Cygwin installer hangs when running post-install scripts

[Kevin Ushey]

Hello,

I'm seeing an issue when attempting to install Cygwin where the
installer hangs while trying to run postinstall scripts (more
specifically, /etc/postinstall/0p_000_autorebase.dash). When the hang
occurs, I see a 'dash.exe' process chewing up 100% of a CPU. If I
attach to the process with WinDbg, I see:

(940.978): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
cygwin1!memmem+0xcb:
00007ffa`490cbb1b 410fb60424      movzx   eax,byte ptr [r12]
ds:00007ffa`552651f7=??
0:000> k
 #   Arch   Child-SP          RetAddr               Call Site
00    AMD64 00000007`ffffc930 00007ffa`48f8028f     cygwin1!memmem+0xcb
01    AMD64 00000007`ffffca90 00007ffa`48f80a49
cygwin1!cygwin_split_path+0x3a0
02    AMD64 00000007`ffffcb20 00007ffa`48f37071
cygwin1!cygwin_split_path+0xb5a
03    AMD64 00000007`ffffcc50 00007ffa`48f35e08
cygwin1!cygwin_dll_init+0x26b
04    AMD64 00000007`ffffcd80 00007ffa`48f35e86     cygwin1!_assert+0x23f0
05    AMD64 00000007`ffffcdd0 00000000`00000000     cygwin1!_assert+0x246e

I'm not sure how much I can trust the stack trace here, though.

If I kill that 'dash.exe' process, then the installer proceeds and
tries running more postinstall scripts, but hangs in a similar way.

I believe this worked quite recently, so I suspect the issue might be
caused by a recent change in Windows. I'm running an insider build of
Windows Pro, inside a Parallels VM on an M1 macOS machine. My Windows
version:

Edition Windows 11 Pro
Version 24H2
Installed on ‎2/‎10/‎2024
OS build 26052.1000
Experience Windows Feature Experience Pack 1000.26052.1000.0

For reference, I first bumped into this when using Git Bash as bundled
with Git for Windows, but it sounds like the underlying issue may be
in Cygwin. See https://github.com/git-for-windows/git/issues/4808 for
more details.

Does any of this sound familiar? Is there anything else I can do to
get more information here; e.g. are there builds of Cygwin with debug
symbols published somewhere, or should I try producing my own debug
build?

Thanks,
Kevin