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diff --git a/startupcache/StartupCache.h b/startupcache/StartupCache.h
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+++ b/startupcache/StartupCache.h
@@ -0,0 +1,289 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef StartupCache_h_
+#define StartupCache_h_
+
+#include <utility>
+
+#include "nsClassHashtable.h"
+#include "nsComponentManagerUtils.h"
+#include "nsTArray.h"
+#include "nsTHashSet.h"
+#include "nsTStringHasher.h"  // mozilla::DefaultHasher<nsCString>
+#include "nsZipArchive.h"
+#include "nsITimer.h"
+#include "nsIMemoryReporter.h"
+#include "nsIObserverService.h"
+#include "nsIObserver.h"
+#include "nsIObjectOutputStream.h"
+#include "nsIFile.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/AutoMemMap.h"
+#include "mozilla/Compression.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/Result.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/UniquePtrExtensions.h"
+
+/**
+ * The StartupCache is a persistent cache of simple key-value pairs,
+ * where the keys are null-terminated c-strings and the values are
+ * arbitrary data, passed as a (char*, size) tuple.
+ *
+ * Clients should use the GetSingleton() static method to access the cache. It
+ * will be available from the end of XPCOM init (NS_InitXPCOM3 in
+ * XPCOMInit.cpp), until XPCOM shutdown begins. The GetSingleton() method will
+ * return null if the cache is unavailable. The cache is only provided for
+ * libxul builds -- it will fail to link in non-libxul builds. The XPCOM
+ * interface is provided only to allow compiled-code tests; clients should avoid
+ * using it.
+ *
+ * The API provided is very simple: GetBuffer() returns a buffer that was
+ * previously stored in the cache (if any), and PutBuffer() inserts a buffer
+ * into the cache. GetBuffer returns a new buffer, and the caller must take
+ * ownership of it. PutBuffer will assert if the client attempts to insert a
+ * buffer with the same name as an existing entry. The cache makes a copy of the
+ * passed-in buffer, so client retains ownership.
+ *
+ * InvalidateCache() may be called if a client suspects data corruption
+ * or wishes to invalidate for any other reason. This will remove all existing
+ * cache data. Additionally, the static method IgnoreDiskCache() can be called
+ * if it is believed that the on-disk cache file is itself corrupt. This call
+ * implicitly calls InvalidateCache (if the singleton has been initialized) to
+ * ensure any data already read from disk is discarded. The cache will not load
+ * data from the disk file until a successful write occurs.
+ *
+ * Finally, getDebugObjectOutputStream() allows debug code to wrap an
+ * objectstream with a debug objectstream, to check for multiply-referenced
+ * objects. These will generally fail to deserialize correctly, unless they are
+ * stateless singletons or the client maintains their own object data map for
+ * deserialization.
+ *
+ * Writes before the final-ui-startup notification are placed in an intermediate
+ * cache in memory, then written out to disk at a later time, to get writes off
+ * the startup path. In any case, clients should not rely on being able to
+ * GetBuffer() data that is written to the cache, since it may not have been
+ * written to disk or another client may have invalidated the cache. In other
+ * words, it should be used as a cache only, and not a reliable persistent
+ * store.
+ *
+ * Some utility functions are provided in StartupCacheUtils. These functions
+ * wrap the buffers into object streams, which may be useful for serializing
+ * objects. Note the above caution about multiply-referenced objects, though --
+ * the streams are just as 'dumb' as the underlying buffers about
+ * multiply-referenced objects. They just provide some convenience in writing
+ * out data.
+ */
+
+namespace mozilla {
+
+namespace scache {
+
+struct StartupCacheEntry {
+  UniqueFreePtr<char[]> mData;
+  uint32_t mOffset;
+  uint32_t mCompressedSize;
+  uint32_t mUncompressedSize;
+  int32_t mHeaderOffsetInFile;
+  int32_t mRequestedOrder;
+  bool mRequested;
+
+  MOZ_IMPLICIT StartupCacheEntry(uint32_t aOffset, uint32_t aCompressedSize,
+                                 uint32_t aUncompressedSize)
+      : mData(nullptr),
+        mOffset(aOffset),
+        mCompressedSize(aCompressedSize),
+        mUncompressedSize(aUncompressedSize),
+        mHeaderOffsetInFile(0),
+        mRequestedOrder(0),
+        mRequested(false) {}
+
+  StartupCacheEntry(UniqueFreePtr<char[]> aData, size_t aLength,
+                    int32_t aRequestedOrder)
+      : mData(std::move(aData)),
+        mOffset(0),
+        mCompressedSize(0),
+        mUncompressedSize(aLength),
+        mHeaderOffsetInFile(0),
+        mRequestedOrder(0),
+        mRequested(true) {}
+
+  struct Comparator {
+    using Value = std::pair<const nsCString*, StartupCacheEntry*>;
+
+    bool Equals(const Value& a, const Value& b) const {
+      return a.second->mRequestedOrder == b.second->mRequestedOrder;
+    }
+
+    bool LessThan(const Value& a, const Value& b) const {
+      return a.second->mRequestedOrder < b.second->mRequestedOrder;
+    }
+  };
+};
+
+// We don't want to refcount StartupCache, and ObserverService wants to
+// refcount its listeners, so we'll let it refcount this instead.
+class StartupCacheListener final : public nsIObserver {
+  ~StartupCacheListener() = default;
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+};
+
+class StartupCache : public nsIMemoryReporter {
+  friend class StartupCacheListener;
+
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIMEMORYREPORTER
+
+  // StartupCache methods. See above comments for a more detailed description.
+
+  // true if the archive has an entry for the buffer or not.
+  bool HasEntry(const char* id);
+
+  // Returns a buffer that was previously stored, caller does not take ownership
+  nsresult GetBuffer(const char* id, const char** outbuf, uint32_t* length);
+
+  // Stores a buffer. Caller yields ownership.
+  nsresult PutBuffer(const char* id, UniqueFreePtr<char[]>&& inbuf,
+                     uint32_t length);
+
+  // Removes the cache file.
+  void InvalidateCache(bool memoryOnly = false);
+
+  // If some event knowingly re-generates the startup cache (like live language
+  // switching) count these events in order to allow them.
+  void CountAllowedInvalidation();
+
+  // For use during shutdown - this will write the startupcache's data
+  // to disk if the timer hasn't already gone off.
+  void MaybeInitShutdownWrite();
+
+  // For use during shutdown - ensure we complete the shutdown write
+  // before shutdown, even in the FastShutdown case.
+  void EnsureShutdownWriteComplete();
+
+  // Signal that data should not be loaded from the cache file
+  static void IgnoreDiskCache();
+
+  static bool GetIgnoreDiskCache();
+
+  // In DEBUG builds, returns a stream that will attempt to check for
+  // and disallow multiple writes of the same object.
+  nsresult GetDebugObjectOutputStream(nsIObjectOutputStream* aStream,
+                                      nsIObjectOutputStream** outStream);
+
+  static StartupCache* GetSingletonNoInit();
+  static StartupCache* GetSingleton();
+  static void DeleteSingleton();
+
+  // This measures all the heap memory used by the StartupCache, i.e. it
+  // excludes the mapping.
+  size_t HeapSizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const
+      MOZ_REQUIRES(mTableLock);
+
+  bool ShouldCompactCache() MOZ_REQUIRES(mTableLock);
+  nsresult ResetStartupWriteTimerCheckingReadCount();
+  nsresult ResetStartupWriteTimerAndLock();
+  nsresult ResetStartupWriteTimer() MOZ_REQUIRES(mTableLock);
+  bool StartupWriteComplete();
+
+ private:
+  StartupCache();
+  virtual ~StartupCache();
+
+  friend class StartupCacheInfo;
+
+  Result<Ok, nsresult> LoadArchive();
+  nsresult Init();
+
+  // Returns a file pointer for the cache file with the given name in the
+  // current profile.
+  Result<nsCOMPtr<nsIFile>, nsresult> GetCacheFile(const nsAString& suffix);
+
+  // Opens the cache file for reading.
+  Result<Ok, nsresult> OpenCache();
+
+  // Writes the cache to disk
+  Result<Ok, nsresult> WriteToDisk() MOZ_REQUIRES(mTableLock);
+
+  void WaitOnPrefetch();
+  void StartPrefetchMemory() MOZ_REQUIRES(mTableLock);
+
+  static nsresult InitSingleton();
+  static void WriteTimeout(nsITimer* aTimer, void* aClosure);
+  void MaybeWriteOffMainThread();
+  void ThreadedPrefetch(uint8_t* aStart, size_t aSize);
+
+  Monitor mPrefetchComplete{"StartupCachePrefetch"};
+  bool mPrefetchInProgress MOZ_GUARDED_BY(mPrefetchComplete){false};
+
+  // This is normally accessed on MainThread, but WriteToDisk() can
+  // access it on other threads
+  HashMap<nsCString, StartupCacheEntry> mTable MOZ_GUARDED_BY(mTableLock);
+  // This owns references to the contents of tables which have been invalidated.
+  // In theory it grows forever if the cache is continually filled and then
+  // invalidated, but this should not happen in practice. Deleting old tables
+  // could create dangling pointers. RefPtrs could be introduced, but it would
+  // be a large amount of error-prone work to change.
+  nsTArray<decltype(mTable)> mOldTables MOZ_GUARDED_BY(mTableLock);
+  size_t mAllowedInvalidationsCount;
+  nsCOMPtr<nsIFile> mFile;
+  loader::AutoMemMap mCacheData MOZ_GUARDED_BY(mTableLock);
+  Mutex mTableLock;
+
+  nsCOMPtr<nsIObserverService> mObserverService;
+  RefPtr<StartupCacheListener> mListener;
+  nsCOMPtr<nsITimer> mTimer;
+
+  bool mDirty MOZ_GUARDED_BY(mTableLock);
+  bool mWrittenOnce MOZ_GUARDED_BY(mTableLock);
+  bool mCurTableReferenced MOZ_GUARDED_BY(mTableLock);
+
+  uint32_t mRequestedCount;
+  size_t mCacheEntriesBaseOffset;
+
+  static StaticRefPtr<StartupCache> gStartupCache;
+  static bool gShutdownInitiated;
+  static bool gIgnoreDiskCache;
+  static bool gFoundDiskCacheOnInit;
+
+  UniquePtr<Compression::LZ4FrameDecompressionContext> mDecompressionContext;
+#ifdef DEBUG
+  nsTHashSet<nsCOMPtr<nsISupports>> mWriteObjectMap;
+#endif
+};
+
+// This debug outputstream attempts to detect if clients are writing multiple
+// references to the same object. We only support that if that object
+// is a singleton.
+#ifdef DEBUG
+class StartupCacheDebugOutputStream final : public nsIObjectOutputStream {
+  ~StartupCacheDebugOutputStream() = default;
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOBJECTOUTPUTSTREAM
+
+  StartupCacheDebugOutputStream(nsIObjectOutputStream* binaryStream,
+                                nsTHashSet<nsCOMPtr<nsISupports>>* objectMap)
+      : mBinaryStream(binaryStream), mObjectMap(objectMap) {}
+
+  NS_FORWARD_SAFE_NSIBINARYOUTPUTSTREAM(mBinaryStream)
+  NS_FORWARD_SAFE_NSIOUTPUTSTREAM(mBinaryStream)
+
+  bool CheckReferences(nsISupports* aObject);
+
+  nsCOMPtr<nsIObjectOutputStream> mBinaryStream;
+  nsTHashSet<nsCOMPtr<nsISupports>>* mObjectMap;
+};
+#endif  // DEBUG
+
+}  // namespace scache
+}  // namespace mozilla
+
+#endif  // StartupCache_h_