Refactor and fix usage of get_session_index() and get_session_info_by_index() (#4735)

* Rename/refactor around get_session_index

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* choose proper head for fetching session

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* revert rename

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* fix comments

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* renaming and more comments

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* review feedback

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* Run Fetch task in correct session

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* Log warning when ancestors unavailable

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* Fixes

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* fix

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

polkadot/node/network/statement-distribution/src/lib.rs

@@ -624,9 +624,9 @@ struct ActiveHeadData {
 	statements: IndexMap<StoredStatementComparator, SignedFullStatement>,
 	/// Large statements we are waiting for with associated meta data.
 	waiting_large_statements: HashMap<CandidateHash, LargeStatementStatus>,
-	/// The validators at this head.
+	/// The parachain validators at the head's child session index.
 	validators: Vec<ValidatorId>,
-	/// The session index this head is at.
+	/// The current session index of this fork.
 	session_index: sp_staking::SessionIndex,
 	/// How many `Seconded` statements we've seen per validator.
 	seconded_counts: HashMap<ValidatorIndex, usize>,
@@ -1798,8 +1798,9 @@ impl StatementDistributionSubsystem {
 						"New active leaf",
 					);
 
+					// Retrieve the parachain validators at the child of the head we track.
 					let session_index =
-						runtime.get_session_index(ctx.sender(), relay_parent).await?;
+						runtime.get_session_index_for_child(ctx.sender(), relay_parent).await?;
 					let info = runtime
 						.get_session_info_by_index(ctx.sender(), relay_parent, session_index)
 						.await?;