refactor: make migration process more intelligent, add test coverage

pkg/db/migration.go

@@ -1,5 +1,14 @@
 package db
 
+import (
+	"fmt"
+
+	"github.com/jmoiron/sqlx"
+
+	"git.offline-twitter.com/offline-labs/gas-stack/pkg/must"
+	"git.offline-twitter.com/offline-labs/gas-stack/pkg/textutils"
+)
+
 type Migration struct {
 	// ID is the version number that the schema will be at, after applying the migration.
 	ID int
@@ -10,3 +19,32 @@ type Migration struct {
 	// SQL is the body of the migration that will be executed.
 	SQL string
 }
+
+// Apply runs the migration in a transaction.  Foreign key checking is delayed until right before committing the transaction.
+func (m Migration) Apply(db *sqlx.DB) error {
+	fmt.Print(textutils.ColorCyan)
+	fmt.Println(m.SQL)
+	fmt.Print(textutils.ColorReset)
+
+	// Temporarily suspend foreign keys until the migration is finished
+	db.MustExec("pragma foreign_keys = off")
+	defer db.MustExec("pragma foreign_keys = on")
+
+	tx := db.MustBegin()
+	defer func() { _ = tx.Rollback() }()
+
+	// Sanity: should initially be on the previous version
+	var prevVersion int
+	must.Do(tx.Get(&prevVersion, `select version from db_version`))
+	if prevVersion != m.ID-1 {
+		return fmt.Errorf("%w: tried to apply migration %d on DB version %d", ErrMigrationOrder, m.ID, prevVersion)
+	}
+
+	// Execute the migration
+	tx.MustExec(m.SQL)
+	tx.MustExec("update db_version set version = ?", m.ID)
+	must.Do(PragmaForeignKeyCheck(tx))
+	must.Do(tx.Commit())
+
+	return nil
+}