Will Executors

A will executor manages a collection of values and associated will procedures. The will procedure for each value is invoked when the value has been proven unreachable -- except through will executors, weak boxes, and custodians -- by the automatic memory manager.

Each will executor e only executes a single will at a time. A will is executed by e

• synchronously when the will-executor-try procedure is applied to e; or,
• asynchronously within a thread that has e as its current will executor (see section 9.4.1.13).

In the latter case, the thread blocks its normal execution as long as e is invoking a will. Note that if two threads have the same will executor, only one thread executes a will at any time, so only one thread blocks during the will executions. When a will is executed via will-executor-try, no threads are blocked except the thread performing the will-executor-try application.

If a value is registered with multiple wills, then the wills are executed in the reverse order in which they were registered. Furthermore, executing a single will procedure makes the value reachable again. The value must be proven unreachable once again before another will is executed for the value.

If the content value of a weak box is registered with a will executor, the weak box's content is not changed to #f until all wills have been executed for the value and the value has been proven unreachable again.

• (make-will-executor) returns a new will executor with no managed values.
• (will-executor? v) returns #t if v is a will executor, #f otherwise.
• (register-will v f [e]) registers the value v with will procedure f in the will executor e. When v is proven unreachable, then the procedure f will be called with v as its argument. The default value for e is the current will executor (see section 9.4.1.13).
• (will-executor-try e) invokes the will for a single ``unreachable'' value registered with the executor e. If no values managed by the will executor have been proven unreachable, or if the will executor is currently executing a will, #f is returned. Otherwise, the value(s) of the will application is returned.