte("batch-replay")
+)
+
+// MigrationConfig is the interface for the migration configuration.
+type MigrationConfig interface {
+	GetDecayedLog() kvdb.Backend
+}
+
+// MigrationConfigImpl is the implementation of the migration configuration.
+type MigrationConfigImpl struct {
+	DecayedLog kvdb.Backend
+}
+
+// GetDecayedLog returns the decayed log backend.
+func (c *MigrationConfigImpl) GetDecayedLog() kvdb.Backend {
+	return c.DecayedLog
+}
+
+// MigrateDecayedLog migrates the decayed log. The migration deletes the
+// `batch-replay` bucket, which is no longer used.
+//
+// NOTE: This migration is idempotent. If the bucket does not exist, then this
+// migration is a no-op.
+func MigrateDecayedLog(db kvdb.Backend, cfg MigrationConfig) error {
+	decayedLog := cfg.GetDecayedLog()
+
+	// Make sure we have a reference to the decayed log.
+	if decayedLog == nil {
+		return fmt.Errorf("decayed log backend is not available")
+	}
+
+	log.Info("Migrating decayed log...")
+	err := decayedLog.Update(func(tx kvdb.RwTx) error {
+		err := tx.DeleteTopLevelBucket(batchReplayBucket)
+		if err != nil && !errors.Is(err, kvdb.ErrBucketNotFound) {
+			return fmt.Errorf("deleting top level bucket %s: %w",
+				batchReplayBucket, err)
+		}
+
+		log.Debugf("top level bucket %s deleted", batchReplayBucket)
+
+		return nil
+	}, func() {})
+
+	if err != nil {
+		return fmt.Errorf("failed to migrate decayed log: %w", err)
+	}
+
+	log.Info("Decayed log migrated successfully")
+
+	return nil
+}