Race conditions

Race conditions, perhaps the most common error when writing concurrent code, happen when concurrent code is written but is expected to behave like sequential code. In more practical terms, it means that there is an assumption that concurrent code will always execute in a particular order.

For example, let's say that you are writing a function that has to concurrently fetch something from a database and call a web service. Then it has to do some computation once both operations are completed. It would be a common mistake to assume that going to the database will be faster, so many people may try to access the result of the database operation as soon as the web service operation is done, thinking that by that time the information from the database will always be ready. Whenever the database operation takes longer than the webservice call, the application will either crash or enter an inconsistent state.

A race condition happens, then, when a concurrent piece of software requires semi-independent operations to complete in a certain order to be able to work correctly. And this is not how concurrent code should be implemented.

Let's see a simple example of this:

data class UserInfo(val name: String, val lastName: String, val id: Int)

lateinit var user: UserInfo

fun main(args: Array<String>) = runBlocking {
    asyncGetUserInfo(1)
    // Do some other operations
    delay(1000)

    println("User ${user.id} is ${user.name}")
}

fun asyncGetUserInfo(id: Int) = async {
    user = UserInfo(id = id, name = "Susan", lastName = "Calvin")
}

The main() function is using a background coroutine to get the information of a user, and after a delay of a second (simulating other tasks), it prints the name of the user. This code will work because of the one second delay. If we either remove that delay or put a higher delay inside asyncGetUserInfo(), the application will crash. Let's replace asyncGetUserInfo() with the following implementation:

fun asyncGetUserInfo(id: Int) = async {
    delay(1100)
    user = UserInfo(id = id, name = "Susan", lastName = "Calvin")
}

Executing this will cause main() to crash while trying to print the information in user, which hasn't been initialized. To fix this race condition, it is important to explicitly wait until the information has been obtained before trying to access it.