Add Servlet and ServerBearerExchangeFilterFunction

[jzheaux]

Fixes gh-5334
Fixes gh-7284

[robotmrv]

maybe it would be better not to create empty RequestContextDataHolder?
by

if (authentication == null || parentContext.hasKey(REQUEST_CONTEXT_DATA_HOLDER_ATTR_NAME)) {}

[robotmrv]

Operators.liftPublisher() will not create unnecessary transformations into Scannable of a publisher

[jzheaux]

Thanks for the tip! I'll update that.

[robotmrv]

maybe would be better to write

return oauth2Token(request.attributes())
    .map(oauth2Token -> bearer(request, oauth2Token))
    .defaultIfEmpty(request)
    .flatMap(next::exchange);

This should create less Publishers/functions and it does not duplicate next::exchange

[robotmrv]

I wonder what case does it cover?
Isn't it covered by Hooks.onLastOperator() from afterPropertiesSet() - it does the same.
It seems to me that this is just legacy solution copied from
https://github.com/spring-projects/spring-security/blob/master/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/web/reactive/function/client/ServletOAuth2AuthorizedClientExchangeFilterFunction.java
Or is it just an altarnative for case when Hook is not enabled and chaining is not necessary?

[jzheaux]

This is an alternative for when more sophisticated chaining is wanted by the application, so oauth2Configuration would not suffice. Order obviously matters with filters, so, for example, I might need to default the request, add my own custom filter, and then add this filter after that.

[robotmrv]

I know there are activities to reduce number of Optional usage and replacement of filter/map/flatMap() to the handle() in the spring core
so it could be rewritten to
may be it is a little bit more imperative, but it creates less operators and does not use create Optional to check attribute presence

public Mono<ClientResponse> filter(ClientRequest request, ExchangeFunction next) {
	return mergeRequestAttributesIfNecessary(request)
			.handle((req, sink) -> {
				Map<String, Object> attributes = req.attributes();
				if (attributes.get(AUTHENTICATION_ATTR_NAME) == null) {
					sink.next(request);
				} else {
					AbstractOAuth2Token token = getOAuth2Token(attributes);
					sink.next(bearer(req, token));
				}
			})
			.flatMap(next::exchange);
}

private Mono<ClientRequest> mergeRequestAttributesIfNecessary(ClientRequest request) {
	if (request.attributes().get(AUTHENTICATION_ATTR_NAME) != null) {
		return Mono.just(request);
	}
	return mergeRequestAttributesFromContext(request);
}

private Mono<ClientRequest> mergeRequestAttributesFromContext(ClientRequest request) {
	return Mono.subscriberContext()
			.map(ctx -> ClientRequest.from(request)
					.attributes(attrs -> populateRequestAttributes(attrs, ctx))
					.build()
			);
}