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The consumer maintains a stack of access keys so that invocations from a callback within a callback are possible. The top of the stack corresponds to the most recent invocation. An empty stack means that the consumer was not yet invoked.

When acquire  is invoked, we first check if access is restricted. It is restricted when the access-key stack is not empty. If it is not empty, the thread-local variable must be equal to the value on the top of the stack. If it is empty, any thread may continue. After this check, we generate a new access-key that can be used inside callbacks. This new access key is pushed on the stack and also stored in the thread-local variable.

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