Remove `GetExtensions` from `component.Host`

[bogdandrutu]

Remove GetExtensions from component.Host. This should probably be an "optional" interface in the Extension package, so that it can return GetExtensions() map[component.ID]extension.Extension.

[mx-psi]

@bogdandrutu I feel like this should stay in component.Host: this feels like the essential functionality a component.Host must have in order to support basic use cases like authentication, encoding or storage configuration.

[jpkrohling]

Isn't this what we use in confighttp and configgrpc in order to get the available auth extensions? We also want to follow the same pattern to implement middleware extensions (#7441).

[mx-psi]

@jpkrohling This would still be available, just as an optional interface.

Right now:

var host component.Host = getComponentHostSomehow()

doSomethingWithExtensions(host.GetExtensions())

After the change proposed by @bogdandrutu we would have an interface like this:

type GetExtensionsInterface interface {
  GetExtensions() map[[component.ID](component.Component)   
}

and you would use it like this:

var host component.Host = getComponentHostSomehow()

getExtHost, ok := host.(GetExtensionsInterface)

if !ok {
  panic("host does not support GetExtensions")
}

doSomethingWithExtensions(host.GetExtensions())

This would be similar to componentstatus.Reporter

[jpkrohling]

Got it -- I'm still leaning towards having it as part of the main interface, unless there's a concrete reason to not have it there. I agree with your previous statement:

this feels like the essential functionality

[jade-guiton-dd]

One possible argument for making this interface optional is that it gives us more flexibility to change the method signature later, even after stabilization, for instance if returning a map no longer seems like the right choice. (Considering we removed GetExporters from Host way back it doesn't seem implausible.) Instead of supporting the old signature forever, it could be removed without completely breaking components relying on it, though at the cost that these components will likely assume there are no extensions set up. On the other hand, if we're fine with that last part, keeping the old signature around with a dummy implementation would not be a difficult alternative either... for that reason, I don't have strong feelings one way or another.

[TylerHelmuth]

This functionality is the is only concrete function on the host interface. I also agree is it the core feature that host is providing to components. If we removed it what would we do with the host interface?

What problem is being solved by removing it?

[bogdandrutu]

What problem is being solved by removing it?

There are multiple problems:

1. We are adding the notion of "extension" within the component package which should be agnostic of different component types (ideally).
2. It is inconsistent experience because GetExtensions() returns a Component not an Extension.