Asynchronous notification mechanism

Section Asynchronous notifications introduces asynchronous notifications (signals) as one of the fundamental inter-component communication mechanisms. The description covers the semantics of the mechanism but the question of how the mechanism relates to core and the underlying kernel remains unanswered. This section complements Section Asynchronous notifications with those implementation details.

Most kernels do not directly support the semantics of asynchronous notifications as presented in Section Asynchronous notifications. As a reminder, the mechanism has the following features:

The gap between this feature set and the mechanisms provided by the underlying kernel is bridged by core as part of the PD service. This service plays the role of a proxy between the producers and receivers of signals. Each component that interacts with signals has a session to this service.

Within core, a signal context is represented as an RPC object. The RPC object maintains a counter of signals pending for this context. Signal contexts can be created and destroyed by the clients of the PD service using the alloc_context and free_context RPC functions. Upon the creation of a signal context, the PD client can specify an integer value called imprint with a client-local meaning. Later, on the reception of signals, the imprint value is delivered along with the signal to enable the client to tell the contexts of the incoming signals apart. As a result of the allocation of a new signal context, the client obtains a signal-context capability. This capability can be delegated to other components using the regular capability-delegation mechanism.

Signal submission

A component in possession of a signal-context capability is able to trigger signals using the submit function of its PD session. The submit function takes the signal context capability of the targeted context and a counter value as arguments. The capability as supplied to the submit function does not need to originate from the called session. It may have been created and delegated by another component. Note that even though a signal context is an RPC object, the submission of a signal is not realized as an invocation of this object. The signal-context capability is merely used as an RPC function argument. This design accounts for the fact that signal-context capabilities may originate from untrusted peers as is the case for servers that deliver asynchronous notifications to their clients. A client of such a server supplies a signal-context capability as argument to one of the server's RPC functions. An example is the input stream of the GUI session (Section GUI) that allows the client to get notified when new user input becomes available. A malicious client may specify a capability that was not created via core's PD service but that instead refers to an RPC object local to the client. If the submit function was an RPC function of the signal context, the server's call of the submit RPC function would eventually invoke the RPC object of the client. This would put the client in a position where it may block the server indefinitely and thereby make the server unavailable to all clients. In contrast to the untrusted signal-context capability, the PD session of a signal producer is by definition trusted. So it is safe to invoke the submit RPC function with the signal-context capability as argument. In the case where an invalid signal-context capability is delegated to the signal producer, core will fail to look up a signal context for the given capability and omit the signal.

Signal reception

For receiving signals, a component needs a way to obtain information about pending signals from core. This involves two steps: First, the component needs a way to block until signals are available. Second, if a signal is pending, the component needs a way to determine the signal context and the signal receiver associated with the signal and wake up the thread that blocks the Signal_receiver::block_for_signal API function.

Both problems are solved by a dedicated thread that is spawned during component startup. This signal thread blocks at core's PD service for incoming signals. The blocking operation is not directly performed on the PD session but on a decoupled RPC object called signal source. In contrast to the PD session interface that is kernel agnostic, the underlying kernel mechanism used for blocking the signal thread at the signal source depends on the used base platform.

The signal-source RPC object implements an RPC interface, on which the PD client issues a blocking wait_for_signal RPC function. This function blocks as long as no signal that refers to the session's signal contexts is pending. If the function returns, the return value contains the imprint that was assigned to the signal context at its creation and the number of signals pending for this context. On most base platforms, the implementation of the blocking RPC interface is realized by processing RPC requests and responses out of order to enable one entrypoint in core to serve all signal sources. Core uses a dedicated entrypoint for the signal-source handling to decouple the delivery of signals from potentially long-taking operations of the other core services.

Given the imprint value returned by the signal source, the signal thread determines the signal context and signal receiver that belongs to the pending signal (using a data structure called Signal_context_registry) and locally submits the signal to the signal-receiver object. This, in turn, unblocks the Signal_receiver::block_for_signal function at the API level.