|V8_INLINE||Unlocker (Isolate *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 thead 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.
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 can not 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.: