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 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_V8_PLATFORM_H_
 #define V8_V8_PLATFORM_H_
 
 #include <math.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>  // For abort.
 
 #include <memory>
 #include <optional>
 #include <string>
 
 #include "v8-source-location.h"  // NOLINT(build/include_directory)
 #include "v8config.h"            // NOLINT(build/include_directory)
 
 namespace v8 {
 
 class Isolate;
 
 // Valid priorities supported by the task scheduling infrastructure.
 enum class TaskPriority : uint8_t {
   /**
    * Best effort tasks are not critical for performance of the application. The
    * platform implementation should preempt such tasks if higher priority tasks
    * arrive.
    */
   kBestEffort,
   /**
    * User visible tasks are long running background tasks that will
    * improve performance and memory usage of the application upon completion.
    * Example: background compilation and garbage collection.
    */
   kUserVisible,
   /**
    * User blocking tasks are highest priority tasks that block the execution
    * thread (e.g. major garbage collection). They must be finished as soon as
    * possible.
    */
   kUserBlocking,
   kMaxPriority = kUserBlocking
 };
 
 /**
  * A Task represents a unit of work.
  */
 class Task {
  public:
   virtual ~Task() = default;
 
   virtual void Run() = 0;
 };
 
 /**
  * An IdleTask represents a unit of work to be performed in idle time.
  * The Run method is invoked with an argument that specifies the deadline in
  * seconds returned by MonotonicallyIncreasingTime().
  * The idle task is expected to complete by this deadline.
  */
 class IdleTask {
  public:
   virtual ~IdleTask() = default;
   virtual void Run(double deadline_in_seconds) = 0;
 };
 
 /**
  * A TaskRunner allows scheduling of tasks. The TaskRunner may still be used to
  * post tasks after the isolate gets destructed, but these tasks may not get
  * executed anymore. All tasks posted to a given TaskRunner will be invoked in
  * sequence. Tasks can be posted from any thread.
  */
 class TaskRunner {
  public:
   /**
    * Schedules a task to be invoked by this TaskRunner. The TaskRunner
    * implementation takes ownership of |task|.
    *
    * Embedders should override PostTaskImpl instead of this.
    */
   void PostTask(std::unique_ptr<Task> task,
                 SourceLocation location = SourceLocation::Current()) {
     PostTaskImpl(std::move(task), location);
   }
 
   /**
    * Schedules a task to be invoked by this TaskRunner. The TaskRunner
    * implementation takes ownership of |task|. The |task| cannot be nested
    * within other task executions.
    *
    * Tasks which shouldn't be interleaved with JS execution must be posted with
    * |PostNonNestableTask| or |PostNonNestableDelayedTask|. This is because the
    * embedder may process tasks in a callback which is called during JS
    * execution.
    *
    * In particular, tasks which execute JS must be non-nestable, since JS
    * execution is not allowed to nest.
    *
    * Requires that |TaskRunner::NonNestableTasksEnabled()| is true.
    *
    * Embedders should override PostNonNestableTaskImpl instead of this.
    */
   void PostNonNestableTask(
       std::unique_ptr<Task> task,
       SourceLocation location = SourceLocation::Current()) {
     PostNonNestableTaskImpl(std::move(task), location);
   }
 
   /**
    * Schedules a task to be invoked by this TaskRunner. The task is scheduled
    * after the given number of seconds |delay_in_seconds|. The TaskRunner
    * implementation takes ownership of |task|.
    *
    * Embedders should override PostDelayedTaskImpl instead of this.
    */
   void PostDelayedTask(std::unique_ptr<Task> task, double delay_in_seconds,
                        SourceLocation location = SourceLocation::Current()) {
     PostDelayedTaskImpl(std::move(task), delay_in_seconds, location);
   }
 
   /**
    * Schedules a task to be invoked by this TaskRunner. The task is scheduled
    * after the given number of seconds |delay_in_seconds|. The TaskRunner
    * implementation takes ownership of |task|. The |task| cannot be nested
    * within other task executions.
    *
    * Tasks which shouldn't be interleaved with JS execution must be posted with
    * |PostNonNestableTask| or |PostNonNestableDelayedTask|. This is because the
    * embedder may process tasks in a callback which is called during JS
    * execution.
    *
    * In particular, tasks which execute JS must be non-nestable, since JS
    * execution is not allowed to nest.
    *
    * Requires that |TaskRunner::NonNestableDelayedTasksEnabled()| is true.
    *
    * Embedders should override PostNonNestableDelayedTaskImpl instead of this.
    */
   void PostNonNestableDelayedTask(
       std::unique_ptr<Task> task, double delay_in_seconds,
       SourceLocation location = SourceLocation::Current()) {
     PostNonNestableDelayedTaskImpl(std::move(task), delay_in_seconds, location);
   }
 
   /**
    * Schedules an idle task to be invoked by this TaskRunner. The task is
    * scheduled when the embedder is idle. Requires that
    * |TaskRunner::IdleTasksEnabled()| is true. Idle tasks may be reordered
    * relative to other task types and may be starved for an arbitrarily long
    * time if no idle time is available. The TaskRunner implementation takes
    * ownership of |task|.
    *
    * Embedders should override PostIdleTaskImpl instead of this.
    */
   void PostIdleTask(std::unique_ptr<IdleTask> task,
                     SourceLocation location = SourceLocation::Current()) {
     PostIdleTaskImpl(std::move(task), location);
   }
 
   /**
    * Returns true if idle tasks are enabled for this TaskRunner.
    */
   virtual bool IdleTasksEnabled() = 0;
 
   /**
    * Returns true if non-nestable tasks are enabled for this TaskRunner.
    */
   virtual bool NonNestableTasksEnabled() const { return false; }
 
   /**
    * Returns true if non-nestable delayed tasks are enabled for this TaskRunner.
    */
   virtual bool NonNestableDelayedTasksEnabled() const { return false; }
 
   TaskRunner() = default;
   virtual ~TaskRunner() = default;
 
   TaskRunner(const TaskRunner&) = delete;
   TaskRunner& operator=(const TaskRunner&) = delete;
 
  protected:
   /**
    * Implementation of above methods with an additional `location` argument.
    */
   virtual void PostTaskImpl(std::unique_ptr<Task> task,
                             const SourceLocation& location) {}
   virtual void PostNonNestableTaskImpl(std::unique_ptr<Task> task,
                                        const SourceLocation& location) {}
   virtual void PostDelayedTaskImpl(std::unique_ptr<Task> task,
                                    double delay_in_seconds,
                                    const SourceLocation& location) {}
   virtual void PostNonNestableDelayedTaskImpl(std::unique_ptr<Task> task,
                                               double delay_in_seconds,
                                               const SourceLocation& location) {}
   virtual void PostIdleTaskImpl(std::unique_ptr<IdleTask> task,
                                 const SourceLocation& location) {}
 };
 
 /**
  * Delegate that's passed to Job's worker task, providing an entry point to
  * communicate with the scheduler.
  */
 class JobDelegate {
  public:
   /**
    * Returns true if this thread *must* return from the worker task on the
    * current thread ASAP. Workers should periodically invoke ShouldYield (or
    * YieldIfNeeded()) as often as is reasonable.
    * After this method returned true, ShouldYield must not be called again.
    */
   virtual bool ShouldYield() = 0;
 
   /**
    * Notifies the scheduler that max concurrency was increased, and the number
    * of worker should be adjusted accordingly. See Platform::PostJob() for more
    * details.
    */
   virtual void NotifyConcurrencyIncrease() = 0;
 
   /**
    * Returns a task_id unique among threads currently running this job, such
    * that GetTaskId() < worker count. To achieve this, the same task_id may be
    * reused by a different thread after a worker_task returns.
    */
   virtual uint8_t GetTaskId() = 0;
 
   /**
    * Returns true if the current task is called from the thread currently
    * running JobHandle::Join().
    */
   virtual bool IsJoiningThread() const = 0;
 };
 
 /**
  * Handle returned when posting a Job. Provides methods to control execution of
  * the posted Job.
  */
 class JobHandle {
  public:
   virtual ~JobHandle() = default;
 
   /**
    * Notifies the scheduler that max concurrency was increased, and the number
    * of worker should be adjusted accordingly. See Platform::PostJob() for more
    * details.
    */
   virtual void NotifyConcurrencyIncrease() = 0;
 
   /**
    * Contributes to the job on this thread. Doesn't return until all tasks have
    * completed and max concurrency becomes 0. When Join() is called and max
    * concurrency reaches 0, it should not increase again. This also promotes
    * this Job's priority to be at least as high as the calling thread's
    * priority.
    */
   virtual void Join() = 0;
 
   /**
    * Forces all existing workers to yield ASAP. Waits until they have all
    * returned from the Job's callback before returning.
    */
   virtual void Cancel() = 0;
 
   /*
    * Forces all existing workers to yield ASAP but doesn’t wait for them.
    * Warning, this is dangerous if the Job's callback is bound to or has access
    * to state which may be deleted after this call.
    */
   virtual void CancelAndDetach() = 0;
 
   /**
    * Returns true if there's any work pending or any worker running.
    */
   virtual bool IsActive() = 0;
 
   /**
    * Returns true if associated with a Job and other methods may be called.
    * Returns false after Join() or Cancel() was called. This may return true
    * even if no workers are running and IsCompleted() returns true
    */
   virtual bool IsValid() = 0;
 
   /**
    * Returns true if job priority can be changed.
    */
   virtual bool UpdatePriorityEnabled() const { return false; }
 
   /**
    *  Update this Job's priority.
    */
   virtual void UpdatePriority(TaskPriority new_priority) {}
 };
 
 /**
  * A JobTask represents work to run in parallel from Platform::PostJob().
  */
 class JobTask {
  public:
   virtual ~JobTask() = default;
 
   virtual void Run(JobDelegate* delegate) = 0;
 
   /**
    * Controls the maximum number of threads calling Run() concurrently, given
    * the number of threads currently assigned to this job and executing Run().
    * Run() is only invoked if the number of threads previously running Run() was
    * less than the value returned. In general, this should return the latest
    * number of incomplete work items (smallest unit of work) left to process,
    * including items that are currently in progress. |worker_count| is the
    * number of threads currently assigned to this job which some callers may
    * need to determine their return value. Since GetMaxConcurrency() is a leaf
    * function, it must not call back any JobHandle methods.
    */
   virtual size_t GetMaxConcurrency(size_t worker_count) const = 0;
 };
 
 // Allows a thread to temporarily boost another thread's priority to match its
 // own priority. The priority is reset when the object is destroyed, which must
 // happens on the boosted thread.
 class ScopedBoostablePriority {
  public:
   ScopedBoostablePriority() = default;
   virtual ~ScopedBoostablePriority() = default;
   ScopedBoostablePriority(const ScopedBoostablePriority&) = delete;
   ScopedBoostablePriority& operator=(const ScopedBoostablePriority& other) =
       delete;
 
   // Boosts the priority of the thread where this ScopedBoostablePriority was
   // created. Can be called from any thread, but requires proper external
   // synchronization with the constructor, destructor and any other call to
   // BoostPriority/Reset(). If called multiple times, only the first call takes
   // effect.
   virtual bool BoostPriority() = 0;
 
   // Resets the priority of the thread where this ScopedBoostablePriority was
   // created to its original priority.
   virtual void Reset() = 0;
 };
 
 /**
  * A "blocking call" refers to any call that causes the calling thread to wait
  * off-CPU. It includes but is not limited to calls that wait on synchronous
  * file I/O operations: read or write a file from disk, interact with a pipe or
  * a socket, rename or delete a file, enumerate files in a directory, etc.
  * Acquiring a low contention lock is not considered a blocking call.
  */
 
 /**
  * BlockingType indicates the likelihood that a blocking call will actually
  * block.
  */
 enum class BlockingType {
   // The call might block (e.g. file I/O that might hit in memory cache).
   kMayBlock,
   // The call will definitely block (e.g. cache already checked and now pinging
   // server synchronously).
   kWillBlock
 };
 
 /**
  * This class is instantiated with CreateBlockingScope() in every scope where a
  * blocking call is made and serves as a precise annotation of the scope that
  * may/will block. May be implemented by an embedder to adjust the thread count.
  * CPU usage should be minimal within that scope. ScopedBlockingCalls can be
  * nested.
  */
 class ScopedBlockingCall {
  public:
   virtual ~ScopedBlockingCall() = default;
 };
 
 /**
  * The interface represents complex arguments to trace events.
  */
 class ConvertableToTraceFormat {
  public:
   virtual ~ConvertableToTraceFormat() = default;
 
   /**
    * Append the class info to the provided |out| string. The appended
    * data must be a valid JSON object. Strings must be properly quoted, and
    * escaped. There is no processing applied to the content after it is
    * appended.
    */
   virtual void AppendAsTraceFormat(std::string* out) const = 0;
 };
 
 /**
  * V8 Tracing controller.
  *
  * Can be implemented by an embedder to record trace events from V8.
  *
  * Will become obsolete in Perfetto build (v8_use_perfetto = true).
  */
 class TracingController {
  public:
   virtual ~TracingController() = default;
 
   // In Perfetto mode, trace events are written using Perfetto's Track Event
   // API directly without going through the embedder. However, it is still
   // possible to observe tracing being enabled and disabled.
 #if !defined(V8_USE_PERFETTO)
   /**
    * Called by TRACE_EVENT* macros, don't call this directly.
    * The name parameter is a category group for example:
    * TRACE_EVENT0("v8,parse", "V8.Parse")
    * The pointer returned points to a value with zero or more of the bits
    * defined in CategoryGroupEnabledFlags.
    **/
   virtual const uint8_t* GetCategoryGroupEnabled(const char* name) {
     static uint8_t no = 0;
     return &no;
   }
 
   /**
    * Adds a trace event to the platform tracing system. These function calls are
    * usually the result of a TRACE_* macro from trace-event-no-perfetto.h when
    * tracing and the category of the particular trace are enabled. It is not
    * advisable to call these functions on their own; they are really only meant
    * to be used by the trace macros. The returned handle can be used by
    * UpdateTraceEventDuration to update the duration of COMPLETE events.
    */
   virtual uint64_t AddTraceEvent(
       char phase, const uint8_t* category_enabled_flag, const char* name,
       const char* scope, uint64_t id, uint64_t bind_id, int32_t num_args,
       const char** arg_names, const uint8_t* arg_types,
       const uint64_t* arg_values,
       std::unique_ptr<ConvertableToTraceFormat>* arg_convertables,
       unsigned int flags) {
     return 0;
   }
   virtual uint64_t AddTraceEventWithTimestamp(
       char phase, const uint8_t* category_enabled_flag, const char* name,
       const char* scope, uint64_t id, uint64_t bind_id, int32_t num_args,
       const char** arg_names, const uint8_t* arg_types,
       const uint64_t* arg_values,
       std::unique_ptr<ConvertableToTraceFormat>* arg_convertables,
       unsigned int flags, int64_t timestamp) {
     return 0;
   }
 
   /**
    * Sets the duration field of a COMPLETE trace event. It must be called with
    * the handle returned from AddTraceEvent().
    **/
   virtual void UpdateTraceEventDuration(const uint8_t* category_enabled_flag,
                                         const char* name, uint64_t handle) {}
 #endif  // !defined(V8_USE_PERFETTO)
 
   class TraceStateObserver {
    public:
     virtual ~TraceStateObserver() = default;
     virtual void OnTraceEnabled() = 0;
     virtual void OnTraceDisabled() = 0;
   };
 
   /**
    * Adds tracing state change observer.
    * Does nothing in Perfetto SDK build (v8_use_perfetto = true).
    */
   virtual void AddTraceStateObserver(TraceStateObserver*) {}
 
   /**
    * Removes tracing state change observer.
    * Does nothing in Perfetto SDK build (v8_use_perfetto = true).
    */
   virtual void RemoveTraceStateObserver(TraceStateObserver*) {}
 };
 
 // Opaque type representing a handle to a shared memory region.
 class SharedMemoryHandle {
  public:
   // For the handle itself, we use the underlying type (e.g. unsigned int)
   // instead of e.g. mach_port_t to avoid pulling in large OS header files into
   // this header file. Instead, the users of these routines are expected to
   // include the respective OS headers in addition to this one.
 
 #if V8_OS_DARWIN
   // A mach_port_t referencing a memory entry object.
   using PlatformHandle = unsigned int;
 #elif V8_OS_FUCHSIA
   // A zx_handle_t to a VMO.
   using PlatformHandle = uint32_t;
 #elif V8_OS_WIN
   // A Windows HANDLE to a file mapping object.
   using PlatformHandle = void*;
 #else
   // A file descriptor.
   using PlatformHandle = int;
 #endif
 
   static constexpr SharedMemoryHandle FromPlatformHandle(
       PlatformHandle handle) {
     return SharedMemoryHandle(handle);
   }
 
   PlatformHandle GetPlatformHandle() const { return handle_; }
 
  private:
   SharedMemoryHandle() = delete;
   explicit constexpr SharedMemoryHandle(PlatformHandle handle)
       : handle_(handle) {}
 
   PlatformHandle handle_;
 };
 
 #define DEFINE_SHARED_MEMORY_HANDLE_WRAPPERS(Wrap, Unwrap)                    \
   V8_DEPRECATE_SOON("Use SharedMemoryHandle::FromPlatformHandle instead")     \
   inline SharedMemoryHandle Wrap(SharedMemoryHandle::PlatformHandle handle) { \
     return SharedMemoryHandle::FromPlatformHandle(handle);                    \
   }                                                                           \
   V8_DEPRECATE_SOON("Use SharedMemoryHandle::GetPlatformHandle instead")      \
   inline SharedMemoryHandle::PlatformHandle Unwrap(                           \
       SharedMemoryHandle handle) {                                            \
     return handle.GetPlatformHandle();                                        \
   }
 
 #if V8_OS_DARWIN
 DEFINE_SHARED_MEMORY_HANDLE_WRAPPERS(SharedMemoryHandleFromMachMemoryEntry,
                                      MachMemoryEntryFromSharedMemoryHandle)
 #elif V8_OS_FUCHSIA
 DEFINE_SHARED_MEMORY_HANDLE_WRAPPERS(SharedMemoryHandleFromVMO,
                                      VMOFromSharedMemoryHandle)
 #elif V8_OS_WIN
 DEFINE_SHARED_MEMORY_HANDLE_WRAPPERS(SharedMemoryHandleFromFileMapping,
                                      FileMappingFromSharedMemoryHandle)
 #else
 DEFINE_SHARED_MEMORY_HANDLE_WRAPPERS(SharedMemoryHandleFromFileDescriptor,
                                      FileDescriptorFromSharedMemoryHandle)
 #endif
 
 #undef DEFINE_SHARED_MEMORY_HANDLE_WRAPPERS
 
 // TODO(https://crbug.com/463925491): Remove this type alias once Chromium's
 // "gin" V8 binding migrates off it.
 using PlatformSharedMemoryHandle = std::optional<SharedMemoryHandle>;
 V8_DEPRECATE_SOON("Use std::nullopt instead")
 static constexpr PlatformSharedMemoryHandle kInvalidSharedMemoryHandle =
     std::nullopt;
 
 /**
  * A V8 memory page allocator.
  *
  * Can be implemented by an embedder to manage large host OS allocations.
  */
 class PageAllocator {
  public:
   virtual ~PageAllocator() = default;
 
   /**
    * Gets the page granularity for AllocatePages and FreePages. Addresses and
    * lengths for those calls should be multiples of AllocatePageSize().
    */
   virtual size_t AllocatePageSize() = 0;
 
   /**
    * Gets the page granularity for SetPermissions and ReleasePages. Addresses
    * and lengths for those calls should be multiples of CommitPageSize().
    */
   virtual size_t CommitPageSize() = 0;
 
   /**
    * Sets the random seed so that GetRandomMmapAddr() will generate repeatable
    * sequences of random mmap addresses.
    */
   virtual void SetRandomMmapSeed(int64_t seed) = 0;
 
   /**
    * Returns a randomized address, suitable for memory allocation under ASLR.
    * The address will be aligned to AllocatePageSize.
    */
   virtual void* GetRandomMmapAddr() = 0;
 
   /**
    * Memory permissions.
    */
   enum Permission {
     kNoAccess,
     kRead,
     kReadWrite,
     kReadWriteExecute,
     kReadExecute,
     // Set this when reserving memory that will later require kReadWriteExecute
     // permissions. The resulting behavior is platform-specific, currently
     // this is used to set the MAP_JIT flag on Apple Silicon.
     // TODO(jkummerow): Remove this when Wasm has a platform-independent
     // w^x implementation.
     // TODO(saelo): Remove this once all JIT pages are allocated through the
     // VirtualAddressSpace API.
     kNoAccessWillJitLater
   };
 
   /**
    * Optional hints for AllocatePages().
    */
   class AllocationHint final {
    public:
     AllocationHint() = default;
 
     V8_WARN_UNUSED_RESULT constexpr AllocationHint WithAddress(
         void* address) const {
       return AllocationHint(address, may_grow_);
     }
 
     V8_WARN_UNUSED_RESULT constexpr AllocationHint WithMayGrow() const {
       return AllocationHint(address_, true);
     }
 
     bool MayGrow() const { return may_grow_; }
     void* Address() const { return address_; }
 
    private:
     constexpr AllocationHint(void* address, bool may_grow)
         : address_(address), may_grow_(may_grow) {}
 
     void* address_ = nullptr;
     bool may_grow_ = false;
   };
 
   /**
    * Allocates memory in range with the given alignment and permission.
    */
   virtual void* AllocatePages(void* address, size_t length, size_t alignment,
                               Permission permissions) = 0;
 
   /**
    * Allocates memory in range with the given alignment and permission. In
    * addition to AllocatePages it allows to pass in allocation hints. The
    * underlying implementation may not make use of hints.
    */
   virtual void* AllocatePages(size_t length, size_t alignment,
                               Permission permissions, AllocationHint hint) {
     return AllocatePages(hint.Address(), length, alignment, permissions);
   }
 
   /**
    * Resizes the previously allocated memory at the given address. Returns true
    * if the allocation could be resized. Returns false if this operation is
    * either not supported or the object could not be resized in-place.
    */
   virtual bool ResizeAllocationAt(void* address, size_t old_length,
                                   size_t new_length, Permission permissions) {
     return false;
   }
 
   /**
    * Frees memory in a range that was allocated by a call to AllocatePages.
    */
   virtual bool FreePages(void* address, size_t length) = 0;
 
   /**
    * Releases memory in a range that was allocated by a call to AllocatePages.
    */
   virtual bool ReleasePages(void* address, size_t length,
                             size_t new_length) = 0;
 
   /**
    * Sets permissions on pages in an allocated range.
    */
   virtual bool SetPermissions(void* address, size_t length,
                               Permission permissions) = 0;
 
   /**
    * Recommits discarded pages in the given range with given permissions.
    * Discarded pages must be recommitted with their original permissions
    * before they are used again.
    */
   virtual bool RecommitPages(void* address, size_t length,
                              Permission permissions) {
     // TODO(v8:12797): make it pure once it's implemented on Chromium side.
     return false;
   }
 
   /**
    * Frees memory in the given [address, address + size) range. address and size
    * should be operating system page-aligned. The next write to this
    * memory area brings the memory transparently back. This should be treated as
    * a hint to the OS that the pages are no longer needed. It does not guarantee
    * that the pages will be discarded immediately or at all.
    */
   virtual bool DiscardSystemPages(void* address, size_t size) { return true; }
 
   /**
    * Decommits any wired memory pages in the given range, allowing the OS to
    * reclaim them, and marks the region as inacessible (kNoAccess). The address
    * range stays reserved and can be accessed again later by changing its
    * permissions. However, in that case the memory content is guaranteed to be
    * zero-initialized again. The memory must have been previously allocated by a
    * call to AllocatePages. Returns true on success, false otherwise.
    */
   virtual bool DecommitPages(void* address, size_t size) = 0;
 
   /**
    * Block any modifications to the given mapping such as changing permissions
    * or unmapping the pages on supported platforms.
    * The address space reservation will exist until the process ends, but it's
    * possible to release the memory using DiscardSystemPages. Note that this
    * might require write permissions to the page as e.g. on Linux, mseal will
    * block discarding sealed anonymous memory.
    */
   virtual bool SealPages(void* address, size_t length) {
     // TODO(360048056): make it pure once it's implemented on Chromium side.
     return false;
   }
 
   /**
    * INTERNAL ONLY: This interface has not been stabilised and may change
    * without notice from one release to another without being deprecated first.
    */
   class SharedMemoryMapping {
    public:
     // Implementations are expected to free the shared memory mapping in the
     // destructor.
     virtual ~SharedMemoryMapping() = default;
     virtual void* GetMemory() const = 0;
   };
 
   /**
    * INTERNAL ONLY: This interface has not been stabilised and may change
    * without notice from one release to another without being deprecated first.
    */
   class SharedMemory {
    public:
     // Implementations are expected to free the shared memory in the destructor.
     virtual ~SharedMemory() = default;
     virtual std::unique_ptr<SharedMemoryMapping> RemapTo(
         void* new_address) const = 0;
     virtual void* GetMemory() const = 0;
     virtual size_t GetSize() const = 0;
   };
 
   /**
    * INTERNAL ONLY: This interface has not been stabilised and may change
    * without notice from one release to another without being deprecated first.
    *
    * Reserve pages at a fixed address returning whether the reservation is
    * possible. The reserved memory is detached from the PageAllocator and so
    * should not be freed by it. It's intended for use with
    * SharedMemory::RemapTo, where ~SharedMemoryMapping would free the memory.
    */
   virtual bool ReserveForSharedMemoryMapping(void* address, size_t size) {
     return false;
   }
 
   /**
    * INTERNAL ONLY: This interface has not been stabilised and may change
    * without notice from one release to another without being deprecated first.
    *
    * Allocates shared memory pages. Not all PageAllocators need support this and
    * so this method need not be overridden.
    * Allocates a new read-only shared memory region of size |length| and copies
    * the memory at |original_address| into it.
    */
   virtual std::unique_ptr<SharedMemory> AllocateSharedPages(
       size_t length, const void* original_address) {
     return {};
   }
 
   /**
    * INTERNAL ONLY: This interface has not been stabilised and may change
    * without notice from one release to another without being deprecated first.
    *
    * If not overridden and changed to return true, V8 will not attempt to call
    * AllocateSharedPages or RemapSharedPages. If overridden, AllocateSharedPages
    * and RemapSharedPages must also be overridden.
    */
   virtual bool CanAllocateSharedPages() { return false; }
 };
 
 /**
  * An allocator that uses per-thread permissions to protect the memory.
  *
  * The implementation is platform/hardware specific, e.g. using pkeys on x64.
  *
  * INTERNAL ONLY: This interface has not been stabilised and may change
  * without notice from one release to another without being deprecated first.
  */
 class ThreadIsolatedAllocator {
  public:
   virtual ~ThreadIsolatedAllocator() = default;
 
   virtual void* Allocate(size_t size) = 0;
 
   virtual void Free(void* object) = 0;
 
   enum class Type {
     kPkey,
   };
 
   virtual Type Type() const = 0;
 
   /**
    * Return the pkey used to implement the thread isolation if Type == kPkey.
    */
   virtual int Pkey() const { return -1; }
 };
 
 /**
  * Possible permissions for memory pages.
  */
 enum class PagePermissions {
   kNoAccess = 0,
   kRead = 1,
   kWrite = 2,
   kExecute = 4,
   kReadWrite = kRead | kWrite,
   kReadExecute = kRead | kExecute,
   kWriteExecute = kWrite | kExecute,
   kReadWriteExecute = kRead | kWrite | kExecute,
 };
 
 inline constexpr PagePermissions operator|(PagePermissions lhs,
                                            PagePermissions rhs) {
   return static_cast<PagePermissions>(static_cast<int>(lhs) |
                                       static_cast<int>(rhs));
 }
 
 inline constexpr PagePermissions operator&(PagePermissions lhs,
                                            PagePermissions rhs) {
   return static_cast<PagePermissions>(static_cast<int>(lhs) &
                                       static_cast<int>(rhs));
 }
 
 inline PagePermissions& operator|=(PagePermissions& lhs, PagePermissions rhs) {
   lhs = lhs | rhs;
   return lhs;
 }
 
 /**
  * Helper routine to determine whether one set of page permissions (the lhs) is
  * a subset of another one (the rhs).
  */
 inline constexpr bool IsSubset(PagePermissions lhs, PagePermissions rhs) {
   return (lhs & rhs) == lhs;
 }
 
 /**
  * Class to manage a virtual memory address space.
  *
  * This class represents a contiguous region of virtual address space in which
  * sub-spaces and (private or shared) memory pages can be allocated, freed, and
  * modified. This interface is meant to eventually replace the PageAllocator
  * interface, and can be used as an alternative in the meantime.
  *
  * This API is not yet stable and may change without notice!
  */
 class VirtualAddressSpace {
  public:
   using Address = uintptr_t;
 
   VirtualAddressSpace(size_t page_size, size_t allocation_granularity,
                       Address base, size_t size,
                       PagePermissions max_page_permissions)
       : page_size_(page_size),
         allocation_granularity_(allocation_granularity),
         base_(base),
         size_(size),
         max_page_permissions_(max_page_permissions) {}
 
   virtual ~VirtualAddressSpace() = default;
 
   /**
    * The page size used inside this space. Guaranteed to be a power of two.
    * Used as granularity for all page-related operations except for allocation,
    * which use the allocation_granularity(), see below.
    *
    * \returns the page size in bytes.
    */
   size_t page_size() const { return page_size_; }
 
   /**
    * The granularity of page allocations and, by extension, of subspace
    * allocations. This is guaranteed to be a power of two and a multiple of the
    * page_size(). In practice, this is equal to the page size on most OSes, but
    * on Windows it is usually 64KB, while the page size is 4KB.
    *
    * \returns the allocation granularity in bytes.
    */
   size_t allocation_granularity() const { return allocation_granularity_; }
 
   /**
    * The base address of the address space managed by this instance.
    *
    * \returns the base address of this address space.
    */
   Address base() const { return base_; }
 
   /**
    * The size of the address space managed by this instance.
    *
    * \returns the size of this address space in bytes.
    */
   size_t size() const { return size_; }
 
   /**
    * The maximum page permissions that pages allocated inside this space can
    * obtain.
    *
    * \returns the maximum page permissions.
    */
   PagePermissions max_page_permissions() const { return max_page_permissions_; }
 
   /**
    * Whether the |address| is inside the address space managed by this instance.
    *
    * \returns true if it is inside the address space, false if not.
    */
   bool Contains(Address address) const {
     return (address >= base()) && (address < base() + size());
   }
 
   /**
    * Sets the random seed so that GetRandomPageAddress() will generate
    * repeatable sequences of random addresses.
    *
    * \param The seed for the PRNG.
    */
   virtual void SetRandomSeed(int64_t seed) = 0;
 
   /**
    * Returns a random address inside this address space, suitable for page
    * allocations hints.
    *
    * \returns a random address aligned to allocation_granularity().
    */
   virtual Address RandomPageAddress() = 0;
 
   /**
    * Allocates private memory pages with the given alignment and permissions.
    *
    * \param hint If nonzero, the allocation is attempted to be placed at the
    * given address first. If that fails, the allocation is attempted to be
    * placed elsewhere, possibly nearby, but that is not guaranteed. Specifying
    * zero for the hint always causes this function to choose a random address.
    * The hint, if specified, must be aligned to the specified alignment.
    *
    * \param size The size of the allocation in bytes. Must be a multiple of the
    * allocation_granularity().
    *
    * \param alignment The alignment of the allocation in bytes. Must be a
    * multiple of the allocation_granularity() and should be a power of two.
    *
    * \param permissions The page permissions of the newly allocated pages.
    *
    * \returns the start address of the allocated pages on success, zero on
    * failure.
    */
   static constexpr Address kNoHint = 0;
   virtual V8_WARN_UNUSED_RESULT Address
   AllocatePages(Address hint, size_t size, size_t alignment,
                 PagePermissions permissions) = 0;
 
   /**
    * Frees previously allocated pages.
    *
    * This function will terminate the process on failure as this implies a bug
    * in the client. As such, there is no return value.
    *
    * \param address The start address of the pages to free. This address must
    * have been obtained through a call to AllocatePages.
    *
    * \param size The size in bytes of the region to free. This must match the
    * size passed to AllocatePages when the pages were allocated.
    */
   virtual void FreePages(Address address, size_t size) = 0;
 
   /**
    * Sets permissions of all allocated pages in the given range.
    *
    * This operation can fail due to OOM, in which case false is returned. If
    * the operation fails for a reason other than OOM, this function will
    * terminate the process as this implies a bug in the client.
    *
    * \param address The start address of the range. Must be aligned to
    * page_size().
    *
    * \param size The size in bytes of the range. Must be a multiple
    * of page_size().
    *
    * \param permissions The new permissions for the range.
    *
    * \returns true on success, false on OOM.
    */
   virtual V8_WARN_UNUSED_RESULT bool SetPagePermissions(
       Address address, size_t size, PagePermissions permissions) = 0;
 
   /**
    * Creates a guard region at the specified address.
    *
    * Guard regions are guaranteed to cause a fault when accessed and generally
    * do not count towards any memory consumption limits. Further, allocating
    * guard regions can usually not fail in subspaces if the region does not
    * overlap with another region, subspace, or page allocation.
    *
    * \param address The start address of the guard region. Must be aligned to
    * the allocation_granularity().
    *
    * \param size The size of the guard region in bytes. Must be a multiple of
    * the allocation_granularity().
    *
    * \returns true on success, false otherwise.
    */
   virtual V8_WARN_UNUSED_RESULT bool AllocateGuardRegion(Address address,
                                                          size_t size) = 0;
 
   /**
    * Frees an existing guard region.
    *
    * This function will terminate the process on failure as this implies a bug
    * in the client. As such, there is no return value.
    *
    * \param address The start address of the guard region to free. This address
    * must have previously been used as address parameter in a successful
    * invocation of AllocateGuardRegion.
    *
    * \param size The size in bytes of the guard region to free. This must match
    * the size passed to AllocateGuardRegion when the region was created.
    */
   virtual void FreeGuardRegion(Address address, size_t size) = 0;
 
   /**
    * Allocates shared memory pages with the given permissions.
    *
    * \param hint Placement hint. See AllocatePages.
    *
    * \param size The size of the allocation in bytes. Must be a multiple of the
    * allocation_granularity().
    *
    * \param permissions The page permissions of the newly allocated pages.
    *
    * \param handle A platform-specific handle to a shared memory object. See
    * the SharedMemoryHandleFromX routines above for ways to obtain these.
    *
    * \param offset The offset in the shared memory object at which the mapping
    * should start. Must be a multiple of the allocation_granularity().
    *
    * \returns the start address of the allocated pages on success, zero on
    * failure.
    */
   virtual V8_WARN_UNUSED_RESULT Address
   AllocateSharedPages(Address hint, size_t size, PagePermissions permissions,
                       SharedMemoryHandle handle, uint64_t offset) = 0;
 
   // TODO(https://crbug.com/463925491): Remove me once API users change from
   // PlatformSharedMemoryHandle to SharedMemoryHandle.
   V8_DEPRECATE_SOON("Use AllocateSharedPages() with SharedMemoryHandle")
   V8_WARN_UNUSED_RESULT Address AllocateSharedPages(
       Address hint, size_t size, PagePermissions permissions,
       std::optional<SharedMemoryHandle> handle, uint64_t offset) {
     return AllocateSharedPages(hint, size, permissions, *handle, offset);
   }
 
   /**
    * Frees previously allocated shared pages.
    *
    * This function will terminate the process on failure as this implies a bug
    * in the client. As such, there is no return value.
    *
    * \param address The start address of the pages to free. This address must
    * have been obtained through a call to AllocateSharedPages.
    *
    * \param size The size in bytes of the region to free. This must match the
    * size passed to AllocateSharedPages when the pages were allocated.
    */
   virtual void FreeSharedPages(Address address, size_t size) = 0;
 
   /**
    * Memory protection key support.
    *
    * If supported by the hardware and operating system, virtual address spaces
    * can use memory protection keys in addition to the regular page
    * permissions. The MemoryProtectionKeyId type identifies a memory protection
    * key and is used by the related APIs in this class.
    *
    * TODO(saelo): consider renaming to just MemoryProtectionKey, but currently
    * there's a naming conflict with base::MemoryProtectionKey.
    */
   using MemoryProtectionKeyId = int;
 
   /**
    * The memory protection key used by this space, if any.
    *
    * If this space uses a memory protection key, then all memory pages in it
    * will have this key set. In that case, this API will return that key.
    *
    * \returns the memory protection key used by this space or std::nullopt.
    */
   virtual std::optional<MemoryProtectionKeyId> ActiveMemoryProtectionKey() = 0;
 
   /**
    * Whether this instance can allocate subspaces or not.
    *
    * \returns true if subspaces can be allocated, false if not.
    */
   virtual bool CanAllocateSubspaces() = 0;
 
   /*
    * Allocate a subspace.
    *
    * The address space of a subspace stays reserved in the parent space for the
    * lifetime of the subspace. As such, it is guaranteed that page allocations
    * on the parent space cannot end up inside a subspace.
    *
    * \param hint Hints where the subspace should be allocated. See
    * AllocatePages() for more details.
    *
    * \param size The size in bytes of the subspace. Must be a multiple of the
    * allocation_granularity().
    *
    * \param alignment The alignment of the subspace in bytes. Must be a multiple
    * of the allocation_granularity() and should be a power of two.
    *
    * \param max_page_permissions The maximum permissions that pages allocated in
    * the subspace can obtain.
    *
    * \param key Optional memory protection key for the subspace. If used, the
    * returned subspace will use this key for all its memory pages.
    *
    * \param handle Optional file descriptor for the subspace. If used, the
    * returned subspace will use this file descriptor with 0 offset as the
    * space's underlying file.
    *
    * \returns a new subspace or nullptr on failure.
    */
   virtual std::unique_ptr<VirtualAddressSpace> AllocateSubspace(
       Address hint, size_t size, size_t alignment,
       PagePermissions max_page_permissions,
       std::optional<MemoryProtectionKeyId> key = std::nullopt,
       std::optional<SharedMemoryHandle> handle = std::nullopt) = 0;
 
   //
   // TODO(v8) maybe refactor the methods below before stabilizing the API. For
   // example by combining them into some form of page operation method that
   // takes a command enum as parameter.
   //
 
   /**
    * Recommits discarded pages in the given range with given permissions.
    * Discarded pages must be recommitted with their original permissions
    * before they are used again.
    *
    * \param address The start address of the range. Must be aligned to
    * page_size().
    *
    * \param size The size in bytes of the range. Must be a multiple
    * of page_size().
    *
    * \param permissions The permissions for the range that the pages must have.
    *
    * \returns true on success, false otherwise.
    */
   virtual V8_WARN_UNUSED_RESULT bool RecommitPages(
       Address address, size_t size, PagePermissions permissions) = 0;
 
   /**
    * Frees memory in the given [address, address + size) range. address and
    * size should be aligned to the page_size(). The next write to this memory
    * area brings the memory transparently back. This should be treated as a
    * hint to the OS that the pages are no longer needed. It does not guarantee
    * that the pages will be discarded immediately or at all.
    *
    * \returns true on success, false otherwise. Since this method is only a
    * hint, a successful invocation does not imply that pages have been removed.
    */
   virtual V8_WARN_UNUSED_RESULT bool DiscardSystemPages(Address address,
                                                         size_t size) {
     return true;
   }
   /**
    * Decommits any wired memory pages in the given range, allowing the OS to
    * reclaim them, and marks the region as inacessible (kNoAccess). The address
    * range stays reserved and can be accessed again later by changing its
    * permissions. However, in that case the memory content is guaranteed to be
    * zero-initialized again. The memory must have been previously allocated by a
    * call to AllocatePages.
    *
    * \returns true on success, false otherwise.
    */
   virtual V8_WARN_UNUSED_RESULT bool DecommitPages(Address address,
                                                    size_t size) = 0;
 
   /**
    * Sets a name for the address space.
    *
    * This is mostly useful for debugging tools. If supported by the system, the
    * name will for example show up in /proc/$pid/maps next to the virtual
    * address reservation:
    *
    *     2ae700000000-2ae700010000 r--p 00000000 00:00 0  [anon:foo-bar]
    *
    * \param name The name of the address space. The name must only contain
    * alphanumeric characters or dashes.
    *
    * \returns true on success, false otherwise.
    */
   virtual bool SetName(const std::string& name) { return false; }
 
  private:
   const size_t page_size_;
   const size_t allocation_granularity_;
   const Address base_;
   const size_t size_;
   const PagePermissions max_page_permissions_;
 };
 
 /**
  * Observer used by V8 to notify the embedder about entering/leaving sections
  * with high throughput of malloc/free operations.
  */
 class HighAllocationThroughputObserver {
  public:
   virtual void EnterSection() {}
   virtual void LeaveSection() {}
 };
 
 /**
  * V8 Platform abstraction layer.
  *
  * The embedder has to provide an implementation of this interface before
  * initializing the rest of V8.
  */
 class Platform {
  public:
   virtual ~Platform() = default;
 
   /**
    * Allows the embedder to manage memory page allocations.
    * Returning nullptr will cause V8 to use the default page allocator.
    */
   virtual PageAllocator* GetPageAllocator() { return nullptr; }
 
   /**
    * Allows the embedder to provide an allocator that uses per-thread memory
    * permissions to protect allocations.
    * Returning nullptr will cause V8 to disable protections that rely on this
    * feature.
    */
   virtual ThreadIsolatedAllocator* GetThreadIsolatedAllocator() {
     return nullptr;
   }
 
   /**
    * Enables the embedder to respond in cases where V8 can't allocate large
    * blocks of memory. V8 retries the failed allocation once after calling this
    * method. On success, execution continues; otherwise V8 exits with a fatal
    * error.
    * Embedder overrides of this function must NOT call back into V8.
    */
   virtual void OnCriticalMemoryPressure() {}
 
   /**
    * Gets the max number of worker threads that may be used to execute
    * concurrent work scheduled for any single TaskPriority by
    * Call(BlockingTask)OnWorkerThread() or PostJob(). This can be used to
    * estimate the number of tasks a work package should be split into. A return
    * value of 0 means that there are no worker threads available. Note that a
    * value of 0 won't prohibit V8 from posting tasks using |CallOnWorkerThread|.
    */
   virtual int NumberOfWorkerThreads() = 0;
 
   /**
    * Returns a TaskRunner which can be used to post a task on the foreground.
    * The TaskRunner's NonNestableTasksEnabled() must be true. This function
    * should only be called from a foreground thread.
    */
   std::shared_ptr<v8::TaskRunner> GetForegroundTaskRunner(Isolate* isolate) {
     return GetForegroundTaskRunner(isolate, TaskPriority::kUserBlocking);
   }
 
   /**
    * Returns a TaskRunner with a specific |priority| which can be used to post a
    * task on the foreground thread. The TaskRunner's NonNestableTasksEnabled()
    * must be true. This function should only be called from a foreground thread.
    */
   virtual std::shared_ptr<v8::TaskRunner> GetForegroundTaskRunner(
       Isolate* isolate, TaskPriority priority) = 0;
 
   /**
    * Schedules a task to be invoked on a worker thread.
    * Embedders should override PostTaskOnWorkerThreadImpl() instead of
    * CallOnWorkerThread().
    */
   V8_DEPRECATE_SOON("Use PostTaskOnWorkerThread instead.")
   void CallOnWorkerThread(std::unique_ptr<Task> task,
                           SourceLocation location = SourceLocation::Current()) {
     PostTaskOnWorkerThreadImpl(TaskPriority::kUserVisible, std::move(task),
                                location);
   }
 
   /**
    * Schedules a task that blocks the main thread to be invoked with
    * high-priority on a worker thread.
    * Embedders should override PostTaskOnWorkerThreadImpl() instead of
    * CallBlockingTaskOnWorkerThread().
    */
   V8_DEPRECATE_SOON("Use PostTaskOnWorkerThread instead.")
   void CallBlockingTaskOnWorkerThread(
       std::unique_ptr<Task> task,
       SourceLocation location = SourceLocation::Current()) {
     // Embedders may optionally override this to process these tasks in a high
     // priority pool.
     PostTaskOnWorkerThreadImpl(TaskPriority::kUserBlocking, std::move(task),
                                location);
   }
 
   /**
    * Schedules a task to be invoked with low-priority on a worker thread.
    * Embedders should override PostTaskOnWorkerThreadImpl() instead of
    * CallLowPriorityTaskOnWorkerThread().
    */
   V8_DEPRECATE_SOON("Use PostTaskOnWorkerThread instead.")
   void CallLowPriorityTaskOnWorkerThread(
       std::unique_ptr<Task> task,
       SourceLocation location = SourceLocation::Current()) {
     // Embedders may optionally override this to process these tasks in a low
     // priority pool.
     PostTaskOnWorkerThreadImpl(TaskPriority::kBestEffort, std::move(task),
                                location);
   }
 
   /**
    * Schedules a task to be invoked on a worker thread after |delay_in_seconds|
    * expires.
    * Embedders should override PostDelayedTaskOnWorkerThreadImpl() instead of
    * CallDelayedOnWorkerThread().
    */
   V8_DEPRECATE_SOON("Use PostDelayedTaskOnWorkerThread instead.")
   void CallDelayedOnWorkerThread(
       std::unique_ptr<Task> task, double delay_in_seconds,
       SourceLocation location = SourceLocation::Current()) {
     PostDelayedTaskOnWorkerThreadImpl(TaskPriority::kUserVisible,
                                       std::move(task), delay_in_seconds,
                                       location);
   }
 
   /**
    * Schedules a task to be invoked on a worker thread.
    * Embedders should override PostTaskOnWorkerThreadImpl() instead of
    * PostTaskOnWorkerThread().
    */
   void PostTaskOnWorkerThread(
       TaskPriority priority, std::unique_ptr<Task> task,
       SourceLocation location = SourceLocation::Current()) {
     PostTaskOnWorkerThreadImpl(priority, std::move(task), location);
   }
 
   /**
    * Schedules a task to be invoked on a worker thread after |delay_in_seconds|
    * expires.
    * Embedders should override PostDelayedTaskOnWorkerThreadImpl() instead of
    * PostDelayedTaskOnWorkerThread().
    */
   void PostDelayedTaskOnWorkerThread(
       TaskPriority priority, std::unique_ptr<Task> task,
       double delay_in_seconds,
       SourceLocation location = SourceLocation::Current()) {
     PostDelayedTaskOnWorkerThreadImpl(priority, std::move(task),
                                       delay_in_seconds, location);
   }
 
   /**
    * Returns true if idle tasks are enabled for the given |isolate|.
    */
   virtual bool IdleTasksEnabled(Isolate* isolate) { return false; }
 
   /**
    * Posts |job_task| to run in parallel. Returns a JobHandle associated with
    * the Job, which can be joined or canceled.
    * This avoids degenerate cases:
    * - Calling CallOnWorkerThread() for each work item, causing significant
    *   overhead.
    * - Fixed number of CallOnWorkerThread() calls that split the work and might
    *   run for a long time. This is problematic when many components post
    *   "num cores" tasks and all expect to use all the cores. In these cases,
    *   the scheduler lacks context to be fair to multiple same-priority requests
    *   and/or ability to request lower priority work to yield when high priority
    *   work comes in.
    * A canonical implementation of |job_task| looks like:
    * class MyJobTask : public JobTask {
    *  public:
    *   MyJobTask(...) : worker_queue_(...) {}
    *   // JobTask:
    *   void Run(JobDelegate* delegate) override {
    *     while (!delegate->ShouldYield()) {
    *       // Smallest unit of work.
    *       auto work_item = worker_queue_.TakeWorkItem(); // Thread safe.
    *       if (!work_item) return;
    *       ProcessWork(work_item);
    *     }
    *   }
    *
    *   size_t GetMaxConcurrency() const override {
    *     return worker_queue_.GetSize(); // Thread safe.
    *   }
    * };
    * auto handle = PostJob(TaskPriority::kUserVisible,
    *                       std::make_unique<MyJobTask>(...));
    * handle->Join();
    *
    * PostJob() and methods of the returned JobHandle/JobDelegate, must never be
    * called while holding a lock that could be acquired by JobTask::Run or
    * JobTask::GetMaxConcurrency -- that could result in a deadlock. This is
    * because [1] JobTask::GetMaxConcurrency may be invoked while holding
    * internal lock (A), hence JobTask::GetMaxConcurrency can only use a lock (B)
    * if that lock is *never* held while calling back into JobHandle from any
    * thread (A=>B/B=>A deadlock) and [2] JobTask::Run or
    * JobTask::GetMaxConcurrency may be invoked synchronously from JobHandle
    * (B=>JobHandle::foo=>B deadlock).
    * Embedders should override CreateJobImpl() instead of PostJob().
    */
   std::unique_ptr<JobHandle> PostJob(
       TaskPriority priority, std::unique_ptr<JobTask> job_task,
       SourceLocation location = SourceLocation::Current()) {
     auto handle = CreateJob(priority, std::move(job_task), location);
     handle->NotifyConcurrencyIncrease();
     return handle;
   }
 
   /**
    * Creates and returns a JobHandle associated with a Job. Unlike PostJob(),
    * this doesn't immediately schedules |worker_task| to run; the Job is then
    * scheduled by calling either NotifyConcurrencyIncrease() or Join().
    *
    * A sufficient CreateJob() implementation that uses the default Job provided
    * in libplatform looks like:
    *  std::unique_ptr<JobHandle> CreateJob(
    *      TaskPriority priority, std::unique_ptr<JobTask> job_task) override {
    *    return v8::platform::NewDefaultJobHandle(
    *        this, priority, std::move(job_task), NumberOfWorkerThreads());
    * }
    *
    * Embedders should override CreateJobImpl() instead of CreateJob().
    */
   std::unique_ptr<JobHandle> CreateJob(
       TaskPriority priority, std::unique_ptr<JobTask> job_task,
       SourceLocation location = SourceLocation::Current()) {
     return CreateJobImpl(priority, std::move(job_task), location);
   }
 
   /**
    * Instantiates a ScopedBoostablePriority to boost a thread's priority.
    */
   virtual std::unique_ptr<ScopedBoostablePriority>
   CreateBoostablePriorityScope() {
     return nullptr;
   }
 
   /**
    * Instantiates a ScopedBlockingCall to annotate a scope that may/will block.
    */
   virtual std::unique_ptr<ScopedBlockingCall> CreateBlockingScope(
       BlockingType blocking_type) {
     return nullptr;
   }
 
   /**
    * Monotonically increasing time in seconds from an arbitrary fixed point in
    * the past. This function is expected to return at least
    * millisecond-precision values. For this reason,
    * it is recommended that the fixed point be no further in the past than
    * the epoch.
    **/
   virtual double MonotonicallyIncreasingTime() = 0;
 
   /**
    * Current wall-clock time in milliseconds since epoch. Use
    * CurrentClockTimeMillisHighResolution() when higher precision is
    * required.
    */
   virtual int64_t CurrentClockTimeMilliseconds() {
     return static_cast<int64_t>(floor(CurrentClockTimeMillis()));
   }
 
   /**
    * This function is deprecated and will be deleted. Use either
    * CurrentClockTimeMilliseconds() or
    * CurrentClockTimeMillisecondsHighResolution().
    */
   virtual double CurrentClockTimeMillis() = 0;
 
   /**
    * Same as CurrentClockTimeMilliseconds(), but with more precision.
    */
   virtual double CurrentClockTimeMillisecondsHighResolution() {
     return CurrentClockTimeMillis();
   }
 
   typedef void (*StackTracePrinter)();
 
   /**
    * Returns a function pointer that print a stack trace of the current stack
    * on invocation. Disables printing of the stack trace if nullptr.
    */
   virtual StackTracePrinter GetStackTracePrinter() { return nullptr; }
 
   /**
    * Returns an instance of a v8::TracingController. This must be non-nullptr.
    */
   virtual TracingController* GetTracingController() = 0;
 
   /**
    * Tells the embedder to generate and upload a crashdump during an unexpected
    * but non-critical scenario.
    */
   virtual void DumpWithoutCrashing() {}
 
   /**
    * Allows the embedder to observe sections with high throughput allocation
    * operations.
    */
   virtual HighAllocationThroughputObserver*
   GetHighAllocationThroughputObserver() {
     static HighAllocationThroughputObserver default_observer;
     return &default_observer;
   }
 
  protected:
   /**
    * Default implementation of current wall-clock time in milliseconds
    * since epoch. Useful for implementing |CurrentClockTimeMillis| if
    * nothing special needed.
    */
   V8_EXPORT static double SystemClockTimeMillis();
 
   /**
    * Creates and returns a JobHandle associated with a Job.
    */
   virtual std::unique_ptr<JobHandle> CreateJobImpl(
       TaskPriority priority, std::unique_ptr<JobTask> job_task,
       const SourceLocation& location) = 0;
 
   /**
    * Schedules a task with |priority| to be invoked on a worker thread.
    */
   virtual void PostTaskOnWorkerThreadImpl(TaskPriority priority,
                                           std::unique_ptr<Task> task,
                                           const SourceLocation& location) = 0;
 
   /**
    * Schedules a task with |priority| to be invoked on a worker thread after
    * |delay_in_seconds| expires.
    */
   virtual void PostDelayedTaskOnWorkerThreadImpl(
       TaskPriority priority, std::unique_ptr<Task> task,
       double delay_in_seconds, const SourceLocation& location) = 0;
 };
 
 }  // namespace v8
 
 #endif  // V8_V8_PLATFORM_H_

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