Bug#983435: Just a wild guess...

[bugsgrid+deb at gmail.com]

Hi mainteners,

Given the observations, I feel it very likely being caused by
misfiring of restore_backup_on_exit(), and looking around the source
tree I found something suspicious:

In the commit 5dec0f2f9cd4d4dd0109c25cd2b399a780179020,
| unix exec: avoid atexit handlers when child exits
| Needed in buster to avoid problems with backup/restore in grub-install.

exit()'s in grub-core/osdep/unix/exec.c:grub_util_exec_redirect_all()
are all replaced with _exit(),
but grub_util_exec_pipe() and grub_util_exec_pipe_stderr() are left
as-is, having exit() right after execvp() which is same to
grub_util_exec_redirect_all() before the patch.  Is this correct?

Sorry this is just a wild guess, I don't know if this actually can
cause the simptom, nor if this path is actually run...

Additional info:
- GPT partitions are just FAT32 EFI system + ext4 single flat root,
there's nothing else.
- It's just a router, nothing fancy is installed.  Yay sysvinit.
- The kernel is built from debian linux-source-4.19 (currently at 4.19.171-2)
-- everything built-in without modules, with a minor patch
(pastebin.com/raw/ikQKniWK).
--- because of the mentioned glitch, the official debian linux-image
doesn't work very well. (sure it boots, it doesn't hang, but...)

Thanks in advance,