New TestOutputReading runtime-testsuite class

runtime-testsuite/test/org/antlr/v4/test/runtime/StreamVacuum.java

@@ -25,12 +25,7 @@ public final class StreamVacuum implements Runnable {
 	@Override
 	public void run() {
 		try {
-			String line = in.readLine();
-			while (line!=null) {
-				buf.append(line);
-				buf.append('\n');
-				line = in.readLine();
-			}
+			TestOutputReading.append(in, buf);
 		}
 		catch (IOException ioe) {
 			System.err.println("can't read output from process");

runtime-testsuite/test/org/antlr/v4/test/runtime/TestOutputReading.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2012-2017 The ANTLR Project. All rights reserved.
+ * Use of this file is governed by the BSD 3-clause license that
+ * can be found in the LICENSE.txt file in the project root.
+ */
+package org.antlr.v4.test.runtime;
+
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.NoSuchFileException;
+import java.nio.file.Path;
+import java.io.BufferedReader;
+import java.io.FileNotFoundException;
+import java.io.IOException;
+
+public abstract class TestOutputReading {
+	public static void append(BufferedReader in, StringBuilder buf) throws IOException {
+		String line = in.readLine();
+		while (line!=null) {
+			buf.append(line);
+			// NOTE: This appends a newline at EOF
+			// regardless of whether or not the
+			// input actually ended with a
+			// newline.
+			//
+			// We should revisit this and read a
+			// block at a time rather than a line
+			// at a time, and change all tests
+			// which rely on this behavior to
+			// remove the trailing newline at EOF.
+			//
+			// When we fix this, we can remove the
+			// TestOutputReading class entirely.
+			buf.append('\n');
+			line = in.readLine();
+		}
+	}
+
+	/**
+	 * Read in the UTF-8 bytes at {@code path}, convert all
+	 * platform-specific line terminators to NL, and append NL
+	 * if the file was non-empty and didn't already end with one.
+	 *
+	 * {@see StreamVacuum#run()} for why this method exists.
+	 *
+	 * Returns {@code null} if the file does not exist or the output
+	 * was empty.
+	 */
+	public static String read(Path path) throws IOException {
+		// Mimic StreamVacuum.run()'s behavior of replacing all platform-specific
+		// EOL sequences with NL.
+		StringBuilder buf = new StringBuilder();
+		try (BufferedReader in = Files.newBufferedReader(path, StandardCharsets.UTF_8)) {
+			append(in, buf);
+		} catch (FileNotFoundException | NoSuchFileException e) {
+			return null;
+		}
+		if (buf.length() > 0) {
+			return buf.toString();
+		} else {
+			return null;
+		}
+	}
+}