feat: validate BufferSize::Fixed against supported range for hosts (#1031)

* feat: validate BufferSize::Fixed against supported range for hosts

Add buffer size validation for ALSA, ASIO, JACK, Emscripten, and WebAudio.
Remove redundant validation for CoreAudio.

* refactor: move JACK buffer size validation into helper method

Author: roderickvd

CHANGELOG.md

@@ -3,7 +3,7 @@
 - Add `Sample::bits_per_sample` method.
 - Update `audio_thread_priority` to 0.34.
 - AAudio: Configure buffer to ensure consistent callback buffer sizes.
-- ALSA: Improve `BufferSize::Fixed` latency precision and audio callback performance.
+- ALSA: Improve `BufferSize::Fixed` precision and audio callback performance.
 - ALSA: Change `BufferSize::Default` to use the device defaults.
 - ALSA: Change card enumeration to work like `aplay -L` does.
 - ALSA: Add `I24` and `U24` sample format support (24-bit samples stored in 4 bytes).
@@ -22,12 +22,15 @@
 - CoreAudio: Update `mach2` to 0.5.
 - CoreAudio: Configure device buffer to ensure predictable callback buffer sizes.
 - CoreAudio: Fix timestamp accuracy.
+- Emscripten: Add `BufferSize::Fixed` validation against supported range.
 - iOS: Fix example by properly activating audio session.
 - iOS: Add complete AVAudioSession integration for device enumeration and buffer size control.
+- JACK: Add `BufferSize::Fixed` validation to reject requests that don't match server buffer size.
 - WASAPI: Expose `IMMDevice` from WASAPI host Device.
 - WASAPI: Add `I24` and `U24` sample format support (24-bit samples stored in 4 bytes).
 - WASAPI: Update `windows` to >= 0.58, <= 0.62.
 - Wasm: Removed optional `wee-alloc` feature for security reasons.
+- WebAudio: Add `BufferSize::Fixed` validation against supported range.
 
 # Version 0.16.0 (2025-06-07)
 

src/host/aaudio/mod.rs

@@ -215,6 +215,9 @@ fn configure_for_device(
         builder
     };
     builder = builder.sample_rate(config.sample_rate.0.try_into().unwrap());
+
+    // Note: Buffer size validation is not needed - the native AAudio API validates buffer sizes
+    // when `open_stream()` is called.
     match &config.buffer_size {
         BufferSize::Default => builder,
         BufferSize::Fixed(size) => builder

src/host/alsa/mod.rs

@@ -279,6 +279,28 @@ impl Device {
         sample_format: SampleFormat,
         stream_type: alsa::Direction,
     ) -> Result<StreamInner, BuildStreamError> {
+        // Validate buffer size if Fixed is specified. This is necessary because
+        // `set_period_size_near()` with `ValueOr::Nearest` will accept ANY value and return the
+        // "nearest" supported value, which could be wildly different (e.g., requesting 4096 frames
+        // might return 512 frames if that's "nearest").
+        if let BufferSize::Fixed(requested_size) = conf.buffer_size {
+            // Note: We use `default_input_config`/`default_output_config` to get the buffer size
+            // range. This queries the CURRENT device (`self.pcm_id`), not the default device. The
+            // buffer size range is the same across all format configurations for a given device
+            // (see `supported_configs()`).
+            let supported_config = match stream_type {
+                alsa::Direction::Capture => self.default_input_config(),
+                alsa::Direction::Playback => self.default_output_config(),
+            };
+            if let Ok(config) = supported_config {
+                if let SupportedBufferSize::Range { min, max } = config.buffer_size {
+                    if !(min..=max).contains(&requested_size) {
+                        return Err(BuildStreamError::StreamConfigNotSupported);
+                    }
+                }
+            }
+        }
+
         let handle_result = self
             .handles
             .lock()

src/host/asio/stream.rs

@@ -661,7 +661,7 @@ fn frames_to_duration(frames: usize, rate: crate::SampleRate) -> std::time::Dura
 
 /// Check whether or not the desired config is supported by the stream.
 ///
-/// Checks sample rate, data type and then finally the number of channels.
+/// Checks sample rate, data type, number of channels, and buffer size.
 fn check_config(
     driver: &sys::Driver,
     config: &StreamConfig,
@@ -671,8 +671,21 @@ fn check_config(
     let StreamConfig {
         channels,
         sample_rate,
-        buffer_size: _,
+        buffer_size,
     } = config;
+
+    // Validate buffer size if `Fixed` is specified. This is necessary because ASIO's
+    // `create_buffers` only validates the upper bound (returns `InvalidBufferSize` if > max) but
+    // does NOT validate the lower bound. Passing a buffer size below min would be accepted but
+    // behavior is unspecified.
+    if let BufferSize::Fixed(requested_size) = buffer_size {
+        let (min, max) = driver.buffersize_range().map_err(build_stream_err)?;
+        let requested_size_i32 = *requested_size as i32;
+        if !(min..=max).contains(&requested_size_i32) {
+            return Err(BuildStreamError::StreamConfigNotSupported);
+        }
+    }
+
     // Try and set the sample rate to what the user selected.
     let sample_rate = sample_rate.0.into();
     if sample_rate != driver.sample_rate().map_err(build_stream_err)? {

src/host/coreaudio/macos/device.rs

@@ -863,7 +863,6 @@ impl Device {
 /// This handles the common setup tasks for both input and output streams:
 /// - Sets the stream format (ASBD)
 /// - Configures buffer size for Fixed buffer size requests
-/// - Validates buffer size ranges
 fn configure_stream_format_and_buffer(
     audio_unit: &mut AudioUnit,
     config: &StreamConfig,
@@ -879,14 +878,6 @@ fn configure_stream_format_and_buffer(
 
     // Configure device buffer size if requested
     if let BufferSize::Fixed(buffer_size) = config.buffer_size {
-        let buffer_size_range = get_io_buffer_frame_size_range(audio_unit)?;
-
-        if let SupportedBufferSize::Range { min, max } = buffer_size_range {
-            if !(min..=max).contains(&buffer_size) {
-                return Err(BuildStreamError::StreamConfigNotSupported);
-            }
-        }
-
         // IMPORTANT: Buffer frame size is a DEVICE-LEVEL property, not stream-specific.
         // Unlike stream format above, we ALWAYS use Scope::Global + Element::Output
         // for device properties, regardless of whether this is an input or output stream.

src/host/emscripten/mod.rs

@@ -197,11 +197,10 @@ impl DeviceTrait for Device {
 
         let buffer_size_frames = match config.buffer_size {
             BufferSize::Fixed(v) => {
-                if v == 0 {
+                if !(MIN_BUFFER_SIZE..=MAX_BUFFER_SIZE).contains(&v) {
                     return Err(BuildStreamError::StreamConfigNotSupported);
-                } else {
-                    v as usize
                 }
+                v as usize
             }
             BufferSize::Default => DEFAULT_BUFFER_SIZE,
         };

src/host/jack/device.rs

@@ -135,6 +135,20 @@ impl Device {
     pub fn is_output(&self) -> bool {
         matches!(self.device_type, DeviceType::OutputDevice)
     }
+
+    /// Validate buffer size if Fixed is specified. This is necessary because JACK buffer size
+    /// is controlled by the JACK server and cannot be changed by clients. Without validation,
+    /// cpal would silently use the server's buffer size even if a different value was requested.
+    fn validate_buffer_size(&self, conf: &StreamConfig) -> Result<(), BuildStreamError> {
+        if let crate::BufferSize::Fixed(requested_size) = conf.buffer_size {
+            if let SupportedBufferSize::Range { min, max } = self.buffer_size {
+                if !(min..=max).contains(&requested_size) {
+                    return Err(BuildStreamError::StreamConfigNotSupported);
+                }
+            }
+        }
+        Ok(())
+    }
 }
 
 impl DeviceTrait for Device {
@@ -191,6 +205,8 @@ impl DeviceTrait for Device {
         if conf.sample_rate != self.sample_rate || sample_format != JACK_SAMPLE_FORMAT {
             return Err(BuildStreamError::StreamConfigNotSupported);
         }
+        self.validate_buffer_size(conf)?;
+
         // The settings should be fine, create a Client
         let client_options = super::get_client_options(self.start_server_automatically);
         let client;
@@ -230,6 +246,7 @@ impl DeviceTrait for Device {
         if conf.sample_rate != self.sample_rate || sample_format != JACK_SAMPLE_FORMAT {
             return Err(BuildStreamError::StreamConfigNotSupported);
         }
+        self.validate_buffer_size(conf)?;
 
         // The settings should be fine, create a Client
         let client_options = super::get_client_options(self.start_server_automatically);

src/host/wasapi/device.rs

@@ -559,6 +559,8 @@ impl Device {
                 }
             };
 
+            // Note: Buffer size validation is not needed here - `IAudioClient::Initialize`
+            // will return `AUDCLNT_E_BUFFER_SIZE_ERROR` if the buffer size is not supported.
             let buffer_duration =
                 buffer_size_to_duration(&config.buffer_size, config.sample_rate.0);
 
@@ -674,6 +676,8 @@ impl Device {
                 .build_audioclient()
                 .map_err(windows_err_to_cpal_err::<BuildStreamError>)?;
 
+            // Note: Buffer size validation is not needed here - `IAudioClient::Initialize`
+            // will return `AUDCLNT_E_BUFFER_SIZE_ERROR` if the buffer size is not supported.
             let buffer_duration =
                 buffer_size_to_duration(&config.buffer_size, config.sample_rate.0);
 

src/host/webaudio/mod.rs

@@ -203,11 +203,10 @@ impl DeviceTrait for Device {
 
         let buffer_size_frames = match config.buffer_size {
             BufferSize::Fixed(v) => {
-                if v == 0 {
+                if !(MIN_BUFFER_SIZE..=MAX_BUFFER_SIZE).contains(&v) {
                     return Err(BuildStreamError::StreamConfigNotSupported);
-                } else {
-                    v as usize
                 }
+                v as usize
             }
             BufferSize::Default => DEFAULT_BUFFER_SIZE,
         };