StackOverflowError when subcommand is subclass

[remkop]

As reported by @petermr on the picocli mailing list, when a subcommand subclasses its parent command, a StackOverflowError is thrown during initialization.

To reproduce:

@Command(name = "parent", subcommands = Sub.class)
static class Parent { }

@Command(name = "sub")
static class Sub extends Parent {}

@Test
public void testSimple() {
    new CommandLine(new Parent());
}

Example stack trace (differs per invocation):

java.lang.StackOverflowError
	at java.util.regex.Pattern.atom(Pattern.java:2200)
	at java.util.regex.Pattern.sequence(Pattern.java:2081)
	at java.util.regex.Pattern.expr(Pattern.java:1998)
	at java.util.regex.Pattern.group0(Pattern.java:2907)
	at java.util.regex.Pattern.sequence(Pattern.java:2053)
	at java.util.regex.Pattern.expr(Pattern.java:1998)
	at java.util.regex.Pattern.group0(Pattern.java:2907)
	at java.util.regex.Pattern.sequence(Pattern.java:2053)
	at java.util.regex.Pattern.expr(Pattern.java:1998)
	at java.util.regex.Pattern.compile(Pattern.java:1698)
	at java.util.regex.Pattern.<init>(Pattern.java:1351)
	at java.util.regex.Pattern.compile(Pattern.java:1028)
	at picocli.CommandLine$RegexTransformer$Builder.addPattern(CommandLine.java:4628)
	at picocli.CommandLine$RegexTransformer.createDefault(CommandLine.java:4555)
	at picocli.CommandLine$Model$CommandSpec.<init>(CommandLine.java:5082)
	at picocli.CommandLine$Model$CommandSpec.wrapWithoutInspection(CommandLine.java:5101)
	at picocli.CommandLine$Model$CommandReflection.extractCommandSpec(CommandLine.java:9527)
	at picocli.CommandLine$Model$CommandSpec.forAnnotatedObject(CommandLine.java:5116)
	at picocli.CommandLine.<init>(CommandLine.java:223)
	at picocli.CommandLine.toCommandLine(CommandLine.java:3212)
	at picocli.CommandLine.access$12900(CommandLine.java:145)
	at picocli.CommandLine$Model$CommandReflection.initSubcommands(CommandLine.java:9603)
	at picocli.CommandLine$Model$CommandReflection.extractCommandSpec(CommandLine.java:9539)
	at picocli.CommandLine$Model$CommandSpec.forAnnotatedObject(CommandLine.java:5116)
	at picocli.CommandLine.<init>(CommandLine.java:223)
	at picocli.CommandLine.toCommandLine(CommandLine.java:3212)
	at picocli.CommandLine.access$12900(CommandLine.java:145)
	at picocli.CommandLine$Model$CommandReflection.initSubcommands(CommandLine.java:9603)
	at picocli.CommandLine$Model$CommandReflection.extractCommandSpec(CommandLine.java:9539)
...
	at picocli.CommandLine.toCommandLine(CommandLine.java:3212)
	at picocli.CommandLine.access$12900(CommandLine.java:145)
	at picocli.CommandLine$Model$CommandReflection.initSubcommands(CommandLine.java:9603)
	at picocli.CommandLine$Model$CommandReflection.extractCommandSpec(CommandLine.java:9539)
	at picocli.CommandLine$Model$CommandSpec.forAnnotatedObject(CommandLine.java:5116)
	at picocli.CommandLine.<init>(CommandLine.java:223)

[remkop]

I believe this is simply a consequence of how reuse works in picocli:
When a command B subclasses another command A, it will get all attributes (and subcommands) of A.
If one of these subcommands is actually command B, then B becomes a subcommand of itself. This is not supported.

I improved things by throwing a more friendly InitializationException when such an eternal loop is detected.

Example 1:

    @Command(name = "self-ref", subcommands = Simplest.class)
    class Simplest {}

This gives the following InitializationException: "self-ref (Simplest) cannot be a subcommand of itself"

Example 2:

    @Command(name = "parent", subcommands = Sub.class)
    class Parent { }

    @Command(name = "sub")
    class Sub extends Parent {}

This gives the following InitializationException: "parent (Parent) has a subcommand (Sub) that is a subclass of itself".

To avoid this, add another level that does not have subclasses, and put the shared functionality there:

    //@Command // optionally define some shared command annotation attributes
    class SharedFunctionality {}

    @Command(name = "parent", subcommands = Sub.class)
    class Parent extends SharedFunctionality { }

    @Command(name = "sub")
    class Sub extends SharedFunctionality {}