From 3d50e6522ee165886e293dc3fe49af3f0789d3a9 Mon Sep 17 00:00:00 2001
From: Per Larsson <p3rl@protonmail.com>
Date: Tue, 30 Nov 2021 19:32:45 +0100
Subject: Fixed crash in cache tracker.

---
 zenserver/cache/cachetracking.cpp | 40 +++++++++++++++++++++++++++------------
 1 file changed, 28 insertions(+), 12 deletions(-)

(limited to 'zenserver/cache/cachetracking.cpp')

diff --git a/zenserver/cache/cachetracking.cpp b/zenserver/cache/cachetracking.cpp
index 6a9b2f403..d1c99a597 100644
--- a/zenserver/cache/cachetracking.cpp
+++ b/zenserver/cache/cachetracking.cpp
@@ -49,22 +49,29 @@ public:
 		Tracker->Track(HashKey);
 	}
 
-	void SerializeSnapshot(CbObjectWriter& Cbo)
+	bool SerializeSnapshot(CbObjectWriter& Cbo)
 	{
+		bool					   Serialized = false;
 		RwLock::ExclusiveLockScope _(m_Lock);
 
 		for (const auto& Kv : m_BucketMapping)
 		{
-			Cbo.BeginArray(Kv.first);
-			m_Buckets[Kv.second]->SerializeSnapshotAndClear(Cbo);
-			Cbo.EndArray();
+			if (m_Buckets[Kv.second]->Size())
+			{
+				Cbo.BeginArray(Kv.first);
+				m_Buckets[Kv.second]->SerializeSnapshotAndClear(Cbo);
+				Cbo.EndArray();
+				Serialized = true;
+			}
 		}
+
+		return Serialized;
 	}
 
 private:
 	struct BucketTracker
 	{
-		RwLock				   Lock;
+		mutable RwLock		   Lock;
 		tsl::robin_set<IoHash> AccessedKeys;
 
 		void Track(const IoHash& HashKey)
@@ -90,6 +97,12 @@ private:
 
 			AccessedKeys.clear();
 		}
+
+		size_t Size() const
+		{
+			RwLock::SharedLockScope _(Lock);
+			return AccessedKeys.size();
+		}
 	};
 
 	BucketTracker* GetBucket(const std::string& BucketName)
@@ -198,15 +211,18 @@ struct ZenCacheTracker::Impl
 	void SaveSnapshot()
 	{
 		CbObjectWriter Cbo;
-		m_CurrentSnapshot.SerializeSnapshot(Cbo);
-		IoBuffer SnapshotBuffer = Cbo.Save().GetBuffer().AsIoBuffer();
 
-		const KeyStruct Key{.TimestampLittleEndian = ToNetworkOrder(GetCurrentCacheTimeStamp())};
-		rocksdb::Slice	KeySlice{(const char*)&Key, sizeof Key};
-		rocksdb::Slice	ValueSlice{(char*)SnapshotBuffer.Data(), SnapshotBuffer.Size()};
+		if (m_CurrentSnapshot.SerializeSnapshot(Cbo))
+		{
+			IoBuffer SnapshotBuffer = Cbo.Save().GetBuffer().AsIoBuffer();
+
+			const KeyStruct Key{.TimestampLittleEndian = ToNetworkOrder(GetCurrentCacheTimeStamp())};
+			rocksdb::Slice	KeySlice{(const char*)&Key, sizeof Key};
+			rocksdb::Slice	ValueSlice{(char*)SnapshotBuffer.Data(), SnapshotBuffer.Size()};
 
-		rocksdb::WriteOptions Wo;
-		m_RocksDb->Put(Wo, KeySlice, ValueSlice);
+			rocksdb::WriteOptions Wo;
+			m_RocksDb->Put(Wo, KeySlice, ValueSlice);
+		}
 	}
 
 	void IterateSnapshots(std::function<void(uint64_t TimeStamp, CbObject Snapshot)>&& Callback)
-- 
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