Better error for spurious cache misses. (#4669)

Happened on Kusama for some not yet known reason.

polkadot/node/network/availability-distribution/src/error.rs

@@ -18,6 +18,7 @@
 //! Error handling related code and Error/Result definitions.
 
 use polkadot_node_network_protocol::request_response::outgoing::RequestError;
+use polkadot_primitives::v1::SessionIndex;
 use thiserror::Error;
 
 use futures::channel::oneshot;
@@ -76,8 +77,8 @@ pub enum NonFatal {
 	QueryAvailableDataResponseChannel(#[source] oneshot::Canceled),
 
 	/// We tried accessing a session that was not cached.
-	#[error("Session is not cached.")]
-	NoSuchCachedSession,
+	#[error("Session {missing_session} is not cached, cached sessions: {available_sessions:?}.")]
+	NoSuchCachedSession { available_sessions: Vec<SessionIndex>, missing_session: SessionIndex },
 
 	/// Sending request response failed (Can happen on timeouts for example).
 	#[error("Sending a request's response failed.")]
@@ -120,7 +121,7 @@ pub fn log_error(result: Result<()>, ctx: &'static str) -> std::result::Result<(
 			match error {
 				NonFatal::UnexpectedPoV |
 				NonFatal::InvalidValidatorIndex |
-				NonFatal::NoSuchCachedSession |
+				NonFatal::NoSuchCachedSession { .. } |
 				NonFatal::QueryAvailableDataResponseChannel(_) |
 				NonFatal::QueryChunkResponseChannel(_) =>
 					tracing::warn!(target: LOG_TARGET, error = %error, ctx),

polkadot/node/network/availability-distribution/src/requester/session_cache.rs

@@ -144,10 +144,13 @@ impl SessionCache {
 	/// We assume validators in a group are tried in reverse order, so the reported bad validators
 	/// will be put at the beginning of the group.
 	pub fn report_bad(&mut self, report: BadValidators) -> crate::Result<()> {
-		let session = self
-			.session_info_cache
-			.get_mut(&report.session_index)
-			.ok_or(NonFatal::NoSuchCachedSession)?;
+		let available_sessions = self.session_info_cache.iter().map(|(k, _)| *k).collect();
+		let session = self.session_info_cache.get_mut(&report.session_index).ok_or(
+			NonFatal::NoSuchCachedSession {
+				available_sessions,
+				missing_session: report.session_index,
+			},
+		)?;
 		let group = session.validator_groups.get_mut(report.group_index.0 as usize).expect(
 			"A bad validator report must contain a valid group for the reported session. qed.",
 		);