Refactors purging old accounts hashes in AccountsHashVerifier (solana…

…-labs#1406)

core/src/accounts_hash_verifier.rs

@@ -220,6 +220,8 @@ impl AccountsHashVerifier {
 
         Self::save_epoch_accounts_hash(&accounts_package, accounts_hash);
 
+        Self::purge_old_accounts_hashes(&accounts_package, snapshot_config);
+
         Self::submit_for_packaging(
             accounts_package,
             snapshot_package_sender,
@@ -324,15 +326,6 @@ impl AccountsHashVerifier {
             })?;
         }
 
-        if accounts_package.package_kind
-            == AccountsPackageKind::Snapshot(SnapshotKind::FullSnapshot)
-        {
-            accounts_package
-                .accounts
-                .accounts_db
-                .purge_old_accounts_hashes(accounts_package.slot);
-        }
-
         // After an accounts package has had its accounts hash calculated and
         // has been reserialized to become a BankSnapshotPost, it is now safe
         // to clean up some older bank snapshots.
@@ -499,6 +492,37 @@ impl AccountsHashVerifier {
         }
     }
 
+    fn purge_old_accounts_hashes(
+        accounts_package: &AccountsPackage,
+        snapshot_config: &SnapshotConfig,
+    ) {
+        let should_purge = match (
+            snapshot_config.should_generate_snapshots(),
+            accounts_package.package_kind,
+        ) {
+            (false, _) => {
+                // If we are *not* generating snapshots, then it is safe to purge every time.
+                true
+            }
+            (true, AccountsPackageKind::Snapshot(SnapshotKind::FullSnapshot)) => {
+                // If we *are* generating snapshots, then only purge old accounts hashes after
+                // handling full snapshot packages.  This is because handling incremental snapshot
+                // packages requires the accounts hash from the latest full snapshot, and if we
+                // purged after every package, we'd remove the accounts hash needed by the next
+                // incremental snapshot.
+                true
+            }
+            (true, _) => false,
+        };
+
+        if should_purge {
+            accounts_package
+                .accounts
+                .accounts_db
+                .purge_old_accounts_hashes(accounts_package.slot);
+        }
+    }
+
     fn submit_for_packaging(
         accounts_package: AccountsPackage,
         snapshot_package_sender: Option<&Sender<SnapshotPackage>>,