Parallel Gateway continues if multiple tokens are received on the same path

[jguthrie100]

Describe the bug
I have a process with a parallel gateway that waits on two branches, one of which has a loop which checks every hour whether a variable is true.
I would expect that the gateway waits for both branches to complete before proceeding, but it actually proceeds if the same branch completes twice.

This can happen because a Message Catch Event can spawn a new token, which gets sent down the same branch again.

Expected behavior
I would expect the parallel gateway to wait for a token from each branch to arrive, rather than just waiting for 2 tokens to arrive from the same branch.

[tijsrademakers]

Thanks for sharing the issue. The parallel gateway join should only complete when both incoming sequence flows are completed, so the one coming from the exclusive gateway and the one coming from the task. Are you saying that if two are coming from the task, the parallel gateway join is still completed?

[jguthrie100]

Exactly. If two tokens arrive from the task branch, then the process ends, even if a token is still in the loop before the exclusive gateway.

I found in the sourcecode that this is a known problem/behaviour..

flowable-engine/modules/flowable5-engine/src/main/java/org/activiti/engine/impl/bpmn/behavior/ParallelGatewayActivityBehavior.java

Line 35 in b1312f3

* Note that there is a slight difference to spec (p. 436): "The parallel gateway is activated if there is at least one Token on each incoming sequence flow." We only check the number of incoming