session: implement Debug for Session

Deriving Debug trait for Session may be helpful in debugging.

Author: wprzytula

examples/custom_load_balancing_policy.rs

@@ -7,6 +7,7 @@ use scylla::{
 use std::{env, sync::Arc};
 
 /// Example load balancing policy that prefers nodes from favorite datacenter
+#[derive(Debug)]
 struct CustomLoadBalancingPolicy {
     fav_datacenter_name: String,
 }

scylla/src/transport/load_balancing/dc_aware_round_robin.rs

@@ -7,6 +7,7 @@ use std::sync::{
 use tracing::trace;
 
 /// A data-center aware Round-robin load balancing policy.
+#[derive(Debug)]
 pub struct DcAwareRoundRobinPolicy {
     index: AtomicUsize,
     local_dc: String,

scylla/src/transport/load_balancing/mod.rs

@@ -35,7 +35,7 @@ impl<'a> Statement<'a> {
 pub type Plan<'a> = Box<dyn Iterator<Item = Arc<Node>> + Send + Sync + 'a>;
 
 /// Policy that decides which nodes to contact for each query
-pub trait LoadBalancingPolicy: Send + Sync {
+pub trait LoadBalancingPolicy: Send + Sync + std::fmt::Debug {
     /// It is used for each query to find which nodes to query first
     fn plan<'a>(&self, statement: &Statement, cluster: &'a ClusterData) -> Plan<'a>;
 

scylla/src/transport/load_balancing/round_robin.rs

@@ -7,6 +7,7 @@ use std::sync::{
 use tracing::trace;
 
 /// A Round-robin load balancing policy.
+#[derive(Debug)]
 pub struct RoundRobinPolicy {
     index: AtomicUsize,
 }

scylla/src/transport/load_balancing/token_aware.rs

@@ -9,6 +9,7 @@ use std::{collections::HashMap, sync::Arc};
 use tracing::trace;
 
 /// A wrapper load balancing policy that adds token awareness to a child policy.
+#[derive(Debug)]
 pub struct TokenAwarePolicy {
     child_policy: Box<dyn ChildLoadBalancingPolicy>,
 }
@@ -430,6 +431,7 @@ mod tests {
 
     // Used as child policy for TokenAwarePolicy tests
     // Forwards plan passed to it in apply_child_policy() method
+    #[derive(Debug)]
     struct DumbPolicy {}
 
     impl LoadBalancingPolicy for DumbPolicy {

scylla/src/transport/session.rs

@@ -25,7 +25,7 @@ use crate::query::Query;
 use crate::routing::Token;
 use crate::statement::{Consistency, SerialConsistency};
 use crate::tracing::{GetTracingConfig, TracingEvent, TracingInfo};
-use crate::transport::cluster::{Cluster, ClusterData};
+use crate::transport::cluster::{Cluster, ClusterData, ClusterNeatDebug};
 use crate::transport::connection::{
     BatchResult, Connection, ConnectionConfig, VerifiedKeyspaceName,
 };
@@ -65,6 +65,29 @@ pub struct Session {
     auto_await_schema_agreement_timeout: Option<Duration>,
 }
 
+/// This implementation deliberately omits some details from Cluster in order
+/// to avoid cluttering the print with much information of little usability.
+impl std::fmt::Debug for Session {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("Session")
+            .field("cluster", &ClusterNeatDebug(&self.cluster))
+            .field("load_balancer", &self.load_balancer)
+            .field("schema_agreement_interval", &self.schema_agreement_interval)
+            .field("retry_policy", &self.retry_policy)
+            .field(
+                "speculative_execution_policy",
+                &self.speculative_execution_policy,
+            )
+            .field("metrics", &self.metrics)
+            .field("default_consistency", &self.default_consistency)
+            .field(
+                "auto_await_schema_agreement_timeout",
+                &self.auto_await_schema_agreement_timeout,
+            )
+            .finish()
+    }
+}
+
 /// Configuration options for [`Session`].
 /// Can be created manually, but usually it's easier to use
 /// [SessionBuilder](super::session_builder::SessionBuilder)