⚡ Optimize AudioPlayer::play by offloading sync file I/O to spawn_blocking (#29)

* perf(audio_player): offload synchronous I/O and decoder init to spawn_blocking

Moved synchronous file system operations (`fs::File::open` and `file_path.exists()`) and CPU-bound decoder initialization (`Decoder::try_from`) inside the async `AudioPlayer::play` method to `tokio::task::spawn_blocking`.
This prevents starving the Tokio worker threads during disk operations and significantly reduces event loop latency.

Co-authored-by: arabianq <55220741+arabianq@users.noreply.github.com>

* perf(audio_player): offload synchronous I/O and decoder init to spawn_blocking

Moved synchronous file system operations (`fs::File::open` and `file_path.exists()`) and CPU-bound decoder initialization (`Decoder::try_from`) inside the async `AudioPlayer::play` method to `tokio::task::spawn_blocking`.
This prevents starving the Tokio worker threads during disk operations and significantly reduces event loop latency.

Co-authored-by: arabianq <55220741+arabianq@users.noreply.github.com>

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

tests/perf_play.rs

@@ -0,0 +1,113 @@
+use rodio::{DeviceSinkBuilder, MixerDeviceSink};
+use std::fs;
+use std::path::Path;
+use std::sync::Arc;
+use std::time::Instant;
+use tokio::sync::Mutex;
+
+// A mock of AudioPlayer to isolate the play method's blocking behavior.
+// We only implement the relevant part of the logic that needs optimizing.
+pub struct AudioPlayerMock {
+    pub tracks: std::collections::HashMap<u32, ()>,
+    pub next_id: u32,
+    pub volume: f32,
+}
+
+impl AudioPlayerMock {
+    pub fn new() -> Self {
+        AudioPlayerMock {
+            tracks: std::collections::HashMap::new(),
+            next_id: 1,
+            volume: 1.0,
+        }
+    }
+
+    pub async fn play(
+        &mut self,
+        file_path: &Path,
+        concurrent: bool,
+    ) -> Result<u32, Box<dyn std::error::Error + Send + Sync>> {
+        if !file_path.exists() {
+            return Err(format!("File does not exist: {}", file_path.display()).into());
+        }
+
+        let path_buf = file_path.to_path_buf();
+        let _file = tokio::task::spawn_blocking(move || {
+            // Simulate some blocking work like Decoder::try_from which reads file headers
+            let _f = fs::File::open(&path_buf).unwrap();
+
+            // Emulate the actual time spent reading file and decoding header (which is what Decoder::try_from does)
+            std::thread::sleep(std::time::Duration::from_millis(100)); // Simulate slow disk/decode
+            _f
+        })
+        .await?;
+
+        if !concurrent {
+            self.tracks.clear();
+        }
+
+        let id = self.next_id;
+        self.next_id += 1;
+        self.tracks.insert(id, ());
+
+        Ok(id)
+    }
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_performance_blocking() {
+    println!("Setting up mock environment...");
+
+    // Create a dummy file to read
+    let test_file = Path::new("test_dummy.wav");
+    fs::write(test_file, "dummy content").unwrap();
+
+    let player = Arc::new(Mutex::new(AudioPlayerMock::new()));
+
+    println!("Starting benchmark for synchronous behavior in async fn...");
+
+    // We launch a background task that measures event loop latency.
+    // If the main tasks block the executor, this task will suffer high latency.
+    let latency_task = tokio::spawn(async {
+        let mut max_latency = std::time::Duration::from_secs(0);
+        for _ in 0..50 {
+            let start = Instant::now();
+            tokio::task::yield_now().await;
+            let elapsed = start.elapsed();
+            if elapsed > max_latency {
+                max_latency = elapsed;
+            }
+            tokio::time::sleep(std::time::Duration::from_millis(5)).await;
+        }
+        max_latency
+    });
+
+    // Launch multiple play operations
+    let mut tasks = vec![];
+    let start_time = Instant::now();
+    for _ in 0..10 {
+        let player_clone = Arc::clone(&player);
+        let file_path = test_file.to_path_buf();
+        tasks.push(tokio::spawn(async move {
+            let mut p = player_clone.lock().await;
+            let _ = p.play(&file_path, true).await;
+        }));
+    }
+
+    // Wait for all tasks to finish
+    for t in tasks {
+        let _ = t.await;
+    }
+    let total_time = start_time.elapsed();
+
+    let max_latency = latency_task.await.unwrap();
+
+    println!("Total execution time: {:?}", total_time);
+    println!(
+        "Max event loop latency (blocking indicator): {:?}",
+        max_latency
+    );
+
+    // Cleanup
+    fs::remove_file(test_file).unwrap();
+}