db-sync: Fix a race condition maintaining ledger state

When blocks and rollback notifications arrive via the chainsync protocol,
they are put in a queue and then a separate thread reads from the queue and
operates on the DB. When ledger-state management was added, code to apply a
block to a ledger state was added at the read end of the queue, but the
rollback handling was incorrectly added at the write end of the queue.

The solution to the problem is trivial, move the ledger state rollback
handling from the write end of the queue to the read end.

Closes: https://github.com/input-output-hk/cardano-db-sync/issue/398

Author: erikd

cardano-db-sync/src/Cardano/DbSync.hs

@@ -237,7 +237,7 @@ dbSyncProtocols trce env plugin queryVar ledgerVar _version codecs _connectionId
                 (cChainSyncCodec codecs)
                 channel
                 (chainSyncClientPeerPipelined
-                    $ chainSyncClient trce env queryVar ledgerVar metrics latestPoints currentTip actionQueue)
+                    $ chainSyncClient trce env queryVar metrics latestPoints currentTip actionQueue)
             )
         atomically $ writeDbActionQueue actionQueue DbFinish
         cancel server
@@ -317,9 +317,9 @@ getCurrentTipBlockNo = do
 chainSyncClient
     :: Trace IO Text -> DbSyncEnv
     -> StateQueryTMVar CardanoBlock (Interpreter (CardanoEras StandardCrypto))
-    -> LedgerStateVar -> Metrics -> [Point CardanoBlock] -> WithOrigin BlockNo -> DbActionQueue
+    -> Metrics -> [Point CardanoBlock] -> WithOrigin BlockNo -> DbActionQueue
     -> ChainSyncClientPipelined CardanoBlock (Tip CardanoBlock) IO ()
-chainSyncClient trce env queryVar ledgerVar metrics latestPoints currentTip actionQueue =
+chainSyncClient trce env queryVar metrics latestPoints currentTip actionQueue =
     ChainSyncClientPipelined $ pure $
       -- Notify the core node about the our latest points at which we are
       -- synchronised.  This client is not persistent and thus it just
@@ -373,7 +373,6 @@ chainSyncClient trce env queryVar ledgerVar metrics latestPoints currentTip acti
                 -- but will only be incorrect for a short time span.
                 let slot = toRollbackSlot point
                 atomically $ writeDbActionQueue actionQueue (mkDbRollback slot)
-                loadLedgerState (envLedgerStateDir env) ledgerVar slot
                 newTip <- getCurrentTipBlockNo
                 pure $ finish newTip tip
         }

cardano-db-sync/src/Cardano/DbSync/Database.hs

@@ -93,8 +93,9 @@ runActions trce env plugin ledgerState actions = do
       case spanDbApply xs of
         ([], DbFinish:_) -> do
             pure Done
-        ([], DbRollBackToPoint pt:ys) -> do
-            runRollbacks trce plugin pt
+        ([], DbRollBackToPoint sn:ys) -> do
+            runRollbacks trce plugin sn
+            liftIO $ loadLedgerState (envLedgerStateDir env) ledgerState sn
             dbAction Continue ys
         (ys, zs) -> do
           insertBlockList trce env ledgerState plugin ys