store/cachekv: use typed types/kv.List instead of container/list.List

[odeke-em]

Reduces CPU burn by using a typed List to avoid the expensive type
assertions from using an interface. This is the only option for now
until Go makes generics generally available.

The change brings time spent on the time assertion cummulatively to:
580ms down from 6.88s

Explanation:
The type assertions were showing up heavily in profiles:

• Before this commit

Total: 42.18s
ROUTINE ======================== github.com/cosmos/cosmos-sdk/store/cachekv.newMemIterator
in /Users/emmanuelodeke/go/src/github.com/cosmos/cosmos-sdk/store/cachekv/memiterator.go
    14.01s     18.87s (flat, cum) 44.74% of Total
         .          .     17:	items      []*kv.Pair
         .          .     18:	ascending  bool
         .          .     19:}
         .          .     20:
         .          .     21:func newMemIterator(start, end []byte, items *list.List, ascending bool) *memIterator {
         .      620ms     22:	itemsInDomain := make([]*kv.Pair, 0, items.Len())
         .          .     23:
         .          .     24:	var entered bool
         .          .     25:
     510ms      870ms     26:	for e := items.Front(); e != nil; e = e.Next() {
     6.85s      6.88s     27:		item := e.Value.(*kv.Pair)
     5.71s      8.19s     28:		if !dbm.IsKeyInDomain(item.Key, start, end) {
     120ms      120ms     29:			if entered {
         .          .     30:				break
         .          .     31:			}
         .          .     32:
         .          .     33:			continue
         .          .     34:		}
         .          .     35:
     820ms      980ms     36:		itemsInDomain = append(itemsInDomain, item)
         .          .     37:		entered = true
         .          .     38:	}
         .          .     39:
         .      1.21s     40:	return &memIterator{
         .          .     41:		start:     start,
         .          .     42:		end:       end,
         .          .     43:		items:     itemsInDomain,
         .          .     44:		ascending: ascending,
         .          .     45:	}

and given that the list only uses that type, it is only right to lift the
code from container/list.List, and only modify Element.Value's type.

For emphasis, the code is basically just a retrofit of
container/list/list.go but with a typing, and we'll keep it as is
until perhaps Go1.17 or Go1.18 or when everyone uses Go1.17+ after
generics have landed.

• After this commit

Total: 45.25s
ROUTINE ======================== github.com/cosmos/cosmos-sdk/store/cachekv.newMemIterator
in /Users/emmanuelodeke/go/src/github.com/cosmos/cosmos-sdk/store/cachekv/memiterator.go
     4.84s      6.77s (flat, cum) 14.96% of Total
         .          .     16:	items      []*kv.Pair
         .          .     17:	ascending  bool
         .          .     18:}
         .          .     19:
         .          .     20:func newMemIterator(start, end []byte, items *kv.List, ascending bool) *memIterator {
         .      330ms     21:	itemsInDomain := make([]*kv.Pair, 0, items.Len())
         .          .     22:
         .          .     23:	var entered bool
         .          .     24:
      60ms      160ms     25:	for e := items.Front(); e != nil; e = e.Next() {
     580ms      580ms     26:		item := e.Value
     3.68s      4.78s     27:		if !dbm.IsKeyInDomain(item.Key, start, end) {
      80ms       80ms     28:			if entered {
         .          .     29:				break
         .          .     30:			}
         .          .     31:
         .          .     32:			continue
         .          .     33:		}
         .          .     34:
     440ms      580ms     35:		itemsInDomain = append(itemsInDomain, item)
         .          .     36:		entered = true
         .          .     37:	}
         .          .     38:
         .      260ms     39:	return &memIterator{
         .          .     40:		start:     start,
         .          .     41:		end:       end,
         .          .     42:		items:     itemsInDomain,
         .          .     43:		ascending: ascending,
         .          .     44:	}

Fixes #8810

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #8811 (fd18e9f) into master (be23295) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #8811   +/-   ##
=======================================
  Coverage   59.45%   59.45%           
=======================================
  Files         563      563           
  Lines       31277    31277           
=======================================
  Hits        18597    18597           
  Misses      10518    10518           
  Partials     2162     2162           

Impacted Files	Coverage Δ
store/cachekv/memiterator.go	65.78% <100.00%> (ø)
store/cachekv/store.go	88.17% <100.00%> (ø)

[alessio]

ACK - pending linter fixes

[odeke-em]

The golint-ci error is just silly

  Error: (*List).move - result 0 (*github.com/cosmos/cosmos-sdk/types/kv.Element) is never used (unparam)

for code that looks like this

// move moves e to next to at and returns e.
func (l *List) move(e, at *Element) *Element {
        if e == at {
                return e
        }
        e.prev.next = e.next
        e.next.prev = e.prev

        e.prev = at
        e.next = at.next
        e.prev.next = e
        e.next.prev = e

        return e
}

just super pedantic and I'd consider that warning a bug on golint-ci itself, the official golint doesn't complain, plus this code is from the standard library.

[tac0turtle]

utACK.

Loving seeing this work land in the sdk. Excited to see more things.

[odeke-em]

Thank you for the reviews @alessio and @marbar3778. Indeed, more coming :-)

[fdymylja]

utACK, amazing work! it's always a pleasure to follow your detailed profiling :)

[odeke-em]

Thank you for the review @fdymylja, and for the kind words.

[odeke-em]

Could we perhaps backport this change?

[alessio]

I'm afraid it's too late for 0.42.0, it will surely make it into v0.42.1 too!

[robert-zaremba]

Good job. Wow, that's a big surprise to me. So the type assertion / cast was a bottleneck here?
It's kind of sad that we need to copy the whole file just to use a concrete type. I always thought that interfaces check is just a dereference + an integer comparison in the runtime.