fix: getting resultset metadata twice could skip row (#323)

* fix: getting resultset metadata twice could skip row

If the client application would call ResultSet#getMetaData() more than once
**before** calling ResultSet#next(), the ResultSet would skip a row when the
rows would be consumed.

Fixes #322

* test: add test for getMetaData twice

src/main/java/com/google/cloud/spanner/jdbc/JdbcResultSet.java

@@ -547,7 +547,7 @@ public InputStream getBinaryStream(String columnLabel) throws SQLException {
   @Override
   public JdbcResultSetMetaData getMetaData() throws SQLException {
     checkClosed();
-    if (isBeforeFirst()) {
+    if (isBeforeFirst() && !nextCalledForMetaData) {
       // do a call to next() on the underlying resultset to initialize metadata
       nextCalledForMetaData = true;
       nextCalledForMetaDataResult = spanner.next();

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

@@ -25,6 +25,7 @@
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 import com.google.cloud.ByteArray;
 import com.google.cloud.Date;
@@ -802,6 +803,32 @@ public void testGetMetaData() throws SQLException {
     assertNotNull(metadata);
   }
 
+  @Test
+  public void testGetMetaDataBeforeNext() throws SQLException {
+    ResultSet spannerResultSet = mock(ResultSet.class);
+    when(spannerResultSet.next()).thenReturn(true, false);
+
+    JdbcResultSet resultSet = JdbcResultSet.of(spannerResultSet);
+    assertNotNull(resultSet.getMetaData());
+    assertTrue(resultSet.next());
+    assertFalse(resultSet.next());
+  }
+
+  @Test
+  public void testGetMetaDataTwiceBeforeNext() throws SQLException {
+    ResultSet spannerResultSet = mock(ResultSet.class);
+    when(spannerResultSet.next()).thenReturn(true, false);
+
+    JdbcResultSet resultSet = JdbcResultSet.of(spannerResultSet);
+    assertNotNull(resultSet.getMetaData());
+    assertNotNull(resultSet.getMetaData());
+
+    // This would have returned false before the fix in
+    // https://github.com/googleapis/java-spanner-jdbc/pull/323
+    assertTrue(resultSet.next());
+    assertFalse(resultSet.next());
+  }
+
   @Test
   public void testFindColumn() throws SQLException {
     assertEquals(2, subject.findColumn(STRING_COL_NOT_NULL));