fix(api): fail fast if has_lua_imp retval looks wrong #39866

src/gen/gen_api_dispatch.lua

@@ -708,6 +708,9 @@ output:write('\n')
 --- @type {binding: string, api:string}[]
 local lua_c_functions = {}
 
+--- Functions which use kRetMultiStack.
+local lua_retstack = { nvim_buf_call = true, nvim_win_call = true }
+
 --- Generates C code to bridge RPC API <=> Lua.
 ---
 --- Inspect the result here:
@@ -844,6 +847,9 @@ local function process_function(fn)
   end
 
   write_shifted_output('    ENTER_LUA_ACTIVE_STATE(lstate);\n')
+  if fn.has_lua_imp and not lua_retstack[fn.name] then
+    write_shifted_output('    const int pretop = lua_gettop(lstate); (void)pretop;\n')
+  end
   local free_at_exit_code = ''
   for i = 1, #free_code do
     local rev_i = #free_code - i + 1
@@ -882,8 +888,14 @@ exit_0:
     local ret_type = real_type(fn.return_type)
     local ret_mode = (ret_type == 'Object') and '&' or ''
     if fn.has_lua_imp then
-      -- it is up to the function to push return values
-      write_shifted_output('    (void)ret;')
+      -- Most has_lua_imp functions are expected to produce a single retval (e.g.
+      -- nvim_buf_get_lines, but not kRetMultiStack callers such as nvim_buf_call). #39851
+      if not lua_retstack[fn.name] then
+        write_shifted_output(
+          '    assert((ERROR_SET(&err) || lua_gettop(lstate) == pretop + 1) && "has_lua_imp function must push exactly one return value");\n'
+        )
+      end
+      write_shifted_output('    (void)ret;\n')
     elseif ret_type:match('^KeyDict_') then
       write_shifted_output('    nlua_push_keydict(lstate, &ret, %s_table);\n', return_type:sub(9))
     else

src/nvim/lua/executor.c

@@ -1808,6 +1808,9 @@ static int mode_ret(LuaRetMode mode)
 Object nlua_call_ref_ctx(bool fast, LuaRef ref, const char *name, Array args, LuaRetMode mode,
                          Arena *arena, Error *err)
 {
+  // Use active_lstate (set by ENTER_LUA_ACTIVE_STATE in gen_api_dispatch.lua) so callbacks run on
+  // the caller's Lua thread. This matters for coroutines: with global_lstate, return values pushed
+  // by kRetMultiStack would land on the main thread's stack, instead of coroutine stack.
   lua_State *const lstate = active_lstate;
   int top = lua_gettop(lstate);
   nlua_pushref(lstate, ref);