No way to affect response when unhandled exceptions thrown from ForwardedHeaderTransformer

[kkroner8451]

Affects: \spring-web:5.2.12

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We are running a service behind a reverse proxy and have set server.forward-headers-strategy=framework. This then registers a ForwardedHeaderTransformer bean. When an exception is thrown from that class it bubbles up the stack and the connection is eventually closed without modifying/sending a response at all. The net result is that the reverse proxy never gets a response and will treat it as a 502 Bad Gateway which is less than ideal.

The exception we are seeing is being thrown in the ForwardedHeaderTransformer.apply(ServerHttpRequest request) method when the UriCompenentsBuilder.build(true) is called. Ultimately, the URI was not properly encoded which is how we discovered the issue. We are fixing our end to better ensure it is encoded correctly when we build the URI to have the user follow, but obviously we can't prevent users from sending bad request in the first place. Also, any unhandled exception that could go wrong in the ForwardedHeaderTransformer or a developer's custom implementation would still result in the connection being closed without an opportunity to write a response. Looking at how this is called from HttpWebHandlerAdapter.handle(ServerHttpRequest request, ServerHttpResponse response), it seems the expectation is that nothing could ever go wrong in applying the transformer.

The preference would be that we could handle the exception, log it and then return a proper response code with a message indicating that the URI was malformed. I would think this could be achieved by throwing an exception such as ResponseStatusException, or a custom exception with the @ResponseStatus annotation present. This was attempted with no affect. Again, with how the exception handling is the only option is the exception is bubbled up until eventually the connection/channel is closed without modifying the response in HttpServerHandle.onStateChanged(Connection connection, State newState). Alternatively, the ServerHttpResponse could be exposed to the transformer but I could see that not being a great idea from a design perspective.

[rstoyanchev]

ForwardedHeaderTransformer is applied ahead of the WebFilter and WebExceptionHandler chain. This is intentional so that the entire chain will see a consistent request but it also means that a ResponseStatusException will not be handled as usual. We can improve this by having the transformer catch and raise a ServerWebInputException (perhaps through a protected method) which would then be handled in HttpWebHandlerAdapter by applying the status code to the response. Does that seem okay?

[kkroner8451]

That seems like it should work.