From 25cc02431b61097b2adfc2fbc493b648b0300c3b Mon Sep 17 00:00:00 2001
From: Mason Reed <mason@vector35.com>
Date: Mon, 10 Mar 2025 11:05:40 -0400
Subject: [SharedCache] Refactor Shared Cache

In absence of a better name, this commit refactors the shared cache code.
---
 view/sharedcache/core/FileAccessorCache.h | 84 +++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100644 view/sharedcache/core/FileAccessorCache.h

(limited to 'view/sharedcache/core/FileAccessorCache.h')

diff --git a/view/sharedcache/core/FileAccessorCache.h b/view/sharedcache/core/FileAccessorCache.h
new file mode 100644
index 00000000..bb2f8084
--- /dev/null
+++ b/view/sharedcache/core/FileAccessorCache.h
@@ -0,0 +1,84 @@
+#pragma once
+
+#include <shared_mutex>
+
+#include "MappedFileAccessor.h"
+
+typedef uint32_t CacheAccessorID;
+
+// TODO: We might want to make this more than just the path, for example
+// TODO: We might want to make it unique to a view session (session id).
+// Get a unique entry id for the given file path.
+CacheAccessorID GetCacheAccessorID(const std::string& filePath);
+
+class WeakFileAccessor;
+
+class FileAccessorCache
+{
+	size_t m_cacheSize;
+	std::mutex m_mutex;
+	// NOTE: If we end up wanting to handle 1000's of files we should consider std::list.
+	std::deque<CacheAccessorID> m_cache;
+	std::unordered_map<CacheAccessorID, std::shared_ptr<MappedFileAccessor>> m_accessors;
+
+	explicit FileAccessorCache(size_t cacheSize = 8);
+
+	void EvictLastUsed();
+
+public:
+	static FileAccessorCache& Global();
+
+	// Get a weak reference to a file accessor, the reference at this point is alive.
+	// The reference is always alive at this point either because it is in the cache or it has been inserted in.
+	// Subsequent calls to this might kill the backing file accessor resulting in the weak ref recreating the file
+	// accessor and inserting itself back into its related cache.
+	WeakFileAccessor Open(const std::string& filePath);
+
+	// Adjust the cache size limit.
+	// This will NOT evict current cache entries, as they are already available.
+	// Any subsequent call to `Open` will assume this cache size, evicting until the size is equal to the cache size.
+	void SetCacheSize(uint64_t size) { m_cacheSize = size; };
+};
+
+// Write log to be used in conjunction with `WeakFileAccessor` to re-apply written data to a "revived" file.
+struct FileAccessorWriteLog
+{
+	// To persist writes to a file accessor being revived (within the lock() function)
+	// we keep a list of writes that will be re-applied in the lock function.
+	std::shared_mutex m_persistedMutex;
+	std::unordered_map<size_t, uint64_t> m_persistedPointers;
+
+	FileAccessorWriteLog() = default;
+
+	// Add the pointer to the persisted pointers.
+	void AddPointer(size_t address, size_t pointer);
+
+	// Apply all logged writes to the given accessor.
+	void ApplyWrites(MappedFileAccessor& accessor);
+};
+
+class WeakFileAccessor
+{
+	// Weak pointer to the mapped file accessor, once this is expired we will re-open.
+	std::weak_ptr<MappedFileAccessor> m_weakPtr;
+	// File path for re-opening if needed
+	std::string m_filePath;
+
+	// Used to re-add writes once the file accessor is "revived".
+	std::shared_ptr<FileAccessorWriteLog> m_writeLog;
+
+	// TODO: Store a weak_ptr/shared_ptr to FileAccessorCache? That way we dont access Global()
+	// TODO: Only need to do the above if we want multiple caches.
+
+public:
+	explicit WeakFileAccessor(std::weak_ptr<MappedFileAccessor> weakPtr, std::string filePath) :
+		m_weakPtr(std::move(weakPtr)), m_filePath(std::move(filePath)),
+		m_writeLog(std::make_shared<FileAccessorWriteLog>())
+	{}
+
+	std::shared_ptr<MappedFileAccessor> lock();
+
+	// Persists the written pointer within this weak file accessor.
+	// This works as we expect the weak file accessor to be stored per virtual memory region.
+	void WritePointer(size_t address, size_t pointer);
+};
-- 
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