[MySQL] Check that the DB schema is compatible (#458)

The version is written at schema creation, and checked on startup. If they are different, it fails. Towards #450.

storage/mysql/DESIGN.md

@@ -15,6 +15,10 @@ The DB layout has been designed such that serving any read request is a point lo
 
 ### Table Schema
 
+#### `Tessera`
+
+A single row that records the current version of the Tessera schema and data compatibility.
+
 #### `Checkpoint`
 
 A single row that records the current published checkpoint.

storage/mysql/mysql.go

@@ -38,6 +38,7 @@ import (
 )
 
 const (
+	selectCompatibilityVersionSQL    = "SELECT `compatibilityVersion` FROM `Tessera` WHERE `id` = 0"
 	selectCheckpointByIDSQL          = "SELECT `note`, `published_at` FROM `Checkpoint` WHERE `id` = ?"
 	selectCheckpointByIDForUpdateSQL = selectCheckpointByIDSQL + " FOR UPDATE"
 	replaceCheckpointSQL             = "REPLACE INTO `Checkpoint` (`id`, `note`, `published_at`) VALUES (?, ?, ?)"
@@ -52,6 +53,8 @@ const (
 	checkpointID = 0
 	treeStateID  = 0
 
+	schemaCompatibilityVersion = 1
+
 	minCheckpointInterval = time.Second
 )
 
@@ -85,6 +88,9 @@ func New(ctx context.Context, db *sql.DB, opts ...func(*options.StorageOptions))
 	if s.newCheckpoint == nil {
 		return nil, errors.New("tessera.WithCheckpointSigner must be provided in New()")
 	}
+	if err := s.ensureVersion(ctx, schemaCompatibilityVersion); err != nil {
+		return nil, fmt.Errorf("incompatible schema version: %v", err)
+	}
 
 	s.queue = storage.NewQueue(ctx, opt.BatchMaxAge, opt.BatchMaxSize, s.sequenceBatch)
 
@@ -110,6 +116,21 @@ func New(ctx context.Context, db *sql.DB, opts ...func(*options.StorageOptions))
 	return s, nil
 }
 
+func (s *Storage) ensureVersion(ctx context.Context, wantVersion uint8) error {
+	row := s.db.QueryRowContext(ctx, selectCompatibilityVersionSQL)
+	if row.Err() != nil {
+		return row.Err()
+	}
+	var gotVersion uint8
+	if err := row.Scan(&gotVersion); err != nil {
+		return fmt.Errorf("failed to read Tessera version from DB: %v", err)
+	}
+	if gotVersion != wantVersion {
+		return fmt.Errorf("DB has Tessera compatibility version of %d, but version %d required", gotVersion, wantVersion)
+	}
+	return nil
+}
+
 // maybeInitTree will insert an initial "empty tree" row into the
 // TreeState table iff no row already exists.
 //

storage/mysql/schema.sql

@@ -14,6 +14,19 @@
 
 -- MySQL version of the Trillian Tessera database schema.
 
+-- "Tessera" table stores a single row that is the version of this schema
+-- and the data formats within it. This is read at startup to prevent Tessera
+-- running against a database with an incompatible format.
+CREATE TABLE IF NOT EXISTS Tessera (
+  -- id is expected to be always 0 to maintain a maximum of a single row.
+  `id`                   TINYINT UNSIGNED NOT NULL,
+  -- compatibilityVersion is the version of this schema and the data within it.
+  `compatibilityVersion` BIGINT UNSIGNED NOT NULL,
+  PRIMARY KEY (`id`)
+);
+
+INSERT IGNORE INTO Tessera (`id`, `compatibilityVersion`) VALUES (0, 1);
+
 -- "Checkpoint" table stores a single row that records the latest _published_ checkpoint for the log.
 -- This is stored separately from the TreeState in order to enable publishing of commitments to updated tree states to happen
 -- on an indepentent timeframe to the internal updating of state.