Custom Coroutine Contexts not propagated in WebClient client filter

[mlukasanderson]

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Affects: 3.2.2

---

It appears when custom coroutine contexts are applied prior to entering a WebClient filter, the custom contexts are no available inside the filter.

When running the below application we add a custom context in the controller which is found in the coroutine context. However when we attempt to fetch the custom context in the filter, it comes back null. You can see this by running the app and hitting

GET http://localhost:8080/test

package com.target.test

import io.netty.channel.ChannelOption
import io.netty.handler.timeout.WriteTimeoutHandler
import kotlinx.coroutines.currentCoroutineContext
import kotlinx.coroutines.reactive.awaitFirstOrNull
import kotlinx.coroutines.withContext
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication
import org.springframework.context.annotation.Bean
import org.springframework.context.annotation.ComponentScan
import org.springframework.context.annotation.Configuration
import org.springframework.http.ResponseEntity
import org.springframework.http.client.reactive.ReactorClientHttpConnector
import org.springframework.stereotype.Component
import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.RestController
import org.springframework.web.reactive.function.client.*
import reactor.netty.http.client.HttpClient
import java.net.URI
import java.time.Duration
import java.util.concurrent.TimeUnit
import kotlin.coroutines.AbstractCoroutineContextElement
import kotlin.coroutines.CoroutineContext

@SpringBootApplication
@ComponentScan(basePackages = ["com.target"])
class TestApplication

fun main(args: Array<String>) {
    runApplication<TestApplication>(*args)
}

@Configuration
class WebClientConfig {

    @Bean
    @Suppress("unused")
    fun webClient(
        filterMissingCustomCoroutineContext: FilterMissingCustomCoroutineContext
    ): WebClient {
        val client = WebClient.builder()
            .clientConnector(ReactorClientHttpConnector(httpClient()))
            .filter(filterMissingCustomCoroutineContext)
            .build()

        return client
    }

    private fun httpClient(): HttpClient {
        return HttpClient.create()
            .responseTimeout(Duration.ofMillis(10000))
            .doOnConnected {
                it.addHandlerFirst(WriteTimeoutHandler(10000, TimeUnit.MILLISECONDS))
            }
            .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000)
    }
}

@Component
class FilterMissingCustomCoroutineContext: CoExchangeFilterFunction() {
    override suspend fun filter(request: ClientRequest, next: CoExchangeFunction): ClientResponse {
        val customContext = currentCoroutineContext()[CustomCoroutineContext]
        println("In client filter, custom context is $customContext")

        try {
            assert(customContext != null)
        }
        catch(t: Throwable) {
            t.printStackTrace()
            throw t
        }
        return next.exchange(request)
    }
}

@RestController
class TestController @Autowired constructor(
    private val webClient: WebClient
) {
    @GetMapping("/test")
    suspend fun test(): ResponseEntity<String>? {
        return withContext(CustomCoroutineContext("test")) {
            val customContext = currentCoroutineContext()[CustomCoroutineContext]
            assert(customContext != null)
            println("In controller, custom context is $customContext")
            webClient.get()
                .uri(URI("https://github.com/spring-projects/spring-framework/issues/26977"))
                .retrieve()
                .toEntity(String::class.java)
                .awaitFirstOrNull()
        }
    }
}

data class CustomCoroutineContext(val value: String) :
    AbstractCoroutineContextElement(Key) {
    companion object Key : CoroutineContext.Key<CustomCoroutineContext>
}

[sdeleuze]

I confirm the issue, likely caused by the CoroutineContext being overridden at CoExchangeFilterFunction level, but I am not sure yet how we can pass properly the CoroutineContext until this point.

Maybe if we provide a specialized awaitEntityOrNull<String>() to replace toEntity(String::class.java).awaitFirstOrNull() where we could store the CoroutineContext in the request attribute and set it explicitly in CoExchangeFilterFunction#filter (and adapt other coroutines extension in a similar way)?

@poutsma Related question: is it possible at DefaultResponseSpec level to get the ClientRequest from the ClientResponse in order to be able to set an attribute? It looks like I can only get the HttpRequest.

Or can we reuse the CoroutineContext injected in the Reactor context when awaitFirstOrNull() is invoked? Not sure.

[poutsma]

Yeah, it looks like HttpRequest is all that's available. I think that's because ClientResponse instances are not necessary created by the connector, but can also be built using the builder for use in interceptors.

[guggens]

We experienced the same issue. Our use case is:

1. We receive some custom http headers and store them in the CoroutineContext using the CoWebFilter, which works fine.
2. We can properly access them later to write some cache entries.
3. We try to call a downstream system and try to forward the http headers by reading from the CoroutineContext with a CoExchangeFilterFunction, but we cannot read out any values set in the CoWebFilter previously. It seems indeed that the CoExchangeFilterFunction has a different CoroutineContext than the code which is calling it, but it should have the same or a child of the context which is created when a rest endpoint is called from outside.