Staging Scheme code to run as one-shot service

[Owen T. Heisler]

service where the entire service functionality is in a procedure. =====
Hi! I am trying to declare a simple one-shot operating-system service
where the entire service functionality is in a procedure. Following is
a minimal reproducible example. The error is `In procedure fport_write:
Broken pipe` (see end of message). What is wrong here? I'm new to
Scheme and Guix, and it's not very clear to me yet how code staging and
g-expressions work, so it's probably something quite obvious. Thanks
for looking!

```scm
;; operating-system declaration
(use-modules (gnu)
             (gnu services shepherd)
             (ice-9 popen))
(use-service-modules networking)
(use-package-modules bootloaders)
(define (say-hello)
  #~(let ((port (open-pipe* OPEN_WRITE "logger" "-plocal0.alert")))
      (display "========say-hello========\n" port)
      (close-pipe port)))
(define say-hello-service
  (simple-service 'say-hello-service shepherd-root-service-type
                  (list (shepherd-service (auto-start? #f)
                                          (documentation "Say hello.")
                                          (one-shot? #t)
                                          (provision '(say-hello-service))
                                          (respawn? #f)
                                          (start #~(lambda ()
                                                     #$(say-hello)))))))
(operating-system
  (bootloader (bootloader-configuration
                (bootloader grub-bootloader)
                (targets '("/dev/vda"))
                (terminal-inputs '(console serial))
                (terminal-outputs '(console serial))
                (timeout 1)))
  (file-systems (cons (file-system
                        (mount-point "/")
                        (device "/dev/vda1")
                        (type "ext4")) %base-file-systems))
  (host-name "test-guix-vm")
  (kernel-arguments (cons "console=ttyS0" %default-kernel-arguments))
  (services
   (append (list say-hello-service) %base-services)))
```

```sh
# Build system image and run with qemu
cp $(guix system image --image-type=qcow2 in.scm) image.qcow2
chmod u+w image.qcow2
qemu-system-x86_64 -enable-kvm -m 2048 -device virtio-blk,drive=myhd \
    -nographic -drive if=none,file=image.qcow2,id=myhd
```

```console
## Log in as root and attempt to start service
# herd start say-hello-service
Starting service say-hello-service...
Service say-hello-service failed to start.
herd: error: exception caught while executing 'start' on service 
'say-hello-service':
In procedure fport_write: Broken pipe
```