libsql: Improve WAL frame push logic

The server returns its maximum frame number so let's use that as a hint
to avoid pushing frames it already knows about. IOW, let's start by
sending the first frame in the WAL, but immediately skip to a the
server's max frame number. Later we should also cache the server max
frame number on client side to make the first push start from the right
frame.

libsql/Cargo.toml

@@ -42,7 +42,7 @@ fallible-iterator = { version = "0.3", optional = true }
 
 libsql_replication = { version = "0.6", path = "../libsql-replication", optional = true }
 async-stream = { version = "0.3.5", optional = true }
-reqwest = { version = "0.12.9", default-features = false, features = [ "rustls-tls" ], optional = true }
+reqwest = { version = "0.12.9", default-features = false, features = [ "rustls-tls", "json" ], optional = true }
 
 [dev-dependencies]
 criterion = { version = "0.5", features = ["html_reports", "async", "async_futures", "async_tokio"] }

libsql/src/local/database.rs

@@ -397,8 +397,16 @@ impl Database {
         let start_frame_no = sync_ctx.durable_frame_num + 1;
         let end_frame_no = max_frame_no;
 
-        for frame_no in start_frame_no..end_frame_no+1 {
-            self.push_one_frame(&conn, &sync_ctx, generation, frame_no, page_size).await?;
+        let mut frame_no = start_frame_no;
+        while frame_no <= end_frame_no {
+            // The server returns its maximum frame number. To avoid resending
+            // frames the server already knows about, we need to update the
+            // frame number to the one returned by the server.
+            let max_frame_no = self.push_one_frame(&conn, &sync_ctx, generation, frame_no, page_size).await?;
+            if max_frame_no > frame_no {
+                frame_no = max_frame_no;
+            }
+            frame_no += 1;
         }
 
         let frame_count = end_frame_no - start_frame_no + 1;
@@ -409,7 +417,7 @@ impl Database {
     }
 
     #[cfg(feature = "sync")]
-    async fn push_one_frame(&self, conn: &Connection, sync_ctx: &SyncContext, generation: u32, frame_no: u32, page_size: u32) -> Result<()> {
+    async fn push_one_frame(&self, conn: &Connection, sync_ctx: &SyncContext, generation: u32, frame_no: u32, page_size: u32) -> Result<u32> {
         let frame_size: usize = 24+page_size as usize;
         let frame = vec![0; frame_size];
         let rc = unsafe {
@@ -419,12 +427,12 @@ impl Database {
             return Err(crate::errors::Error::SqliteFailure(rc as std::ffi::c_int, format!("Failed to get frame: {}", frame_no)));
         }
         let uri = format!("{}/sync/{}/{}/{}", sync_ctx.sync_url, generation, frame_no, frame_no+1);
-        self.push_with_retry(uri, &sync_ctx.auth_token, frame.to_vec(), sync_ctx.max_retries).await?;
-        Ok(())
+        let max_frame_no = self.push_with_retry(uri, &sync_ctx.auth_token, frame.to_vec(), sync_ctx.max_retries).await?;
+        Ok(max_frame_no)
     }
 
     #[cfg(feature = "sync")]
-    async fn push_with_retry(&self, uri: String, auth_token: &Option<String>, frame: Vec<u8>, max_retries: usize) -> Result<()> {
+    async fn push_with_retry(&self, uri: String, auth_token: &Option<String>, frame: Vec<u8>, max_retries: usize) -> Result<u32> {
         let mut nr_retries = 0;
         loop {
             let client = reqwest::Client::new();
@@ -437,7 +445,9 @@ impl Database {
             }
             let res = builder.body(frame.to_vec()).send().await.unwrap();
             if res.status().is_success() {
-                return Ok(());
+                let resp = res.json::<serde_json::Value>().await.unwrap();
+                let max_frame_no = resp.get("max_frame_no").unwrap().as_u64().unwrap();
+                return Ok(max_frame_no as u32);
             }
             if nr_retries > max_retries {
                 return Err(crate::errors::Error::ConnectionFailed(format!("Failed to push frame: {}", res.status())));