Support HTTP trailing headers

With the current WebClient implementation its not possible to get the trailer data of a chunked http response. For example for the response:

HTTP/1.1 200 OK
Transfer-Encoding: chunked
Content-Type: text/binary
Connection: Keep-Alive
Keep-Alive: timeout-20
Date: Fri, 03 May 2024 14:26:43 GMT
Cache-Control: max-age=1
x-ee-cameraid: 100f9718
accept-ranges: bytes
trailer: x-result,x-error

0
x-result: 1
x-error: Failed to determine asset size

The ResponseEntity object has no way to access the trailer. The data you get from the body doesn't contain the trailer info, and neither does the headers object. I debugged the issue a little and found that netty correctly parses and makes the data available:

But the info is "thrown away" in the reactor.netty.ByteBufFlux

    final static Function<Object, ByteBuf> bytebufExtractor = o -> {
        if (o instanceof ByteBuf) {
            return (ByteBuf) o;
        }
        if (o instanceof ByteBufHolder) {
            return ((ByteBufHolder) o).content();
        }
        if (o instanceof byte[]) {
            return Unpooled.wrappedBuffer((byte[]) o);
        }
        throw new IllegalArgumentException("Object " + o + " of type " + o.getClass() + " " + "cannot be converted to ByteBuf");
    };

As this only looks at the content of the LastHttpContent, and ignores the trailerHeaders.

That said, i'm also not really sure how to properly make the trailers available, best i could come up with is an extra mono in the response entity object, as the info would only be available once the full body has been read. But hopefully you can think of something better :)

spring-projects / spring-framework

Support HTTP trailing headers #32890