Deadlocks

Often, in order to guarantee that concurrent code is synchronized correctly, it's necessary to suspend or block execution while a task is completed in a different thread. But due to the complexity of these situations, it isn't uncommon to end up in a situation where the execution of the complete application is halted because of circular dependencies:

lateinit var jobA : Job
lateinit var jobB : Job

fun main(args: Array<String>) = runBlocking {
    jobA = launch {
        delay(1000)
        // wait for JobB to finish
        jobB.join()
     }

    jobB = launch {
        // wait for JobA to finish
        jobA.join()
    }

    // wait for JobA to finish
    jobA.join()
    println("Finished")
}

Let's take a look at a simple flow diagram of jobA.

In this example, jobA is waiting for jobB to finish its execution; meanwhile jobB is waiting for jobA to finish. Since both are waiting for each other, none of them is ever going to end; hence the message Finished will never be printed:

This example is, of course, intended to be as simple as possible, but in real-life scenarios deadlocks are more difficult to spot and correct. They are commonly caused by intricate networks of locks, and often happen hand-in-hand with race conditions. For example, a race condition can create an unexpected state in which the deadlock can happen.