Race condition with two pickers filling the same machine at the same time #5
Description
When there's two pickers feeding a machine with the same item, there's a small chance both will try to insert the item at the same time. This results in one of the pickers getting stuck if there's no more available space in the machine. This is best reproduced with the catalyzer, where ADP getting stuck on input can deadlock the whole machine if the stuck picker means no more glucose can get in to start another batch of processing.
The image shows the setup:
Note that there's another input from the left, which is why this doesn't get blocked more often - there's actually two opportunities to get glucose and NAD+ in. At least one of the pickers gets "blocked" in this way fairly often, but the third one gives an opportunity for recovery. With just two input pickers, this happens much more often - in my case, to get the full deadlock, I one picker blocked with ADP and the other with NAD+, with no way for the glucose to go in.
But the deadlock is just the worst case scenario - the core problem is that pickers do not reserve the slots they put an item into, so multiple pickers can attempt to overfill the target machine.