fix: clear statement tag before auto rollback (#147)

Clear any statement tag before executing an automatic rollback when a
transaction is aborted, as statement tags are not allowed for rollback
requests.

Fixes #146

src/main/java/com/google/cloud/spanner/pgadapter/wireprotocol/QueryMessage.java

@@ -140,6 +140,8 @@ public void handleQuery() throws Exception {
       if (connection.getStatus() == ConnectionStatus.TRANSACTION_ABORTED) {
         transactionStatus = Status.FAILED;
         // Actively rollback the aborted transaction but still block clients
+        // Clear any statement tags, as these are not allowed for rollbacks.
+        connection.getSpannerConnection().setStatementTag(null);
         connection.getSpannerConnection().rollback();
       } else {
         transactionStatus = Status.TRANSACTION;

src/test/java/com/google/cloud/spanner/pgadapter/JdbcSimpleModeMockServerTest.java

@@ -842,4 +842,20 @@ public void testAbortedTransactionRollbackInAutocommit() throws SQLException {
     List<RollbackRequest> rollbackRequests = mockSpanner.getRequestsOfType(RollbackRequest.class);
     assertEquals(1, rollbackRequests.size());
   }
+
+  @Test
+  public void testStatementTagError() throws SQLException {
+    try (Connection connection = DriverManager.getConnection(createUrl())) {
+      try (java.sql.Statement statement = connection.createStatement()) {
+        assertFalse(statement.execute("begin"));
+        assertFalse(statement.execute(INSERT_STATEMENT.getSql()));
+        assertFalse(statement.execute("set spanner.statement_tag='foo'"));
+        // Execute an invalid statement.
+        assertThrows(SQLException.class, () -> statement.execute("set statement_timeout=2s"));
+        // Make sure that we actually received a Rollback statement. The rollback was initiated by
+        // PGAdapter when the transaction was aborted.
+        assertEquals(1, mockSpanner.countRequestsOfType(RollbackRequest.class));
+      }
+    }
+  }
 }