Report why a task is not starting

[benoit74]

We regularly get questions about why a given requested task is not starting.

While this is easy for the machine to know, it consumes significant developer resources to check all existing reasons, and it will probably only get worse in the future where we will probably add more complex scheduling constraints.

As a user, it would be nice to know why a given requested task is not starting.

I don't know yet how feasible it is to compute this information for all requested tasks, refresh it every 5 minutes or so, and hence display this information on all requested tasks.

Or maybe this could be an "on-demand" feature where the user will click a button and get a reason.

Probably the situation is so complex that we cannot give a single reason, but we will have to display one reason per worker in many situations. Meaning the reason is not just a phrase but a list of constraints blocking the start of the task.