If no rollout percentage is provided, then we currently default to rolling out to 100% of users.

However, it's possible to accidentally omit the rollout value. For example, if the rollout value is parameterised, but the parameter is not entered at build time for whatever reason.

Rolling out to 100% of users is irreversible — if you roll out to < 100%, you can later edit the percentage upwards or downwards; but setting the rollout to 100% means the rollout is "complete", and you can no longer change the value.

So it would be safer to prevent the upload from taking place in such situations, i.e. require that the rollout percentage is explicitly specified at build time.

Changing the default rollout percentage value from 100% to 0% would also be an option, now that we have the ability to upload releases as a draft. But this would be a silent behaviour change — users running builds, expecting to release to 100%, would have to investigate why their releases aren't becoming visible to users. By removing the default and throwing a configuration error, users will see that they have to make the configuration change — or will realise that their parameterised value was not working as expected.