unblock PreCache (#164)

Don't lock disk cache buckets from writing when scanning records for attachment references

4 files changed, 10 insertions, 7 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index fe5a96c9c..6ccf6d9ac 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 - Feature: Added `zen cache-gen` command to generate large amount of cache data for testing
 - Improvement: Optimizations to GC leading to 50-100% performance increase when handling large data sets
 - Bugfix: Symbol resolution could fail on Windows depending on the process' current working directory
+- Bugfix: Don't block disk cache buckets from writing when scanning records for attachment references
 
 ## 5.5.7
 - Bugfix: Fix race condition in zenserver during batched fetch of small cache chunks (<=1024b) resulting in missing rawhash/rawsize in the response. UE-223703
diff --git a/src/zenstore/cache/cachedisklayer.cpp b/src/zenstore/cache/cachedisklayer.cpp
index 110acba9e..63f6d708a 100644
--- a/src/zenstore/cache/cachedisklayer.cpp
+++ b/src/zenstore/cache/cachedisklayer.cpp
@@ -3587,7 +3587,7 @@ ZenCacheDiskLayer::CacheBucket::ReadAttachmentsFromMetaData(uint32_t					 BlockI
 }
 
 bool
-ZenCacheDiskLayer::CacheBucket::GetReferencesLocked(GcCtx& Ctx, std::vector<IoHash>& OutReferences)
+ZenCacheDiskLayer::CacheBucket::GetReferences(GcCtx& Ctx, bool StateIsAlreadyLocked, std::vector<IoHash>& OutReferences)
 {
 	ZEN_TRACE_CPU("Z$::Bucket::GetReferencesLocked");
 
@@ -3611,7 +3611,11 @@ ZenCacheDiskLayer::CacheBucket::GetReferencesLocked(GcCtx& Ctx, std::vector<IoHa
 
 		{
 			std::unordered_map<uint32_t, std::size_t> BlockIndexToChunkIndexes;
-
+			std::unique_ptr<RwLock::SharedLockScope>  StateLock;
+			if (!StateIsAlreadyLocked)
+			{
+				StateLock = std::make_unique<RwLock::SharedLockScope>(m_IndexLock);
+			}
 			for (const auto& Entry : m_Index)
 			{
 				if (Ctx.IsCancelledFlag.load())
@@ -3784,9 +3788,7 @@ public:
 
 		m_CacheBucket.m_IndexLock.WithExclusiveLock([&]() { m_CacheBucket.m_TrackedReferences = std::make_unique<HashSet>(); });
 
-		RwLock::SharedLockScope IndexLock(m_CacheBucket.m_IndexLock);
-		bool					Continue = m_CacheBucket.GetReferencesLocked(Ctx, m_References);
-		IndexLock.ReleaseNow();
+		bool Continue = m_CacheBucket.GetReferences(Ctx, /*StateIsAlreadyLocked*/ false, m_References);
 		if (!Continue)
 		{
 			m_CacheBucket.m_IndexLock.WithExclusiveLock([&]() { m_CacheBucket.m_TrackedReferences.reset(); });
diff --git a/src/zenstore/cache/structuredcachestore.cpp b/src/zenstore/cache/structuredcachestore.cpp
index c98561ebb..7794d025f 100644
--- a/src/zenstore/cache/structuredcachestore.cpp
+++ b/src/zenstore/cache/structuredcachestore.cpp
@@ -1183,7 +1183,7 @@ public:
 
 		for (ZenCacheDiskLayer::CacheBucket* Bucket : AddedBuckets)
 		{
-			bool Continue = Bucket->GetReferencesLocked(Ctx, m_References);
+			bool Continue = Bucket->GetReferences(Ctx, /*StateIsAlreadyLocked*/ true, m_References);
 			if (!Continue)
 			{
 				break;
diff --git a/src/zenstore/include/zenstore/cache/cachedisklayer.h b/src/zenstore/include/zenstore/cache/cachedisklayer.h
index b0b8b4cf3..a735893a1 100644
--- a/src/zenstore/include/zenstore/cache/cachedisklayer.h
+++ b/src/zenstore/include/zenstore/cache/cachedisklayer.h
@@ -235,7 +235,7 @@ public:
 		void	 GatherReferences(GcContext& GcCtx);
 		void	 CollectGarbage(GcContext& GcCtx);
 		RwLock::SharedLockScope GetGcReferencerLock();
-		bool					GetReferencesLocked(GcCtx& Ctx, std::vector<IoHash>& OutReferences);
+		bool					GetReferences(GcCtx& Ctx, bool StateIsAlreadyLocked, std::vector<IoHash>& OutReferences);
 
 		bool ReadAttachmentsFromMetaData(uint32_t					  BlockIndex,
 										 std::span<const IoHash>	  InlineKeys,