bug#46256: [feature/native-comp] AOT eln files ignored if run from build tree

[Eli Zaretskii <eliz@gnu.org>]

> From: Andrea Corallo <akrl@sdf.org>
> Cc: pipcet@gmail.com, 46256@debbugs.gnu.org, andrewjmoreton@gmail.com
> Date: Tue, 09 Mar 2021 14:55:40 +0000
> 
> cc-*.el files for instance have more than one direct call to load.  IIRC
> one of the analyzed cases was cc-mode related (certanly one I observed).
> 
> >> > What are the reasons for unloading a .eln file which was once loaded
> >> > into a session?
> >>
> >> All the functions defined in it are not anymore reachable (read all
> >> symbols functions are makunbound or defined to some other function).
> >
> > That means someone did unload-feature, right?
> 
> Also loading for example a new version freshly compiled of the same file
> should present the same scenario.

I see a problem in this area.  Consider this code in
native-elisp-load:

  if (!NILP (Fgethash (filename, all_loaded_comp_units_h, Qnil))
      && !file_in_eln_sys_dir (filename)
      && !NILP (Ffile_writable_p (filename)))
    {
      /* If in this session there was ever a file loaded with this
	 name, rename it before loading, to make sure we always get a
	 new handle!  */
      Lisp_Object tmp_filename =
	Fmake_temp_file_internal (filename, Qnil, build_string (".eln.tmp"),
				  Qnil);
      if (NILP (Ffile_writable_p (tmp_filename)))
	comp_u->handle = dynlib_open (SSDATA (encoded_filename));
      else
	{
	  Frename_file (filename, tmp_filename, Qt);
	  comp_u->handle = dynlib_open (SSDATA (ENCODE_FILE (tmp_filename)));
	  Frename_file (tmp_filename, filename, Qnil);
	}

The last 'else' branch momentarily renames the .eln file, then loads
it under the modified name, with the assumption that this would force
dynlib_open to produce a new handle.  But in the case that the same
.eln file is loaded more than once, i.e. the .eln file was not
modified since the previous load, dynlib_open returns the same handle
regardless of the file name, at least on MS-Windows.  (Does this work
as intended on GNU/Linux?)

The problem with returning the same handle is that the refcount of the
handle is incremented, which means unload-feature will be unable to
unload the library.

Is this renaming dance for the case that the .eln file was updated
since the last load, or do we need it even if it wasn't updated?  If
the former, then I guess we could dynlib_close the handle immediately
if we discover that it's identical to the one we had from the previous
load.