Fix infinite loop in PackedInputStream when the supplied ArrayInputStream is smaller than needed

Additionally:
* ArrayInputStream is no longer final, as there is no reason to stop consumers from extending and adjusting the behaviour
*  The wrapped ByteBuffer is not private, as according to the (previously) undocumented contract, it should be only accessible through #getReadBuffer().

runtime/src/main/java/org/capnproto/ArrayInputStream.java

@@ -23,11 +23,10 @@ package org.capnproto;
 
 import java.io.IOException;
 import java.nio.ByteBuffer;
-import java.nio.channels.ReadableByteChannel;
 
-public final class ArrayInputStream implements BufferedInputStream {
+public class ArrayInputStream implements BufferedInputStream {
 
-    public final ByteBuffer buf;
+    private final ByteBuffer buf;
 
     public ArrayInputStream(ByteBuffer buf) {
         this.buf = buf.asReadOnlyBuffer();
@@ -52,7 +51,11 @@ public final class ArrayInputStream implements BufferedInputStream {
 
     @Override
     public final ByteBuffer getReadBuffer() {
-        return this.buf;
+        if (buf.remaining() > 0) {
+            return buf;
+        } else {
+            throw new DecodeException("Premature EOF while reading buffer");
+        }
     }
 
     @Override

runtime/src/main/java/org/capnproto/BufferedInputStream.java

@@ -25,5 +25,14 @@ import java.nio.ByteBuffer;
 import java.nio.channels.ReadableByteChannel;
 
 public interface BufferedInputStream extends ReadableByteChannel {
-    public ByteBuffer getReadBuffer() throws java.io.IOException;
+
+    /**
+     * Returns a {@link ByteBuffer} to read data from.
+     * If there is no more data to read, throws a {@link DecodeException}.
+     *
+     * @return a {@link ByteBuffer} with data to read from, if any available
+     * @throws java.io.IOException when an I/O error occurs
+     * @throws DecodeException     when trying to read more data than available
+     */
+    ByteBuffer getReadBuffer() throws java.io.IOException;
 }

runtime/src/test/java/org/capnproto/SerializePackedTest.java

@@ -82,4 +82,16 @@ public class SerializePackedTest {
             Assert.assertTrue(Arrays.equals(bytes, unpacked));
         }
     }
+
+    @Test(timeout = 1000, expected = DecodeException.class)
+    public void read_shouldThrowDecodingExceptionOnEmptyArrayInputStream() throws IOException {
+        byte[] emptyByteArray = {};
+        MessageReader reader = SerializePacked.read(new ArrayInputStream(ByteBuffer.wrap(emptyByteArray)), ReaderOptions.DEFAULT_READER_OPTIONS);
+    }
+
+    @Test(timeout = 1000, expected = DecodeException.class)
+    public void read_shouldThrowDecodingExceptionWhenTryingToReadMoreThanAvailableFromArrayInputStream() throws IOException {
+        byte[] bytes = {17, 0, 127, 0, 0, 0, 0}; //segment0 size of 127 words, which is way larger than the tiny 7 byte input
+        MessageReader reader = SerializePacked.read(new ArrayInputStream(ByteBuffer.wrap(bytes)), ReaderOptions.DEFAULT_READER_OPTIONS);
+    }
 }