I'm using `suspendCancellableCoroutine` to suspend...

# coroutines

[Jeff Lockhart]

I'm using suspendCancellableCoroutine to suspend a coroutine awaiting the results of a callback API to resume the coroutine with:

val result = suspendCancellableCoroutine { continuation ->
    callbackApi {
        continuation.resume(it)
    }
}

This works as expected the first time the code is called, but the second time the code is executed continuation.resume() doesn't work. continuation.isActive is false, continuation.isCompleted and continuation.isCancelled are true. Is it wrong to expect an active continuation after calling suspendCancellableCoroutine? Even after wrapping the code in async, launching a fresh coroutine, the continuation still isn't active the second time the code executes. What am I doing wrong?

[efemoney]

Check your callback api, it might be capturing a stale lambda

[efemoney]

Also show the surrounding code and or how callbackApi is implemented

[Jeff Lockhart]

I had the same thought, so I'm logging the other state captured in the lambda to confirm it is the new lambda. I'm hoping after getting some sleep, I'll see something I missed this morning. 😅 I'll share more of the code.

[Jeff Lockhart]

My code and tests are based on the SQLDelight paging extension. The suspendCancellableCoroutine is used to suspend awaiting callback here. The failing tests were all calling this.

If I replace runTest with runBlocking and the withPagingDataDiffer function with this:

suspend fun <T : Any> PagingData<T>.withPagingDataDiffer(
    testScope: CoroutineScope,
    diffCallback: DiffUtil.ItemCallback<T>,
    block: AsyncPagingDataDiffer<T>.() -> Unit,
) {
    val pagingDataDiffer = AsyncPagingDataDiffer(
        diffCallback,
        NoopListCallback,
        mainDispatcher = Dispatchers.Default,
        workerDispatcher = Dispatchers.Default,
    )
    val job = testScope.launch {
        pagingDataDiffer.submitData(this@withPagingDataDiffer)
    }
    delay(100)
    block(pagingDataDiffer)
    job.cancel()
}

the tests now pass with the continuations behaving as expected.

[Jeff Lockhart]

Obviously I don't like the arbitrary delay(100) awaiting the database operations to be completed. I'll have to see if there's a better way to replace this.

[Jeff Lockhart]

I created an issue to track this.