Add Event::modify + reflector::store helpers

[clux]

• A convenience reflector::store fn that creates a (reader,writer) pair in default cases
• An Event::modify fn that can be used to downsize the memory use of a watcher stream
• Example usage updated (pod_reflector reduces internal memory use by modifying out properties)

orginal idea - scrapped

An extension written into `Writer` to take a mutator (defaults to the identity)

The first part is the most substantial, and it required tweaking the Default + Debug implementations (which were done via Derivative before). There might be some ways to get around this with less trait restrictions on F but it always complained without this, and it seems to work well in a real word setting in example with a straight closure.

The use case for the mutator is memory improvements. Currently you can customize structs with partially typed resources, but you cannot drop the significant contributions from ObjectMeta via its managed fields. Additionally, it feels a bit wrong to me to recommend rewriting structs when what we really want is just to drop what parts of it we store, and that can be done with a mutator. By the time we use it we have already extracted the key parts for the keys in the state so it should be pretty user-safe as well.

[codecov-commenter]

Codecov Report

Merging #907 (ee617be) into master (d0bf02f) will decrease coverage by 0.11%.
The diff coverage is 53.33%.

@@            Coverage Diff             @@
##           master     #907      +/-   ##
==========================================
- Coverage   70.56%   70.44%   -0.12%     
==========================================
  Files          64       64              
  Lines        4331     4341      +10     
==========================================
+ Hits         3056     3058       +2     
- Misses       1275     1283       +8     

Impacted Files	Coverage Δ
kube-runtime/src/watcher.rs	50.64% <0.00%> (-5.07%)	⬇️
kube-runtime/src/reflector/mod.rs	100.00% <100.00%> (ø)
kube-runtime/src/reflector/store.rs	95.16% <100.00%> (+0.24%)	⬆️
kube-runtime/src/wait.rs	68.00% <0.00%> (-2.00%)	⬇️

[nightkr]

Hm.. personally I'd be more inclined to put this into the reflector, or to have this be a map step in between the watcher and reflector. It feels pretty weird to have the Store mutate things on its own when it's presented as a dumb hashmap-style data structure. Boxing the callback also becomes an optimization barrier for the compiler, and requires a virtual call.

Something along the lines of..

watcher(...)
  .map_ok(|event| event.map_objects(|mut obj| {
    obj.metadata.managed_fields = None;
    obj
  }))
  .reflector(...)

Now that we have WatchStreamExt we could even define a WatchStreamExt::map_event_objects() or something.

On the other hand, 👍 on reflector::store().

[clux]

Yeah, that makes perfect sense. I will get back to this and try something with CustomResourceExt later if i have time. Might put this on hold before 0.72.0.

[clux]

Actually not too bad to get working. This is only the Event::map_event though, that's actually fine for now. Names are still a bit awkward, but gotta step away for a bit now.

[nightkr]

mutate_objects, perhaps? map implies that it operates on a fn(T) -> T rather than fn(&mut T) -> ()

[nightkr]

I believe this could also be FnMut rather than Fn since we apply them sequentially (unless you plan on introducing parallelism later?)

[clux]

AFAIKT the benefit to having it be FnMut is that it can capture mutable variables from a closure?

[nightkr]

Yes. Or rather, it can capture closure variables mutably. You could still capture mutable variables immutably since they just get downgraded to immutable during the borrow.