If you have a list: [producer1, consumer1, consumer2, consumer3]
and you update in order, starting at supply = 0, you'll get this:
producer1 produces 5 mass. You now have 5 mass
consumer1 wants 3 mass. Success, you now have 2 mass
consumer2 wants 3 mass. Fail
consumer3 wants 3 mass. Fail
[next tick]
producer1 produces 5 mass. You now have 7 mass
consumer1 wants 3 mass. Success, you now have 4 mass
consumer2 wants 3 mass. Success, you now have 1 mass
consumer3 wants 3 mass. Fail
... consumers 2 and 3 starve until consumer 1 has been satisfied