Allow writing to hidden files on Windows (#1653)

* feat(system): add system.ftruncate

* fix: EPERM writing to hidden files on Windows

* chore(doc): fix capitalization

* refactor(system): make ftruncate length optional

* refactor(doc): don't specify length

data/core/doc/init.lua

@@ -103,7 +103,23 @@ function Doc:save(filename, abs_filename)
   else
     assert(self.filename or abs_filename, "calling save on unnamed doc without absolute path")
   end
-  local fp = assert(io.open(filename, "wb"))
+
+  local fp
+  if PLATFORM == "Windows" then
+    -- On Windows, opening a hidden file with wb fails with a permission error.
+    -- To get around this, we must open the file as r+b and truncate.
+    -- Since r+b fails if file doesn't exist, fall back to wb.
+    fp = io.open(filename, "r+b")
+    if fp then
+      system.ftruncate(fp)
+    else
+      -- file probably doesn't exist, create one
+      fp = assert ( io.open(filename, "wb") )
+    end
+  else
+    fp = assert ( io.open(filename, "wb") )
+  end
+
   for _, line in ipairs(self.lines) do
     if self.crlf then line = line:gsub("\n", "\r\n") end
     fp:write(line)

docs/api/system.lua

@@ -190,6 +190,15 @@ function system.rmdir(path) end
 ---@param path string
 function system.chdir(path) end
 
+---
+---Truncates a file to a set length.
+---
+---@param file file* A file handle returned by io.open().
+---@param length integer? Number of bytes to truncate to. Defaults to 0.
+---@return boolean success True if the operation suceeded, false otherwise
+---@return string? message An error message if the operation failed.
+function system.ftruncate(file, length) end
+
 ---
 ---Create a new directory, note that this function doesn't recursively
 ---creates the directories on the given path.

src/api/system.c

@@ -27,6 +27,9 @@
   #if !defined(S_ISDIR) && defined(S_IFMT) && defined(S_IFDIR)
     #define S_ISDIR(m) (((m) & S_IFMT) == S_IFDIR)
   #endif
+
+  #define fileno _fileno
+  #define ftruncate _chsize
 #else
 
 #include <dirent.h>
@@ -847,6 +850,26 @@ static int f_get_fs_type(lua_State *L) {
 }
 
 
+static int f_ftruncate(lua_State *L) {
+#if LUA_VERSION_NUM < 503
+  // note: it is possible to support pre 5.3 and JIT
+  //       since file handles are just FILE*  wrapped in a userdata;
+  //       but it is not standardized. YMMV.
+  #error luaL_Stream is not supported in this version of Lua.
+#endif
+  luaL_Stream *stream = luaL_checkudata(L, 1, LUA_FILEHANDLE);
+  lua_Integer len = luaL_optinteger(L, 2, 0);
+  if (ftruncate(fileno(stream->f), len) != 0) {
+    lua_pushboolean(L, 0);
+    lua_pushfstring(L, "ftruncate(): %s", strerror(errno));
+    return 2;
+  }
+
+  lua_pushboolean(L, 1);
+  return 1;
+}
+
+
 static int f_mkdir(lua_State *L) {
   const char *path = luaL_checkstring(L, 1);
 
@@ -1285,6 +1308,7 @@ static const luaL_Reg lib[] = {
   { "get_fs_type",           f_get_fs_type           },
   { "text_input",            f_text_input            },
   { "setenv",                f_setenv                },
+  { "ftruncate",             f_ftruncate             },
   { NULL, NULL }
 };