Ktor: Error Ktor running on background thread on iOS

[aleksey-chugaev]

Ktor Version and Engine Used (client or server and name)
Ktor client version 1.3.0-rc2
build.gradle:

ext.ktor_version = '1.3.0-rc2'
ext.kotlin_coroutines_version = '1.3.3-native-mt'

sourceSets {
        commonMain {
            dependencies {
                implementation kotlin('stdlib-common')
                implementation ("io.ktor:ktor-client-core:$ktor_version") {
                    exclude group: "org.jetbrains.kotlinx", module: "kotlinx-coroutines-core-common"
                }
                implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core-common:$kotlin_coroutines_version"
            }
        }
        iosMain {
            dependencies {
                implementation ("io.ktor:ktor-client-ios:$ktor_version") {
                    exclude group: "org.jetbrains.kotlinx", module: "kotlinx-coroutines-core-native"
                    exclude group: "org.jetbrains.kotlinx", module: "kotlinx-coroutines-core-common"
                }
                implementation ("io.ktor:ktor-client-core-native:$ktor_version") {
                    exclude group: "org.jetbrains.kotlinx", module: "kotlinx-coroutines-core-native"
                    exclude group: "org.jetbrains.kotlinx", module: "kotlinx-coroutines-core-common"
                }
                implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core-native:$kotlin_coroutines_version"
            }
        }
    }

Describe the bug
Executing an API request on the background thread results in kotlin.native.concurrent.InvalidMutabilityException
Full stacktrace

kotlin.native.concurrent.InvalidMutabilityException: mutation attempt of frozen io.ktor.client.request.HttpRequestPipeline@b5b748
        at 0   app                                 0x0000000105688a87 kfun:kotlin.Throwable.<init>(kotlin.String?)kotlin.Throwable + 87
        at 1   app                                 0x0000000105682215 kfun:kotlin.Exception.<init>(kotlin.String?)kotlin.Exception + 85
        at 2   app                                 0x0000000105681dd5 kfun:kotlin.RuntimeException.<init>(kotlin.String?)kotlin.RuntimeException + 85
        at 3   app                                 0x00000001056b6005 kfun:kotlin.native.concurrent.InvalidMutabilityException.<init>(kotlin.String)kotlin.native.concurrent.InvalidMutabilityException + 85
        at 4   app                                 0x00000001056b77d8 ThrowInvalidMutabilityException + 680
        at 5   app                                 0x00000001059a8c58 MutationCheck + 104
        at 6   app                                 0x00000001058c9400 kfun:io.ktor.util.pipeline.Pipeline.<set-interceptors>#internal + 96
        at 7   app                                 0x00000001058c98f5 kfun:io.ktor.util.pipeline.Pipeline.notSharedInterceptorsList#internal + 85
        at 8   app                                 0x00000001058c8a8a kfun:io.ktor.util.pipeline.Pipeline.cacheInterceptors#internal + 650
        at 9   app                                 0x00000001058c97a4 kfun:io.ktor.util.pipeline.Pipeline.sharedInterceptorsList#internal + 340
        at 10  app                                 0x00000001058c5f57 kfun:io.ktor.util.pipeline.Pipeline.createContext$ktor-utils(#GENERIC_kotlin.Any;#GENERIC_kotlin.Any)io.ktor.util.pipeline.PipelineExecutor<#GENERIC_kotlin.Any> + 263
        at 11  app                                 0x00000001058c5d71 kfun:io.ktor.util.pipeline.Pipeline.execute(#GENERIC_kotlin.Any;#GENERIC_kotlin.Any)#GENERIC_kotlin.Any + 273
        at 12  app                                 0x000000010590797a kfun:io.ktor.client.HttpClient.$executeCOROUTINE$6.invokeSuspend(kotlin.Result<kotlin.Any?>)kotlin.Any? + 666
        at 13  app                                 0x0000000105907cc4 kfun:io.ktor.client.HttpClient.execute(io.ktor.client.request.HttpRequestBuilder)io.ktor.client.call.HttpClientCall + 308
        at 14  app                                 0x000000010593bb7c kfun:io.ktor.client.statement.HttpStatement.$executeUnsafeCOROUTINE$35.invokeSuspend(kotlin.Result<kotlin.Any?>)kotlin.Any? + 956
        at 15  app                                 0x000000010593be8b kfun:io.ktor.client.statement.HttpStatement.executeUnsafe$ktor-client-core()io.ktor.client.statement.HttpResponse + 235
        at 16  app                                 0x000000010594b557 kfun:sample.ClientApi.$getQuote$lambda-0COROUTINE$0.invokeSuspend#internal + 6775
        at 17  app                                 0x00000001056a9ec8 kfun:kotlin.coroutines.native.internal.BaseContinuationImpl.resumeWith(kotlin.Result<kotlin.Any?>) + 712
        at 18  app                                 0x00000001057ca07c kfun:kotlinx.coroutines.DispatchedTask.run() + 2780
        at 19  app                                 0x000000010577fa18 kfun:kotlinx.coroutines.EventLoopImplBase.processNextEvent()ValueType + 792
        at 20  app                                 0x00000001057e3e31 kfun:kotlinx.coroutines.runEventLoop$kotlinx-coroutines-core(kotlinx.coroutines.EventLoop?;kotlin.Function0<kotlin.Boolean>) + 881
        at 21  app                                 0x00000001057eb412 kfun:kotlinx.coroutines.WorkerCoroutineDispatcherImpl.start$lambda-0#internal + 402
        at 22  app                                 0x00000001057eb5fb kfun:kotlinx.coroutines.WorkerCoroutineDispatcherImpl.$start$lambda-0$FUNCTION_REFERENCE$71.invoke#internal + 59
        at 23  app                                 0x00000001057eb65b kfun:kotlinx.coroutines.WorkerCoroutineDispatcherImpl.$start$lambda-0$FUNCTION_REFERENCE$71.$<bridge-UNN>invoke()#internal + 59
        at 24  app                                 0x00000001056b73a1 WorkerLaunchpad + 177
        at 25  app                                 0x00000001059b1909 _ZN6Worker19processQueueElementEb + 2569
        at 26  app                                 0x00000001059b1eb6 _ZN12_GLOBAL__N_113workerRoutineEPv + 54
        at 27  libsystem_pthread.dylib             0x00007fff51bfe2eb _pthread_body + 126
        at 28  libsystem_pthread.dylib             0x00007fff51c01249 _pthread_start + 66
        at 29  libsystem_pthread.dylib             0x00007fff51bfd40d thread_start + 13

To Reproduce
Steps to reproduce the behavior:

1. Clone sample app from https://github.com/aleksey-chugaev/ktor-mt-app
2. Run iosApp in emulator
3. See error

Expected behavior
Should successfully execute the request.

[kpgalligan]

@e5l Any updates on the native mt support? Would love to test this. I've been working on alternatives in the interim, but if this is happening soon, I'd rather not.

[joreilly]

Any update on this in light of 1.4 related changes?

[oleg-larshin]

Please check the following ticket on YouTrack for follow-ups to this issue. GitHub issues will be closed in the coming weeks.

[e5l]

Yep, can't reproduce the problem with recent update.