Documentation: GameServer updates are not supported. No pods are created after switching image for GameServer

[aLekSer]

What happened:
No pod got restarted and rescheduled after updating GameServer to a new image.

What you expected to happen:
Create a new pod for GameServer if its image parameter was updated.

How to reproduce it (as minimally and precisely as possible):

kubectl create -f https://raw.githubusercontent.com/googleforgames/agones/release-1.7.0/examples/simple-udp/gameserver.yaml
kubectl edit gameservers <name>
// Change image from udp-server:0.21 to udp-server:0.20
kubectl describe gameserver <name>
// See 0.20
kubectl describe pod <name>
// See 0.21

Anything else we need to know?:

Environment:

• Agones version: 1.8.0-dev
• Kubernetes version (use kubectl version): 1.15
• Cloud provider or hardware configuration: GKE
• Install method (yaml/helm): helm
• Troubleshooting guide log(s):
• Others:

[markmandel]

We really don't actually support editing GameServer spec (although a controller does make a small change to port values on update, because of old reasons).

So not sure if this is really a bug per se. I'm also concerned that if we add an extra validation webhook to GameServer, it may be a performance issue, since GameServers are on the hotpath - it would require some performance testing to confirm or not.

[aLekSer]

@markmandel As a side issue - I found an infinite loop in the code. Which I am able to reproduce with a simple test:

	t.Parallel()
	fixture := dynamicGameServerFixture()

	t.Run("test allocated port counts", func(t *testing.T) {
... line 64:
		// single port dynamic
		fd := fixture.DeepCopy()
		fd.Spec.Ports[0].PortPolicy = ""
		pa.Allocate(fd)
		assert.Nil(t, err)
...
}

Will make a PR to fix this soon. We can have an empty string PortPolicy and no validations on Update. Could lead to this issue.
And internal allocate() would be called recursively forever.
Not sure how to reproduce this using kubectl only.
@akremsa can you also take a look since you have created a PR recently on this topic?

[aLekSer]

I assume at least docs should contain that gameservers could not be updated.

[markmandel]

I assume at least docs should contain that gameservers could not be updated.

Yeah, that totally makes sense!