Snapshot optimizations (#5518)

* Limit slots_since_snapshot size, only package latest snapshot, refactor tests

* Add test checking status_cache.roots == bank_forks.slots_since_snapshot after bank_forks.set_root()

runtime/src/bank.rs

@@ -45,7 +45,7 @@ use solana_sdk::{
     timing::{duration_as_ns, get_segment_from_slot, Epoch, Slot, MAX_RECENT_BLOCKHASHES},
     transaction::{Result, Transaction, TransactionError},
 };
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet};
 use std::io::{BufReader, Cursor, Error as IOError, Read};
 use std::path::Path;
 use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
@@ -124,7 +124,7 @@ impl Serialize for BankRc {
 pub struct StatusCacheRc {
     /// where all the Accounts are stored
     /// A cache of signature statuses
-    status_cache: Arc<RwLock<BankStatusCache>>,
+    pub status_cache: Arc<RwLock<BankStatusCache>>,
 }
 
 impl StatusCacheRc {
@@ -133,6 +133,10 @@ impl StatusCacheRc {
         sc.slot_deltas(slots)
     }
 
+    pub fn roots(&self) -> HashSet<u64> {
+        self.status_cache.read().unwrap().roots().clone()
+    }
+
     pub fn append(&self, slot_deltas: &[SlotDelta<Result<()>>]) {
         let mut sc = self.status_cache.write().unwrap();
         sc.append(slot_deltas);

runtime/src/status_cache.rs

@@ -124,6 +124,10 @@ impl<T: Serialize + Clone> StatusCache<T> {
         self.purge_roots();
     }
 
+    pub fn roots(&self) -> &HashSet<u64> {
+        &self.roots
+    }
+
     /// Insert a new signature for a specific slot.
     pub fn insert(&mut self, transaction_blockhash: &Hash, sig: &Signature, slot: Slot, res: T) {
         let sig_index: usize;