Merge pull request #225 from labthings/call_async_task

Add `call_async_task` to `thing_server_interface` allowing access to `BlockingPortal.call`

Author: julianstirling

src/labthings_fastapi/thing_server_interface.py

@@ -106,6 +106,31 @@ def start_async_task_soon(
             raise ServerNotRunningError("Can't run async code without an event loop.")
         return portal.start_task_soon(async_function, *args)
 
+    def call_async_task(
+        self, async_function: Callable[Params, Awaitable[ReturnType]], *args: Any
+    ) -> ReturnType:
+        r"""Run an asynchronous task in the server's event loop in a blocking manner.
+
+        This function wraps `anyio.from_thread.BlockingPortal.call` to
+        provide a way of calling asynchronous code from threaded code. It will
+        block the current thread while it calls the provided async function in the
+        server's event loop.
+
+        Do not call this from the event loop or it may lead to a deadlock.
+
+        :param async_function: the asynchronous function to call.
+        :param \*args: positional arguments to be provided to the function.
+
+        :returns: The return value from the asynchronous function.
+
+        :raises ServerNotRunningError: if the server is not running
+            (i.e. there is no event loop).
+        """
+        portal = self._get_server().blocking_portal
+        if portal is None:
+            raise ServerNotRunningError("Can't run async code without an event loop.")
+        return portal.call(async_function, *args)
+
     @property
     def settings_folder(self) -> str:
         """The path to a folder where persistent files may be saved."""

tests/test_thing_server_interface.py

@@ -133,6 +133,28 @@ async def set_mutable(val):
     assert mutable[0] is True
 
 
+def test_call_async_task(server, interface):
+    """Check async tasks may be run in a blocking fashion."""
+
+    async def async_shout(input: str):
+        """A function that shouts back at you.
+
+        This is only async to check it can be called.
+        """
+        return input.upper()
+
+    with pytest.raises(ServerNotRunningError):
+        # You can't run async code unless the server
+        # is running: this should raise a helpful
+        # error.
+        interface.call_async_task(async_shout, "foobar")
+
+    with TestClient(server.app) as _:
+        # TestClient starts an event loop in the background
+        # so this should work
+        assert interface.call_async_task(async_shout, "foobar") == "FOOBAR"
+
+
 def test_settings_folder(server, interface):
     """Check the interface returns the right settings folder."""
     assert interface.settings_folder == os.path.join(server.settings_folder, NAME)