From b2d471ab337e56f660eb7c89ae24f447f7b7a165 Mon Sep 17 00:00:00 2001
From: Gregory Anders <8965202+gpanders@users.noreply.github.com>
Date: Thu, 7 Dec 2023 08:01:27 -0800
Subject: [PATCH] fix(lua): allow nil values in serialized Lua arrays (#26329)

When we convert a Lua table to an Object, we consider the table a
"dictionary" if it contains only string keys, and an array if it
contains all numeric indices with no gaps. While rare, Lua tables can
have both strictly numeric indices and gaps (e.g. { [2] = 2 }). These
currently cannot be serialized because it is not considered an array.

However, we know the maximum index of the table and as long as all of
the keys in the table are numeric, it is still possible to serialize
this table as an array. The missing indices will have nil values.
---
 src/nvim/lua/converter.c         | 6 +-----
 test/functional/lua/api_spec.lua | 4 ++++
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/nvim/lua/converter.c b/src/nvim/lua/converter.c
index fd2bdbd677..ca0be28fac 100644
--- a/src/nvim/lua/converter.c
+++ b/src/nvim/lua/converter.c
@@ -148,7 +148,7 @@ static LuaTableProps nlua_traverse_table(lua_State *const lstate)
     }
   } else {
     if (tsize == 0
-        || (tsize == ret.maxidx
+        || (tsize <= ret.maxidx
             && other_keys_num == 0
             && ret.string_keys_num == 0)) {
       ret.type = kObjectTypeArray;
@@ -1129,10 +1129,6 @@ Object nlua_pop_Object(lua_State *const lstate, bool ref, Error *const err)
         }
         const size_t idx = cur.obj->data.array.size++;
         lua_rawgeti(lstate, -1, (int)idx + 1);
-        if (lua_isnil(lstate, -1)) {
-          lua_pop(lstate, 2);
-          continue;
-        }
         kvi_push(stack, cur);
         cur = (ObjPopStackItem) {
           .obj = &cur.obj->data.array.items[idx],
diff --git a/test/functional/lua/api_spec.lua b/test/functional/lua/api_spec.lua
index d808693a9e..8c03eb60ec 100644
--- a/test/functional/lua/api_spec.lua
+++ b/test/functional/lua/api_spec.lua
@@ -245,4 +245,8 @@ describe('luaeval(vim.api.…)', function()
     eq('', funcs.luaeval('vim.api.nvim_replace_termcodes("", 0, 1.5, "test")'))
     eq('', funcs.luaeval('vim.api.nvim_replace_termcodes("", true, {}, {[vim.type_idx]=vim.types.array})'))
   end)
+
+  it('serializes sparse arrays in Lua', function()
+    eq({ [1] = vim.NIL, [2] = 2 }, exec_lua [[ return { [2] = 2 } ]])
+  end)
 end)