test/jdk/java/io/BufferedInputStream/TransferToTrusted.java

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import java.io.*;
import java.util.Arrays;
import java.util.Objects;
import java.util.Random;

/*
 * @test
 * @bug 8320971
 * @summary Verify BufferedInputStream.buf is used directly by
 *          BufferedInputStream.implTransferTo() only when its OutputStream
 *          parameter is trusted
 * @key randomness
 * @run main/othervm --add-opens=java.base/java.io=ALL-UNNAMED TransferToTrusted
 */
public class TransferToTrusted {

    private static final Random RND = new Random(System.nanoTime());

    private static final class UntrustedOutputStream extends OutputStream {

        UntrustedOutputStream() {
            super();
        }

        @Override
        public void write(byte[] b, int off, int len) {
            Objects.checkFromIndexSize(off, len, b.length);
            byte[] tmp = new byte[len];
            RND.nextBytes(tmp);
            System.arraycopy(tmp, 0, b, off, len);
        }

        @Override
        public void write(int b) throws IOException {
            write(new byte[]{(byte) b});
        }
    }

    public static void main(String[] args) throws Exception {
        final int length = 128;
        byte[] buf = new byte[length];
        RND.nextBytes(buf);

        var outputStreams = new OutputStream[]{
                new ByteArrayOutputStream(),
                new FileOutputStream(File.createTempFile(TransferToTrusted.class.getName(), null)),
                new PipedOutputStream(new PipedInputStream(length)),
                new UntrustedOutputStream()
        };

        for (var out : outputStreams) {
            System.err.println("out: " + out.getClass().getName());

            var bis = new BufferedInputStream(new ByteArrayInputStream(buf.clone()));
            try (out; bis) {
                bis.read();//need this to fill the BIS.buf in
                bis.transferTo(out);
                var internalBuffer = bis.getClass().getDeclaredField("buf");
                internalBuffer.setAccessible(true);
                if (!Arrays.equals(buf, Arrays.copyOf((byte[]) internalBuffer.get(bis), length))) {
                    throw new RuntimeException("Internal buffer was modified");
                }
            }
        }
    }
}