validator: Forge a confirmed root before halting for RPC inspection (bp #10061) (#10066)

* Forge a confirmed root before halting for RPC inspection (#10061)

(cherry picked from commit 1da1667920)

# Conflicts:
#	core/src/commitment.rs

* Update commitment.rs

Co-authored-by: Michael Vines <mvines@gmail.com>

core/src/commitment.rs

@@ -162,7 +162,6 @@ impl BlockCommitmentCache {
         }
     }
 
-    #[cfg(test)]
     pub(crate) fn set_get_largest_confirmed_root(&mut self, root: Slot) {
         self.largest_confirmed_root = root;
     }

core/src/validator.rs

@@ -314,6 +314,13 @@ impl Validator {
         );
 
         if config.dev_halt_at_slot.is_some() {
+            // Simulate a confirmed root to avoid RPC errors with CommitmentmentConfig::max() and
+            // to ensure RPC endpoints like getConfirmedBlock, which require a confirmed root, work
+            block_commitment_cache
+                .write()
+                .unwrap()
+                .set_get_largest_confirmed_root(bank_forks.read().unwrap().root());
+
             // Park with the RPC service running, ready for inspection!
             warn!("Validator halted");
             std::thread::park();