Parent coroutine shall wait for its children to complete

[elizarov]

The logic behind parent-child relations between coroutines shall be changed so that parent coroutine automatically waits for its children to complete.

This is needed for several use-cases:

1. Predictable exception propagation in parent-child relations. The use-case for child coroutine is that the child carries out a subtask on parent's behalf and if something goes wrong (child crashes), then the parent should predictably crash, too. Right now, a child that was fired with launch cancels its parent on crash, but if parent had already completed, then there is nothing else a child can do. So, it is somewhat random behavior now -- if a child crashes before parent's completion, then exception gets propagated, otherwise it does not.

2. We'd like for Rx-inspired merge operation to have simple implementation. The current implementation given in the reactive guide is not correct under existing parent-child semantics. It stops publishing as soon as the source channel is over, without waiting to merge all the received channels. With the change to the parent-child relation (parent waits) it starts to work as expected automagically:

fun <T> Publisher<Publisher<T>>.merge(context: CoroutineContext) = publish<T>(context) {
  consumeEach { pub ->                 // for each publisher received on the source channel
      launch(coroutineContext) {       // launch a child coroutine
          pub.consumeEach { send(it) } // resend all element from this publisher
      }
  }
}

3. Reduce surprising behavior. The fact that children are cancelled when parent completes normally is surprising. It is hard to trouble-shoot and to figure out what is going on.

[elizarov]

This is now fixed in develop branch.

[elizarov]

Released in version 0.19

[mgroth0]

Related: https://youtrack.jetbrains.com/issue/KT-60182/Launching-a-new-coroutine-from-a-completed-scope-leads-to-a-silent-but-deadly-state-coroutine-never-launches-yet-there-is-no