feat: support fetching a REF_CURSOR using getObject

Since DatabaseMetaData#supportsRefCursors now reports true
frameworks like Hibernate now use
callableStatement.getObject(int, ResultSet.class) to fetch the
REF_CURSOR.

Only support ResultSet.class as an argument to keep the patch simple.

Closes: #794

pgjdbc/src/main/java/org/postgresql/jdbc/PgCallableStatement.java

@@ -691,7 +691,11 @@ public Reader getCharacterStream(String parameterName) throws SQLException {
  }

  public <T> T getObject(int parameterIndex, Class<T> type) throws SQLException {
    throw Driver.notImplemented(this.getClass(), "getObject(int, Class<T>)");
    if (type == ResultSet.class) {
      return type.cast(getObject(parameterIndex));
    }
    throw new PSQLException(GT.tr("Unsupported type conversion to {1}.", type),
            PSQLState.INVALID_PARAMETER_VALUE);
  }

  public <T> T getObject(String parameterName, Class<T> type) throws SQLException {

pgjdbc/src/test/java/org/postgresql/test/jdbc42/Jdbc42CallableStatementTest.java

@@ -0,0 +1,94 @@
/*
 * Copyright (c) 2017, PostgreSQL Global Development Group
 * See the LICENSE file in the project root for more information.
 */

package org.postgresql.test.jdbc42;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.fail;

import org.postgresql.test.jdbc2.BaseTest4;

import org.junit.Test;

import java.sql.CallableStatement;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;
import java.sql.Types;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;


/**
 * Tests for JDBC 4.2 features in {@link org.postgresql.jdbc.PgCallableStatement}.
 */
public class Jdbc42CallableStatementTest extends BaseTest4 {

  @Override
  public void setUp() throws Exception {
    super.setUp();

    try (Statement stmt = con.createStatement();) {
      stmt.execute(
              "CREATE OR REPLACE FUNCTION testspg__getResultSetWithoutArg() "
                      + "RETURNS refcursor AS '  "
                      + "declare ref refcursor;"
                      + "begin OPEN ref FOR SELECT 1; RETURN ref; end; ' LANGUAGE plpgsql;");
    }
  }


  final String func = "{ ? = call ";
  final String pkgName = "testspg__";

  @Override
  public void tearDown() throws SQLException {
    try (Statement stmt = con.createStatement()) {
      stmt.execute("drop FUNCTION testspg__getResultSetWithoutArg ();");
    }
    super.tearDown();
  }

  @Test
  public void testGetResultSetWithoutArg() throws SQLException {
    assumeCallableStatementsSupported();
    try (CallableStatement call = con.prepareCall(func + pkgName + "getResultSetWithoutArg () }")) {
      con.setAutoCommit(false); // ref cursors only work if auto commit is off
      call.registerOutParameter(1, Types.REF_CURSOR);
      call.execute();
      List<Integer> values = new ArrayList<>(1);
      try (ResultSet rs = call.getObject(1, ResultSet.class)) {
        while (rs.next()) {
          values.add(rs.getInt(1));
        }
      }
      assertEquals(Collections.singletonList(1), values);
    } finally {
      con.setAutoCommit(true);
    }
  }

  @Test
  public void testGetResultSetWithoutArgUnsupportedConversion() throws SQLException {
    assumeCallableStatementsSupported();
    try (CallableStatement call = con.prepareCall(func + pkgName + "getResultSetWithoutArg () }")) {
      con.setAutoCommit(false); // ref cursors only work if auto commit is off
      call.registerOutParameter(1, Types.REF_CURSOR);
      call.execute();
      try {
        // this should never be allowed even if more types will be implemented in the future
        call.getObject(1, ResultSetMetaData.class);
        fail("conversion from ResultSet to ResultSetMetaData should not be supported");
      } catch (SQLException e) {
        // should reach
      }
    } finally {
      con.setAutoCommit(true);
    }
  }

}

pgjdbc/src/test/java/org/postgresql/test/jdbc42/Jdbc42TestSuite.java

@@ -15,6 +15,7 @@
    CustomizeDefaultFetchSizeTest.class,
    GetObject310Test.class,
    PreparedStatementTest.class,
    Jdbc42CallableStatementTest.class,
    GetObject310BinaryTest.class,
    SetObject310Test.class})
public class Jdbc42TestSuite {