Ledger-tool recreate snapshot of starting snapshots

Co-authored-by: Carl Lin <carl@solana.com>

ledger/src/blockstore_processor.rs

@@ -820,6 +820,7 @@ fn load_frozen_forks(
     )?;
 
     let dev_halt_at_slot = opts.dev_halt_at_slot.unwrap_or(std::u64::MAX);
+    if root_bank.slot() != dev_halt_at_slot {
         while !pending_slots.is_empty() {
             let (meta, bank, last_entry_hash) = pending_slots.pop().unwrap();
             let slot = bank.slot();
@@ -907,7 +908,8 @@ fn load_frozen_forks(
                 }
 
                 // Filter out all non descendants of the new root
-            pending_slots.retain(|(_, pending_bank, _)| pending_bank.ancestors.contains_key(root));
+                pending_slots
+                    .retain(|(_, pending_bank, _)| pending_bank.ancestors.contains_key(root));
                 initial_forks.retain(|_, fork_tip_bank| fork_tip_bank.ancestors.contains_key(root));
                 all_banks.retain(|_, bank| bank.ancestors.contains_key(root));
             }
@@ -934,6 +936,7 @@ fn load_frozen_forks(
                 break;
             }
         }
+    }
 
     Ok(initial_forks.values().cloned().collect::<Vec<_>>())
 }
@@ -2607,6 +2610,37 @@ pub mod tests {
         assert_eq!(bank.process_transaction(&fail_tx), Ok(()));
     }
 
+    #[test]
+    fn test_halt_at_slot_starting_snapshot_root() {
+        let GenesisConfigInfo { genesis_config, .. } = create_genesis_config(123);
+
+        // Create roots at slots 0, 1
+        let forks = tr(0) / tr(1);
+        let ledger_path = get_tmp_ledger_path!();
+        let blockstore = Blockstore::open(&ledger_path).unwrap();
+        blockstore.add_tree(
+            forks,
+            false,
+            true,
+            genesis_config.ticks_per_slot,
+            genesis_config.hash(),
+        );
+        blockstore.set_roots(&[0, 1]).unwrap();
+
+        // Specify halting at slot 0
+        let opts = ProcessOptions {
+            poh_verify: true,
+            dev_halt_at_slot: Some(0),
+            ..ProcessOptions::default()
+        };
+        let (bank_forks, _leader_schedule) =
+            process_blockstore(&genesis_config, &blockstore, Vec::new(), opts).unwrap();
+
+        // Should be able to fetch slot 0 because we specified halting at slot 0, even
+        // if there is a greater root at slot 1.
+        assert!(bank_forks.get(0).is_some());
+    }
+
     #[test]
     fn test_process_blockstore_from_root() {
         let GenesisConfigInfo {