sound: add NullBackend skeleton

crates/sound/Cargo.toml

@@ -9,6 +9,10 @@ keywords = ["vhost", "sound", "virtio-sound"]
 license = "Apache-2.0 OR BSD-3-Clause"
 edition = "2018"
 
+[features]
+default = ["null-backend"]
+null-backend = []
+
 [dependencies]
 clap = { version = "4.1",  features = ["derive"] }
 env_logger = "0.10"

crates/sound/src/audio_backends.rs

@@ -1,5 +1,10 @@
 // SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
 
+#[cfg(feature = "null-backend")]
+mod null;
+
+#[cfg(feature = "null-backend")]
+use self::null::NullBackend;
 use crate::{Error, Result, SoundRequest};
 
 pub trait AudioBackend {
@@ -10,6 +15,8 @@ pub trait AudioBackend {
 
 pub fn allocate_audio_backend(name: String) -> Result<Box<dyn AudioBackend + Send + Sync>> {
     match name.as_str() {
+        #[cfg(feature = "null-backend")]
+        "null" => Ok(Box::new(NullBackend::new())),
         _ => Err(Error::AudioBackendNotSupported),
     }
 }

crates/sound/src/audio_backends/null.rs

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
+
+use super::AudioBackend;
+use crate::{Error, Result, SoundRequest};
+
+pub struct NullBackend {}
+
+impl NullBackend {
+    pub fn new() -> Self {
+        NullBackend {}
+    }
+}
+
+impl AudioBackend for NullBackend {
+    fn write(&self, _req: &SoundRequest) -> Result<()> {
+        Ok(())
+    }
+
+    fn read(&self, req: &mut SoundRequest) -> Result<()> {
+        let buf = req.data_slice().ok_or(Error::SoundReqMissingData)?;
+        let zero_mem = vec![0u8; buf.len()];
+
+        buf.copy_from(&zero_mem);
+
+        Ok(())
+    }
+}