host: add support for emitting signals

Add support for emitting signals to host.Service which can be used to
transmit data back to the client during an ongoing method call. This
provides the possibility for the services to send information to their
client counterpart while running. The signal can take file descriptors
as extra parameters to send data on separate files.

test/mod/test_host.py

@@ -61,6 +61,18 @@ class ServiceTest(host.Service):
                         continue
                     raise
                 raise ValueError(f"fd '{fd}' was not closed")
+        elif method == "signal_me_3_times":
+            self.emit_signal(0)
+            self.emit_signal(1)
+            self.emit_signal(2)
+        elif method == "signal_me_on_fd":
+            with tempfile.TemporaryFile("w+") as f:
+                with os.fdopen(fds.steal(0)) as d:
+                    f.write(d.read())
+                f.seek(0)
+                fds = [os.dup(f.fileno())]
+                self.register_fds(fds)
+                self.emit_signal("that should do it", fds)
         else:
             raise host.ProtocolError("unknown method:", method)
 
@@ -135,6 +147,42 @@ def test_exception():
             client.call("exception")
 
 
+def test_signals():
+    with host.ServiceManager() as mgr:
+        exec_callback = 0
+
+        def check_value(item, _fds):
+            nonlocal exec_callback
+            assert item == exec_callback
+            exec_callback += 1
+        client = mgr.start("test_signal_me_3_times", __file__)
+        client.call_with_fds("signal_me_3_times", on_signal=check_value)
+        assert exec_callback == 3
+
+
+def test_signals_on_separate_fd():
+    with host.ServiceManager() as mgr:
+
+        data = "osbuild\n"
+        exec_callback = False
+
+        def check_value(item, fds):
+            nonlocal exec_callback
+            exec_callback = True
+            assert item == "that should do it"
+            with os.fdopen(fds.steal(0)) as d:
+                assert data == d.read()
+
+        client = mgr.start("test_signal_me_on_fd", __file__)
+
+        with tempfile.TemporaryFile("w+") as f:
+            f.write(data)
+            f.seek(0)
+
+            client.call_with_fds("signal_me_on_fd", fds=[f.fileno()], on_signal=check_value)
+        assert exec_callback
+
+
 def main():
     service = ServiceTest.from_args(sys.argv[1:])
     service.main()