By convention, calls to suspending functions should not block the thread, eliminating the requirement of calling suspending functions on background threads. Any long-running blocking operations should be moved to background threads within the suspending function that performs these operations. If suspending functions do block, this is breaking the Kotlin coroutines conventions (also see https://github.com/SonarSource/rspec/pull/173[S6307]).

As suspending functions can generally be called directly within other suspending functions, there is no need to move such a call to a background thread. This does not bring much benefit while adding complexity and making the code harder to understand.

In fact, various libraries explicitly provide suspending APIs for otherwise long-running blocking operations. The complexity of moving the appropriate tasks to background threads is already taken care of within the library and does not have to be considered when calling the library's suspending API.

Note also, however, that suspending functions do not block by convention. This behavior is not enforced on a technical level, leaving it to the developer to ensure the conventions are actually followed. If a suspending library function not under the control of a developer is actually blocking, then calling it in a different thread can work around the issues caused by the poorly written suspending function. It should be preferred to fix the callee, however, and not make such workarounds common practice.

This rule raises an issue when a suspending function is called in a way that executes the call in a new thread.

== Noncompliant Code Example

----
suspend fun caller() = coroutineScope {
    async(Dispatchers.IO) {
        callee()
    }
}

suspend fun callee() {
    // Do some work
}
----

== Compliant Solution

----
suspend fun caller() = coroutineScope {
    async {
        callee()
    }
}

suspend fun callee() {
    // Do some work
}
----

== See

* https://kotlinlang.org/docs/composing-suspending-functions.html[Composing suspending functions] in the Kotlin docs
* https://github.com/SonarSource/rspec/pull/173[S6307: Suspending functions should be main-safe]
