The law governing licenses (of all kinds.)
> If you grant rights to someone, in what legal way can you simply revoke those rights?
If the grant is gratuitous (not contracted for) you generally have no legal obligation not to simply revoke those rights (it is exactly the same case as inviting someone into your house and then ejecting them when you decide they've worn out their welcome, which is also a case of gratuitous license.)
> Real question. I am not a lawyer, but would assume arbitrary revocation would essentially be a breach of contract.
Right, if there was a contract, then revocation inconsistent with its terms would generally be a breach, which would still least present a cause of action for damages and may permit an equitable remedy extending the license.
Most F/OSS license scenarios are not contract licenses, and even when they are, publication with an offer of a license isn't a contract, the offer must be accepted without the offer having expired or having been revoked first for a contract to exist.
If I agree that my neighbor can have 5 feet on my side of the property line in perpetuity and put the offer in writing, I can simply change my mind later and take it back?
But if you instead gave your neighbor that 5 feet then you would not be able to revoke it.
A key part of most open source licenses is that the author is not giving up ownership of the code. This is why many projects require a copyright reassignments for contributions to avoid being hamstrung by an author revoking their right to use that contribution sometime in the future.
Real question. I am not a lawyer, but would assume arbitrary revocation would essentially be a breach of contract. (But I guess it's not technically a contract since it's one sided?)