Add `BackendSpecificError`. Add new `DevicesError`.

See the documentation for both new errors for details.

The new `DevicesError` has been added to allow for returning errors when
enumerating devices. This has allowed to remove multiple potential
`panic!`s in each of the alsa, coreaudio and wasapi backends.

examples/enumerate.rs

@@ -4,7 +4,7 @@ fn main() {
     println!("Default Input Device:\n  {:?}", cpal::default_input_device().map(|e| e.name()));
     println!("Default Output Device:\n  {:?}", cpal::default_output_device().map(|e| e.name()));
 
-    let devices = cpal::devices();
+    let devices = cpal::devices().expect("failed to enumerate devices");
     println!("Devices: ");
     for (device_index, device) in devices.enumerate() {
         println!("{}. \"{}\"",

src/alsa/enumerate.rs

@@ -1,3 +1,4 @@
+use {BackendSpecificError, DevicesError};
 use super::Device;
 use super::alsa;
 use super::check_errors;
@@ -13,6 +14,28 @@ pub struct Devices {
     next_str: *const *const u8,
 }
 
+impl Devices {
+    pub fn new() -> Result<Self, DevicesError> {
+        unsafe {
+            // TODO: check in which situation this can fail.
+            let card = -1; // -1 means all cards.
+            let iface = b"pcm\0"; // Interface identification.
+            let mut hints = mem::uninitialized(); // Array of device name hints.
+            let res = alsa::snd_device_name_hint(card, iface.as_ptr() as *const _, &mut hints);
+            if let Err(description) = check_errors(res) {
+                let err = BackendSpecificError { description };
+                return Err(err.into());
+            }
+            let hints = hints as *const *const u8;
+            let devices = Devices {
+                global_list: hints,
+                next_str: hints,
+            };
+            Ok(devices)
+        }
+    }
+}
+
 unsafe impl Send for Devices {
 }
 unsafe impl Sync for Devices {
@@ -27,24 +50,6 @@ impl Drop for Devices {
     }
 }
 
-impl Default for Devices {
-    fn default() -> Devices {
-        unsafe {
-            // TODO: check in which situation this can fail.
-            let card = -1; // -1 means all cards.
-            let iface = b"pcm\0"; // Interface identification.
-            let mut hints = mem::uninitialized(); // Array of device name hints.
-            let res = alsa::snd_device_name_hint(card, iface.as_ptr() as *const _, &mut hints);
-            check_errors(res).unwrap();
-            let hints = hints as *const *const u8;
-            Devices {
-                global_list: hints,
-                next_str: hints,
-            }
-        }
-    }
-}
-
 impl Iterator for Devices {
     type Item = Device;
 

src/coreaudio/enumerate.rs

@@ -1,4 +1,4 @@
-use SupportedFormat;
+use {BackendSpecificError, DevicesError, SupportedFormat};
 use std::mem;
 use std::ptr::null;
 use std::vec::IntoIter as VecIntoIter;
@@ -64,20 +64,27 @@ unsafe fn audio_devices() -> Result<Vec<AudioDeviceID>, OSStatus> {
 
 pub struct Devices(VecIntoIter<AudioDeviceID>);
 
+impl Devices {
+    pub fn new() -> Result<Self, DevicesError> {
+        let devices = unsafe {
+            match audio_devices() {
+                Ok(devices) => devices,
+                Err(os_status) => {
+                    let description = format!("{}", os_status);
+                    let err = BackendSpecificError { description };
+                    return Err(err.into());
+                }
+            }
+        };
+        Ok(Devices(devices.into_iter()))
+    }
+}
+
 unsafe impl Send for Devices {
 }
 unsafe impl Sync for Devices {
 }
 
-impl Default for Devices {
-    fn default() -> Self {
-        let devices = unsafe {
-            audio_devices().expect("failed to get audio output devices")
-        };
-        Devices(devices.into_iter())
-    }
-}
-
 impl Iterator for Devices {
     type Item = Device;
     fn next(&mut self) -> Option<Device> {

src/emscripten/mod.rs

@@ -10,6 +10,7 @@ use stdweb::web::set_timeout;
 
 use BuildStreamError;
 use DefaultFormatError;
+use DevicesError;
 use Format;
 use SupportedFormatsError;
 use StreamData;
@@ -183,6 +184,13 @@ fn is_webaudio_available() -> bool {
 
 // Content is false if the iterator is empty.
 pub struct Devices(bool);
+
+impl Devices {
+    pub fn new() -> Result<Self, DevicesError> {
+        Ok(Self::default())
+    }
+}
+
 impl Default for Devices {
     fn default() -> Devices {
         // We produce an empty iterator if the WebAudio API isn't available.

src/lib.rs

@@ -279,6 +279,37 @@ pub struct SupportedInputFormats(cpal_impl::SupportedInputFormats);
 /// See [`Device::supported_output_formats()`](struct.Device.html#method.supported_output_formats).
 pub struct SupportedOutputFormats(cpal_impl::SupportedOutputFormats);
 
+/// Some error has occurred that is specific to the backend from which it was produced.
+///
+/// This error is often used as a catch-all in cases where:
+///
+/// - It is unclear exactly what error might be produced by the backend API.
+/// - It does not make sense to add a variant to the enclosing error type.
+/// - No error was expected to occur at all, but we return an error to avoid the possibility of a
+///   `panic!` caused by some unforseen or unknown reason.
+///
+/// **Note:** If you notice a `BackendSpecificError` that you believe could be better handled in a
+/// cross-platform manner, please create an issue or submit a pull request with a patch that adds
+/// the necessary error variant to the appropriate error enum.
+#[derive(Debug, Fail)]
+#[fail(display = "A backend-specific error has occurred: {}", description)]
+pub struct BackendSpecificError {
+    pub description: String
+}
+
+/// An error that might occur while attempting to enumerate the available devices on a system.
+#[derive(Debug, Fail)]
+pub enum DevicesError {
+    /// Some error that is specific to the backend from which it was produced.
+    ///
+    /// Note: This error is often used when
+    #[fail(display = "{}", err)]
+    BackendSpecific {
+        #[fail(cause)]
+        err: BackendSpecificError,
+    }
+}
+
 /// Error that can happen when enumerating the list of supported formats.
 #[derive(Debug, Fail)]
 pub enum SupportedFormatsError {
@@ -331,34 +362,34 @@ pub enum BuildStreamError {
 ///
 /// Can be empty if the system does not support audio in general.
 #[inline]
-pub fn devices() -> Devices {
-    Devices(Default::default())
+pub fn devices() -> Result<Devices, DevicesError> {
+    Ok(Devices(cpal_impl::Devices::new()?))
 }
 
 /// An iterator yielding all `Device`s currently available to the system that support one or more
 /// input stream formats.
 ///
 /// Can be empty if the system does not support audio input.
-pub fn input_devices() -> InputDevices {
+pub fn input_devices() -> Result<InputDevices, DevicesError> {
     fn supports_input(device: &Device) -> bool {
         device.supported_input_formats()
             .map(|mut iter| iter.next().is_some())
             .unwrap_or(false)
     }
-    devices().filter(supports_input)
+    Ok(devices()?.filter(supports_input))
 }
 
 /// An iterator yielding all `Device`s currently available to the system that support one or more
 /// output stream formats.
 ///
 /// Can be empty if the system does not support audio output.
-pub fn output_devices() -> OutputDevices {
+pub fn output_devices() -> Result<OutputDevices, DevicesError> {
     fn supports_output(device: &Device) -> bool {
         device.supported_output_formats()
             .map(|mut iter| iter.next().is_some())
             .unwrap_or(false)
     }
-    devices().filter(supports_output)
+    Ok(devices()?.filter(supports_output))
 }
 
 /// The default input audio device on the system.
@@ -704,6 +735,12 @@ impl Iterator for SupportedOutputFormats {
     }
 }
 
+impl From<BackendSpecificError> for DevicesError {
+    fn from(err: BackendSpecificError) -> Self {
+        DevicesError::BackendSpecific { err }
+    }
+}
+
 // If a backend does not provide an API for retrieving supported formats, we query it with a bunch
 // of commonly used rates. This is always the case for wasapi and is sometimes the case for alsa.
 //

src/null/mod.rs

@@ -4,6 +4,7 @@ use std::marker::PhantomData;
 
 use BuildStreamError;
 use DefaultFormatError;
+use DevicesError;
 use Format;
 use SupportedFormatsError;
 use StreamData;
@@ -56,6 +57,12 @@ pub struct StreamId;
 #[derive(Default)]
 pub struct Devices;
 
+impl Devices {
+    pub fn new() -> Result<Self, DevicesError> {
+        Ok(Devices)
+    }
+}
+
 impl Iterator for Devices {
     type Item = Device;
 

src/wasapi/device.rs

@@ -9,7 +9,9 @@ use std::ptr;
 use std::slice;
 use std::sync::{Arc, Mutex, MutexGuard};
 
+use BackendSpecificError;
 use DefaultFormatError;
+use DevicesError;
 use Format;
 use SupportedFormatsError;
 use SampleFormat;
@@ -18,6 +20,7 @@ use SupportedFormat;
 use COMMON_SAMPLE_RATES;
 
 use super::check_result;
+use super::check_result_backend_specific;
 use super::com;
 use super::winapi::Interface;
 use super::winapi::ctypes::c_void;
@@ -688,6 +691,32 @@ pub struct Devices {
     next_item: u32,
 }
 
+impl Devices {
+    pub fn new() -> Result<Self, DevicesError> {
+        unsafe {
+            let mut collection: *mut IMMDeviceCollection = mem::uninitialized();
+            // can fail because of wrong parameters (should never happen) or out of memory
+            check_result_backend_specific(
+                (*ENUMERATOR.0).EnumAudioEndpoints(
+                    eAll,
+                    DEVICE_STATE_ACTIVE,
+                    &mut collection,
+                )
+            )?;
+
+            let mut count = mem::uninitialized();
+            // can fail if the parameter is null, which should never happen
+            check_result_backend_specific((*collection).GetCount(&mut count))?;
+
+            Devices {
+                collection: collection,
+                total_count: count,
+                next_item: 0,
+            }
+        }
+    }
+}
+
 unsafe impl Send for Devices {
 }
 unsafe impl Sync for Devices {
@@ -702,32 +731,6 @@ impl Drop for Devices {
     }
 }
 
-impl Default for Devices {
-    fn default() -> Devices {
-        unsafe {
-            let mut collection: *mut IMMDeviceCollection = mem::uninitialized();
-            // can fail because of wrong parameters (should never happen) or out of memory
-            check_result(
-                (*ENUMERATOR.0).EnumAudioEndpoints(
-                    eAll,
-                    DEVICE_STATE_ACTIVE,
-                    &mut collection,
-                )
-            ).unwrap();
-
-            let mut count = mem::uninitialized();
-            // can fail if the parameter is null, which should never happen
-            check_result((*collection).GetCount(&mut count)).unwrap();
-
-            Devices {
-                collection: collection,
-                total_count: count,
-                next_item: 0,
-            }
-        }
-    }
-}
-
 impl Iterator for Devices {
     type Item = Device;
 

src/wasapi/mod.rs

@@ -1,10 +1,10 @@
 extern crate winapi;
 
+use BackendSpecificError;
+use self::winapi::um::winnt::HRESULT;
 use std::io::Error as IoError;
-
 pub use self::device::{Device, Devices, SupportedInputFormats, SupportedOutputFormats, default_input_device, default_output_device};
 pub use self::stream::{EventLoop, StreamId};
-use self::winapi::um::winnt::HRESULT;
 
 mod com;
 mod device;
@@ -18,3 +18,13 @@ fn check_result(result: HRESULT) -> Result<(), IoError> {
         Ok(())
     }
 }
+
+fn check_result_backend_specific(result: HRESULT) -> Result<(), BackendSpecificError> {
+    match check_result(result) {
+        Ok(()) => Ok(())
+        Err(err) => {
+            let description = format!("{}", err);
+            return BackendSpecificError { description }
+        }
+    }
+}