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#include "FileAccessorCache.h"
#include <cassert>
CacheAccessorID GetCacheAccessorID(const std::string& filePath)
{
constexpr std::hash<std::string> hasher;
return static_cast<CacheAccessorID>(hasher(filePath));
}
FileAccessorCache::FileAccessorCache(size_t cacheSize)
{
m_cacheSize = cacheSize;
m_accessors = {};
}
void FileAccessorCache::EvictLastUsed()
{
if (m_cache.empty())
return;
// Evict the least recently used element.
const auto lruID = m_cache.front();
m_cache.pop_front();
// Ensure the least recently used ID actually exists in the accessors map.
assert(m_accessors.find(lruID) != m_accessors.end() && "Evicting non-existent ID from accessors map");
m_accessors.erase(lruID);
}
FileAccessorCache& FileAccessorCache::Global()
{
static FileAccessorCache cache {};
return cache;
}
WeakFileAccessor FileAccessorCache::Open(const std::string& filePath)
{
const auto id = GetCacheAccessorID(filePath);
std::unique_lock lock(m_mutex);
// Check if the file is already in the cache.
if (const auto it = m_accessors.find(id); it != m_accessors.end())
{
// Move the accessed ID to the back so we keep it in the cache.
auto pos = std::find(m_cache.begin(), m_cache.end(), id);
if (pos != m_cache.end())
m_cache.erase(pos);
m_cache.push_back(id);
return WeakFileAccessor(it->second, filePath);
}
// Evict if we are going to go above the limit.
if (m_cache.size() >= m_cacheSize)
EvictLastUsed();
// Create a new file accessor and add it to the cache.
auto accessor = MappedFileAccessor::Open(filePath);
if (accessor == nullptr)
{
// We failed to open the file, we must throw hard!
// TODO: Make this mechanism more thought out...
throw std::runtime_error("Failed to open file: " + filePath);
}
auto sharedAccessor = std::make_shared<MappedFileAccessor>(std::move(*accessor));
m_accessors.insert_or_assign(id, sharedAccessor);
m_cache.push_back(id);
return WeakFileAccessor(sharedAccessor, filePath);
}
void FileAccessorWriteLog::AddPointer(const size_t address, const size_t pointer)
{
// TODO: A sharded map here would be better. see: rust dashmap
std::unique_lock<std::shared_mutex> lock(m_persistedMutex);
m_persistedPointers.emplace_back(address, pointer);
}
void FileAccessorWriteLog::ApplyWrites(MappedFileAccessor& accessor)
{
std::shared_lock<std::shared_mutex> lock(m_persistedMutex);
for (const auto& [address, pointer] : m_persistedPointers)
accessor.WritePointer(address, pointer);
}
std::shared_ptr<MappedFileAccessor> WeakFileAccessor::lock()
{
auto sharedPtr = m_weakPtr.lock();
if (!sharedPtr)
{
// This will revive other weak pointers to the same shared ptr.
// Update the weak pointer to the newly created shared instance
m_weakPtr = FileAccessorCache::Global().Open(m_filePath).m_weakPtr;
sharedPtr = m_weakPtr.lock();
// Apply any previously written pointers back.
if (sharedPtr)
m_writeLog->ApplyWrites(*sharedPtr);
}
return sharedPtr;
}
void WeakFileAccessor::WritePointer(const size_t address, const size_t pointer)
{
// Persist the pointer after the file accessor is revived.
m_writeLog->AddPointer(address, pointer);
// And then actually apply the written pointer...
if (auto sharedPtr = m_weakPtr.lock())
sharedPtr->WritePointer(address, pointer);
}
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