enable prePuller.hook.pullOnlyOnChanges by default

[consideRatio]

By enabling this feature by default, those with prePuller.hook.enabled=true will not need to wait ~10 seconds on hook-image-awaiter and hook-image-puller pods to startup and shut down if no new image has been configured.

In practice, this feature works by inspecting the rendered Helm template of the hook-image-puller DaemonSet for a change to anything besides labels since the last helm upgrade was made.

This feature also relies on the ability for us to lookup a value in a ConfigMap using the Helm 3 feature called lookup.

[yuvipanda]

Yay!

What's the minimum helm version for this? I had to update from 3.2 to get this to work - but not sure what's the minimum version

[consideRatio]

Oh helm 3.1 introduced the lookup function.

[yuvipanda]

oh, hmm - not sure why I needed at least 3.3... Either way, do we mention that somewhere? Happy to merge this once we have that...

[consideRatio]

Hmmm i have not mentioned 3.3, i have no clear signal on whats required besides at least 3.1. we only test against different k8s versions, not helm versions.

If you could conclude 3.3 was required that is good for us to know. Note that you will always need to make one upgrade with the feature bugfix, and only after that this flag will make a difference. (A checksum must be updated correctly and stored in the hub configmap, which later can be found to be the same)

When you said "i needed" does it mean you got an error before, or you still found the image-puller to show up?

[yuvipanda]

@consideRatio yeah, with 3.2 I got an error in rendering the daemonset helper. I migraetd to latest (3.5) and it went away. Unfortunately I don't have those logs anymore

[consideRatio]

@yuvipanda I've concluded that 3.4.X doesn't work but 3.5.0 works. As upgrading to a recent helm 3 binary is easy, I suggest we accept a requirement on helm 3.5 and above. I'm making sure this is documented and we test against the minimally known supported helm version going onwards.

See #2181

---

It turns out 3.5.0 is now required because helm have given the built in context .Chart special treatment that caused issues after #2174 where my solution was to manipulate .Chart.Version to avoid modifications of a checksum.

I think it may be possible to retain support for a lower version by bashing my head against this a while but I suggest we simply require 3.5+ instead of supporting 3.x (where x < 5).

[yuvipanda]

+1 on a minimum helm 3.5 requirement.

[yuvipanda]

yay! :)