ProxyExchange fails with "IOException: insufficient data written" with boot 3.2

[jkuipers]

The ProxyExchange type in spring-cloud-gateway-mvc copies over the headers from the incoming request, unless they're marked as sensitive. In addition, it also uses the HTTP message converters to unmarshal the payload of the incoming request and marshal it again when sending.
What we found is that in the header copying, it also includes the Content-Length header of the incoming request. However, b/o the unmarshalling and then marshalling the content length of the proxied request can be different: especially in the case of JSON where whitespace is removed, it can become less.

This causes problems with some downstream servers, as they keep waiting for additional request contents based on the erroneous Content-Length header. When not using smth like Apache Component HTTP client but using the default Java client instead, this even leads to errors in the client itself (showcased in attached sample).

This behavior was not there in previous versions: actually, with Spring Boot 3.1.6 and the underlying Spring web version this works. When you check what's being sent then you can see that a Content-Length header is always set, with the correct value.
It looks like the latest Spring web code (Jackson HTTP message converter?) no longer sets its own Content-Length header when marshalling to JSON. Therefore, spring-cloud-gateway-mvc should never copy over this header from the incoming request, but should rather leave it up to the RestTemplate to either leave it out or set this to the correct value for the outgoing request.

BTW, although this isn't strictly related to this issue, I believe that the Host header of the incoming request shouldn't be copied either by default: this can also lead to issues.

Sample
I've attached a project with two integration tests that showcase the problem. By configuring the content-length to be a sensitive header you can work around the issue, but users of the framework should not be required to do this.
gateway-mvc-contentlength.zip

[spencergibb]

Posting for those encountering, the workaround is to set

spring.cloud.gateway.proxy.sensitive=content-length

[spencergibb]

Why don't you pass the body in the ProxyController?

[jkuipers]

The body isn’t modified by my code, why would I pass it myself? This is done automatically.
The way that we use this, we just want to pass the request with some modified headers and path to the downstream service, which I think is the typical use case for a proxy solution. The controller doesn't care about the type of a potential payload of the request, nor about the response payload type: that's why we use ProxyExchange<byte[]>.

We could explicitly add an @RequestBody byte[] body parameter to the controller to force the conversion to not use the Jackson message converter, but to me that feels like just another workaround: I think it's confusing to explicitly ask for input that you don't actually process in your controller method just to prevent the ProxyExchange from producing an invalid HTTP request (invalid in the sense that its Content-Length value is too low).
In our current use case we don't really mind the fact that the JSON message conversion is done: it also provides an additional layer of validation to ensure that the payload is in fact valid JSON. In general, since this is all just Spring MVC, other uses might involve unmarshalling the request body to some type (even outside of the controller) without the proxying code being interested in the body, and you'd expect that to work without having to compensate for headers with invalid values I think.