introduce Processor::flush method

Introduce a `Processor::flush` method for handling events without a
corresponding set of audio buffers. Update the VST3 and CLAP backends to
call `flush` wherever they currently call `process` with a zero-length
buffer.

Author: micahrj

examples/gain/src/lib.rs

@@ -113,16 +113,28 @@ pub struct GainProcessor {
     params: GainParams,
 }
 
+impl GainProcessor {
+    fn handle_event(&mut self, event: &Event) {
+        if let Data::ParamChange { id, value } = event.data {
+            self.params.set_param(id, value);
+        }
+    }
+}
+
 impl Processor for GainProcessor {
     fn reset(&mut self) {}
 
+    fn flush(&mut self, events: Events) {
+        for event in events {
+            self.handle_event(event);
+        }
+    }
+
     fn process(&mut self, buffers: Buffers, events: Events) {
         let mut buffers: (BufferMut,) = buffers.try_into().unwrap();
         for (mut buffer, events) in buffers.0.split_at_events(events) {
             for event in events {
-                if let Data::ParamChange { id, value } = event.data {
-                    self.params.set_param(id, value);
-                }
+                self.handle_event(event);
             }
 
             for sample in buffer.samples() {

src/format/clap/instance.rs

@@ -394,16 +394,7 @@ impl<P: Plugin> Instance<P> {
             instance.sync_processor(&mut process_state.events);
 
             if !process_state.events.is_empty() {
-                process_state.buffer_ptrs.fill(NonNull::dangling().as_ptr());
-                processor.process(
-                    Buffers::from_raw_parts(
-                        &process_state.buffer_data,
-                        &process_state.buffer_ptrs,
-                        0,
-                        0,
-                    ),
-                    Events::new(&process_state.events),
-                );
+                processor.flush(Events::new(&process_state.events));
             }
 
             processor.reset();
@@ -810,8 +801,6 @@ impl<P: Plugin> Instance<P> {
 
         // If we are in the active state, flush will be called on the audio thread.
         if let Some(processor) = &mut process_state.processor {
-            process_state.buffer_ptrs.fill(NonNull::dangling().as_ptr());
-
             process_state.events.clear();
             instance.sync_processor(&mut process_state.events);
             instance.process_param_events(in_, &mut process_state.events);
@@ -822,15 +811,7 @@ impl<P: Plugin> Instance<P> {
                 0,
             );
 
-            processor.process(
-                Buffers::from_raw_parts(
-                    &process_state.buffer_data,
-                    &process_state.buffer_ptrs,
-                    0,
-                    0,
-                ),
-                Events::new(&process_state.events),
-            );
+            processor.flush(Events::new(&process_state.events));
         }
         // Otherwise, flush will be called on the main thread.
         else {

src/format/clap/tests.rs

@@ -77,6 +77,7 @@ struct TestProcessor;
 
 impl Processor for TestProcessor {
     fn reset(&mut self) {}
+    fn flush(&mut self, _events: Events) {}
     fn process(&mut self, _buffers: Buffers, _events: Events) {}
 }
 

src/format/vst3/buffers.rs

@@ -87,14 +87,26 @@ impl ScratchBuffers {
         self.moves.reserve(in_out_channels);
     }
 
+    /// Set up buffer pointers for the processor given a VST3 `ProcessData` struct.
+    ///
+    /// This method is responsible for detecting if any of the buffers for an input bus are aliased
+    /// by a output buffer and, if so, copying those inputs to scratch buffers. It is also
+    /// responsible for detecting if separate input and output buffers have been passed for an
+    /// in-out bus and copying those inputs to the corresponding outputs.
+    ///
+    /// This method will return `Err` if the channel counts do not match the current layout or if
+    /// the buffer's length exceeds the maximum buffer size. It will return `Ok(None)` if the
+    /// buffer's length is 0, as hosts are not guaranteed to provide the correct number of inputs
+    /// and outputs in that case, and we don't need to construct a `Buffers` object as we will be
+    /// calling `Processor::flush` instead of `Processor::process`.
     pub unsafe fn get_buffers(
         &mut self,
         buses: &[BusInfo],
         input_bus_map: &[usize],
         output_bus_map: &[usize],
         config: &Config,
         data: &ProcessData,
-    ) -> Result<Buffers, ()> {
+    ) -> Result<Option<Buffers>, ()> {
         let len = data.numSamples as usize;
         if len > config.max_buffer_size {
             return Err(());
@@ -103,7 +115,7 @@ impl ScratchBuffers {
         let mut scratch = &mut self.buffers[..];
 
         if len == 0 {
-            return Ok(self.get_empty_buffers());
+            return Ok(None);
         }
 
         let input_count = data.numInputs as usize;
@@ -242,11 +254,8 @@ impl ScratchBuffers {
 
         self.output_ptrs.clear();
 
-        Ok(Buffers::from_raw_parts(&self.data, &self.ptrs, 0, len))
-    }
-
-    pub fn get_empty_buffers(&mut self) -> Buffers {
-        self.ptrs.fill(NonNull::dangling().as_ptr());
-        unsafe { Buffers::from_raw_parts(&self.data, &self.ptrs, 0, 0) }
+        Ok(Some(Buffers::from_raw_parts(
+            &self.data, &self.ptrs, 0, len,
+        )))
     }
 }

src/format/vst3/component.rs

@@ -487,10 +487,7 @@ impl<P: Plugin> IAudioProcessorTrait for Component<P> {
             }
 
             if !process_state.events.is_empty() {
-                processor.process(
-                    process_state.scratch_buffers.get_empty_buffers(),
-                    Events::new(&process_state.events),
-                );
+                processor.flush(Events::new(&process_state.events));
             }
 
             processor.reset();
@@ -563,7 +560,12 @@ impl<P: Plugin> IAudioProcessorTrait for Component<P> {
             }
         }
 
-        processor.process(buffers, Events::new(&process_state.events));
+        let events = Events::new(&process_state.events);
+        if let Some(buffers) = buffers {
+            processor.process(buffers, events);
+        } else {
+            processor.flush(events);
+        }
 
         kResultOk
     }

src/format/vst3/tests.rs

@@ -86,6 +86,7 @@ struct TestProcessor;
 
 impl Processor for TestProcessor {
     fn reset(&mut self) {}
+    fn flush(&mut self, _events: Events) {}
     fn process(&mut self, _buffers: Buffers, _events: Events) {}
 }
 

src/process.rs

@@ -11,5 +11,6 @@ pub struct Config {
 
 pub trait Processor: Send + Sized + 'static {
     fn reset(&mut self);
+    fn flush(&mut self, events: Events);
     fn process(&mut self, buffers: Buffers, events: Events);
 }