Escape newlines in single-instance argument passing

Arguments containing newlines are now properly escaped when sent over
the wire protocol. Backslashes, newlines (\n), and carriage returns
(\r) are escaped to prevent them from being interpreted as line
separators in the one-arg-per-line protocol.

Author: ctrueden

src/main/java/org/scijava/launcher/SingleInstance.java

@@ -142,7 +142,7 @@ static boolean tryHandoff(String[] args) {
 					lockFile.toFile().delete();
 					return false;
 				}
-				for (String arg : args) out.println(arg);
+				for (String arg : args) out.println(escapeNewlines(arg));
 			}
 			Log.debug("[SingleInstance] Args handed off to existing instance.");
 			return true;
@@ -179,7 +179,7 @@ private static void handleConnection(Socket client, String secretGreeting, Strin
 			out.println(secretResponse);
 			List<String> args = new ArrayList<>();
 			String line;
-			while ((line = in.readLine()) != null) args.add(line);
+			while ((line = in.readLine()) != null) args.add(unescapeNewlines(line));
 			if (!args.isEmpty()) argReceiver.accept(args.toArray(new String[0]));
 		}
 		catch (IOException e) {
@@ -206,4 +206,26 @@ private static String toHex(byte[] bytes) {
 		for (byte b : bytes) sb.append(String.format("%02x", b));
 		return sb.toString();
 	}
+
+	private static String escapeNewlines(String s) {
+		return s.replace("\\", "\\\\").replace("\n", "\\n").replace("\r", "\\r");
+	}
+
+	private static String unescapeNewlines(String s) {
+		StringBuilder sb = new StringBuilder();
+		for (int i = 0; i < s.length(); i++) {
+			char c = s.charAt(i);
+			if (c == '\\' && i + 1 < s.length()) {
+				char next = s.charAt(i + 1);
+				if (next == 'n') { sb.append('\n'); i++; }
+				else if (next == 'r') { sb.append('\r'); i++; }
+				else if (next == '\\') { sb.append('\\'); i++; }
+				else sb.append(c);
+			}
+			else {
+				sb.append(c);
+			}
+		}
+		return sb.toString();
+	}
 }