8216140: Correct UnicodeDecoder U+FFFE handling

Reviewed-by: rriggs

src/java.base/share/classes/sun/nio/cs/UnicodeDecoder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2000, 2006, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -91,11 +91,6 @@ abstract class UnicodeDecoder extends CharsetDecoder {
 
                 char c = decode(b1, b2);
 
-                if (c == REVERSED_MARK) {
-                    // A reversed BOM cannot occur within middle of stream
-                    return CoderResult.malformedForLength(2);
-                }
-
                 // Surrogates
                 if (Character.isSurrogate(c)) {
                     if (Character.isHighSurrogate(c)) {

test/jdk/sun/nio/cs/TestUnicodeReversedBOM.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8216140
+ * @summary Test reversed BOM (U+FFFE) in the middle of a byte buffer
+ *      passes through during decoding with UnicodeDecoder.
+ * @run testng TestUnicodeReversedBOM
+ */
+import java.nio.charset.*;
+import java.nio.*;
+import java.util.*;
+
+import org.testng.annotations.DataProvider;
+import org.testng.annotations.Test;
+
+@Test
+public class TestUnicodeReversedBOM {
+    private static byte[] A_REVERSED_BE =
+        {(byte)0x0, (byte)'A', (byte)0xff, (byte)0xfe};
+    private static byte[] A_REVERSED_LE =
+        {(byte)'A', (byte)0x0, (byte)0xfe, (byte)0xff};
+    private static byte[] BOM_REVERSED_BE =
+        {(byte)0xfe, (byte)0xff, (byte)0xff, (byte)0xfe};
+    private static byte[] BOM_REVERSED_LE =
+        {(byte)0xff, (byte)0xfe, (byte)0xfe, (byte)0xff};
+
+    @DataProvider
+    // [(byte[])byte array, (Charset)cs]
+    public static Object[][] ReversedBOM() {
+        return new Object[][] {
+            {A_REVERSED_BE, StandardCharsets.UTF_16},
+            {A_REVERSED_LE, StandardCharsets.UTF_16},
+            {A_REVERSED_BE, StandardCharsets.UTF_16BE},
+            {A_REVERSED_LE, StandardCharsets.UTF_16BE},
+            {A_REVERSED_BE, StandardCharsets.UTF_16LE},
+            {A_REVERSED_LE, StandardCharsets.UTF_16LE},
+            {BOM_REVERSED_BE, StandardCharsets.UTF_16},
+            {BOM_REVERSED_LE, StandardCharsets.UTF_16},
+            {BOM_REVERSED_BE, StandardCharsets.UTF_16BE},
+            {BOM_REVERSED_LE, StandardCharsets.UTF_16BE},
+            {BOM_REVERSED_BE, StandardCharsets.UTF_16LE},
+            {BOM_REVERSED_LE, StandardCharsets.UTF_16LE},
+        };
+    }
+
+    @Test(dataProvider = "ReversedBOM")
+    public void testReversedBOM(byte[] ba, Charset cs) throws CharacterCodingException {
+        cs.newDecoder()
+            .onMalformedInput(CodingErrorAction.REPORT)
+            .onUnmappableCharacter(CodingErrorAction.REPORT)
+            .decode(ByteBuffer.wrap(ba));
+    }
+}