ClientResponse.mutate() builder can modify originalResponse headers.

[sparecycles]

** Bug Reports **

Not sure if this is a bug, but

var mutatedClientResponse = originalClientResponse.mutate().header("xyz", "pqrst").build();

updates the headers in the original originalClientResponse, too.

---

Doc says

Return a builder to mutate this response, for example to change
the status, headers, cookies, and replace or transform the body.

So maybe is this expected behavior. IMO mutate() returning a builder looks like immutable semantics apply.

analysis

ClientResponse.mutate() returns a DefaultClientResponseBuilder with a null headers field.

spring-framework/spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultClientResponseBuilder.java

Lines 101 to 115 in 9bdeadc

	DefaultClientResponseBuilder(ClientResponse other, boolean mutate) {
	Assert.notNull(other, "ClientResponse must not be null");
	this.strategies = other.strategies();
	this.statusCode = other.statusCode();
	if (mutate) {
	this.body = other.bodyToFlux(DataBuffer.class);
	}
	else {
	this.headers = new HttpHeaders();
	this.headers.addAll(other.headers().asHttpHeaders());
	}
	this.originalResponse = other;
	this.request = (other instanceof DefaultClientResponse defaultClientResponse ?
	defaultClientResponse.request() : EMPTY_REQUEST);
	}

Headers are implied from this.originalResponse = other; and lazily created via getHeaders:

spring-framework/spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultClientResponseBuilder.java

Lines 144 to 150 in 9bdeadc

	@SuppressWarnings({"ConstantConditions", "NullAway"})
	private HttpHeaders getHeaders() {
	if (this.headers == null) {
	this.headers = new HttpHeaders(this.originalResponse.headers().asHttpHeaders());
	}
	return this.headers;
	}

This refers to the original headers because this.headers = unwrap(httpHeaders) just copies the backing map reference.

spring-framework/spring-web/src/main/java/org/springframework/http/HttpHeaders.java

Lines 477 to 482 in 9bdeadc

	public HttpHeaders(HttpHeaders httpHeaders) {
	Assert.notNull(httpHeaders, "HttpHeaders must not be null");
	this.headers = (httpHeaders == EMPTY ?
	CollectionUtils.toMultiValueMap(new LinkedCaseInsensitiveMap<>(8, Locale.ENGLISH)) :
	unwrap(httpHeaders));
	}

So updates to the headers goes to the original (shared) backing map reference.