State locking: why it prevents corruption, which backends support it, `-lock-timeout` and `terraform force-unlock`

Imagine two engineers running terraform apply against the same state at the same moment. Both read the same starting state, both make changes in the cloud and both try to write a new state. Whichever writes second overwrites the first, and the state no longer matches reality. State locking prevents this by letting only one operation that could write state run at a time.