go() is called on a non-initiator node when there's no initiator in a phase

[freekvw]

I hoped I could code my own initiator outside of the framework and just call begin(), pull() and end() on a pull node, but that was of course not going to work. Instead of telling me there's a phase without an initiator (namely the one containing the output node of a pull-sorter), it apparently tried to call go() on the pull sorter output.

[Mortal]

By definition, an initiator of a phase is a node with no ingoing edges in the actor graph (that is, the node that nobody claims to push to or pull from). It is an error for a phase to have more than one initiator node - in that case, a buffer or other phase boundary is necessary. It is an error for a phase to have no initiators, but in this case you have a cycle in your actor graph, which I don't think we should permit either.

By extension, it is an error to have a passive sorter in a pipeline where nobody pulls the output out of the passive sorter. In this case the output node is treated as a phase initiator which does not make sense.

The current error message in this case is "node subclass [cryptic name] is not an initiator node". Perhaps this should be changed to something along the lines of "go() called on non-initiator - phase has no suitable initiator". What do you think?

I hope you figured out how to write your initiator for your use case.

[freekvw]

I can of course just write a trivial end-node... But shouldn't the graph framework itself instead of the pull sorter output find out that something is wrong?

[Mortal]

In a sense it does due to the default implementation of node::go() which the pull sort output does not override. In almost all cases, a phase will have a node that fits the definition of an initiator node. There is no reflection or flag that indicates that the initiator determined by the definition is the right initiator or not - it finds out by trying to call go() and failing when that has not been overridden.

Is that not also the case for your failing program?

[freekvw]

I don't really get that -- if I would have a proper pull pipeline with an initiator at the end, if the graph doesn't know what is what, how can it determine that it should call go() on the last node?

[Mortal]

It is sort of a special case when you use a passive_sorter and don't use the .output() for anything - in that case, a final pipelining phase is constructed not from a pipe_begin/pipe_end expression, but implicitly from the passive_sorter object on automatic storage (aka on the stack).

I suppose in this case the pull output node from the passive sorter should print a specialized error message (but still throw the same exception).

[freekvw]

It's not a big deal, you can close the bug if you want.