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// Copyright 2021 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 INCLUDE_V8_LOCKER_H_
#define INCLUDE_V8_LOCKER_H_
#include "v8config.h" // NOLINT(build/include_directory)
namespace v8 {
namespace internal {
class Isolate;
} // namespace internal
class Isolate;
/**
* Multiple threads in V8 are allowed, but only one thread at a time is allowed
* to use any given V8 isolate, see the comments in the Isolate class. The
* definition of 'using a V8 isolate' includes accessing handles or holding onto
* object pointers obtained from V8 handles while in the particular V8 isolate.
* It is up to the user of V8 to ensure, perhaps with locking, that this
* constraint is not violated. In addition to any other synchronization
* mechanism that may be used, the v8::Locker and v8::Unlocker classes must be
* used to signal thread switches to V8.
*
* v8::Locker is a scoped lock object. While it's active, i.e. between its
* construction and destruction, the current thread is allowed to use the locked
* isolate. V8 guarantees that an isolate can be locked by at most one thread at
* any time. In other words, the scope of a v8::Locker is a critical section.
*
* Sample usage:
* \code
* ...
* {
* v8::Locker locker(isolate);
* v8::Isolate::Scope isolate_scope(isolate);
* ...
* // Code using V8 and isolate goes here.
* ...
* } // Destructor called here
* \endcode
*
* If you wish to stop using V8 in a thread A you can do this either by
* destroying the v8::Locker object as above or by constructing a v8::Unlocker
* object:
*
* \code
* {
* isolate->Exit();
* v8::Unlocker unlocker(isolate);
* ...
* // Code not using V8 goes here while V8 can run in another thread.
* ...
* } // Destructor called here.
* isolate->Enter();
* \endcode
*
* The Unlocker object is intended for use in a long-running callback from V8,
* where you want to release the V8 lock for other threads to use.
*
* The v8::Locker is a recursive lock, i.e. you can lock more than once in a
* given thread. This can be useful if you have code that can be called either
* from code that holds the lock or from code that does not. The Unlocker is
* not recursive so you can not have several Unlockers on the stack at once, and
* you cannot use an Unlocker in a thread that is not inside a Locker's scope.
*
* An unlocker will unlock several lockers if it has to and reinstate the
* correct depth of locking on its destruction, e.g.:
*
* \code
* // V8 not locked.
* {
* v8::Locker locker(isolate);
* Isolate::Scope isolate_scope(isolate);
* // V8 locked.
* {
* v8::Locker another_locker(isolate);
* // V8 still locked (2 levels).
* {
* isolate->Exit();
* v8::Unlocker unlocker(isolate);
* // V8 not locked.
* }
* isolate->Enter();
* // V8 locked again (2 levels).
* }
* // V8 still locked (1 level).
* }
* // V8 Now no longer locked.
* \endcode
*/
class V8_EXPORT Unlocker {
public:
/**
* Initialize Unlocker for a given Isolate.
*/
V8_INLINE explicit Unlocker(Isolate* isolate) { Initialize(isolate); }
~Unlocker();
private:
void Initialize(Isolate* isolate);
internal::Isolate* isolate_;
};
class V8_EXPORT Locker {
public:
/**
* Initialize Locker for a given Isolate.
*/
V8_INLINE explicit Locker(Isolate* isolate) { Initialize(isolate); }
~Locker();
/**
* Returns whether or not the locker for a given isolate, is locked by the
* current thread.
*/
static bool IsLocked(Isolate* isolate);
// Disallow copying and assigning.
Locker(const Locker&) = delete;
void operator=(const Locker&) = delete;
private:
void Initialize(Isolate* isolate);
bool has_lock_;
bool top_level_;
internal::Isolate* isolate_;
};
} // namespace v8
#endif // INCLUDE_V8_LOCKER_H_
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