Add new database migration system

database/migration/error.go

@@ -0,0 +1,56 @@
+package migration
+
+import "errors"
+
+type DiagnosticStep struct {
+	Version     string
+	Description string
+}
+
+type Diagnostics struct {
+	// Message holds a human readable message of the encountered
+	// error.
+	Message string
+	// Wrapped must be set to the underlying error that was encountered
+	// while preparing or executing migrations.
+	Wrapped error
+	// StartOfMigration is set to the version of the database before
+	// any migrations are applied.
+	StartOfMigration string
+	// LastSuccessfulMigration is set to the version of the database
+	// which has been applied successfully before the error happened.
+	LastSuccessfulMigration string
+	// TargetVersion is set to the version of the database that the
+	// migration run aimed for. That is, it's the last available version
+	// added to the registry.
+	TargetVersion string
+	// ExecutionPlan is a list of migration steps that were planned to
+	// be executed.
+	ExecutionPlan []DiagnosticStep
+	// FailedMigration is the description of the migration that has
+	// failed.
+	FailedMigration string
+}
+
+// Error returns a string representation of the migration error.
+func (err *Diagnostics) Error() string {
+	msg := ""
+	if err.FailedMigration != "" {
+		msg = err.FailedMigration + ": "
+	}
+	if err.Message != "" {
+		msg += err.Message + ": "
+	}
+	msg += err.Wrapped.Error()
+	return msg
+}
+
+// Unwrap returns the actual error that happend when executing
+// a migration. It implements the interface required by the stdlib
+// errors package to support errors.Is and errors.As
+func (err *Diagnostics) Unwrap() error {
+	if u := errors.Unwrap(err.Wrapped); u != nil {
+		return u
+	}
+	return err.Wrapped
+}