Re: [PR PATCH] [Merged]: sane: add libcurl for escl backend, add runit service

[ericonr <ericonr@users.noreply.github.com>]

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There's a merged pull request on the void-packages repository

sane: add libcurl for escl backend, add runit service
https://github.com/void-linux/void-packages/pull/31851

Description:
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#### General
- [ ] This is a new package and it conforms to the [quality requirements](https://github.com/void-linux/void-packages/blob/master/Manual.md#quality-requirements)

#### Have the results of the proposed changes been tested?
- [x] I use the packages affected by the proposed changes on a regular basis and confirm this PR works for me
- [ ] I generally don't use the affected packages but briefly tested this PR

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Changes

* Make Avahi support optional
* Add libcurl for eSCL (AirScan) backend when Avahi is enabled
* Add `_saned` user/group
* Add a runit service for `saned`

There is an issue with the runit service when SANE is built with Avahi support. `saned` will spawn a second process (pid 1429) which can be seen below:
```
$ ps
1428 _saned   _saned    0.0  0.0  16164  6088 ?        S    18:57:06 00:00:00 saned -l -u _saned -d128
1429 _saned   _saned    0.0  0.0  16164   548 ?        S    18:57:06 00:00:00 saned -l -u _saned -d128
```
```
$ svlogtail
2021-07-08T08:57:06.35901 daemon.debug: Jul  8 18:57:06 saned[1428]: read_config: searching for config file
2021-07-08T08:57:06.35906 daemon.debug: Jul  8 18:57:06 saned[1428]: read_config: done reading config
2021-07-08T08:57:06.35909 daemon.debug: Jul  8 18:57:06 saned[1428]: saned (AF-indep+IPv6) from sane-backends 1.0.32 starting up
2021-07-08T08:57:06.35911 daemon.debug: Jul  8 18:57:06 saned[1428]: do_bindings: trying to get port for service "sane-port" (getaddrinfo)
2021-07-08T08:57:06.36066 daemon.debug: Jul  8 18:57:06 saned[1428]: do_bindings: [1] socket () using IPv6
2021-07-08T08:57:06.36069 daemon.debug: Jul  8 18:57:06 saned[1428]: do_bindings: [1] setsockopt ()
2021-07-08T08:57:06.36066 daemon.debug: Jul  8 18:57:06 saned[1428]: do_bindings: [1] socket () using IPv6
2021-07-08T08:57:06.36069 daemon.debug: Jul  8 18:57:06 saned[1428]: do_bindings: [1] setsockopt ()
2021-07-08T08:57:06.36070 daemon.debug: Jul  8 18:57:06 saned[1428]: do_bindings: [1] bind () to port 6566
2021-07-08T08:57:06.36072 daemon.debug: Jul  8 18:57:06 saned[1428]: do_bindings: [1] listen ()
2021-07-08T08:57:06.36076 daemon.debug: Jul  8 18:57:06 saned[1428]: do_bindings: [0] socket () using IPv4
2021-07-08T08:57:06.36077 daemon.debug: Jul  8 18:57:06 saned[1428]: do_bindings: [0] setsockopt ()
2021-07-08T08:57:06.36079 daemon.debug: Jul  8 18:57:06 saned[1428]: do_bindings: [0] bind () to port 6566
2021-07-08T08:57:06.36080 daemon.debug: Jul  8 18:57:06 saned[1428]: do_bindings: [0] bind failed: Address already in use
2021-07-08T08:57:06.36083 daemon.debug: Jul  8 18:57:06 saned[1428]: Dropped privileges to uid 993 gid 990
2021-07-08T08:57:06.36084 daemon.debug: Jul  8 18:57:06 saned[1428]: run_standalone: spawning Avahi process
2021-07-08T08:57:06.36101 daemon.debug: Jul  8 18:57:06 saned[1428]: run_standalone: waiting for control connection
2021-07-08T08:57:06.36204 daemon.debug: Jul  8 18:57:06 saned[1429]: saned_avahi_callback: AVAHI_CLIENT_CONNECTING
```
The second process is related to Avahi functionality. `sv` will stop the main process (pid 1428) but the second process remains. This doesn’t occur when SANE is built without Avahi support. This is why I made Avahi support optional.

I don't know how to properly fix this other than to have a `./finish` script clean up any lingering `saned` processes, but I’m not sure how safe that is. If there is no acceptable solution I can drop the runit service from this PR.