add pipe as a backend (#1284)

add pipe as a backend, which sends audio to stdout or the file handle
given to --device

Fixes #354

Cargo.toml

@@ -54,7 +54,8 @@ env_logger = "0.10"
 alsa_backend = ["librespot-playback/alsa-backend", "alsa"]
 dbus_keyring = ["keyring"]
 dbus_mpris = ["dbus", "dbus-tokio", "dbus-crossroads", "rspotify"]
-default = ["alsa_backend"]
+pipe_backend = []
+default = ["alsa_backend", "pipe_backend"]
 portaudio_backend = ["librespot-playback/portaudio-backend"]
 pulseaudio_backend = ["librespot-playback/pulseaudio-backend"]
 rodio_backend = ["librespot-playback/rodio-backend"]

src/config.rs

@@ -25,6 +25,7 @@ const CONFIG_FILE_NAME: &str = "spotifyd.conf";
     feature = "pulseaudio_backend",
     feature = "portaudio_backend",
     feature = "alsa_backend",
+    feature = "pipe_backend",
     feature = "rodio_backend",
     feature = "rodiojack_backend",
 )))]
@@ -38,6 +39,8 @@ static BACKEND_VALUES: &[&str] = &[
     "portaudio",
     #[cfg(feature = "rodio_backend")]
     "rodio",
+    #[cfg(feature = "pipe_backend")]
+    "pipe",
     #[cfg(feature = "rodiojack_backend")]
     "rodiojack",
 ];
@@ -50,6 +53,7 @@ pub enum Backend {
     PortAudio,
     PulseAudio,
     Rodio,
+    Pipe,
     RodioJack,
 }
 
@@ -66,6 +70,7 @@ impl FromStr for Backend {
             "portaudio" => Ok(Backend::PortAudio),
             "pulseaudio" => Ok(Backend::PulseAudio),
             "rodio" => Ok(Backend::Rodio),
+            "pipe" => Ok(Backend::Pipe),
             "rodiojack" => Ok(Backend::RodioJack),
             _ => unreachable!(),
         }
@@ -79,6 +84,7 @@ impl fmt::Display for Backend {
             Backend::PortAudio => write!(f, "portaudio"),
             Backend::PulseAudio => write!(f, "pulseaudio"),
             Backend::Rodio => write!(f, "rodio"),
+            Backend::Pipe => write!(f, "pipe"),
             Backend::RodioJack => write!(f, "rodiojack"),
         }
     }
@@ -444,7 +450,7 @@ pub struct SharedConfigValues {
     #[serde(alias = "volume-control")]
     volume_controller: Option<VolumeController>,
 
-    /// The audio device
+    /// The audio device (or file handle if using pipe backend)
     #[structopt(long, value_name = "string")]
     device: Option<String>,