user@hostname ~$ guile
GNU Guile 3.0.5
Copyright (C) 1995-2021 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> blabla [C-c C-c]
While reading expression:
User interrupt
scheme@(guile-user)> (move->fdes (fdes->inport 0) 9)
$1 = #<input: file /dev/pts/8>
scheme@(guile-user)> [C-c C-C]
user@hostname ~$

What I expected:
While reading expression:
User interrupt

Your file descriptor may vary.

My suspicion: look at libguile/scmsigs.c (start_signal_delivery_thread),
in particular:

  if (pipe2 (signal_pipe, O_CLOEXEC) != 0)
    scm_syserror (NULL);

signal_pipe[0] and signal_pipe[1] don't seem to be added to the
port table, so primitive-move->fdes does not know about the signal pipes
and will silently overwrite them.

Greetings,
Maxime