Subject: Re: bug#58601: 29.0.50; Infinite loop in byte-compile--first-symbol-with-pos

From: Alan Mackenzie <[email-masked]>
Date: Tue, 18 Oct 2022 15:01:49 +0000

Hello, Basil.

On Tue, Oct 18, 2022 at 02:24:02 +0300, Basil L. Contovounesios wrote:
> Given this sample input file:

[ .... ]

> Byte-compiling it as follows hangs indefinitely:
  
>   emacs -Q -batch -f batch-byte-compile my.el

> While it's hanging, might as well poke it:

>   pkill -f 'batch-byte-compile my\.el' -SIGUSR2  

> To which it yields the following backtrace:

[ .... ]

> I.e. the 'pure' compile-time form has a cycle, and the DFS of
> byte-compile--first-symbol-with-pos gets lost.

What's a DFS?

> Paraphrasing Stefan, it's acceptable if the compiler mishandles
> circular source code, but it should be able to handle circular data.

The problem here seems to be feeding macroexp-warn-and-return with data
as the FORM argument.  FORM is intended only to be an executable lisp
form.  The doc string, perhaps, could be clearer on this.

> I don't know why, but I can reproduce the hang only when the form is inside
> ert-deftest+should, and not inside a plain defun.

There is a huge concentration of "advanced" features inside those ~20
lines of Lisp code.  There's eval-and-compile, nconc, a compiler-macro,
and ert.  ;-)

> Perhaps byte-compile--first-symbol-with-pos needs to employ something
> like byte-run--ssp-seen?  Or does ERT somehow come into play?

It wouldn't be difficult, just tedious, to add checking for circular
lists into byte-compile--first-symbol-with-pos.  But a circular list is
an invalid argument for FORM in macorexp-warn-and-return, see above.
There are surely lots of places in Emacs where feeding a circular list as
an argument to a function will cause a hang.

At the moment, I'm not in favour of doing anything here.  I don't think
there's a bug.

> Thanks,

> -- 
> Basil

-- 
Alan Mackenzie (Nuremberg, Germany).