From fb6aeaba2d3a38c7febd0a39cabd89685de11b9d Mon Sep 17 00:00:00 2001
From: Barrett Ruth <62671086+barrettruth@users.noreply.github.com>
Date: Wed, 22 Apr 2026 13:36:43 -0400
Subject: [PATCH] feat(eval): treat Lua string as "blob" in writefile() #39098

Problem:
vim.fn.writefile() treats Lua strings as Vimscript strings instead of a "binary clean" string.

Solution:
Treat Lua-originated strings as blob data.
---
 src/nvim/eval/fs.c                           | 23 ++++++++++++++++----
 src/nvim/lua/executor.c                      |  2 ++
 test/functional/vimscript/writefile_spec.lua |  6 +++++
 3 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/src/nvim/eval/fs.c b/src/nvim/eval/fs.c
index 1235d78783..7e4dc1801b 100644
--- a/src/nvim/eval/fs.c
+++ b/src/nvim/eval/fs.c
@@ -42,6 +42,7 @@
 #include "nvim/os/os.h"
 #include "nvim/path.h"
 #include "nvim/pos_defs.h"
+#include "nvim/runtime.h"
 #include "nvim/strings.h"
 #include "nvim/types_defs.h"
 #include "nvim/vim_defs.h"
@@ -1716,13 +1717,12 @@ write_list_error:
 /// @param[in]  blob  Blob to write.
 ///
 /// @return true on success, or false on failure.
-static bool write_blob(FileDescriptor *const fp, const blob_T *const blob)
+static bool write_data(FileDescriptor *const fp, const char *const data, const size_t len)
   FUNC_ATTR_NONNULL_ARG(1)
 {
   int error = 0;
-  const int len = tv_blob_len(blob);
   if (len > 0) {
-    const ptrdiff_t written = file_write(fp, blob->bv_ga.ga_data, (size_t)len);
+    const ptrdiff_t written = file_write(fp, data, len);
     if (written < (ptrdiff_t)len) {
       error = (int)written;
       goto write_blob_error;
@@ -1738,6 +1738,18 @@ write_blob_error:
   return false;
 }
 
+static bool write_blob(FileDescriptor *const fp, const blob_T *const blob)
+  FUNC_ATTR_NONNULL_ARG(1)
+{
+  return write_data(fp, blob->bv_ga.ga_data, (size_t)tv_blob_len(blob));
+}
+
+static bool write_string(FileDescriptor *const fp, const char *const data)
+  FUNC_ATTR_NONNULL_ARG(1)
+{
+  return write_data(fp, data, strlen(data));
+}
+
 /// "writefile()" function
 void f_writefile(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 {
@@ -1753,7 +1765,8 @@ void f_writefile(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
         return;
       }
     });
-  } else if (argvars[0].v_type != VAR_BLOB) {
+  } else if (argvars[0].v_type != VAR_BLOB
+             && !(argvars[0].v_type == VAR_STRING && script_is_lua(current_sctx.sc_sid))) {
     semsg(_(e_invarg2),
           _("writefile() first argument must be a List or a Blob"));
     return;
@@ -1823,6 +1836,8 @@ void f_writefile(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     bool write_ok;
     if (argvars[0].v_type == VAR_BLOB) {
       write_ok = write_blob(&fp, argvars[0].vval.v_blob);
+    } else if (argvars[0].v_type == VAR_STRING) {
+      write_ok = write_string(&fp, argvars[0].vval.v_string);
     } else {
       write_ok = write_list(&fp, argvars[0].vval.v_list, binary);
     }
diff --git a/src/nvim/lua/executor.c b/src/nvim/lua/executor.c
index 1771a01565..02a95d5f6d 100644
--- a/src/nvim/lua/executor.c
+++ b/src/nvim/lua/executor.c
@@ -1366,12 +1366,14 @@ int nlua_call(lua_State *lstate)
   funcexe.fe_firstline = curwin->w_cursor.lnum;
   funcexe.fe_lastline = curwin->w_cursor.lnum;
   funcexe.fe_evaluate = true;
+  const sctx_T save_current_sctx = api_set_sctx(LUA_INTERNAL_CALL);
 
   TRY_WRAP(&err, {
     // call_func() retval is deceptive, ignore it.  Instead we set `msg_list`
     // (TRY_WRAP) to capture abort-causing non-exception errors.
     (void)call_func(name, (int)name_len, &rettv, nargs, vim_args, &funcexe);
   });
+  current_sctx = save_current_sctx;
 
   if (!ERROR_SET(&err)) {
     nlua_push_typval(lstate, &rettv, 0);
diff --git a/test/functional/vimscript/writefile_spec.lua b/test/functional/vimscript/writefile_spec.lua
index b81cb43691..f406e5a469 100644
--- a/test/functional/vimscript/writefile_spec.lua
+++ b/test/functional/vimscript/writefile_spec.lua
@@ -6,6 +6,7 @@ local clear = n.clear
 local eq = t.eq
 local fn = n.fn
 local api = n.api
+local exec_lua = n.exec_lua
 local read_file = t.read_file
 local write_file = t.write_file
 local pcall_err = t.pcall_err
@@ -99,6 +100,11 @@ describe('writefile()', function()
     eq('a\0', read_file(fname))
   end)
 
+  it('writes Lua strings to a file', function()
+    eq(0, exec_lua([[return vim.fn.writefile('foo\0bar', ..., 'b')]], fname))
+    eq('foo\0bar', read_file(fname))
+  end)
+
   it('shows correct file name when supplied numbers', function()
     api.nvim_set_current_dir(dname)
     eq(