A necessary element of a riot under P.C. 42.02 is an unlawful assembly. (per the case law) For purposes of that statue are inmates of the county jail, all housed in the same tank, unlawfully assembled when they show their true colors and smash the place up together? I tend to think not, but there is not much case law on riots, and the law that does exist is quite old. So I'd appreciate any thoughts.

42.02(c) gives a defense to prosecution if the assemblage was first lawful and the actor retired from the assembly when someone first manifests the intent to start the riot. So I would say that your lawful assembly can turn unlawful as soon as the participants start any of the conduct enumerated in 42.02(a), and anyone who doesn't withdraw then is part of the riot.

In other words, an assembly becomes unlawful -- no matter its original status -- as soon as its members start trashing the place. I think your jail example would count.