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diff --git a/dom/ipc/SharedMap.h b/dom/ipc/SharedMap.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 dom_ipc_SharedMap_h
+#define dom_ipc_SharedMap_h
+
+#include "mozilla/dom/MozSharedMapBinding.h"
+
+#include "mozilla/AutoMemMap.h"
+#include "mozilla/dom/ipc/StructuredCloneData.h"
+#include "mozilla/DOMEventTargetHelper.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/Variant.h"
+#include "nsClassHashtable.h"
+#include "nsTArray.h"
+
+class nsIGlobalObject;
+
+namespace mozilla::dom {
+
+class ContentParent;
+
+namespace ipc {
+
+/**
+ * Together, the SharedMap and WritableSharedMap classes allow sharing a
+ * dynamically-updated, shared-memory key-value store across processes.
+ *
+ * The maps may only ever be updated in the parent process, via
+ * WritableSharedMap instances. When that map changes, its entire contents are
+ * serialized into a contiguous shared memory buffer, and broadcast to all child
+ * processes, which in turn update their entire map contents wholesale.
+ *
+ * Keys are arbitrary UTF-8 strings (currently exposed to JavaScript as UTF-16),
+ * and values are structured clone buffers. Values are eagerly encoded whenever
+ * they are updated, and lazily decoded each time they're read.
+ *
+ * Updates are batched. Rather than each key change triggering an immediate
+ * update, combined updates are broadcast after a delay. Changes are flushed
+ * immediately any time a new process is created. Additionally, any time a key
+ * is changed, a flush task is scheduled for the next time the event loop
+ * becomes idle. Changes can be flushed immediately by calling the flush()
+ * method.
+ *
+ *
+ * Whenever a read-only SharedMap is updated, it dispatches a "change" event.
+ * The event contains a "changedKeys" property with a list of all keys which
+ * were changed in the last update batch. Change events are never dispatched to
+ * WritableSharedMap instances.
+ */
+class SharedMap : public DOMEventTargetHelper {
+  using FileDescriptor = mozilla::ipc::FileDescriptor;
+
+ public:
+  SharedMap();
+
+  SharedMap(nsIGlobalObject* aGlobal, const FileDescriptor&, size_t,
+            nsTArray<RefPtr<BlobImpl>>&& aBlobs);
+
+  // Returns true if the map contains the given (UTF-8) key.
+  bool Has(const nsACString& name);
+
+  // If the map contains the given (UTF-8) key, decodes and returns a new copy
+  // of its value. Otherwise returns null.
+  void Get(JSContext* cx, const nsACString& name,
+           JS::MutableHandle<JS::Value> aRetVal, ErrorResult& aRv);
+
+  // Conversion helpers for WebIDL callers
+  bool Has(const nsAString& aName) { return Has(NS_ConvertUTF16toUTF8(aName)); }
+
+  void Get(JSContext* aCx, const nsAString& aName,
+           JS::MutableHandle<JS::Value> aRetVal, ErrorResult& aRv) {
+    return Get(aCx, NS_ConvertUTF16toUTF8(aName), aRetVal, aRv);
+  }
+
+  /**
+   * WebIDL iterator glue.
+   */
+  uint32_t GetIterableLength() const { return EntryArray().Length(); }
+
+  /**
+   * These functions return the key or value, respectively, at the given index.
+   * The index *must* be less than the value returned by GetIterableLength(), or
+   * the program will crash.
+   */
+  const nsString GetKeyAtIndex(uint32_t aIndex) const;
+  bool GetValueAtIndex(JSContext* aCx, uint32_t aIndex,
+                       JS::MutableHandle<JS::Value> aResult) const;
+
+  /**
+   * Returns a copy of the read-only file descriptor which backs the shared
+   * memory region for this map. The file descriptor may be passed between
+   * processes, and used to update corresponding instances in child processes.
+   */
+  FileDescriptor CloneMapFile() const;
+
+  /**
+   * Returns the size of the memory mapped region that backs this map. Must be
+   * passed to the SharedMap() constructor or Update() method along with the
+   * descriptor returned by CloneMapFile() in order to initialize or update a
+   * child SharedMap.
+   */
+  size_t MapSize() const { return mMap.size(); }
+
+  /**
+   * Updates this instance to reflect the contents of the shared memory region
+   * in the given map file, and broadcasts a change event for the given set of
+   * changed (UTF-8-encoded) keys.
+   */
+  void Update(const FileDescriptor& aMapFile, size_t aMapSize,
+              nsTArray<RefPtr<BlobImpl>>&& aBlobs,
+              nsTArray<nsCString>&& aChangedKeys);
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+ protected:
+  ~SharedMap() override = default;
+
+  class Entry {
+   public:
+    Entry(Entry&&) = delete;
+
+    explicit Entry(SharedMap& aMap, const nsACString& aName = ""_ns)
+        : mMap(aMap), mName(aName), mData(AsVariant(uint32_t(0))) {}
+
+    ~Entry() = default;
+
+    /**
+     * Encodes or decodes this entry into or from the given OutputBuffer or
+     * InputBuffer.
+     */
+    template <typename Buffer>
+    void Code(Buffer& buffer) {
+      DebugOnly<size_t> startOffset = buffer.cursor();
+
+      buffer.codeString(mName);
+      buffer.codeUint32(DataOffset());
+      buffer.codeUint32(mSize);
+      buffer.codeUint16(mBlobOffset);
+      buffer.codeUint16(mBlobCount);
+
+      MOZ_ASSERT(buffer.cursor() == startOffset + HeaderSize());
+    }
+
+    /**
+     * Returns the size that this entry will take up in the map header. This
+     * must be equal to the number of bytes encoded by Code().
+     */
+    size_t HeaderSize() const {
+      return (sizeof(uint16_t) + mName.Length() + sizeof(DataOffset()) +
+              sizeof(mSize) + sizeof(mBlobOffset) + sizeof(mBlobCount));
+    }
+
+    /**
+     * Updates the value of this entry to the given structured clone data, of
+     * which it takes ownership. The passed StructuredCloneData object must not
+     * be used after this call.
+     */
+    void TakeData(StructuredCloneData&&);
+
+    /**
+     * This is called while building a new snapshot of the SharedMap. aDestPtr
+     * must point to a buffer within the new snapshot with Size() bytes reserved
+     * for it, and `aNewOffset` must be the offset of that buffer from the start
+     * of the snapshot's memory region.
+     *
+     * This function copies the raw structured clone data for the entry's value
+     * to the new buffer, and updates its internal state for use with the new
+     * data. Its offset is updated to aNewOffset, and any StructuredCloneData
+     * object it holds is destroyed.
+     *
+     * After this call, the entry is only valid in reference to the new
+     * snapshot, and must not be accessed again until the SharedMap mMap has
+     * been updated to point to it.
+     */
+    void ExtractData(char* aDestPtr, uint32_t aNewOffset,
+                     uint16_t aNewBlobOffset);
+
+    // Returns the UTF-8-encoded name of the entry, which is used as its key in
+    // the map.
+    const nsCString& Name() const { return mName; }
+
+    // Decodes the entry's value into the current Realm of the given JS context
+    // and puts the result in aRetVal on success.
+    void Read(JSContext* aCx, JS::MutableHandle<JS::Value> aRetVal,
+              ErrorResult& aRv);
+
+    // Returns the byte size of the entry's raw structured clone data.
+    uint32_t Size() const { return mSize; }
+
+   private:
+    // Returns a pointer to the entry value's structured clone data within the
+    // SharedMap's mapped memory region. This is *only* valid shen mData
+    // contains a uint32_t.
+    const char* Data() const { return mMap.Data() + DataOffset(); }
+
+    // Returns the offset of the entry value's structured clone data within the
+    // SharedMap's mapped memory region. This is *only* valid shen mData
+    // contains a uint32_t.
+    uint32_t& DataOffset() { return mData.as<uint32_t>(); }
+    const uint32_t& DataOffset() const { return mData.as<uint32_t>(); }
+
+   public:
+    uint16_t BlobOffset() const { return mBlobOffset; }
+    uint16_t BlobCount() const { return mBlobCount; }
+
+    Span<const RefPtr<BlobImpl>> Blobs() {
+      if (mData.is<StructuredCloneData>()) {
+        return mData.as<StructuredCloneData>().BlobImpls();
+      }
+      return {&mMap.mBlobImpls[mBlobOffset], BlobCount()};
+    }
+
+   private:
+    // Returns the temporary StructuredCloneData object containing the entry's
+    // value. This is *only* value when mData contains a StructuredCloneDAta
+    // object.
+    const StructuredCloneData& Holder() const {
+      return mData.as<StructuredCloneData>();
+    }
+
+    SharedMap& mMap;
+
+    // The entry's (UTF-8 encoded) name, which serves as its key in the map.
+    nsCString mName;
+
+    /**
+     * This member provides a reference to the entry's structured clone data.
+     * Its type varies depending on the state of the entry:
+     *
+     * - For entries which have been snapshotted into a shared memory region,
+     *   this is a uint32_t offset into the parent SharedMap's Data() buffer.
+     *
+     * - For entries which have been changed in a WritableSharedMap instance,
+     *   but not serialized to a shared memory snapshot yet, this is a
+     *   StructuredCloneData instance, containing a process-local copy of the
+     *   data. This will be discarded the next time the map is serialized, and
+     *   replaced with a buffer offset, as described above.
+     */
+    Variant<uint32_t, StructuredCloneData> mData;
+
+    // The size, in bytes, of the entry's structured clone data.
+    uint32_t mSize = 0;
+
+    uint16_t mBlobOffset = 0;
+    uint16_t mBlobCount = 0;
+  };
+
+  const nsTArray<Entry*>& EntryArray() const;
+
+  nsTArray<RefPtr<BlobImpl>> mBlobImpls;
+
+  // Rebuilds the entry hashtable mEntries from the values serialized in the
+  // current snapshot, if necessary. The hashtable is rebuilt lazily after
+  // construction and after every Update() call, so this function must be called
+  // before any attempt to access mEntries.
+  Result<Ok, nsresult> MaybeRebuild();
+  void MaybeRebuild() const;
+
+  // Note: This header is included by WebIDL binding headers, and therefore
+  // can't include "windows.h". Since FileDescriptor.h does include "windows.h"
+  // on Windows, we can only forward declare FileDescriptor, and can't include
+  // it as an inline member.
+  UniquePtr<FileDescriptor> mMapFile;
+  // The size of the memory-mapped region backed by mMapFile, in bytes.
+  size_t mMapSize = 0;
+
+  mutable nsClassHashtable<nsCStringHashKey, Entry> mEntries;
+  mutable Maybe<nsTArray<Entry*>> mEntryArray;
+
+  // Manages the memory mapping of the current snapshot. This is initialized
+  // lazily after each SharedMap construction or updated, based on the values in
+  // mMapFile and mMapSize.
+  loader::AutoMemMap mMap;
+
+  bool mWritable = false;
+
+  // Returns a pointer to the beginning of the memory mapped snapshot. Entry
+  // offsets are relative to this pointer, and Entry objects access their
+  // structured clone data by indexing this pointer.
+  char* Data() { return mMap.get<char>().get(); }
+};
+
+class WritableSharedMap final : public SharedMap {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(WritableSharedMap, SharedMap)
+
+  WritableSharedMap();
+
+  // Sets the value of the given (UTF-8 encoded) key to a structured clone
+  // snapshot of the given value.
+  void Set(JSContext* cx, const nsACString& name, JS::Handle<JS::Value> value,
+           ErrorResult& aRv);
+
+  // Deletes the given (UTF-8 encoded) key from the map.
+  void Delete(const nsACString& name);
+
+  // Conversion helpers for WebIDL callers
+  void Set(JSContext* aCx, const nsAString& aName, JS::Handle<JS::Value> aValue,
+           ErrorResult& aRv) {
+    return Set(aCx, NS_ConvertUTF16toUTF8(aName), aValue, aRv);
+  }
+
+  void Delete(const nsAString& aName) {
+    return Delete(NS_ConvertUTF16toUTF8(aName));
+  }
+
+  // Flushes any queued changes to a new snapshot, and broadcasts it to all
+  // child SharedMap instances.
+  void Flush();
+
+  // Sends the current set of shared map data to the given content process.
+  void SendTo(ContentParent* aContentParent) const;
+
+  /**
+   * Returns the read-only SharedMap instance corresponding to this
+   * WritableSharedMap for use in the parent process.
+   */
+  SharedMap* GetReadOnly();
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+ protected:
+  ~WritableSharedMap() override = default;
+
+ private:
+  // The set of (UTF-8 encoded) keys which have changed, or been deleted, since
+  // the last snapshot.
+  nsTArray<nsCString> mChangedKeys;
+
+  RefPtr<SharedMap> mReadOnly;
+
+  bool mPendingFlush = false;
+
+  // Creates a new snapshot of the map, and updates all Entry instance to
+  // reference its data.
+  Result<Ok, nsresult> Serialize();
+
+  void IdleFlush();
+
+  // If there have been any changes since the last snapshot, creates a new
+  // serialization and broadcasts it to all child SharedMap instances.
+  void BroadcastChanges();
+
+  // Marks the given (UTF-8 encoded) key as having changed. This adds it to
+  // mChangedKeys, if not already present, and schedules a flush for the next
+  // time the event loop is idle.
+  nsresult KeyChanged(const nsACString& aName);
+};
+
+}  // namespace ipc
+}  // namespace mozilla::dom
+
+#endif  // dom_ipc_SharedMap_h