spanset: fix bug in mergeSpans

Epic: none
Release note: none

Author: pav-kv

pkg/kv/kvserver/spanset/merge.go

@@ -78,8 +78,9 @@ func mergeSpans(latches []Span) []Span {
 				// [a, b] merge [b, nil]
 				if cur.Timestamp != prev.Timestamp {
 					r = append(r, cur)
+				} else {
+					prev.EndKey = cur.Key.Next()
 				}
-				prev.EndKey = cur.Key.Next()
 			} else {
 				// [a, c] merge [b, nil]
 				if cur.Timestamp != prev.Timestamp {

pkg/kv/kvserver/spanset/merge_test.go

@@ -47,7 +47,7 @@ func TestMergeSpans(t *testing.T) {
 		{"a@10,a-b@10", "a-b@10"},
 		{"a@10,a-b@20", "a@10,a-b@20"},
 		{"a-b@20,b@20", "a-b\x00@20"},
-		{"a-b@20,b@30", "a-b\x00@20,b@30"}, // FIXME: this is a bug
+		{"a-b@20,b@30", "a-b@20,b@30"},
 		{"a-c@20,m-n@10,c-o@10,o-z@20", "a-c@20,c-o@10,o-z@20"},
 	} {
 		t.Run("", func(t *testing.T) {