Adding an afterForkHook in streamly

I've been trying to use hs-opentelemetry in a project. The library provides some combinators to wrap sections of your code in a span for telemetry.

Every span has a certain context it needs to work with. hs-opentelemetry stores that context in the haskell thread-local storage.

The combinators in hs-opentelemetry derive the parent context from the thread-local storage. This context is used for propagating the parent context so that the parent-child relationship is preserved in the telemetry.

But when we fork a thread, the context is lost as the parent context isn't propagated. The solution to this is to manually set the context in the child thread after the fork.

We need to extend streamly to accommodate this kind of a functionality.

Requirements

  • We need a user-defined action to run after a fork.
  • This user defined action should be configurable in a clean way.

Proposal

We can extend the concurrent configuration to have the afterForkHook.

API: 

afterForkHook :: IO a -> Config -> Config

We need to modify streamly accordingly so that this option will take effect.

Update

Adding an afterForkHook might create problems. This seems a little hacky and the behaviour can be hard to define in situations with deep parent-child relationship.

Instead, we should model this problem in terms of inheritance.

  • There is a parent thread and there is a child thread
  • Some properties of the parent thread should be inherited by the child thread when forked.

This is a good start and seems like a cleaner way to share properties between threads.