bug#65463: Herd `fport_write: Broken pipe` error when running `guix home reconfigure`

[Tanguy LE CARROUR <tanguy@bioneland.org>]

Hi Guix,

I think this has been happening for quite a long time, but I never paid
attention to it, for everything seemed to be running just fine.
When I run `guix home reconfigure`, it completes with the following message:

```
# […]
Finished updating symlinks.

SSLoading /gnu/store/zbfyaxxigns5lqyxhxzxhm92w54ns1cz-shepherd.conf.
herd: error: exception caught while executing 'load' on service 'root':
In procedure fport_write: Broken pipe
Comparing /gnu/store/2sr90smj33k686394krjx1klhlpipd7z-home/profile/share/fonts and
          /gnu/store/2sr90smj33k686394krjx1klhlpipd7z-home/profile/share/fonts... done (same)
Evaluating on-change gexps.

On-change gexps evaluation finished.
```

I don't know if it's related, but since my last `guix pull`,
`herd status` has been reporting strange statuses:

```
$ herd status
Started:
 + root
Starting:
 ^ mcron
 ^ mpd
Stopped:
 - tor
 - transmission
```

`mcron` and `mpd` (a home service that I will eventually submit) are listed as
`starting`, but they are actually started and they work just fine!?

I had a look at this morning's log. This is what I see right after logging in:

```
2023-08-23 08:21:36 Starting service root...
2023-08-23 08:21:36 Service root started.
2023-08-23 08:21:36 Service root running with value #t.
2023-08-23 08:21:36 Service root has been started.
2023-08-23 08:21:36 Daemonizing...
2023-08-23 08:21:36 Restarting signal handler.
2023-08-23 08:21:36 Now running as process 567.
2023-08-23 08:21:36 Starting services...
2023-08-23 08:21:36 Configuration successfully loaded from '/gnu/store/zbfyaxxigns5lqyxhxzxhm92w54ns1cz-shepherd.conf'.
2023-08-23 08:21:36 Starting service mpd...
2023-08-23 08:21:36 Starting service mcron...
2023-08-23 08:21:36 Service mpd has been started.
2023-08-23 08:21:37 [mpd] Aug 23 08:21 : server_socket: bind to '0.0.0.0:6600' failed (continuing anyway, because binding to '[::]:6600' succeeded): Failed to bind socket: Address already in use
```

The last line is strange. It looks like a second instance of `mpd`
started, tried to bind to the same port and failed!?

After `guix home reconfigure` I get:

```
2023-08-23 08:31:51 Service mcron has been started.
2023-08-23 08:31:51 SSL2023-08-23 08:31:51 oading /gnu/store/zbfyaxxigns5lqyxhxzxhm92w54ns1cz-shepherd.conf.
```

There is no mention to `mpd` and there's a timestamp right in the middle of
the second line?!

When I try to `herd stop SOMETHING`, it hangs forever and I have to
`ctrl+c` it.

Any idea what might be going wrong?!

-- 
Tanguy