Declared Object in ConstructingObjectParser is ignored in case target object is already built.

[bizybot]

We found a somewhat unexpected behavior of the ConstructingObjectParser.
While parsing JSON object { "role_mapping" : { "created" : "true" } }, if we declare
inner field "create" as a constructorArg along with the field "role_mapping" for which the parser is a no-op, the target object is still constructed.

static {
        PARSER.declareBoolean(constructorArg(), new ParseField("created"));
        PARSER.declareObject((a,b) -> {}, (parser, context) -> null, new ParseField("role_mapping"));
}

We continue to sub parse the object for role_mapping and once we encounter constructorArg we build the target object. The field role_mapping object is queued but later ignored.

The behaviour seems to be similar to what we do for unknown fields or optional constructor args, but
not sure about it.

[elasticmachine]

Pinging @elastic/es-core-infra

[bizybot]

Adding @nik9000 for more insights. Thank you.

[nik9000]

I think I understand what is up here. I'll open up something to detect it.

[nik9000]

Ok - the issue is that any parser that attempts to parse an object or a list is obligated to move to the end of that object or list. If it doesn't then all kinds of crazy things happen. You can work around this with (parser, context) -> parser.skipChildren().

[nik9000]

I'm most of the way through a change to detect "bad parsers" but fighting with some test failures.

[nik9000]

Ah! I see what you are doing now. I'll include a fix when i open a PR.