Support for dynamic proxy configuration at the HTTP protocol level

[raccoonback]

Motivation

The existing proxy(...) and noProxy() methods are static, applying the same configuration to all requests. In scenarios where proxy behavior needs to change dynamically based on the request (e.g., host-specific proxies, conditional proxy usage), this static approach is insufficient.

The proxyWhen(...) method fills this gap by allowing developers to provide a function that resolves proxy settings dynamically during request execution.

Description

• Added support for the proxyWhen(BiConsumer<HttpClientConfig, ? super ProxyProvider.TypeSpec>) method to HttpClient.
  • This method enables users to dynamically determine the proxy configuration on a per-request basis using a provided HttpClientConfig function.
  • proxyWhen(...) evaluates the configuration function for each HTTP request, allowing fine-grained control over proxy settings.
• If proxyWhen(...) is used, any static proxy configurations applied via proxy(...) or noProxy() will be ignored.
• Internally, it uses deferredConfig to set the proxyProvider at the HttpClientConnect.connect() stage.

HttpClient client = HttpClient.create()
    .proxyWhen((HttpClientConfig config, ProxyProvider.TypeSpec spec) -> {
        if (config.uri().equals("example.com")) {
            spec.type(ProxyProvider.Proxy.HTTP)
                .host("proxy.example.com")
                .port(8080);
        }
    });

client.get()
    .uri("http://example.com")
    .response()
    .block();

Backward Compatibility

• The new proxyWhen(...) method is additive and does not interfere with existing static configurations unless explicitly used.
• Static configurations (proxy(...) and noProxy()) remain unchanged and fully functional.

Related Issue

• HttpClient proxy configuration per request #2746

[violetagg]

@raccoonback Thanks for the PR!

[violetagg]

What do you think if we use Mono for the proxy configuration? Imagine that you need to resolve something else before providing the proxy configuration.
Something similar to what we have for the headers e.g. this test case:

reactor-netty/reactor-netty-http/src/test/java/reactor/netty/http/client/WebsocketClientOperationsTest.java

Line 85 in c5e4a96

.headersWhen(h -> login(disposableServer.port()).map(token -> h.set("Authorization", token)))

Suggested change

	public final HttpClient proxyWhen(BiConsumer<HttpClientConfig, ? super ProxyProvider.TypeSpec> proxyBuilder) {
	Objects.requireNonNull(proxyBuilder, "proxyBuilder");
	HttpClient dup = duplicate();
	dup.configuration()
	.deferredConf(
	config -> {
	ProxyProvider.TypeSpec spec = ProxyProvider.builder();
	proxyBuilder.accept(config, spec);
	if (((ProxyProvider.Validator) spec).isConfiguredCorrectly()) {
	ProxyProvider proxyProvider = ((ProxyProvider.Builder) spec).build();
	config.proxyProvider(proxyProvider);
	}
	return Mono.just(config);
	}
	);
	return dup;
	}
	public final HttpClient proxyWhen(BiFunction<HttpClientConfig, ? super ProxyProvider.TypeSpec,
	Mono<? extends ProxyProvider.Builder>> proxyBuilder) {
	Objects.requireNonNull(proxyBuilder, "proxyBuilder");
	HttpClient dup = duplicate();
	dup.configuration().deferredConf(config -> {
	Mono<? extends ProxyProvider.Builder> mono = proxyBuilder.apply(config, ProxyProvider.builder());
	return mono == Mono.<ProxyProvider.Builder>empty() ?
	Mono.just(config) :
	mono.map(builder -> {
	config.proxyProvider(builder.build());
	return config;
	});
	});
	return dup;
	}