[CCing Philipp as the author of module assertions.]
       
Eli Zaretskii <eliz@gnu.org> writes:

>> From: "Basil L. Contovounesios" <contovob@tcd.ie>
>> Date: Mon, 25 Feb 2019 21:00:41 +0000
>> 
>> Starting program: /home/blc/.local/src/emacs26/src/emacs -Q --module-assertions
>> [Thread debugging using libthread_db enabled]
>> Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
>> [New Thread 0x7ffff01cb700 (LWP 8299)]
>> [New Thread 0x7fffef9ac700 (LWP 8300)]
>> [New Thread 0x7fffef1ab700 (LWP 8301)]
>> 
>> Thread 1 "emacs" received signal SIGSEGV, Segmentation fault.
>> re_search_2 (bufp=0xbf5d00 <searchbufs+384>, str1=0x0, size1=0, str2=0x0, size2=18, startpos=0, 
>>     range=18, regs=0x0, stop=18) at regex.c:4354
>> 4354				buf_ch = STRING_CHAR_AND_LENGTH (d, buf_charlen);
>> #0  0x0000000000608594 in re_search_2
>>     (bufp=0xbf5d00 <searchbufs+384>, str1=0x0, size1=0, str2=0x0, size2=18, startpos=0, range=18, regs=0x0, stop=18) at regex.c:4354
>>         buf_charlen = 0
>>         irange = 18
>>         lim = 0
>>         d = 0x0
>>         buf_ch = 18
>>         val = 691541629
>>         string1 = 0x0
>>         string2 = 0x0
>>         fastmap = 0xbf5d38 <searchbufs+440> ""
>>         translate = make_number(0)
>>         total_size = 18
>>         endpos = 18
>>         anchored_start = 0 '\000'
>>         multibyte = 1 '\001'
>> #1  0x0000000000607f91 in re_search
>>     (bufp=0xbf5d00 <searchbufs+384>, string=0x0, size=18, startpos=0, range=18, regs=0x0)
>>     at regex.c:4181
>> #2  0x00000000005f3fd0 in fast_string_match_internal
>>     (regexp=XIL(0x8c761c), string=XIL(0x3036ec4), table=XIL(0)) at search.c:485
>>         val = 140737488336288
>>         bufp = 0xbf5d00 <searchbufs+384>
>
> Here's your problem: fast_string_match_internal got a Lisp
> string=XIL(0x3036ec4), but its data passed to re_search as the 2nd arg
> is a NULL pointer.  You need to find out how this happens, e.g. by
> setting a watchpoint on string's data inside Ffile_name_as_directory.
> Or maybe the string is already corrupted there?

The file name string is already corrupted when Ffile_name_as_directory
is called; see below.

First, I rewrote the dynamic module[1] to add nonlocal exit checks after
almost every call to the module API.

[1]: https://gitlab.com/basil-conto/realpath

I also added the following assertions to src/emacs-module.c: