watchtower/wtdb/tower_db: add TowerDB and db versioning

watchtower/wtdb/version.go

@@ -0,0 +1,84 @@
+package wtdb
+
+import "github.com/coreos/bbolt"
+
+// migration is a function which takes a prior outdated version of the database
+// instances and mutates the key/bucket structure to arrive at a more
+// up-to-date version of the database.
+type migration func(tx *bbolt.Tx) error
+
+// version pairs a version number with the migration that would need to be
+// applied from the prior version to upgrade.
+type version struct {
+	number    uint32
+	migration migration
+}
+
+// dbVersions stores all versions and migrations of the database. This list will
+// be used when opening the database to determine if any migrations must be
+// applied.
+var dbVersions = []version{
+	{
+		// Initial version requires no migration.
+		number:    0,
+		migration: nil,
+	},
+}
+
+// getLatestDBVersion returns the last known database version.
+func getLatestDBVersion(versions []version) uint32 {
+	return versions[len(versions)-1].number
+}
+
+// getMigrations returns a slice of all updates with a greater number that
+// curVersion that need to be applied to sync up with the latest version.
+func getMigrations(versions []version, curVersion uint32) []version {
+	var updates []version
+	for _, v := range versions {
+		if v.number > curVersion {
+			updates = append(updates, v)
+		}
+	}
+
+	return updates
+}
+
+// getDBVersion retrieves the current database version from the metadata bucket
+// using the dbVersionKey.
+func getDBVersion(tx *bbolt.Tx) (uint32, error) {
+	metadata := tx.Bucket(metadataBkt)
+	if metadata == nil {
+		return 0, ErrUninitializedDB
+	}
+
+	versionBytes := metadata.Get(dbVersionKey)
+	if len(versionBytes) != 4 {
+		return 0, ErrNoDBVersion
+	}
+
+	return byteOrder.Uint32(versionBytes), nil
+}
+
+// initDBVersion initializes the top-level metadata bucket and writes the passed
+// version number as the current version.
+func initDBVersion(tx *bbolt.Tx, version uint32) error {
+	_, err := tx.CreateBucketIfNotExists(metadataBkt)
+	if err != nil {
+		return err
+	}
+
+	return putDBVersion(tx, version)
+}
+
+// putDBVersion stores the passed database version in the metadata bucket under
+// the dbVersionKey.
+func putDBVersion(tx *bbolt.Tx, version uint32) error {
+	metadata := tx.Bucket(metadataBkt)
+	if metadata == nil {
+		return ErrUninitializedDB
+	}
+
+	versionBytes := make([]byte, 4)
+	byteOrder.PutUint32(versionBytes, version)
+	return metadata.Put(dbVersionKey, versionBytes)
+}