Support alternative read-replica PgPool (#4374)

* Add ReadOnlyPgPool

* Clippy, fmt

apps/labrinth/src/database/postgres_database.rs

@@ -2,10 +2,35 @@ use prometheus::{IntGauge, Registry};
 use sqlx::migrate::MigrateDatabase;
 use sqlx::postgres::{PgPool, PgPoolOptions};
 use sqlx::{Connection, PgConnection, Postgres};
+use std::ops::{Deref, DerefMut};
 use std::time::Duration;
 use tracing::info;
 
-pub async fn connect() -> Result<PgPool, sqlx::Error> {
+#[derive(Clone)]
+#[repr(transparent)]
+pub struct ReadOnlyPgPool(PgPool);
+
+impl From<PgPool> for ReadOnlyPgPool {
+    fn from(pool: PgPool) -> Self {
+        ReadOnlyPgPool(pool)
+    }
+}
+
+impl Deref for ReadOnlyPgPool {
+    type Target = PgPool;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl DerefMut for ReadOnlyPgPool {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+pub async fn connect_all() -> Result<(PgPool, ReadOnlyPgPool), sqlx::Error> {
     info!("Initializing database connection");
     let database_url =
         dotenvy::var("DATABASE_URL").expect("`DATABASE_URL` not in .env");
@@ -26,7 +51,29 @@ pub async fn connect() -> Result<PgPool, sqlx::Error> {
         .connect(&database_url)
         .await?;
 
-    Ok(pool)
+    if let Ok(url) = dotenvy::var("READONLY_DATABASE_URL") {
+        let ro_pool = PgPoolOptions::new()
+            .min_connections(
+                dotenvy::var("READONLY_DATABASE_MIN_CONNECTIONS")
+                    .ok()
+                    .and_then(|x| x.parse().ok())
+                    .unwrap_or(0),
+            )
+            .max_connections(
+                dotenvy::var("READONLY_DATABASE_MAX_CONNECTIONS")
+                    .ok()
+                    .and_then(|x| x.parse().ok())
+                    .unwrap_or(1),
+            )
+            .max_lifetime(Some(Duration::from_secs(60 * 60)))
+            .connect(&url)
+            .await?;
+
+        Ok((pool, ReadOnlyPgPool(ro_pool)))
+    } else {
+        let ro = ReadOnlyPgPool(pool.clone());
+        Ok((pool, ro))
+    }
 }
 pub async fn check_for_migrations() -> Result<(), sqlx::Error> {
     let uri = dotenvy::var("DATABASE_URL").expect("`DATABASE_URL` not in .env");