4Falcon Virtual Machine Preemptibility

Falcon Virtual Machine supports bidirectional communication with the embedding applications and with extension modules that allows external code to ask for the VM to suspend or terminate its execution, and allows the VM to request for the embedding application to perform idle operations.

Namely, this communication is currently supported through three mechanisms:

Wait Interruption Protocol

Blocking operations performed by the Virtual Machine can be asynchronously interrupted from another thread of the application where the virtual machine runs. Whenever an interruption request is issued, if the VM is already engaged in or is approaches a compliant blocking operation, it will raise an InterruptedError. Scripts may intercept this error to for provide some cleanup, or just let it unfiltered.

Note: Unfiltered errors doesn't reach the script layer and cause immediate termination of the signaled VM. Also, a full Falcon error instance is not created for them, and embedding application can access the exception that terminated the execution through a standard Falcon::Error C++ interface.

Not all the blocking waits are instrumented to respect interrupt requets and raise this error, as some waiting operations may be caused by external module that are not required to adopt the vm interruption protocol.

Compliant waiting functions are mainly sleep, Stream.readAvailable, Stream.writeAvailable and a few others. Compliancy to interruption protocol is specified in the item descriptions.

Suspension on sleep request.

Embedding applications may also require the VM to suspend its execution on sleep requests coming from the scripts or from extension modules. It is usefull to have the VM instruct the embedding application on how much sleep time has been required, and let the embedding application manage this idle time.

The Embedding application may also decide to ignore the request, to reset the VM and start a new execution or to destroy it. After a return in sleep request state, the VM is clear for inspection, modification, concurrent execution and termination.

Synchronous periodic callback.

The VM provides a callback hook that will be periodically called. This hook can be used by embedding applications to check for program status change, and it may inject in the VM quit or suspension requests.

In example, a Falcon VM may run in a thread of an embedding application; an asynchronous request to stop the VM may be posted to the runner thread, and the VM may peek this request through the periodic callback. This hook may also be used to dequeue incoming interthread requests that may then be safely injected in the VM and possibly dispatched to coroutines that may handle them without alter the state of the running VM.

Frequency of the periodic callback is expressed in count of virtual instructions executed, and can be regulated and changed live.

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