fix: broken FilterInputStream in StreamConnectionProvider#forwardCopyTo

Author: sebthom

org.eclipse.lsp4e.test/src/org/eclipse/lsp4e/test/server/StreamConnectionProviderTest.java

@@ -0,0 +1,84 @@
+/*******************************************************************************
+ * Copyright (c) 2025
+ * This program and the accompanying materials are made
+ * available under the terms of the Eclipse Public License 2.0
+ * which is available at https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *******************************************************************************/
+package org.eclipse.lsp4e.test.server;
+
+import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.junit.Assert.*;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.net.URI;
+
+import org.eclipse.lsp4e.server.StreamConnectionProvider;
+import org.eclipse.lsp4j.jsonrpc.messages.Message;
+import org.eclipse.lsp4j.services.LanguageServer;
+import org.junit.Test;
+
+public class StreamConnectionProviderTest {
+
+	private static StreamConnectionProvider newProvider() {
+		return new StreamConnectionProvider() {
+			@Override
+			public void start() throws IOException {
+			}
+
+			@Override
+			public InputStream getInputStream() {
+				return null;
+			}
+
+			@Override
+			public OutputStream getOutputStream() {
+				return null;
+			}
+
+			@Override
+			public InputStream getErrorStream() {
+				return null;
+			}
+
+			@Override
+			public void stop() {
+			}
+
+			@Override
+			public void handleMessage(Message message, LanguageServer languageServer, URI rootURI) {
+			}
+		};
+	}
+
+	@Test
+	public void test_forwardCopy_singleByteRead_writesToProvidedOutput() throws Exception {
+		final var input = new ByteArrayInputStream("ABC".getBytes(UTF_8));
+		final var sink = new ByteArrayOutputStream();
+
+		try (InputStream forwarding = newProvider().forwardCopyTo(input, sink)) {
+			while ((forwarding.read()) != -1) {
+				// read one byte at a time to exercise single-byte read path
+			}
+		}
+		assertEquals("expected input to be forwarded to provided OutputStream", "ABC", sink.toString(UTF_8));
+	}
+
+	@Test
+	public void test_forwardCopy_readArray_onEOF_returnsMinusOne_noException() throws Exception {
+		final var emptyInput = new ByteArrayInputStream(new byte[0]);
+		final var sink = new ByteArrayOutputStream();
+
+		try (final var forwarding = newProvider().forwardCopyTo(emptyInput, sink)) {
+			final var buf = new byte[8];
+			int n = forwarding.read(buf); // should be -1 and not throw
+			assertEquals("expected EOF (-1) on empty stream", -1, n);
+		}
+		assertArrayEquals(new byte[0], sink.toByteArray());
+	}
+}

org.eclipse.lsp4e/src/org/eclipse/lsp4e/server/StreamConnectionProvider.java

@@ -75,25 +75,23 @@ public interface StreamConnectionProvider {
 			@Override
 			public int read() throws IOException {
 				int res = super.read();
-				System.err.print((char) res);
+				if (res == -1) {
+					return -1;
+				}
+				output.write(res);
 				return res;
 			}
 
 			@Override
 			public int read(byte[] b, int off, int len) throws IOException {
 				int bytes = super.read(b, off, len);
-				final var payload = new byte[bytes];
-				System.arraycopy(b, off, payload, 0, bytes);
-				output.write(payload, 0, payload.length);
-				return bytes;
-			}
-
-			@Override
-			public int read(byte[] b) throws IOException {
-				int bytes = super.read(b);
-				final var payload = new byte[bytes];
-				System.arraycopy(b, 0, payload, 0, bytes);
-				output.write(payload, 0, payload.length);
+				if (bytes == -1) {
+					return -1;
+				}
+				if (bytes == 0) {
+					return 0;
+				}
+				output.write(b, off, bytes);
 				return bytes;
 			}
 		};