From e009728858272c44a80c4e77c4aec63eb538b11f Mon Sep 17 00:00:00 2001
From: Jacek Sieka <jacek@status.im>
Date: Thu, 24 Mar 2022 15:37:37 +0100
Subject: [PATCH] work around Nim assignment bug that breaks state pruning
 (#3545)

See https://github.com/nim-lang/Nim/issues/19613
---
 .../consensus_object_pools/blockchain_dag.nim |  4 ++-
 tests/test_blockchain_dag.nim                 | 30 +++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/beacon_chain/consensus_object_pools/blockchain_dag.nim b/beacon_chain/consensus_object_pools/blockchain_dag.nim
index a2689986e..6c186b88e 100644
--- a/beacon_chain/consensus_object_pools/blockchain_dag.nim
+++ b/beacon_chain/consensus_object_pools/blockchain_dag.nim
@@ -1499,7 +1499,9 @@ proc pruneStateCachesDAG*(dag: ChainDAGRef) =
       if not isFinalizedStateSnapshot(cur.get().slot) and
           cur.get().slot != dag.tail.slot:
         dag.delState(cur.get())
-      cur = dag.parentOrSlot(cur.get())
+      # TODO https://github.com/nim-lang/Nim/issues/19613
+      let tmp = cur.get()
+      cur = dag.parentOrSlot(tmp)
 
   let statePruneTick = Moment.now()
 
diff --git a/tests/test_blockchain_dag.nim b/tests/test_blockchain_dag.nim
index a530d3041..3af755cee 100644
--- a/tests/test_blockchain_dag.nim
+++ b/tests/test_blockchain_dag.nim
@@ -415,6 +415,34 @@ suite "chain DAG finalization tests" & preset():
     check:
       dag.db.immutableValidators.len() == getStateField(dag.headState, validators).len()
 
+    block:
+      var cur = dag.head.bid
+      while true:
+        let parent = dag.parent(cur)
+        if cur.slot > 0:
+          check:
+            parent.isSome and parent.get().slot < cur.slot
+          cur = parent.get()
+        else:
+          check:
+            parent.isErr()
+          break
+      check: cur.slot == 0
+
+    block:
+      var cur = dag.head.bid.atSlot()
+      while true:
+        let parent = dag.parentOrSlot(cur)
+        if cur.slot > 0:
+          check:
+            parent.isSome and (parent.get().slot < cur.slot or parent.get().bid != cur.bid)
+          cur = parent.get()
+        else:
+          check:
+            parent.isErr()
+          break
+      check: cur.slot == 0
+
     let
       finalER = dag.getEpochRef(
         dag.finalizedHead.blck, dag.finalizedHead.slot.epoch, false)
@@ -458,9 +486,11 @@ suite "chain DAG finalization tests" & preset():
       let
         finalizedCheckpoint = dag.stateCheckpoint(dag.finalizedHead.toBlockSlotId().get())
         headCheckpoint = dag.stateCheckpoint(dag.head.bid.atSlot())
+        prunedCheckpoint = dag.stateCheckpoint(dag.parent(dag.finalizedHead.blck.bid).get().atSlot())
       check:
         db.getStateRoot(headCheckpoint.bid.root, headCheckpoint.slot).isSome
         db.getStateRoot(finalizedCheckpoint.bid.root, finalizedCheckpoint.slot).isSome
+        db.getStateRoot(prunedCheckpoint.bid.root, prunedCheckpoint.slot).isNone
 
     let
       validatorMonitor2 = newClone(ValidatorMonitor.init())