bug#48342: native-comp emacs gets into an infinite loop at startup if no .el files are available

[Andrea Corallo]

Dima Kogan <dima@secretsauce.net> writes:

> Hi. I maintain Debian packages of bleeding-edge emacs builds:
>
>   http://emacs.secretsauce.net/
>
> Recently I enabled the native-comp flags for these packages. The results
> mostly work, but a user sent me a bug report whose cause I just tracked
> down. Some of this is probably my fault, as the packager, but there's an
> emacs bug here too.
>
> To tickle the bug the user needs to install the emacs-snapshot package,
> but NOT the emacs-snapshot-el package. This results in the .elc files
> being shipped, but NOT the .el files. This is a valid way to do it
> without the native-comp patches, and the Debian emacs packages have
> allowed this since forever.
>
> If you install emacs like this, this happens:
>
>   $ emacs-snapshot -Q -batch -f batch-native-compile empty.el
>
>   Fatal error 11: Segmentation fault
>
> The C backtrace is 6033 frames long, which can't be good. xbacktrace
> says this:
>
>   (gdb) xbacktrace
>   "display-warning" (0xffebcfd8)
>   "display-warning" (0xffebd5d8)
>   .... lots more "display-warning"
>   "locate-file" (0xffffe498)
>   "command-line" (0xffffe5a0)
>   "normal-top-level" (0xffffe640)
>
> And the warning itself is:
>
>   "Cannot look-up eln file as no source file was found for 
> /usr/share/emacs/28.0.50/lisp/emacs-lisp/warnings.elc"
>
> So something somewhere wanted to throw a warning, and this warning tried
> to find its own sources, couldn't do it, and threw another warning. And
> so on.
>
> So first off, it'd be great if emacs could handle this without such a
> recursive failure mode. It took me a long time to figure out what's
> happening, and a plain-text error message on the console would have been
> nice.

Hi Dima,

could you share the Lisp backtrace?  If you have loaded the .gdbinit
shipped with the repot this will be at the bottom of the gdb backtrace.

> And second: how should this be packaged? Is shipping the .el files a
> hard requirement now?

Yes if you want the native compiler to be able to compile files,
otherwise you should either native compile all lisp files Ahead of Time
or set `comp-deferred-compilation' to nil in early init so that Emacs
will not try to native compile bytecode being loaded.

Thanks

  Andrea