From 11bbc660823246b9fc25e4b994121e30a9f17306 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?P=C3=A9ter=20Szil=C3=A1gyi?= <peterke@gmail.com>
Date: Wed, 8 Aug 2018 17:16:38 +0300
Subject: [PATCH] eth, trie: fix tracer GC which accidentally pruned the
 metaroot

---
 eth/api_tracer.go | 8 ++++++--
 trie/database.go  | 5 +++++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/eth/api_tracer.go b/eth/api_tracer.go
index 623e5ed1b..722e2a6e3 100644
--- a/eth/api_tracer.go
+++ b/eth/api_tracer.go
@@ -297,7 +297,9 @@ func (api *PrivateDebugAPI) traceChain(ctx context.Context, start, end *types.Bl
 				database.TrieDB().Reference(root, common.Hash{})
 			}
 			// Dereference all past tries we ourselves are done working with
-			database.TrieDB().Dereference(proot)
+			if proot != (common.Hash{}) {
+				database.TrieDB().Dereference(proot)
+			}
 			proot = root
 
 			// TODO(karalabe): Do we need the preimages? Won't they accumulate too much?
@@ -526,7 +528,9 @@ func (api *PrivateDebugAPI) computeStateDB(block *types.Block, reexec uint64) (*
 			return nil, err
 		}
 		database.TrieDB().Reference(root, common.Hash{})
-		database.TrieDB().Dereference(proot)
+		if proot != (common.Hash{}) {
+			database.TrieDB().Dereference(proot)
+		}
 		proot = root
 	}
 	nodes, imgs := database.TrieDB().Size()
diff --git a/trie/database.go b/trie/database.go
index 7df45fe2d..d0691b637 100644
--- a/trie/database.go
+++ b/trie/database.go
@@ -431,6 +431,11 @@ func (db *Database) reference(child common.Hash, parent common.Hash) {
 
 // Dereference removes an existing reference from a root node.
 func (db *Database) Dereference(root common.Hash) {
+	// Sanity check to ensure that the meta-root is not removed
+	if root == (common.Hash{}) {
+		log.Error("Attempted to dereference the trie cache meta root")
+		return
+	}
 	db.lock.Lock()
 	defer db.lock.Unlock()