Cond: Waiting for a Condition

Several customers wait at a bakery counter for bread. The shelf is empty, so they sit down instead of checking the shelf every few seconds. When the baker puts out one loaf, the baker calls one customer. If the baker brings out a whole tray, the baker calls everyone. A customer who is called still looks at the shelf before taking anything, because another customer may have taken the last loaf first, and in that case the customer sits down again.

Goroutines sometimes need to wait in the same way