Implement protocol to notice server rollback

sync-server/src/app_state/database/queries.rs

@@ -9,6 +9,8 @@ use anyhow::{Context as _, Result, ensure};
 
 use crate::consts::MAX_EVENTS_PAGE_BYTES;
 
+const EMPTY_HISTORY_CHECKPOINT: &str = "0:empty";
+
 impl Database {
     pub async fn get_missing_document(
         tx: &mut Transaction<'_>,
@@ -236,3 +238,45 @@ impl Database {
         })
     }
 }
+
+impl Database {
+    pub async fn history_checkpoint(&self, vault: &VaultId) -> Result<String> {
+        let mut tx = self.create_readonly_transaction(vault).await?;
+
+        let row: Option<(i64, String)> = sqlx::query_as(
+            "SELECT event_id, event_token FROM events ORDER BY event_id DESC LIMIT 1",
+        )
+        .fetch_optional(&mut *tx)
+        .await?;
+
+        Ok(row.map_or_else(
+            || EMPTY_HISTORY_CHECKPOINT.to_owned(),
+            |(id, token)| format!("{id}:{token}"),
+        ))
+    }
+
+    pub async fn contains_checkpoint(&self, vault: &VaultId, checkpoint: &str) -> Result<bool> {
+        if checkpoint == EMPTY_HISTORY_CHECKPOINT {
+            return Ok(true);
+        }
+
+        let Some((id, token)) = checkpoint.split_once(':') else {
+            return Ok(false);
+        };
+
+        let Ok(id) = id.parse::<i64>() else {
+            return Ok(false);
+        };
+
+        let mut tx = self.create_readonly_transaction(vault).await?;
+        let exists: bool = sqlx::query_scalar(
+            "SELECT EXISTS(SELECT 1 FROM events WHERE event_id = ? AND event_token = ?)",
+        )
+        .bind(id)
+        .bind(token)
+        .fetch_one(&mut *tx)
+        .await?;
+
+        Ok(exists)
+    }
+}