fix: include pagination.key at reverse mode (backport #20939) (#20954)

Co-authored-by: beer-1 <147697694+beer-1@users.noreply.github.com>
Co-authored-by: Facundo <facundomedica@gmail.com>
Co-authored-by: Julien Robert <julien@rbrt.fr>

CHANGELOG.md

@@ -38,6 +38,10 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ## [Unreleased]
 
+## Bug Fixes
+
+* [#20939](https://github.com/cosmos/cosmos-sdk/pull/20939) Fix collection reverse iterator to include `pagination.key` in the result.
+
 ## [v0.50.8](https://github.com/cosmos/cosmos-sdk/releases/tag/v0.50.8) - 2024-07-15
 
 ## Features

types/query/collections_pagination.go

@@ -321,11 +321,11 @@ func encodeCollKey[K, V any, C Collection[K, V]](coll C, key K) ([]byte, error)
 func getCollIter[K, V any, C Collection[K, V]](ctx context.Context, coll C, prefix, start []byte, reverse bool) (collections.Iterator[K, V], error) {
 	// TODO: maybe can be simplified
 	if reverse {
-		var end []byte
-		if prefix != nil {
-			start = storetypes.PrefixEndBytes(append(prefix, start...))
-			end = prefix
-		}
+		// if we are in reverse mode, we need to increase the start key
+		// to include the start key in the iteration.
+		start = storetypes.PrefixEndBytes(append(prefix, start...))
+		end := prefix
+
 		return coll.IterateRaw(ctx, end, start, collections.OrderDescending)
 	}
 	var end []byte

types/query/collections_pagination_test.go

@@ -83,6 +83,16 @@ func TestCollectionPagination(t *testing.T) {
 			},
 			expResults: createResults(299, 200),
 		},
+		"with key and reverse": {
+			req: &PageRequest{
+				Key:     encodeKey(199),
+				Reverse: true,
+			},
+			expResp: &PageResponse{
+				NextKey: encodeKey(99),
+			},
+			expResults: createResults(199, 100),
+		},
 		"with offset and count total": {
 			req: &PageRequest{
 				Offset:     50,