spring.cloud.stream.function.autodetect=true (default setting) for spring-webflux app with server.forward-headers-strategy=FRAMEWORK can't work

[jonenst]

Describe the issue
Using a spring-webflux app with server.forward-headers-strategy=FRAMEWORK, autoconfiguration provides a ForwardedHeaderTransformer bean which breaks all use cases of spring.cloud.stream.function.autodetect :

• if you have only output-bindings, a queue is createded and a consumer is registered on this queue which noone ever wants. If you send a message to this queue, you get

java.lang.ClassCastException: class [B cannot be cast to class org.springframework.http.server.reactive.ServerHttpRequest ([B is in module java.base of loader 'bootstrap'; org.springframework.http.server.reactive.ServerHttpRequest is in unnamed module of loader 'app')

• if you had a single java.util.function.* bean and relied on autodetection, it doesn't work.:

17:59:09.253 [main] WARN  org.springframework.cloud.function.context.catalog.BeanFactoryAwareFunctionRegistry - Multiple functional beans were found [forwardedHeaderTransformer, myConsumer], thus can't determine default function definition. Please use 'spring.cloud.function.definition' property to explicitly define it. 

To Reproduce
Steps to reproduce the behavior:
see above
all versions

Additional context
In the docs there is the following note:

In the event you only have single bean of type java.util.function.[Supplier/Function/Consumer], you can skip the spring.cloud.function.definition property, since such functional bean will be auto-discovered. However, it is considered best practice to use such property to avoid any confusion. Some time this auto-discovery can get in the way, since single bean of type java.util.function.[Supplier/Function/Consumer] could be there for purposes other then handling messages, yet being single it is auto-discovered and auto-bound. For these rare scenarios you can disable auto-discovery by providing spring.cloud.stream.function.autodetect property with value set to false.

To me, this would have been a good idea if there were really no easy cases where autodection breaks down. It depends on what "rare" means of course, but just a plain webflux springboot app with a single nonexotic property "server.forward-headers-strategy" doesn't seem to be that rare.

So maybe just make autodetection opt-in ? But then it doesn't make much sense, autodetection is for zero config (onboarding), so it's not interesting to add explicit autodetection=true configuration just to avoid some other configuration..

So maybe just delete autodetection entirely? You would lose the easy zero config onboarding.

Or make the autodection really reliable:

• maybe the autodetection should only happen when there is zero spring.cloud.stream.bindings or output-bindings configuration ? zeroconfig onboarding would still work (unless the user chose webflux+forward-headers-strategy=FRAMEWORK to try it out) and normal applications with only output-bindings wouldn't get the spurious consumer
• maybe find a way to exclude all "standard" beans. Based on the package ? Based on the function Signature (who would use Function<HttpServletRequest, HttpServletRequest>) ? Based on something else ?
• another idea ?

Cheers,
Jon