How to step through suspend function calls when debugging Kotlin coroutines

Question

This page shows some general techniques. In short, run with enabled assertions (-ea JVM flag).

kotlinx-coroutines-debug module is specifically designed for what its name says. This is how I use it in unit tests;

runBlocking {
    DebugProbes.install()
    val deferred = async { methodUnderTest() }
    delay(3000)
    DebugProbes.dumpCoroutines()
    println("\nDumping only deferred")
    DebugProbes.printJob(deferred)
    DebugProbes.uninstall()
    cleanup()
}

There’s a JUnit4 rule to reduce the boilerplate, but you shouldn’t be using JUnit4 in late 2020.

Also, Kotlin 1.4.0-RC and later has support for debugging Coroutines from inside the IDE, but just like everything new from JetBrains, I found it half-baked and not always showing suspended coroutines; kotlinx-coroutines-debug works better.
https://blog.jetbrains.com/kotlin/2020/07/kotlin-1-4-rc-debugging-coroutines/

Edit Oct 2020:

I created a JUnit 5 Extension that can dump coroutines on timeout.
https://github.com/asarkar/coroutines-test

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