core: fix temp memory blowup caused by defers holding on to state

core/blockchain.go

@@ -1805,6 +1805,17 @@ func (bc *BlockChain) insertChain(chain types.Blocks, verifySeals bool) (int, er
 		return it.index, err
 	}
 	// No validation errors for the first block (or chain prefix skipped)
+	var activeState *state.StateDB
+	defer func() {
+		// The chain importer is starting and stopping trie prefetchers. If a bad
+		// block or other error is hit however, an early return may not properly
+		// terminate the background threads. This defer ensures that we clean up
+		// and dangling prefetcher, without defering each and holding on live refs.
+		if activeState != nil {
+			activeState.StopPrefetcher()
+		}
+	}()
+
 	for ; block != nil && err == nil || err == ErrKnownBlock; block, err = it.next() {
 		// If the chain is terminating, stop processing blocks
 		if bc.insertStopped() {
@@ -1867,7 +1878,7 @@ func (bc *BlockChain) insertChain(chain types.Blocks, verifySeals bool) (int, er
 		}
 		// Enable prefetching to pull in trie node paths while processing transactions
 		statedb.StartPrefetcher("chain")
-		defer statedb.StopPrefetcher() // stopped on write anyway, defer meant to catch early error returns
+		activeState = statedb
 
 		// If we have a followup block, run that against the current state to pre-cache
 		// transactions and probabilistically some of the account/storage trie nodes.