[ECO-1997] Add transaction version in MQTT price levels topic payload

doc/doc-site/docs/off-chain/dss/mqtt.md

@@ -45,10 +45,15 @@ A payload for this event looks like this:
 ```json
 {
   "price": 12345,
-  "size": 12345
+  "size": 12345,
+  "txn_version": 12345
 }
 ```
 
+*Remember to set `MQTT_PRICE_LEVELS=yes` in the `.env` file for docker compose
+(if running with docker compose, otherwise the `mqtt-publisher` executable must
+have that variable exported) if you wish to get price levels over MQTT.*
+
 ### PlaceLimitOrderEvents
 
 `place_limit_order/MARKET_ID/USER_ADDRESS/CUSTODIAN_ID/INTEGRATOR`

src/rust/mqtt-publisher/sqlx_queries/get_user_history_last_indexed_txn.sql

@@ -0,0 +1,4 @@
+SELECT
+    txn_version
+FROM
+    aggregator.user_history_last_indexed_txn;

src/rust/mqtt-publisher/src/main.rs

@@ -5,6 +5,7 @@ use bigdecimal::ToPrimitive;
 use chrono::{DateTime, Utc};
 use rumqttc::{AsyncClient, EventLoop, MqttOptions, QoS, Transport};
 use serde::{Deserialize, Serialize};
+use sqlx::{Executor, PgConnection};
 use sqlx_postgres::{PgListener, PgPool};
 use tokio::sync::RwLock;
 
@@ -136,16 +137,26 @@ async fn eventpoll_loop(mut eventloop: EventLoop) -> Result<()> {
 struct PriceLevel {
     price: u128,
     size: u128,
+    txn_version: u128,
 }
 
 async fn price_level_loop(db_url: &str, mqtt_client: Arc<RwLock<AsyncClient>>) -> Result<()> {
     let pool = PgPool::connect(&db_url).await?;
 
     loop {
         tokio::time::sleep(Duration::from_millis(50)).await;
+        let mut tx = pool.begin().await?;
+        tx.execute("SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;").await?;
         let data = sqlx::query_file!("sqlx_queries/get_price_levels.sql")
-        .fetch_all(&pool)
-        .await?;
+            .fetch_all(&mut tx as &mut PgConnection)
+            .await?;
+        let txn_version = sqlx::query_file!("sqlx_queries/get_user_history_last_indexed_txn.sql")
+            .fetch_one(&mut tx as &mut PgConnection)
+            .await?
+            .txn_version
+            .to_u128()
+            .ok_or(anyhow!("txn_version is too big"))?;
+        tx.rollback().await?;
         let mqtt_client = mqtt_client.read().await;
         for row in data {
             let topic = format!(
@@ -165,6 +176,7 @@ async fn price_level_loop(db_url: &str, mqtt_client: Arc<RwLock<AsyncClient>>) -
                     .ok_or(anyhow!("total_size is None"))?
                     .to_u128()
                     .ok_or(anyhow!("total_size is too big"))?,
+                txn_version,
             };
             mqtt_client
                 .publish(