bug#34655: 26.1.92; Segfault in module with --module-assertions

[Basil L. Contovounesios]

gdb.txt
Description: GDB log

I have written a dynamic module which provides a function
realpath-truename similar to file-truename, but which uses
canonicalize_file_name(3) internally and which doesn't respect file name
handlers.  It is hosted at the following URL:

https://gitlab.com/basil-conto/realpath

Repeatedly calling realpath-truename on a directory name in an Emacs
started with --module-assertions segfaults in a call to
file-name-as-directory.  The precise recipe I am using is the following:

0. git clone https://gitlab.com/basil-conto/realpath.git
1. cd realpath
2. make
3. cd /path/to/emacs/src
4. gdb emacs
5. set logging on
6. run -Q --module-assertions
7. (progn (module-load "/path/to/realpath.so")
          (dotimes (_ 1000)
            (realpath-truename user-emacs-directory)))
   C-j
9. bt full

I attach the corresponding GDB log.

I am not confident that the module is written properly, in particular
w.r.t. nonlocal exits, but I don't see what, if anything, I am doing
wrong, so any help would be greatly appreciated.

Thanks,

-- 
Basil

In GNU Emacs 26.1.92 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
 of 2019-02-25 built on thunk
Repository revision: 1dff09739346037a588a3b9290800c09a9b3409a
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description:     Debian GNU/Linux buster/sid

Configured using:
 'configure 'CC=ccache gcc' 'CFLAGS=-O0 -g3 -ggdb -gdwarf-4 -pipe'
 --config-cache --prefix=/home/blc/.local --program-suffix=26
 --enable-checking=yes,glyphs --enable-check-lisp-object-type
 --with-mailutils --with-x-toolkit=lucid --with-modules
 --with-file-notification=yes --with-x'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS
GLIB NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT
ZLIB TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM MODULES THREADS LIBSYSTEMD
LCMS2

Important settings:
  value of $LANG: en_IE.UTF-8
  locale-coding-system: utf-8-unix