Rolling updates do not progress if all game servers in the fleet are in the Allocated state

[GonzaloSaez]

Within the agones Fleet controller logic, we do not allow scaling the replica count above the specified amount of replicas. See

agones/pkg/fleets/controller.go

Line 501 in 3e18f4f

if replicas+sumAllocated > fleet.Spec.Replicas {

. This is not consistent with how MaxSurge works in other k8s entities such as Deployment, since in the latter the number of replicas can scale up while the redeployment is happening. Also, not being able to scale up the replica count during the rolling update implies that, if all the GameServers are in allocated mode, the RollingUpdate will not happen until at least one of the GameServers dies or transitions back into Ready state. In cases in which the users of Agones need to use the overflow event during the RollingUpdate to control the allocations based on certain labels, the allocations will constantly fail since the RollingUpdate is not capable of creating more replicas and the overflow event has been processed by the GameServers.

Adding a flag to allow scaling up the fleet replica count would not be a lot of work (since I guess we do not want to change the meaning of the RollingUpdate parameters for backwards compatibility reasons). That said, I'm not familiar enough with the codebase to understand the consequences of the the fleet replica count increasing above the specified count. This could also impact the FleetAutoscaler.