ModifyRequestBodyGatewayFilterFactory creates Content-Type header even when there is no body.

[HarryHsuzx]

Describe the bug
Using ModifyRequestBodyGatewayFilterFactory with a rewrite function that returns Mono.empty();, a content-type header is still added to the downstream request which bricks the downstream API contract.
Reproduced on spring cloud gateway 4.1.1 and 3.1.1

Expected behaviour
When the return value is Mono.empty() (cannot be null / Mono.just(null)), the content-type header should not be set. This ensures bodyless requests are not assigned the wrong headers.

Sample
Received request:

2024-01-26T09:30:41.373+08:00 DEBUG 15344 --- [gateway] [ctor-http-nio-3] reactor.netty.http.server.HttpServer     : [86124063, L:/[0:0:0:0:0:0:0:1]:8080 - R:/[0:0:0:0:0:0:0:1]:35221] READ: 162B
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 47 45 54 20 2f 61 70 69 2f 64 65 62 75 67 31 2f |GET /api/debug1/|
|00000010| 31 32 33 20 48 54 54 50 2f 31 2e 31 0d 0a 55 73 |123 HTTP/1.1..Us|
|00000020| 65 72 2d 41 67 65 6e 74 3a 20 50 6f 73 74 6d 61 |er-Agent: Postma|
|00000030| 6e 52 75 6e 74 69 6d 65 2f 37 2e 32 38 2e 34 0d |nRuntime/7.28.4.|
|00000040| 0a 41 63 63 65 70 74 3a 20 2a 2f 2a 0d 0a 48 6f |.Accept: */*..Ho|
|00000050| 73 74 3a 20 6c 6f 63 61 6c 68 6f 73 74 3a 38 30 |st: localhost:80|
|00000060| 38 30 0d 0a 41 63 63 65 70 74 2d 45 6e 63 6f 64 |80..Accept-Encod|
|00000070| 69 6e 67 3a 20 67 7a 69 70 2c 20 64 65 66 6c 61 |ing: gzip, defla|
|00000080| 74 65 2c 20 62 72 0d 0a 43 6f 6e 6e 65 63 74 69 |te, br..Connecti|
|00000090| 6f 6e 3a 20 6b 65 65 70 2d 61 6c 69 76 65 0d 0a |on: keep-alive..|
|000000a0| 0d 0a                                           |..              |
+--------+-------------------------------------------------+----------------+

Modified request:

2024-01-26T09:30:41.375+08:00  WARN 15344 --- [gateway] [ctor-http-nio-3] d.SecureHttpPostSignGatewayFilterFactory : MONO.EMPTY returned!
2024-01-26T09:30:41.376+08:00 DEBUG 15344 --- [gateway] [ctor-http-nio-3] reactor.netty.http.client.HttpClient     : [59584b99-4, L:/127.0.0.1:35223 - R:/127.0.0.1:8080] WRITE: 380B
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 47 45 54 20 2f 31 2f 31 32 33 20 48 54 54 50 2f |GET /1/123 HTTP/|
|00000010| 31 2e 31 0d 0a 55 73 65 72 2d 41 67 65 6e 74 3a |1.1..User-Agent:|
|00000020| 20 50 6f 73 74 6d 61 6e 52 75 6e 74 69 6d 65 2f | PostmanRuntime/|
|00000030| 37 2e 32 38 2e 34 0d 0a 41 63 63 65 70 74 3a 20 |7.28.4..Accept: |
|00000040| 2a 2f 2a 0d 0a 41 63 63 65 70 74 2d 45 6e 63 6f |*/*..Accept-Enco|
|00000050| 64 69 6e 67 3a 20 67 7a 69 70 2c 20 64 65 66 6c |ding: gzip, defl|
|00000060| 61 74 65 2c 20 62 72 0d 0a 43 6f 6e 74 65 6e 74 |ate, br..Content|
|00000070| 2d 54 79 70 65 3a 20 74 65 78 74 2f 70 6c 61 69 |-Type: text/plai|
|00000080| 6e 3b 63 68 61 72 73 65 74 3d 55 54 46 2d 38 0d |n;charset=UTF-8.|
|00000090| 0a 46 6f 72 77 61 72 64 65 64 3a 20 70 72 6f 74 |.Forwarded: prot|
|000000a0| 6f 3d 68 74 74 70 3b 68 6f 73 74 3d 22 6c 6f 63 |o=http;host="loc|
|000000b0| 61 6c 68 6f 73 74 3a 38 30 38 30 22 3b 66 6f 72 |alhost:8080";for|
|000000c0| 3d 22 5b 30 3a 30 3a 30 3a 30 3a 30 3a 30 3a 30 |="[0:0:0:0:0:0:0|
|000000d0| 3a 31 5d 3a 33 35 32 32 31 22 0d 0a 58 2d 46 6f |:1]:35221"..X-Fo|
|000000e0| 72 77 61 72 64 65 64 2d 46 6f 72 3a 20 30 3a 30 |rwarded-For: 0:0|
|000000f0| 3a 30 3a 30 3a 30 3a 30 3a 30 3a 31 0d 0a 58 2d |:0:0:0:0:0:1..X-|
|00000100| 46 6f 72 77 61 72 64 65 64 2d 50 72 6f 74 6f 3a |Forwarded-Proto:|
|00000110| 20 68 74 74 70 0d 0a 58 2d 46 6f 72 77 61 72 64 | http..X-Forward|
|00000120| 65 64 2d 50 6f 72 74 3a 20 38 30 38 30 0d 0a 58 |ed-Port: 8080..X|
|00000130| 2d 46 6f 72 77 61 72 64 65 64 2d 48 6f 73 74 3a |-Forwarded-Host:|
|00000140| 20 6c 6f 63 61 6c 68 6f 73 74 3a 38 30 38 30 0d | localhost:8080.|
|00000150| 0a 68 6f 73 74 3a 20 31 32 37 2e 30 2e 30 2e 31 |.host: 127.0.0.1|
|00000160| 3a 38 30 38 30 0d 0a 63 6f 6e 74 65 6e 74 2d 6c |:8080..content-l|
|00000170| 65 6e 67 74 68 3a 20 30 0d 0a 0d 0a             |ength: 0....    |
+--------+-------------------------------------------------+----------------+

Note there is a Content-Type: text/plainlcharset=UTF-8 header in the downstream request.

RewriteFunction

@Override
    public Publisher<String> apply(ServerWebExchange exchange, String body) {
        // Escape if not POST
        if (exchange.getRequest().getMethod() != HttpMethod.POST) {
            logger.warn("MONO.EMPTY returned!");
            return Mono.empty();
        }

        //.....
    }

I've made a mre with the version 4.1.1, but this behaviour also exists in the 3.x versions. The repo is as below:
https://github.com/HarryHsuzx/scgRewriteFunctionContentTypeMRE/blob/master/src/main/resources/application.yml

[spencergibb]

I suppose we could condition the setting of a different content type only if content length is greater than 0 here

spring-cloud-gateway/spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/filter/factory/rewrite/ModifyRequestBodyGatewayFilterFactory.java

Lines 86 to 88 in 67b612d

	if (config.getContentType() != null) {
	headers.set(HttpHeaders.CONTENT_TYPE, config.getContentType());
	}

[vasantteja]

Hi can I work on this?

[spencergibb]

It's been assigned to you

[NadChel]

I suppose we could condition the setting of a different content type only if content length is greater than 0 here

@spencergibb We don't know anything about the actual content length before the body Mono is subscribed to (which happens beyond the scope of the filter() method)

I reproduced it, wrote a quick fix, but then realized org.springframework.http.codec.EncoderHttpMessageWriter sets a default content type anyway (which is in my case text/plain;charset=UTF-8)

	@Nullable
	private MediaType updateContentType(ReactiveHttpOutputMessage message, @Nullable MediaType mediaType) {
		MediaType result = message.getHeaders().getContentType();
		if (result != null) {
			return result;
		}
		MediaType fallback = this.defaultMediaType;
		result = (useFallback(mediaType, fallback) ? fallback : mediaType);
		if (result != null) {
			result = addDefaultCharset(result, fallback);
			message.getHeaders().setContentType(result);
		}
		return result;
	}

I figure it means it's blocked for now

I created a separate issue for the blocker as well as a PR that fixes it. I hope it will be merged soon

[NadChel]

@spencergibb also, if I may ask, are you the only member of the Spring Cloud Gateway team? I don't see anyone else responding. Aren't there other members, like Olga?

[NadChel]

@spencergibb my PR was merged a few days ago. As soon as we're past an upcoming 6.2M2 milestone, I'm all good to submit a fix for this issue (as long as @vasantteja doesn't mind as an assignee). Feel free to tag me when it's the time

[chrisholly-skyuk]

@NadChel any update on fixing this issue?