Fix share sync update loss on flush failure

src/cortex-share/src/sync.rs

@@ -123,14 +123,33 @@ impl ShareSync {
 
         let count = updates.len();
 
-        for update in updates {
-            self.send_update(update).await?;
+        let mut sent = 0;
+        let mut remaining = updates.into_iter();
+        while let Some(update) = remaining.next() {
+            if let Err(error) = self.send_update(update.clone()).await {
+                let mut unsent = Vec::with_capacity(count - sent);
+                unsent.push(update);
+                unsent.extend(remaining);
+                self.requeue_front(unsent).await;
+                return Err(error);
+            }
+            sent += 1;
         }
 
         debug!("Flushed {} sync updates", count);
         Ok(count)
     }
 
+    async fn requeue_front(&self, mut updates: Vec<SyncUpdate>) {
+        if updates.is_empty() {
+            return;
+        }
+
+        let mut pending = self.pending.lock().await;
+        updates.extend(std::mem::take(&mut *pending));
+        *pending = updates;
+    }
+
     /// Send a single update.
     async fn send_update(&self, update: SyncUpdate) -> Result<()> {
         let response = self
@@ -192,6 +211,7 @@ impl Default for ShareSync {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use tokio::io::{AsyncReadExt, AsyncWriteExt};
 
     #[tokio::test]
     async fn test_queue_update() {
@@ -208,4 +228,47 @@ mod tests {
         let pending = sync.pending.lock().await;
         assert_eq!(pending.len(), 1);
     }
+
+    #[tokio::test]
+    async fn test_flush_requeues_failed_and_unsent_updates() {
+        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let addr = listener.local_addr().unwrap();
+
+        let server = tokio::spawn(async move {
+            let (mut stream, _) = listener.accept().await.unwrap();
+            let mut buf = [0; 2048];
+            let _ = stream.read(&mut buf).await.unwrap();
+            stream
+                .write_all(b"HTTP/1.1 200 OK\r\ncontent-length: 2\r\nconnection: close\r\n\r\nOK")
+                .await
+                .unwrap();
+
+            let (mut stream, _) = listener.accept().await.unwrap();
+            let mut buf = [0; 2048];
+            let _ = stream.read(&mut buf).await.unwrap();
+        });
+
+        let sync = ShareSync::new().with_api_url(format!("http://{}", addr));
+        let share = SharedSession::new(
+            "test-session".to_string(),
+            "https://example.com/share/123".to_string(),
+            "secret123".to_string(),
+        );
+
+        sync.queue_update(&share, "first", serde_json::json!({"order": 1}))
+            .await;
+        sync.queue_update(&share, "second", serde_json::json!({"order": 2}))
+            .await;
+        sync.queue_update(&share, "third", serde_json::json!({"order": 3}))
+            .await;
+
+        let result = sync.flush().await;
+        assert!(result.is_err(), "flush should report the failed update");
+
+        let pending = sync.pending.lock().await;
+        let keys: Vec<_> = pending.iter().map(|update| update.key.as_str()).collect();
+        assert_eq!(keys, vec!["second", "third"]);
+
+        server.await.unwrap();
+    }
 }