Fix exclude primary with no replicas

mise.toml

@@ -1,3 +1,4 @@
 [tools]
+"rust" = { version = "1.91.0" }
 "cargo:cargo-nextest" = "latest"
 "cargo:cargo-watch" = "latest"

pgdog/src/backend/pool/lb/mod.rs

@@ -277,7 +277,10 @@ impl LoadBalancer {
         let primary_reads = match self.rw_split {
             IncludePrimary => true,
             IncludePrimaryIfReplicaBanned => candidates.iter().any(|target| target.ban.banned()),
-            ExcludePrimary => false,
+            // we read from the primary if we have no replicas
+            ExcludePrimary => !candidates
+                .iter()
+                .any(|target| target.role() == Role::Replica),
         };
 
         if !primary_reads {

pgdog/src/backend/pool/lb/test.rs

@@ -393,6 +393,42 @@ async fn test_read_write_split_include_primary() {
     replicas.shutdown();
 }
 
+#[tokio::test]
+async fn test_read_write_split_exclude_primary_no_replicas() {
+    let primary_config = create_test_pool_config("127.0.0.1", 5432);
+    let primary_pool = Pool::new(&primary_config);
+    primary_pool.launch();
+
+    let replica_configs = [];
+
+    let replicas = LoadBalancer::new(
+        &Some(primary_pool),
+        &replica_configs,
+        LoadBalancingStrategy::RoundRobin,
+        ReadWriteSplit::ExcludePrimary,
+    );
+    replicas.launch();
+
+    let request = Request::default();
+
+    // Try getting connections multiple times and we have primary in the set
+    let mut used_pool_ids = HashSet::new();
+    for _ in 0..2 {
+        let conn = replicas.get(&request).await.unwrap();
+        used_pool_ids.insert(conn.pool.id());
+    }
+
+    // Should use only primary
+    assert_eq!(used_pool_ids.len(), 1);
+
+    // Verify primary pool ID is in the set of used pools
+    let primary_id = replicas.primary().unwrap().id();
+    assert!(used_pool_ids.contains(&primary_id));
+
+    // Shutdown
+    replicas.shutdown();
+}
+
 #[tokio::test]
 async fn test_read_write_split_exclude_primary_no_primary() {
     // Test exclude primary setting when no primary exists