Comments clarifying non-emptiness of threadset (#35388)

core/src/banking_stage/transaction_scheduler/prio_graph_scheduler.rs

@@ -388,7 +388,8 @@ impl PrioGraphScheduler {
     /// If the `chain_thread` is available, this thread will be selected, regardless of
     /// load-balancing.
     ///
-    /// Panics if the `thread_set` is empty.
+    /// Panics if the `thread_set` is empty. This should never happen, see comment
+    /// on `ThreadAwareAccountLocks::try_lock_accounts`.
     fn select_thread(
         thread_set: ThreadSet,
         batches_per_thread: &[Vec<SanitizedTransaction>],

core/src/banking_stage/transaction_scheduler/thread_aware_account_locks.rs

@@ -65,6 +65,8 @@ impl ThreadAwareAccountLocks {
     /// `allowed_threads` is a set of threads that the caller restricts locking to.
     /// If accounts are schedulable, then they are locked for the thread
     /// selected by the `thread_selector` function.
+    /// `thread_selector` is only called if all accounts are schdulable, meaning
+    /// that the `thread_set` passed to `thread_selector` is non-empty.
     pub(crate) fn try_lock_accounts<'a>(
         &mut self,
         write_account_locks: impl Iterator<Item = &'a Pubkey> + Clone,