fix(store): handle nil end in cachekv/iterator (#12945)

## Description

Closes: #12661 

Adds support for nil end semantics to iterators in cachekv store, addressing [this workaround](https://github.com/osmosis-labs/osmosis/blob/4176b287d48338870bfda3029bfa20a6e45ac126/osmoutils/store_helper.go#L37-L41).  

Note that this has the effect of sorting the dirty cache into the BTree cache store in the bounds `[startIndex, end-of-cache-space]`



---

### Author Checklist

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please add links to any relevant follow up issues.*

I have...

- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#pr-targeting))
- [x] provided a link to the relevant issue or specification
- [x] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/main/docs/building-modules)
- [x] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [x] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

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your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

CHANGELOG.md

@@ -122,6 +122,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * (x/group) [#12888](https://github.com/cosmos/cosmos-sdk/pull/12888) Fix event propagation to the current context of `x/group` message execution `[]sdk.Result`.
 * (sdk/dec_coins) [#12903](https://github.com/cosmos/cosmos-sdk/pull/12903) Fix nil `DecCoin` creation when converting `Coins` to `DecCoins`
 * (x/upgrade) [#12906](https://github.com/cosmos/cosmos-sdk/pull/12906) Fix upgrade failure by moving downgrade verification logic after store migration.
+* (store) [#12945](https://github.com/cosmos/cosmos-sdk/pull/12945) Fix nil end semantics in store/cachekv/iterator when iterating a dirty cache.
 
 ### Deprecated
 

store/cachekv/store.go

@@ -281,7 +281,7 @@ const (
 // Constructs a slice of dirty items, to use w/ memIterator.
 func (store *Store) dirtyItems(start, end []byte) {
 	startStr, endStr := conv.UnsafeBytesToStr(start), conv.UnsafeBytesToStr(end)
-	if startStr > endStr {
+	if end != nil && startStr > endStr {
 		// Nothing to do here.
 		return
 	}
@@ -296,6 +296,7 @@ func (store *Store) dirtyItems(start, end []byte) {
 	// than just not having the cache.
 	if n < 1024 {
 		for key := range store.unsortedCache {
+			// dbm.IsKeyInDomain is nil safe and returns true iff key is greater than start
 			if dbm.IsKeyInDomain(conv.UnsafeStrToBytes(key), start, end) {
 				cacheValue := store.cache[key]
 				unsorted = append(unsorted, &kv.Pair{Key: []byte(key), Value: cacheValue.value})
@@ -316,13 +317,18 @@ func (store *Store) dirtyItems(start, end []byte) {
 	// Now find the values within the domain
 	//  [start, end)
 	startIndex := findStartIndex(strL, startStr)
-	endIndex := findEndIndex(strL, endStr)
+	if startIndex < 0 {
+		startIndex = 0
+	}
 
-	if endIndex < 0 {
+	var endIndex int
+	if end == nil {
 		endIndex = len(strL) - 1
+	} else {
+		endIndex = findEndIndex(strL, endStr)
 	}
-	if startIndex < 0 {
-		startIndex = 0
+	if endIndex < 0 {
+		endIndex = len(strL) - 1
 	}
 
 	kvL := make([]*kv.Pair, 0)

store/cachekv/store_test.go

@@ -306,6 +306,50 @@ func TestCacheKVMergeIteratorRandom(t *testing.T) {
 	}
 }
 
+func TestNilEndIterator(t *testing.T) {
+	const SIZE = 3000
+
+	tests := []struct {
+		name       string
+		write      bool
+		startIndex int
+		end        []byte
+	}{
+		{name: "write=false, end=nil", write: false, end: nil, startIndex: 1000},
+		{name: "write=false, end=nil; full key scan", write: false, end: nil, startIndex: 2000},
+		{name: "write=true, end=nil", write: true, end: nil, startIndex: 1000},
+		{name: "write=false, end=non-nil", write: false, end: keyFmt(3000), startIndex: 1000},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			st := newCacheKVStore()
+
+			for i := 0; i < SIZE; i++ {
+				kstr := keyFmt(i)
+				st.Set(kstr, valFmt(i))
+			}
+
+			if tt.write {
+				st.Write()
+			}
+
+			itr := st.Iterator(keyFmt(tt.startIndex), tt.end)
+			i := tt.startIndex
+			j := 0
+			for itr.Valid() {
+				require.Equal(t, keyFmt(i), itr.Key())
+				require.Equal(t, valFmt(i), itr.Value())
+				itr.Next()
+				i++
+				j++
+			}
+
+			require.Equal(t, SIZE-tt.startIndex, j)
+		})
+	}
+}
+
 //-------------------------------------------------------------------------------------------
 // do some random ops