bug#53164: (After an ELC+ELN build, don't load the source file into a buffer.) fixed.

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 10 Jan 2022 19:34:58 +0000
> Cc: 53164@debbugs.gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > FTR, I didn't see any build problems due to this issue, not even
> > once.  So I wonder what exactly did you see and in what scenario.
> > Please elaborate.
> 
> It took me practically three days solid to diagnose this bug.  The
> symptom, whilst bootstrapping the scratch/correct-warning-pos, was the
> following message on stderr:
> 
>     Error: (invalid-function #<symbol = at 18944>)
> 
> ..  I quickly tracked down the source of the `=' symbol to lisp/subr.el,
> in the code for `zerop'.  Further progress was elusive.  In the course
> of debugging it, I ended up writing my own backtrace routine, free of
> dependencies, which can work early in the bootstrap, unlike the standard
> backtrace.
> 
> The course of events which led to that error message is this:
> 
> 1. During the bootstrap with native compile, ELC+ELN compiles subr.el.
>   It does so by getting the LAP code from bytecomp.el via a side
>   channel.  It completes the compilation.
> 2. bootstrap-emacs now contains the native-compiled version of subr.el.
>   Unfortunately, the macro `zerop--anon-cmacro' a complier macro for
>   `zerop', still contains symbols with position.
> 3. Having finished the native compilation, ELC+ELN visited subr.el in a
>   buffer, due to this bug.
> 4. As part of visiting subr.el, Emacs calls after-find-file, which
>   invokes find-file-hook.
> 5. One of the entries in find-file-hook is vc-refresh-state, which gets
>   called.
> 6. This causes vc-git.elc to be loaded.  Eventually, vc-git--out-ok gets
>   called.
> 7. vc-git--out-ok invokes `zerop', or more precisely the code generated
>   by the macro `zerop--anon-cmacro'.  This contains #<symbol = at
>   18944>.
> 8. eval signals an error, since symbols-with-pos-enabled is nil and it
>   thus can't handle the symbol with position.  This error gets blocked
>   from reaching a backtrace function by an inconsiderate condition-case,
>   which just dumps the message onto stderr.
> 
> The most obvious cause of the error seems to be at step 3, where
> bootstrap-emacs spuriously visits the source file.

After reading this, I still don't understand how come you bump into
this problem, whereas I don't, and neither does anyone else who builds
the release branch with native-compilation.  Is this something
specific to that branch you are working on?  If so, why is it urgent
to have the fix on the release branch?  The branch on which you ware
working will land on master.

> As I said above, please consider putting the fix to this bug back.  I do
> not want anybody else to have to go through what I had to to track the
> bug down.  Leaving stale arguments on the command line, later to be
> visited, cannot possibly be the Right Thing.  I'm pretty sure it was not
> done deliberately, it was just a minor oversight which didn't seem to
> matter.

There's always one more bug.  When we are so far into the pretest
process, problems that take unusual steps to reproduce are not
important enough to delay the release.  The judgment call I need to
make is how important is it to have this in the release branch, and
that's only after I understand how to trigger the problem in the first
place.

Thanks.