Test the command line handling in janus and print help plainly

shell/src/main/java/org/unigrid/janus/shell/Janus.java

@@ -57,7 +57,16 @@ private Janus() {
 	}
 
 	public static void main(final String[] args) throws Exception {
-		final Optional<LaunchOptions> options = options(args);
+		final CommandLine command = command();
+		final Optional<LaunchOptions> options;
+
+		try {
+			options = options(command, args);
+		} catch (IllegalArgumentException | CommandLine.ParameterException e) {
+			refuse(command, e);
+			System.exit(USAGE_ERROR);
+			return;
+		}
 
 		if (options.isPresent()) {
 			start(application(), options.get());
@@ -66,28 +75,36 @@ public static void main(final String[] args) throws Exception {
 
 	/*
 	 * The command line is Janus's: Oriel takes out the window system and Janus reads what is left, refusing what
-	 * it does not know so that a mistyped option is never quietly ignored. Empty when the help was asked for.
+	 * it does not know so that a mistyped option is never quietly ignored. Empty when the help was asked for,
+	 * which is printed plainly on the command's own output; an option Janus or Oriel does not take is thrown,
+	 * so that main alone decides how Janus ends.
 	 */
-	private static Optional<LaunchOptions> options(final String... args) {
-		final CommandLine command = new CommandLine(new Arguments());
+	static Optional<LaunchOptions> options(final CommandLine command, final String... args) {
+		final LaunchOptions options = LaunchOptions.of(args);
 
-		LaunchOptions.help().ifPresent(line -> command.getCommandSpec().usageMessage().footer("", "Window:",
-			"  " + line));
+		if (command.parseArgs(options.remaining().toArray(String[]::new)).isUsageHelpRequested()) {
+			command.usage(command.getOut());
+			command.getOut().flush();
+			return Optional.empty();
+		}
 
-		try {
-			final LaunchOptions options = LaunchOptions.of(args);
+		return Optional.of(options);
+	}
 
-			if (command.parseArgs(options.remaining().toArray(String[]::new)).isUsageHelpRequested()) {
-				LOG.info("{}", command.getUsageMessage());
-				return Optional.empty();
-			}
+	/* A refused command line is answered on the command's error output, the way picocli answers it itself. */
+	static void refuse(final CommandLine command, final RuntimeException refused) {
+		command.getErr().println(refused.getMessage());
+		command.usage(command.getErr());
+		command.getErr().flush();
+	}
 
-			return Optional.of(options);
-		} catch (IllegalArgumentException | CommandLine.ParameterException e) {
-			LOG.error("{}{}{}", e.getMessage(), System.lineSeparator(), command.getUsageMessage());
-			System.exit(USAGE_ERROR);
-			return Optional.empty();
-		}
+	/* What Janus takes, with the window system Oriel takes listed in the help where it can be chosen. */
+	static CommandLine command() {
+		final CommandLine command = new CommandLine(new Arguments());
+
+		LaunchOptions.help().ifPresent(line -> command.getCommandSpec().usageMessage().footer("", "Window:",
+			"  " + line));
+		return command;
 	}
 
 	/* The window and the message shown when it cannot open both go by this name. */

shell/src/test/java/org/unigrid/janus/shell/JanusTest.java

@@ -0,0 +1,74 @@
+/*
+    The Janus Wallet
+    Copyright © 2021-2026 Stiftelsen The Unigrid Foundation
+
+    This program is free software: you can redistribute it and/or modify it under the terms of the
+    addended GNU Affero General Public License as published by the Free Software Foundation, version 3
+    of the License (see COPYING and COPYING.addendum).
+
+    This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
+    even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+    GNU Affero General Public License for more details.
+
+    You should have received an addended copy of the GNU Affero General Public License with this program.
+    If not, see <http://www.gnu.org/licenses/> and <https://github.com/unigrid-project/janus-java>.
+ */
+
+package org.unigrid.janus.shell;
+
+import java.io.PrintWriter;
+import java.io.StringWriter;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import picocli.CommandLine;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class JanusTest {
+	/* What a log line starts with, so that plain output can be told from what went through the logger. */
+	private static final String LOG_PREFIX = "^\\d{2}:\\d{2}:\\d{2}\\.\\d{3} .*";
+
+	private final StringWriter out = new StringWriter();
+	private final StringWriter err = new StringWriter();
+	private CommandLine command;
+
+	/* picocli colours the help where it takes the console to understand colours, as on the Windows runners, which
+	   puts escape codes inside the text, so the tests read it without them. */
+	@BeforeEach
+	public void captureWhatJanusPrints() {
+		command = Janus.command();
+		command.setColorScheme(CommandLine.Help.defaultColorScheme(CommandLine.Help.Ansi.OFF));
+		command.setOut(new PrintWriter(out));
+		command.setErr(new PrintWriter(err));
+	}
+
+	@Test
+	public void shouldPrintTheHelpPlainlyAndNotStart() {
+		assertTrue(Janus.options(command, "--help").isEmpty(), "Janus would start after showing the help");
+		assertTrue(out.toString().startsWith("Usage: Unigrid"), out.toString());
+		assertFalse(out.toString().lines().anyMatch(line -> line.matches(LOG_PREFIX)), out.toString());
+	}
+
+	@Test
+	public void shouldRefuseAnOptionItDoesNotTakeAndSayWhyPlainly() {
+		final CommandLine.ParameterException refused = assertThrows(CommandLine.ParameterException.class,
+			() -> Janus.options(command, "--bogus")
+		);
+
+		Janus.refuse(command, refused);
+		assertTrue(err.toString().startsWith(refused.getMessage()), err.toString());
+		assertTrue(err.toString().contains("--bogus"), err.toString());
+		assertTrue(err.toString().contains("Usage: Unigrid"), err.toString());
+	}
+
+	/* Oriel reads the window system on every desktop, even where it hides it from the help, so this holds anywhere. */
+	@Test
+	public void shouldRefuseAWindowSystemOrielDoesNotKnow() {
+		final IllegalArgumentException refused = assertThrows(IllegalArgumentException.class,
+			() -> Janus.options(command, "--window-system=bogus")
+		);
+
+		assertTrue(refused.getMessage().contains("bogus"), refused.getMessage());
+	}
+}