lbp_config: make LBP optional

In this commit, we make `load_balancing_kind` optional in
`LoadBalancingConfig`. Now, there are two cases:
- for cluster's LBP, there is a default value (round-robin) which
  driver's going to use if user did not provide LBP. Thus, the `unwrap_or`
  call in `LoadBalancingConfig::build`
- for exec profile's LBP, if user did not provide an LBP kind (load_balancing_kind is None),
  then, according to cpp-driver, cluster's LBP should be used (instead of some predefined default).

After this commit, we are finally consistent with cpp-driver.
Now, if user did not specify an LBP for given execution profile,
the cluster's default LBP (session-wide LBP) will be assigned to the profile.

scylla-rust-wrapper/src/cluster.rs

@@ -51,21 +51,26 @@ const DRIVER_VERSION: &str = env!("CARGO_PKG_VERSION");
 pub(crate) struct LoadBalancingConfig {
     pub(crate) token_awareness_enabled: bool,
     pub(crate) token_aware_shuffling_replicas_enabled: bool,
-    pub(crate) load_balancing_kind: LoadBalancingKind,
+    pub(crate) load_balancing_kind: Option<LoadBalancingKind>,
     pub(crate) latency_awareness_enabled: bool,
     pub(crate) latency_awareness_builder: LatencyAwarenessBuilder,
 }
 impl LoadBalancingConfig {
     // This is `async` to prevent running this function from beyond tokio context,
     // as it results in panic due to DefaultPolicyBuilder::build() spawning a tokio task.
     pub(crate) async fn build(self) -> Arc<dyn LoadBalancingPolicy> {
+        let load_balancing_kind = self
+            .load_balancing_kind
+            // Round robin is chosen by default for cluster wide LBP.
+            .unwrap_or(LoadBalancingKind::RoundRobin);
+
         let mut builder = DefaultPolicyBuilder::new().token_aware(self.token_awareness_enabled);
         if self.token_awareness_enabled {
             // Cpp-driver enables shuffling replicas only if token aware routing is enabled.
             builder =
                 builder.enable_shuffling_replicas(self.token_aware_shuffling_replicas_enabled);
         }
-        if let LoadBalancingKind::DcAware { local_dc } = self.load_balancing_kind {
+        if let LoadBalancingKind::DcAware { local_dc } = load_balancing_kind {
             builder = builder.prefer_datacenter(local_dc).permit_dc_failover(true)
         }
         if self.latency_awareness_enabled {
@@ -79,7 +84,7 @@ impl Default for LoadBalancingConfig {
         Self {
             token_awareness_enabled: true,
             token_aware_shuffling_replicas_enabled: true,
-            load_balancing_kind: LoadBalancingKind::RoundRobin,
+            load_balancing_kind: None,
             latency_awareness_enabled: false,
             latency_awareness_builder: Default::default(),
         }
@@ -456,7 +461,7 @@ pub unsafe extern "C" fn cass_cluster_set_credentials_n(
 #[no_mangle]
 pub unsafe extern "C" fn cass_cluster_set_load_balance_round_robin(cluster_raw: *mut CassCluster) {
     let cluster = ptr_to_ref_mut(cluster_raw);
-    cluster.load_balancing_config.load_balancing_kind = LoadBalancingKind::RoundRobin;
+    cluster.load_balancing_config.load_balancing_kind = Some(LoadBalancingKind::RoundRobin);
 }
 
 #[no_mangle]
@@ -495,7 +500,7 @@ pub(crate) unsafe fn set_load_balance_dc_aware_n(
         .unwrap()
         .to_string();
 
-    load_balancing_config.load_balancing_kind = LoadBalancingKind::DcAware { local_dc };
+    load_balancing_config.load_balancing_kind = Some(LoadBalancingKind::DcAware { local_dc });
 
     CassError::CASS_OK
 }
@@ -850,10 +855,7 @@ mod tests {
                 /* Test valid configurations */
                 let cluster = ptr_to_ref(cluster_raw);
                 {
-                    assert_matches!(
-                        cluster.load_balancing_config.load_balancing_kind,
-                        LoadBalancingKind::RoundRobin
-                    );
+                    assert_matches!(cluster.load_balancing_config.load_balancing_kind, None);
                     assert!(cluster.load_balancing_config.token_awareness_enabled);
                     assert!(!cluster.load_balancing_config.latency_awareness_enabled);
                 }
@@ -882,7 +884,7 @@ mod tests {
 
                     let load_balancing_kind = &cluster.load_balancing_config.load_balancing_kind;
                     match load_balancing_kind {
-                        LoadBalancingKind::DcAware { local_dc } => {
+                        Some(LoadBalancingKind::DcAware { local_dc }) => {
                             assert_eq!(local_dc, "eu")
                         }
                         _ => panic!("Expected preferred dc"),

scylla-rust-wrapper/src/exec_profile.rs

@@ -42,10 +42,19 @@ impl CassExecProfile {
         }
     }
 
-    pub(crate) async fn build(self) -> ExecutionProfile {
-        self.inner
-            .load_balancing_policy(self.load_balancing_config.build().await)
-            .build()
+    pub(crate) async fn build(
+        self,
+        cluster_default_profile: &ExecutionProfile,
+    ) -> ExecutionProfile {
+        let load_balacing = if self.load_balancing_config.load_balancing_kind.is_some() {
+            self.load_balancing_config.build().await
+        } else {
+            // If load balancing config does not have LB kind defined,
+            // we make use of cluster's LBP.
+            cluster_default_profile.get_load_balancing_policy().clone()
+        };
+
+        self.inner.load_balancing_policy(load_balacing).build()
     }
 }
 
@@ -353,7 +362,7 @@ pub unsafe extern "C" fn cass_execution_profile_set_load_balance_round_robin(
     profile: *mut CassExecProfile,
 ) -> CassError {
     let profile_builder = ptr_to_ref_mut(profile);
-    profile_builder.load_balancing_config.load_balancing_kind = LoadBalancingKind::RoundRobin;
+    profile_builder.load_balancing_config.load_balancing_kind = Some(LoadBalancingKind::RoundRobin);
 
     CassError::CASS_OK
 }
@@ -473,10 +482,7 @@ mod tests {
                 /* Test valid configurations */
                 let profile = ptr_to_ref(profile_raw);
                 {
-                    assert_matches!(
-                        profile.load_balancing_config.load_balancing_kind,
-                        LoadBalancingKind::RoundRobin
-                    );
+                    assert_matches!(profile.load_balancing_config.load_balancing_kind, None);
                     assert!(profile.load_balancing_config.token_awareness_enabled);
                     assert!(!profile.load_balancing_config.latency_awareness_enabled);
                 }
@@ -505,7 +511,7 @@ mod tests {
 
                     let load_balancing_kind = &profile.load_balancing_config.load_balancing_kind;
                     match load_balancing_kind {
-                        LoadBalancingKind::DcAware { local_dc } => {
+                        Some(LoadBalancingKind::DcAware { local_dc }) => {
                             assert_eq!(local_dc, "eu")
                         }
                         _ => panic!("Expected preferred dc"),

scylla-rust-wrapper/src/session.rs

@@ -109,12 +109,18 @@ impl CassSessionInner {
                 "Already connecting, closing, or connected".msg(),
             ));
         }
+
+        let mut session_builder = session_builder_fut.await;
+        let default_profile = session_builder
+            .config
+            .default_execution_profile_handle
+            .to_profile();
+
         let mut exec_profile_map = HashMap::with_capacity(exec_profile_builder_map.len());
         for (name, builder) in exec_profile_builder_map {
-            exec_profile_map.insert(name, builder.build().await.into_handle());
+            exec_profile_map.insert(name, builder.build(&default_profile).await.into_handle());
         }
 
-        let mut session_builder = session_builder_fut.await;
         if let Some(keyspace) = keyspace {
             session_builder = session_builder.use_keyspace(keyspace, false);
         }