Genode: add scheduleCallbacks to asyncdispatch (#20708)

* Genode: add native signal handler

* Genode: add scheduleCallbacks to asyncdispatch

This resolves some awkwardness where an RPC server may or may not
use callSoon while dispatching RPC but without scheduling timers
or I/O.

lib/genode/constructibles.nim

@@ -0,0 +1,21 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2022 Emery Hemingway
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+type Constructible*[T] {.
+  importcpp: "Genode::Constructible",
+  header: "<util/reconstructible.h>", byref, pure.} = object
+
+proc construct*[T](x: Constructible[T]) {.importcpp.}
+  ## Construct a constructible C++ object.
+
+proc destruct*[T](x: Constructible[T]) {.importcpp.}
+  ## Destruct a constructible C++ object.
+
+proc constructed*[T](x: Constructible[T]): bool {.importcpp.}
+  ## Test if an object is constructed.

lib/genode/entrypoints.nim

@@ -0,0 +1,22 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2022 Emery Hemingway
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## See `Genode Foundations - Entrypoint <https://genode.org/documentation/genode-foundations/21.05/functional_specification/Entrypoint.html>`
+## for a description of Entrypoints.
+
+type
+  EntrypointObj {.
+    importcpp: "Genode::Entrypoint",
+    header: "<base/entrypoint.h>",
+    pure.} = object
+  Entrypoint* = ptr EntrypointObj
+    ## Opaque Entrypoint object.
+
+proc ep*(env: GenodeEnv): Entrypoint {.importcpp: "(&#->ep())".}
+  ## Access the entrypoint associated with `env`.

lib/genode/signals.nim

@@ -0,0 +1,77 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2022 Emery Hemingway
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## See `Genode Foundations - Asynchronous notifications <https://genode.org/documentation/genode-foundations/21.05/architecture/Inter-component_communication.html#Asynchronous_notifications>`
+## for a description of Genode signals.
+
+when not defined(genode) or defined(nimdoc):
+  {.error: "Genode only module".}
+
+import ./entrypoints, ./constructibles
+
+export ep # Entrypoint accessor on GenodeEnv
+
+type
+  SignalContextCapability* {.
+    importcpp: "Genode::Signal_context_capability",
+    header: "<base/signal.h>", pure.} = object
+    ## Capability to an asynchronous signal context.
+
+proc isValid*(cap: SignalContextCapability): bool {.importcpp: "#.valid()".}
+  ## Call the Genode core to check if this `SignalContextCapability` is valid.
+  # TODO: RpcEffect
+
+type
+  HandlerProc = proc () {.closure, gcsafe.}
+
+  SignalHandlerBase {.
+    importcpp: "Nim::SignalHandler",
+    header: "genode_cpp/signals.h",
+    pure.} = object
+
+  SignalHandlerCpp = Constructible[SignalHandlerBase]
+
+  SignalHandlerObj = object
+    cpp: SignalHandlerCpp
+    cb: HandlerProc
+      ## Signal handling procedure called during dispatch.
+
+  SignalHandler* = ref SignalHandlerObj
+    ## Nim object enclosing a Genode signal handler.
+
+proc construct(cpp: SignalHandlerCpp; ep: Entrypoint; sh: SignalHandler) {.importcpp.}
+
+proc cap(cpp: SignalHandlerCpp): SignalContextCapability {.importcpp: "#->cap()".}
+
+proc newSignalHandler*(ep: Entrypoint; cb: HandlerProc): SignalHandler =
+  ## Create a new signal handler. A label is recommended for
+  ## debugging purposes. A signal handler will not be garbage
+  ## collected until after it has been dissolved.
+  result = SignalHandler(cb: cb)
+  result.cpp.construct(ep, result)
+  GCref result
+
+proc dissolve*(sig: SignalHandler) =
+  ## Dissolve signal dispatcher from entrypoint.
+  # TODO: =destroy?
+  destruct sig.cpp
+  sig.cb = nil # lose the callback
+  GCunref sig
+
+proc cap*(sig: SignalHandler): SignalContextCapability =
+  ## Signal context capability. Can be delegated to external components.
+  sig.cpp.cap
+
+proc submit*(cap: SignalContextCapability) {.
+    importcpp: "Genode::Signal_transmitter(#).submit()".}
+  ## Submit a signal to a context capability.
+
+proc nimHandleSignal(p: pointer) {.exportc.} =
+  ## C symbol invoked by entrypoint during signal dispatch.
+  cast[SignalHandler](p).cb()