tests/modules/performance/byte-compile-lua: improve bytecode detection

Previously, to determine if a file is byte-compiled, a simple binary
file detection was used, specifically checking if it contained any null
bytes. This commit updates the check to read the file header and compare
it with a known LuaJIT header.

tests/platforms/hm-extra-files-byte-compiling.nix

@@ -57,17 +57,19 @@ let
     }).config.home-files;
 in
 pkgs.runCommand "home-manager-extra-files-byte-compiling" { } ''
-  is_binary() {
+  is_byte_compiled() {
-      ! grep -qI . "$1"
+      # LuaJIT bytecode header is: ESC L J version
+      # https://github.com/LuaJIT/LuaJIT/blob/v2.1/src/lj_bcdump.h
+      [[ $(head -c3 "$1") = $'\x1bLJ' ]]
   }
   test_byte_compiled() {
-      if ! is_binary "$home_files/.config/nvim/$1"; then
+      if ! is_byte_compiled "$home_files/.config/nvim/$1"; then
           echo "File $1 is expected to be byte compiled, but it's not"
           exit 1
       fi
   }
   test_not_byte_compiled() {
-      if is_binary "$home_files/.config/nvim/$1"; then
+      if is_byte_compiled "$home_files/.config/nvim/$1"; then
           echo "File $1 is not expected to be byte compiled, but it is"
           exit 1
       fi

tests/test-sources/modules/performance/byte-compile-lua.nix

@@ -1,27 +1,27 @@
 { pkgs, ... }:
 let
-  isByteCompiledFun = ''
+  isByteCompiledFun = # lua
-    local function is_byte_compiled(filename)
+    ''
-      local f = assert(io.open(filename, "rb"))
+      -- LuaJIT bytecode header is: ESC L J version
-      local data = assert(f:read("*a"))
+      -- https://github.com/LuaJIT/LuaJIT/blob/v2.1/src/lj_bcdump.h
-      -- Assume that file is binary if it contains null bytes
+      -- We are comparing the first three bytes of the file (until version)
-      for i = 1, #data do
+      local expected_header = string.char(0x1b, 0x4c, 0x4a)
-        if data:byte(i) == 0 then
+      local function is_byte_compiled(filename)
-          return true
+        local f = assert(io.open(filename, "rb"))
+        local data = assert(f:read(3))
+        f:close()
+        return data == expected_header
+      end
+
+      local function test_rtp_file(name, is_compiled)
+        local file = assert(vim.api.nvim_get_runtime_file(name, false)[1], "file " .. name .. " not found in runtime")
+        if is_compiled then
+          assert(is_byte_compiled(file), name .. " is expected to be byte compiled, but it's not")
+        else
+          assert(not is_byte_compiled(file), name .. " is not expected to be byte compiled, but it is")
         end
       end
-      return false
+    '';
-    end
-
-    local function test_rtp_file(name, is_compiled)
-      local file = assert(vim.api.nvim_get_runtime_file(name, false)[1], "file " .. name .. " not found in runtime")
-      if is_compiled then
-        assert(is_byte_compiled(file), name .. " is expected to be byte compiled, but it's not")
-      else
-        assert(not is_byte_compiled(file), name .. " is not expected to be byte compiled, but it is")
-      end
-    end
-  '';
 in
 {
   default =