I m confused about the following quote from the `Job` docume

Question

I m confused about the following quote from the `Job` documentation <https kotlin github io kotlinx coroutines kotlinx coroutines core kotlinx coroutines job index html> gt Normal cancellation of a job is distinguished from its failure by the type of its cancellation exception cause If the cause of cancellation is CancellationException then the job is considered to be cancelled normally This usually happens when cancel is invoked without additional parameters If the cause of cancellation is a different exception then the job is considered to have failed This usually happens when the code of the job encounters some problem and throws an exception From the first sentence I conclude that a coroutine s `CancellationException` may have a `cause` From the second sentence I conclude that the `cause` may be yet another `CancellationException` confused am I right And from the fourth sentence I conclude that if failed I will receive a `CancellationException` with a `cause` that is not a `CancellationException` am I right So normally can a `CancellationException` have another `CancellationException` as its `cause` If so can this lead to deep nesting of `CancellationException` instances And if so should traverse this chain of cancellation exceptions in search for a different type of exception to determine if this was a failure or normal cancellation

Zach Klippenstein (he/him) [MOD] · Answer

Yes typically to find the real favor you need to traverse the chain I use this a lot ``` generateSequence e it cause firstOrNull it is CancellationException ```

Erik · Answer

Do you have an example that would lead to a nested `CancellationException` being caught I tried creating an example but I only was able to catch `CancellationException`s without `CancellationException` causes

Zach Klippenstein (he/him) [MOD] · Answer

`cancel CancellationException Oops cause ` where cause is any other type of exception

Erik · Answer

Well yeah you can always cancel manually with a `CancellationException` but that s not what I normally do Do you have an example where this would normally occur

Erik · Answer

It s hard to describe because I can t come up with an example I can imagine some coroutine structure with many parents and children where one fails and the rest would be cancelled with the failure as the cause But I ve never seen a failure in a nested `CancellationException` So does this actually happen if you don t manually nest `CancellationException`s in one another

Fatih · Answer

If your question is about work manager job I remember you asked earlier you won t get a reason Job cancel by work manager will raise a CancellationException without a reason

Erik · Answer

Well remembered However this is not about that particular use case I have another case where I check for failure or normal cancellation

Erik · Answer

The more I read about the subject the more I think that `CancellationException`s are normally never nested in a cause chain unless you create your own chain of nested cancellation exceptions and cancel a coroutine with it Furthermore I log various exceptions caught in coroutines to Crashlytics and I haven t seen a single chain of `CancellationException` they re all just a single cancellation with an optional cause

elizarov · Answer

Yes The coroutines infrastructure always unwaps CE so that they don t nest

Erik · Answer

Well if master coroutine himself says it then it must be the truth wink Thank you Roman

elizarov · Answer

Here s my attempt to write better docs on that part <https github com Kotlin kotlinx coroutines pull 2035>

Erik · Answer

Wow thanks I was about to write something myself or at least create an issue I ll take a look