fix: refuse DDL during transactions (#41)

DDL statements cannot be executed as part of a transaction on Cloud Spanner.
Although it would be technically possible to execute a DDL statement while a
transaction is active, it could cause confusion whether the statement is part
of the transaction or not. The driver therefore should return an error if an
application tries to execute a DDL statement during an active transaction.

Fixes #31

driver.go

@@ -352,10 +352,12 @@ func (c *conn) ExecContext(ctx context.Context, query string, args []driver.Name
 	}
 
 	if isDdl {
-		// TODO: Determine whether we want to return an error if a transaction
-		// is active. Cloud Spanner does not support DDL in transactions, but
-		// this makes it seem like the DDL statement is executed on the
-		// transaction on this connection if it has a transaction.
+		// Spanner does not support DDL in transactions, and although it is technically possible to execute DDL
+		// statements while a transaction is active, we return an error to avoid any confusion whether the DDL
+		// statement is executed as part of the active transaction or not.
+		if c.inTransaction() {
+			return nil, spanner.ToSpannerError(status.Errorf(codes.FailedPrecondition, "cannot execute DDL as part of a transaction"))
+		}
 		op, err := c.adminClient.UpdateDatabaseDdl(ctx, &adminpb.UpdateDatabaseDdlRequest{
 			Database:   c.database,
 			Statements: []string{query},

driver_with_mockserver_test.go

@@ -953,45 +953,18 @@ func TestDdlInTransaction(t *testing.T) {
 	db, server, teardown := setupTestDBConnection(t)
 	defer teardown()
 
-	var expectedResponse = &emptypb.Empty{}
-	any, _ := ptypes.MarshalAny(expectedResponse)
-	server.TestDatabaseAdmin.SetResps([]proto.Message{
-		&longrunningpb.Operation{
-			Done:   true,
-			Result: &longrunningpb.Operation_Response{Response: any},
-			Name:   "test-operation",
-		},
-	})
 	query := "CREATE TABLE Singers (SingerId INT64, FirstName STRING(100), LastName STRING(100)) PRIMARY KEY (SingerId)"
 	tx, err := db.BeginTx(context.Background(), &sql.TxOptions{})
 	if err != nil {
 		t.Fatal(err)
 	}
-	res, err := tx.ExecContext(context.Background(), query)
-	if err != nil {
-		t.Fatal(err)
-	}
-	affected, err := res.RowsAffected()
-	if err != nil {
-		t.Fatal(err)
-	}
-	if affected != 0 {
-		t.Fatalf("affected rows count mismatch\nGot: %v\nWant: %v", affected, 0)
+	if _, err := tx.ExecContext(context.Background(), query); spanner.ErrCode(err) != codes.FailedPrecondition {
+		t.Fatalf("error mismatch\nGot:  %v\nWant: %v", spanner.ErrCode(err), codes.FailedPrecondition)
 	}
 	requests := server.TestDatabaseAdmin.Reqs()
-	if g, w := len(requests), 1; g != w {
+	if g, w := len(requests), 0; g != w {
 		t.Fatalf("requests count mismatch\nGot: %v\nWant: %v", g, w)
 	}
-	if req, ok := requests[0].(*databasepb.UpdateDatabaseDdlRequest); ok {
-		if g, w := len(req.Statements), 1; g != w {
-			t.Fatalf("statement count mismatch\nGot: %v\nWant: %v", g, w)
-		}
-		if g, w := req.Statements[0], query; g != w {
-			t.Fatalf("statement mismatch\nGot: %v\nWant: %v", g, w)
-		}
-	} else {
-		t.Fatalf("request type mismatch, got %v", requests[0])
-	}
 }
 
 func TestBegin(t *testing.T) {