Server only presents GHC versions that all workers support

[tomsmeding]

Currently, the server computes the list of available GHC versions to be presented to the user as the intersection of the availability sets of all workers currently registered on the server. This is sound, but rather pessimistic if there are multiple workers with unequal sets.

However, it is unclear whether this really needs addressing immediately: if different workers have different available version sets, and a some popular version is available only on a fraction of the workers, then those workers would (in the fixed system) get a disproportionate number of jobs. Hence you probably want to ensure your workers have consistent version sets anyway.

[Kleidukos]

Ensuring the same version set across all workers sounds like the best approach indeed.