Prefetch next segmented chunk to reduce boundary lag

src/player.rs

@@ -550,19 +550,24 @@ struct SegmentedStreamSource {
     key: Option<[u8; 16]>,
     pending: VecDeque<u8>,
     finished: bool,
+    prefetch_segment: Option<usize>,
+    prefetch_result: Option<Arc<std::sync::Mutex<Option<io::Result<Vec<u8>>>>>>,
+    prefetch_ready: Option<Arc<AtomicBool>>,
 }
 
-impl SegmentedStreamSource {
-    fn fetch_segment(&self, segment: usize) -> io::Result<Vec<u8>> {
-        let url = self.url_template.replace("$SEGMENT$", &segment.to_string());
+fn fetch_segment_with_client(
+    client: &reqwest::blocking::Client,
+    url_template: &str,
+    segment: usize,
+) -> io::Result<Vec<u8>> {
+    let url = url_template.replace("$SEGMENT$", &segment.to_string());
     for attempt in 0..3 {
         if attempt > 0 {
             std::thread::sleep(std::time::Duration::from_millis(300 * attempt as u64));
         }
-            match self.client.get(&url).send() {
+        match client.get(&url).send() {
             Ok(mut response) if response.status().is_success() => {
-                    let mut data =
-                        Vec::with_capacity(response.content_length().unwrap_or(0) as usize);
+                let mut data = Vec::with_capacity(response.content_length().unwrap_or(0) as usize);
                 response
                     .copy_to(&mut data)
                     .map_err(|e| io::Error::other(format!("segment read failed: {}", e)))?;
@@ -585,6 +590,48 @@ impl SegmentedStreamSource {
         }
     }
     Err(io::Error::other("segment fetch retries exhausted"))
+}
+
+impl SegmentedStreamSource {
+    fn fetch_segment(&self, segment: usize) -> io::Result<Vec<u8>> {
+        fetch_segment_with_client(&self.client, &self.url_template, segment)
+    }
+
+    fn next_segment_candidate(&self) -> Option<usize> {
+        if self.include_segment_one {
+            Some(1)
+        } else if self.next_segment <= self.n_segments {
+            Some(self.next_segment)
+        } else {
+            None
+        }
+    }
+
+    fn ensure_prefetch(&mut self) {
+        if self.prefetch_result.is_some() {
+            return;
+        }
+        let Some(segment) = self.next_segment_candidate() else {
+            return;
+        };
+
+        let client = self.client.clone();
+        let url_template = self.url_template.clone();
+        let result_slot = Arc::new(std::sync::Mutex::new(None));
+        let ready = Arc::new(AtomicBool::new(false));
+        let result_slot_bg = result_slot.clone();
+        let ready_bg = ready.clone();
+        std::thread::spawn(move || {
+            let result = fetch_segment_with_client(&client, &url_template, segment);
+            if let Ok(mut slot) = result_slot_bg.lock() {
+                *slot = Some(result);
+            }
+            ready_bg.store(true, Ordering::Release);
+        });
+
+        self.prefetch_segment = Some(segment);
+        self.prefetch_result = Some(result_slot);
+        self.prefetch_ready = Some(ready);
     }
 
     fn fill_pending(&mut self) -> io::Result<()> {
@@ -605,13 +652,36 @@ impl SegmentedStreamSource {
             seg
         };
 
-        let mut data = self.fetch_segment(segment_to_fetch)?;
+        let mut data = if self.prefetch_segment == Some(segment_to_fetch) {
+            let result_slot = self.prefetch_result.take();
+            let ready = self.prefetch_ready.take();
+            self.prefetch_segment = None;
+            match (result_slot, ready) {
+                (Some(slot), Some(ready_flag)) => {
+                    while !ready_flag.load(Ordering::Acquire) {
+                        std::thread::sleep(std::time::Duration::from_millis(2));
+                    }
+                    let mut guard = slot
+                        .lock()
+                        .map_err(|_| io::Error::other("segment prefetch lock poisoned"))?;
+                    guard
+                        .take()
+                        .ok_or_else(|| io::Error::other("segment prefetch missing result"))??
+                }
+                (None, _) => self.fetch_segment(segment_to_fetch)?,
+                _ => self.fetch_segment(segment_to_fetch)?,
+            }
+        } else {
+            self.fetch_segment(segment_to_fetch)?
+        };
         let frames = decrypt_and_extract_frames(&mut data, self.key.as_ref());
         if frames.is_empty() {
             self.pending.extend(data);
         } else {
             self.pending.extend(frames);
         }
+
+        self.ensure_prefetch();
         Ok(())
     }
 }
@@ -988,7 +1058,7 @@ fn play_segmented_stream(
         skip_within_segment_ms = skip_within_segment_ms.saturating_add(segment_duration_ms);
     }
 
-    let source = SegmentedStreamSource {
+    let mut source = SegmentedStreamSource {
         client,
         url_template: url_template.to_string(),
         n_segments: n_segments as usize,
@@ -997,7 +1067,11 @@ fn play_segmented_stream(
         key,
         pending,
         finished: false,
+        prefetch_segment: None,
+        prefetch_result: None,
+        prefetch_ready: None,
     };
+    source.ensure_prefetch();
     let mss = MediaSourceStream::new(Box::new(source), Default::default());
 
     let mut hint = Hint::new();