Discovery: Refactor and add Avahi DBus backend (#1347)

* discovery: use opaque error type for DnsSdError

This helps to decouple discovery and core by not leaking implementation
details of the zeroconf backend into Error conversion impls in core.

* discovery: map all MDNS/DNS-SD errors to DiscoveryError::DnsSdError

previously, libmdns errors would use a generic conversion
from std::io::Error to core::Error

* discovery: use an opaque type for the handle to the DNS-SD service

* discovery: make features additive

i.e. add with-libmdns instead of using not(with-dns-sd).

The logic is such that enabling with-dns-sd in addition to the default
with-libmdns will still end up using dns-sd, as before.
If only with-libmdns is enabled, that will be the default.
If none of the features is enabled, attempting to build a `Discovery`
will yield an error.

* discovery: add --zeroconf-backend CLI flag

* discovery: Add minimal Avahi zeroconf backend

* bump MSRV to 1.75

required by zbus >= 4

* discovery: ensure that server and dns-sd backend shutdown gracefully

Previously, on drop the the shutdown_tx/close_tx, it wasn't guaranteed
the corresponding tasks would continue to be polled until they actually
completed their shutdown.

Since dns_sd::Service is not Send and non-async, and because libmdns is
non-async, put them on their own threads.

* discovery: use a shared channel for server and zeroconf status messages

* discovery: add Avahi reconnection logic

This deals gracefully with the case where the Avahi daemon is restarted
or not running initially.

* discovery: allow running when compiled without zeroconf backend...

...but exit with an error if there's no way to authenticate

* better error messages for invalid options with no short flag

.devcontainer/Dockerfile

@@ -1,6 +1,6 @@
 # syntax=docker/dockerfile:1
 ARG debian_version=slim-bookworm
-ARG rust_version=1.74.0
+ARG rust_version=1.75.0
 FROM rust:${rust_version}-${debian_version}
 
 ARG DEBIAN_FRONTEND=noninteractive

.github/workflows/test.yml

@@ -109,7 +109,7 @@ jobs:
       matrix:
         os: [ubuntu-latest]
         toolchain:
-          - "1.74" # MSRV (Minimum supported rust version)
+          - "1.75" # MSRV (Minimum supported rust version)
           - stable
         experimental: [false]
         # Ignore failures in beta
@@ -164,7 +164,7 @@ jobs:
       matrix:
         os: [windows-latest]
         toolchain:
-          - "1.74" # MSRV (Minimum supported rust version)
+          - "1.75" # MSRV (Minimum supported rust version)
           - stable
     steps:
       - name: Checkout code
@@ -215,7 +215,7 @@ jobs:
           - aarch64-unknown-linux-gnu
           - riscv64gc-unknown-linux-gnu
         toolchain:
-          - "1.74" # MSRV (Minimum supported rust version)
+          - "1.75" # MSRV (Minimum supported rust version)
           - stable
     steps:
       - name: Checkout code

CHANGELOG.md

@@ -10,11 +10,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - [core] The `access_token` for http requests is now acquired by `login5`
+- [core] MSRV is now 1.75 (breaking)
+- [discovery] librespot can now be compiled with multiple MDNS/DNS-SD backends
+  (avahi, dns_sd, libmdns) which can be selected using a CLI flag. The defaults
+  are unchanged (breaking).
 
 ### Added
 
 - [core] Add `login` (mobile) and `auth_token` retrieval via login5
 - [core] Add `OS` and `os_version` to `config.rs`
+- [discovery] Added a new MDNS/DNS-SD backend which connects to Avahi via D-Bus.
 
 ### Removed
 

COMPILING.md

@@ -56,6 +56,17 @@ On Fedora systems:
 sudo dnf install alsa-lib-devel
 ```
 
+### Zeroconf library dependencies
+Depending on the chosen backend, specific development libraries are required.
+
+*_Note this is an non-exhaustive list, open a PR to add to it!_*
+
+| Zeroconf backend   | Debian/Ubuntu                | Fedora                            | macOS       |
+|--------------------|------------------------------|-----------------------------------|-------------|
+|avahi               |                              |                                   |             |
+|dns_sd              | `libavahi-compat-libdnssd-dev pkg-config` | `avahi-compat-libdns_sd-devel` |   |
+|libmdns (default)   |                              |                                   |             |
+
 ### Getting the Source
 
 The recommended method is to first fork the repo, so that you have a copy that you have read/write access to. After that, it’s a simple case of cloning your fork.