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 {