feat: allow DDL with autocommit=false (#1600)

Adds support for running DDL statements when a connection is in autocommit=false mode. By default, DDL statements are only allowed when no transaction is active. That is; no query or DML statement has been executed which activated a read/write transaction.

A new flag is added that can be used to revert the behavior back to the original behavior where DDL is always refused when autocommit=false. The same flag can also be used to make the API behave the same as MySQL and Oracle, where any active transaction is automatically committed whenever a DDL statement is encountered.

Concretely this means that the following is now allowed:

```
set autocommit=false;
create table Singers (SingerId INT64, Name STRING(MAX)) PRIMARY KEY (SingerId);
```

The following is by default NOT allowed, unless
ddlInTransactionMode=AUTO_COMMIT_TRANSACTION

```
set autocommit=false;
select * from singers; -- This starts a transaction
create table Albums (AlbumId INT64) PRIMARY KEY (AlbumId); -- This is not allowed
```

src/test/java/com/google/cloud/spanner/jdbc/DdlMockServerTest.java

@@ -0,0 +1,101 @@
+/*
+ * Copyright 2024 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.cloud.spanner.jdbc;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertThrows;
+import static org.junit.Assert.assertTrue;
+
+import com.google.cloud.spanner.MockSpannerServiceImpl.StatementResult;
+import com.google.cloud.spanner.Statement;
+import com.google.cloud.spanner.connection.AbstractMockServerTest;
+import com.google.longrunning.Operation;
+import com.google.protobuf.Any;
+import com.google.protobuf.Empty;
+import com.google.rpc.Code;
+import com.google.spanner.admin.database.v1.UpdateDatabaseDdlMetadata;
+import java.sql.Connection;
+import java.sql.DriverManager;
+import java.sql.SQLException;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class DdlMockServerTest extends AbstractMockServerTest {
+
+  private String createUrl(boolean autoCommit) {
+    return String.format(
+        "jdbc:cloudspanner://localhost:%d/projects/%s/instances/%s/databases/%s?usePlainText=true;autoCommit=%s",
+        getPort(), "proj", "inst", "db", autoCommit);
+  }
+
+  private Connection createConnection(boolean autoCommit) throws SQLException {
+    return DriverManager.getConnection(createUrl(autoCommit));
+  }
+
+  @Test
+  public void testDdlInAutoCommitIsTrue_succeeds() throws SQLException {
+    mockDatabaseAdmin.addResponse(
+        Operation.newBuilder()
+            .setDone(true)
+            .setResponse(Any.pack(Empty.getDefaultInstance()))
+            .setMetadata(Any.pack(UpdateDatabaseDdlMetadata.getDefaultInstance()))
+            .build());
+
+    try (Connection connection = createConnection(/* autoCommit = */ true)) {
+      assertFalse(
+          connection.createStatement().execute("create table foo (id int64) primary key (id)"));
+    }
+  }
+
+  @Test
+  public void testDdlInAutoCommitIsFalse_succeedsWithNoActiveTransaction() throws SQLException {
+    mockDatabaseAdmin.addResponse(
+        Operation.newBuilder()
+            .setDone(true)
+            .setResponse(Any.pack(Empty.getDefaultInstance()))
+            .setMetadata(Any.pack(UpdateDatabaseDdlMetadata.getDefaultInstance()))
+            .build());
+
+    try (Connection connection = createConnection(/* autoCommit = */ false)) {
+      assertFalse(
+          connection.createStatement().execute("create table foo (id int64) primary key (id)"));
+    }
+  }
+
+  @Test
+  public void testDdlInAutoCommitIsFalse_failsWithActiveTransaction() throws SQLException {
+    mockSpanner.putStatementResult(
+        StatementResult.update(Statement.of("update foo set bar=1 where true"), 1L));
+
+    try (Connection connection = createConnection(/* autoCommit = */ false)) {
+      assertFalse(connection.createStatement().execute("update foo set bar=1 where true"));
+      SQLException exception =
+          assertThrows(
+              SQLException.class,
+              () ->
+                  connection
+                      .createStatement()
+                      .execute("create table foo (id int64) primary key (id)"));
+      assertTrue(exception instanceof JdbcSqlException);
+      JdbcSqlException jdbcSqlException = (JdbcSqlException) exception;
+      assertEquals(Code.FAILED_PRECONDITION, jdbcSqlException.getCode());
+    }
+  }
+}