Avoid crashing when Avahi is not available

When librespot is built with Avahi turned on, it will crash if Avahi is
later not available at runtime.

This change avoids it crashing hard when Avahi is not available;
librespot will merely warn of the issue.

This affects some distribution packages too, where the maintainer might
prefer leaving Avahi support enabled, but many setups don't (or can't)
run Avahi.

Co-authored-by: Nick Steel <nick@nsteel.co.uk>

CHANGELOG.md

@@ -35,6 +35,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - [playback] Adhere to ReplayGain spec when calculating gain normalisation factor.
 - [playback] `alsa`: Use `--volume-range` overrides for softvol controls
 - [connect] Don't panic when activating shuffle without previous interaction.
+- [main] Fix crash when built with Avahi support but Avahi is locally unavailable.
 
 ### Removed
 - [playback] `alsamixer`: previously deprecated option `mixer-card` has been removed.

discovery/src/lib.rs

@@ -111,8 +111,7 @@ impl Builder {
                     None,
                     port,
                     &["VERSION=1.0", "CPath=/"],
-                )
-                .unwrap();
+                ).map_err(|e| Error::DnsSdError(io::Error::new(io::ErrorKind::Unsupported, e)))?;
 
             } else {
                 let responder = libmdns::Responder::spawn(&tokio::runtime::Handle::current())?;

src/main.rs

@@ -1581,19 +1581,15 @@ async fn main() {
 
     if setup.enable_discovery {
         let device_id = setup.session_config.device_id.clone();
-
-        discovery = match librespot::discovery::Discovery::builder(device_id)
+        match librespot::discovery::Discovery::builder(device_id)
             .name(setup.connect_config.name.clone())
             .device_type(setup.connect_config.device_type)
             .port(setup.zeroconf_port)
             .launch()
         {
-            Ok(d) => Some(d),
-            Err(e) => {
-                error!("Discovery Error: {}", e);
-                exit(1);
-            }
-        }
+            Ok(d) => discovery = Some(d),
+            Err(err) => warn!("Could not initialise discovery: {}.", err),
+        };
     }
 
     if let Some(credentials) = setup.credentials {
@@ -1606,6 +1602,11 @@ async fn main() {
             )
             .fuse(),
         );
+    } else if discovery.is_none() {
+        error!(
+            "Discovery is unavailable and no credentials provided. Authentication is not possible."
+        );
+        exit(1);
     }
 
     loop {