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Module kotlinx-coroutines-debug

Debugging facilities for kotlinx.coroutines on JVM.

Overview

This module provides a debug JVM agent that allows to track and trace existing coroutines. The main entry point to debug facilities is DebugProbes API. Call to DebugProbes.install installs debug agent via ByteBuddy and starts spying on coroutines when they are created, suspended and resumed.

After that, you can use DebugProbes.dumpCoroutines to print all active (suspended or running) coroutines, including their state, creation and suspension stacktraces. Additionally, it is possible to process the list of such coroutines via DebugProbes.dumpCoroutinesInfo or dump isolated parts of coroutines hierarchy referenced by a Job or CoroutineScope instances using DebugProbes.printJob and DebugProbes.printScope respectively.

This module also provides an automatic BlockHound integration that detects when a blocking operation was called in a coroutine context that prohibits it. In order to use it, please follow the BlockHound quick start guide.

Using in your project

Add kotlinx-coroutines-debug to your project test dependencies:

dependencies {
    testImplementation 'org.jetbrains.kotlinx:kotlinx-coroutines-debug:1.3.9'
}

Using in unit tests

For JUnit4 debug module provides special test rule, CoroutinesTimeout, for installing debug probes and to dump coroutines on timeout to simplify tests debugging.

Its usage is better demonstrated by the example (runnable code is here):

class TestRuleExample {
    @get:Rule
    public val timeout = CoroutinesTimeout.seconds(1)

    private suspend fun someFunctionDeepInTheStack() {
        withContext(Dispatchers.IO) {
            delay(Long.MAX_VALUE) // Hang method
        }  
    }

    @Test
    fun hangingTest() = runBlocking {
        val job = launch {
            someFunctionDeepInTheStack()
        }
        job.join() // Join will hang
    }
}

After 1 second, test will fail with TestTimeoutException and all coroutines (runBlocking and launch) and their stacktraces will be dumped to the console.

Using as JVM agent

Debug module can also be used as a standalone JVM agent to enable debug probes on the application startup. You can run your application with an additional argument: -javaagent:kotlinx-coroutines-debug-1.3.9.jar. Additionally, on Linux and Mac OS X you can use kill -5 $pid command in order to force your application to print all alive coroutines. When used as Java agent, "kotlinx.coroutines.debug.enable.creation.stack.trace" system property can be used to control DebugProbes.enableCreationStackTraces along with agent startup.

Using in production environment

It is possible to run an application in production environments with debug probes in order to monitor its state and improve its observability. For that, it is strongly recommended to switch off DebugProbes.enableCreationStackTraces property to significantly reduce the overhead of debug probes and make it insignificant. With creation stack-traces disabled, the typical overhead of enabled debug probes is a single-digit percentage of the total application throughput.

Example of usage

Capabilities of this module can be demonstrated by the following example (runnable code is here):

suspend fun computeValue(): String = coroutineScope {
    val one = async { computeOne() }
    val two = async { computeTwo() }
    combineResults(one, two)
}

suspend fun combineResults(one: Deferred<String>, two: Deferred<String>): String =
    one.await() + two.await()

suspend fun computeOne(): String {
    delay(5000)
    return "4"
}

suspend fun computeTwo(): String {
    delay(5000)
    return "2"
}

fun main() = runBlocking {
    DebugProbes.install()
    val deferred = async { computeValue() }
    // Delay for some time
    delay(1000)
    // Dump running coroutines
    DebugProbes.dumpCoroutines()
    println("\nDumping only deferred")
    DebugProbes.printJob(deferred)
}

Printed result will be:

Coroutines dump 2018/11/12 21:44:02

Coroutine "coroutine#2":DeferredCoroutine{Active}@289d1c02, state: SUSPENDED
	at kotlinx.coroutines.DeferredCoroutine.await$suspendImpl(Builders.common.kt:99)
	at ExampleKt.combineResults(Example.kt:11)
	at ExampleKt$computeValue$2.invokeSuspend(Example.kt:7)
	at ExampleKt$main$1$deferred$1.invokeSuspend(Example.kt:25)
	(Coroutine creation stacktrace)
	at kotlin.coroutines.intrinsics.IntrinsicsKt__IntrinsicsJvmKt.createCoroutineUnintercepted(IntrinsicsJvm.kt:116)
	at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable(Cancellable.kt:25)
	at kotlinx.coroutines.BuildersKt.async$default(Unknown Source)
	at ExampleKt$main$1.invokeSuspend(Example.kt:25)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:32)
	at kotlinx.coroutines.DispatchedTask.run(Dispatched.kt:233)
	at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
	at ExampleKt.main(Example.kt:23)
	at ExampleKt.main(Example.kt)

... More coroutines here ...

Dumping only deferred
"coroutine#2":DeferredCoroutine{Active}, continuation is SUSPENDED at line kotlinx.coroutines.DeferredCoroutine.await$suspendImpl(Builders.common.kt:99)
			"coroutine#3":DeferredCoroutine{Active}, continuation is SUSPENDED at line ExampleKt.computeOne(Example.kt:14)
		"coroutine#4":DeferredCoroutine{Active}, continuation is SUSPENDED at line ExampleKt.computeTwo(Example.kt:19)

Status of the API

API is experimental, and it is not guaranteed it won't be changed (while it is marked as @ExperimentalCoroutinesApi). Like the rest of experimental API, DebugProbes is carefully designed, tested and ready to use in both test and production environments. It is marked as experimental to leave us the room to enrich the output data in a potentially backwards incompatible manner to further improve diagnostics and debugging experience.

The output format of DebugProbes can be changed in the future and it is not recommended to rely on the string representation of the dump programmatically.

Debug agent and Android

Unfortunately, Android runtime does not support Instrument API necessary for kotlinx-coroutines-debug to function, triggering java.lang.NoClassDefFoundError: Failed resolution of: Ljava/lang/management/ManagementFactory;.

Nevertheless, it will be possible to support debug agent on Android as soon as GradleAspectJ-Android will support android-gradle 3.3