libsql: Fix WAL pull logic on last frame

The WAL sync protocol only allows you to fetch frames for a range.
While the protocol does not have mechanism to determine what is the
latest frame on the server for pull, it does signal with HTTP status
code 400 when you attempt to read a frame that does not exists.

Let's use this signal to fix WAL pull logic not to fail always. In the
future, we might want to consider extending the protocol to allow
clients to proble for latest frame. That, however, will make the
protocol a bit more chatty, so I am trying to avoid that as long as I
can.

libsql/src/local/database.rs

@@ -476,10 +476,13 @@ impl Database {
 
         loop {
             match sync_ctx.pull_one_frame(generation, frame_no).await {
-                Ok(frame) => {
+                Ok(Some(frame)) => {
                     conn.wal_insert_frame(&frame)?;
                     frame_no += 1;
                 }
+                Ok(None) => {
+                    break;
+                }
                 Err(e) => {
                     tracing::debug!("pull_one_frame error: {:?}", e);
                     err.replace(e);

libsql/src/sync.rs

@@ -107,7 +107,7 @@ impl SyncContext {
     }
 
     #[tracing::instrument(skip(self))]
-    pub(crate) async fn pull_one_frame(&mut self, generation: u32, frame_no: u32) -> Result<Bytes> {
+    pub(crate) async fn pull_one_frame(&mut self, generation: u32, frame_no: u32) -> Result<Option<Bytes>> {
         let uri = format!(
             "{}/sync/{}/{}/{}",
             self.sync_url,
@@ -232,7 +232,7 @@ impl SyncContext {
         }
     }
 
-    async fn pull_with_retry(&self, uri: String, max_retries: usize) -> Result<Bytes> {
+    async fn pull_with_retry(&self, uri: String, max_retries: usize) -> Result<Option<Bytes>> {
         let mut nr_retries = 0;
         loop {
             let mut req = http::Request::builder().method("GET").uri(uri.clone());
@@ -256,7 +256,10 @@ impl SyncContext {
                 let frame = hyper::body::to_bytes(res.into_body())
                     .await
                     .map_err(SyncError::HttpBody)?;
-                return Ok(frame);
+                return Ok(Some(frame));
+            }
+            if res.status() == StatusCode::BAD_REQUEST {
+                return Ok(None);
             }
             // If we've retried too many times or the error is not a server error,
             // return the error.