Add `mngrs=(<gen_comprehension>)` test

tests/test_clustering.py

@@ -1,5 +1,6 @@
 import itertools
 
+import pytest
 import trio
 import tractor
 from tractor import open_actor_cluster
@@ -11,26 +12,72 @@ from conftest import tractor_test
 MESSAGE = 'tractoring at full speed'
 
 
+def test_empty_mngrs_input_raises() -> None:
+
+    async def main():
+        with trio.fail_after(1):
+            async with (
+                open_actor_cluster(
+                    modules=[__name__],
+
+                    # NOTE: ensure we can passthrough runtime opts
+                    loglevel='info',
+                    # debug_mode=True,
+
+                ) as portals,
+
+                gather_contexts(
+                    # NOTE: it's the use of inline-generator syntax
+                    # here that causes the empty input.
+                    mngrs=(
+                        p.open_context(worker) for p in portals.values()
+                    ),
+                ),
+            ):
+                assert 0
+
+    with pytest.raises(ValueError):
+        trio.run(main)
+
+
 @tractor.context
-async def worker(ctx: tractor.Context) -> None:
+async def worker(
+    ctx: tractor.Context,
+
+) -> None:
+
     await ctx.started()
-    async with ctx.open_stream(backpressure=True) as stream:
+
+    async with ctx.open_stream(
+        backpressure=True,
+    ) as stream:
+
+        # TODO: this with the below assert causes a hang bug?
+        # with trio.move_on_after(1):
+
         async for msg in stream:
             # do something with msg
             print(msg)
             assert msg == MESSAGE
 
+        # TODO: does this ever cause a hang
+        # assert 0
+
 
 @tractor_test
 async def test_streaming_to_actor_cluster() -> None:
+
     async with (
         open_actor_cluster(modules=[__name__]) as portals,
+
         gather_contexts(
             mngrs=[p.open_context(worker) for p in portals.values()],
         ) as contexts,
+
         gather_contexts(
             mngrs=[ctx[0].open_stream() for ctx in contexts],
         ) as streams,
+
     ):
         with trio.move_on_after(1):
             for stream in itertools.cycle(streams):