bug#70914: 29.3; Crashes often on Windows

[Eli Zaretskii <eliz@gnu.org>]

> From: Simen Endsjø <simendsjo@gmail.com>
> Date: Fri, 24 May 2024 12:07:23 +0200
> Cc: 70914@debbugs.gnu.org
> 
> I still get these. Should I open a new issue for this as the bug
> report has changed towards the path issue?
> The path issue was only visible to me when debugging in gdb, so these
> crashes (my original case) is still ongoing.

Are you running with the source of parse_root fixed, as in the current
emacs-29 branch?  If not, please rebuild Emacs with that patch before
doing anything else.  If you want, you install the patch locally, see
the patch below.

> #0  0x00007ff9672dd313 in KERNELBASE!DebugBreak () from
> C:\WINDOWS\System32\KernelBase.dll
> #1  0x00007ff719eb0f28 in emacs_abort ()
> #2  0x00007ff719d7a389 in terminate_due_to_signal ()
> #3  0x00007ff719d9b249 in deliver_fatal_thread_signal ()
> #4  0x00007ff719f16672 in _gnu_exception_handler
> (exception_data=0xb3a4dfb400) at
> C:/M/B/src/mingw-w64/mingw-w64-crt/crt/crt_handler.c:213
> #5  0x00007ff968bbb248 in msvcrt!__C_specific_handler () from
> C:\WINDOWS\System32\msvcrt.dll
> #6  0x00007ff969b549ff in ntdll!.chkstk () from C:\WINDOWS\SYSTEM32\ntdll.dll
> #7  0x00007ff969ace466 in ntdll!RtlFindCharInUnicodeString () from
> C:\WINDOWS\SYSTEM32\ntdll.dll
> #8  0x00007ff969b539ee in ntdll!KiUserExceptionDispatcher () from
> C:\WINDOWS\SYSTEM32\ntdll.dll
> #9  0x00007ff719dda17d in rpl_re_compile_pattern ()
> #10 0x00007ff719dcaf10 in compile_pattern ()
> #11 0x00007ff719dcb34d in looking_at_1 ()
> #12 0x00007ff719e10032 in Ffuncall ()
> #13 0x00007ff8e7591da7 in
> F73702d736b69702d666f72776172642d746f2d73796d626f6c_sp_skip_forward_to_symbol_0
> ()
>    from d:\.emacs.d\.local\cache\eln\29.3-2771a4de\smartparens-7ac9a6ec-f08b49fa.eln

This seems like a different issue, so if rebuilding with the patch
below doesn't help, please do open a new issue.  And I will tell you
already what we need to know for investigating: the arguments to
looking_at_1.  You should be able to show them like this:

  (gdb) frame 11
  (gdb) p string
  (gdb) xstring

The "frame 11" above assumes that the call to looking_at_1 is at
call-stack frame #11, as in the above backtraces; if not, adjust the
number accordingly.  The command "xstring" is defined in src/.gdbinit,
so if GDB says it doesn't know about it, do

  (gdb) source /path/to/emacs/src/.gdbinit

and then repeat the above commands.

Here's the patch which prevents crashes due to "//" file names:

diff --git a/src/w32.c b/src/w32.c
index d463962..a78d556 100644
--- a/src/w32.c
+++ b/src/w32.c
@@ -2572,7 +2572,7 @@ parse_root (const char * name, const char ** pPath)
       name += 2;
       do
         {
-	  if (IS_DIRECTORY_SEP (*name) && --slashes == 0)
+	  if (!*name || (IS_DIRECTORY_SEP (*name) && --slashes == 0))
 	    break;
 	  name++;
 	}