>>> Take GPLv2. Suppose I license some code I own to Alice under GPLv2. Later Alice gives a copy to Bob. Where does Bob's license come from?

There is some case law around that in France. A software company makes GPL software or library, a contracting firm uses/modifies it, reselling the result to some clients.

In 2 cases I am aware of, the final client was the government and sued the contracting firm for breach of contract (the GPL is a contract) and won. (Winning is relative of course because the client can't use the software they paid for anymore and years are passing by in court).

The contract creates a chain of contractual responsibility, which I am not an expert in, but as far as I understand it opens any party to being sued by any other party.

Yes it means that GPL developers, by giving an open contract to anybody who is willing to take it, are pretty much opening themselves to be sued by the entire world.

In practice you sue for a purpose (maximizing damages :D) so the purpose guides whom to pursue on which grounds.

Now to answer your question... what happens if you decide to cancel the license? Well, you can't unilaterally cancel a contract on a whim.