Add some safety comments about using bootstrap

Author: avinassh

libsql/src/local/database.rs

@@ -465,6 +465,9 @@ impl Database {
     /// Sync WAL frames to remote.
     pub async fn sync_offline(&self) -> Result<crate::database::Replicated> {
         let mut sync_ctx = self.sync_ctx.as_ref().unwrap().lock().await;
+        // it is important we call `bootstrap` before we `sync`. Because sync uses a connection
+        // to the db and during bootstrap we replace the sqlite db file. This can lead to
+        // inconsistencies and data corruption.
         crate::sync::bootstrap_db(&mut sync_ctx).await?;
         let conn = self.connect()?;
         crate::sync::sync_offline(&mut sync_ctx, &conn).await

libsql/src/sync.rs

@@ -686,6 +686,8 @@ async fn atomic_write<P: AsRef<Path>>(path: P, data: &[u8]) -> Result<()> {
 
 /// bootstrap_db brings the .db file from remote, if required. If the .db file already exists, then
 /// it does nothing. Calling this function multiple times is safe.
+/// However, make sure there are no existing active connections to the db file as this method can
+/// replace it
 pub async fn bootstrap_db(sync_ctx: &mut SyncContext) -> Result<()> {
     // todo: we are checking with the remote server only during initialisation. ideally,
     // we need to do this when we notice a large gap in generations, when bootstrapping is cheaper