Document the CPAL buffer size handling

CHANGELOG.md

@@ -10,6 +10,14 @@ Since there is no stable release yet, the changes are organized per day in
 reverse chronological order. The main purpose of this document in its current
 state is to list breaking changes.
 
+## [2024-05-05]
+
+### Fixed
+
+- The CPAL backend now correctly handles situations where it receives fewer
+  samples than configured.
+- Fixed the handling of multichannel audio in the CPAL backend.
+
 ## [2024-05-04]
 
 ### Fixed

src/wrapper/standalone/backend/cpal.rs

@@ -702,6 +702,8 @@ impl CpalMidir {
             .main_input_channels
             .map(NonZeroU32::get)
             .unwrap_or(0) as usize;
+        // This may contain excess unused space at the end if we get fewer samples than configured
+        // from CPAL
         let mut main_io_storage = vec![vec![0.0f32; buffer_size]; num_output_channels];
 
         // This backend does not support auxiliary inputs and outputs, so in order to have the same
@@ -824,10 +826,16 @@ impl CpalMidir {
             }
 
             {
-                let sample_count = data.len() / num_output_channels;
-                assert!(sample_count <= buffer_size);
+                // Even though we told CPAL that we wanted `buffer_size` samples, it may still give
+                // us fewer. If we receive more than what we configured, then this will panic.
+                let actual_sample_count = data.len() / num_output_channels;
+                assert!(
+                    actual_sample_count <= buffer_size,
+                    "Received {actual_sample_count} samples, while the configured buffer size is \
+                     {buffer_size}"
+                );
                 let buffers = unsafe {
-                    buffer_manager.create_buffers(0, sample_count, |buffer_sources| {
+                    buffer_manager.create_buffers(0, actual_sample_count, |buffer_sources| {
                         *buffer_sources.main_output_channel_pointers = Some(ChannelPointers {
                             ptrs: NonNull::new(main_io_channel_pointers.get().as_mut_ptr())
                                 .unwrap(),