WIP tinkering with strict-eg-tns and cluster API

Seems that the way the actor-nursery interacts with the
`.trionics.gather_contexts()` API on cancellation makes our
`.trionics.collapse_eg()` not work as intended?

I need to dig into how `ActorNursery.cancel()` and `.__aexit__()` might
be causing this discrepancy..

Consider this a commit-of-my-index type save for rn.

tests/test_clustering.py

@@ -25,14 +25,17 @@ def test_empty_mngrs_input_raises() -> None:
                 ) 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()
                     ),
+                  # ^^NOTE XXX ^^^
+                    # it's the use of inline-generator syntax here
+                    # that causes the "empty input" -> ValueError,
+                    # see `._clustering` impl.
                 ),
             ):
-                assert 0
+                # test should fail if we mk it here!
+                assert 0, 'Should have raised val-err !?'
 
     with pytest.raises(ValueError):
         trio.run(main)

tractor/_clustering.py

@@ -55,10 +55,17 @@ async def open_actor_cluster(
         raise ValueError(
             'Number of names is {len(names)} but count it {count}')
 
-    async with tractor.open_nursery(
-        **runtime_kwargs,
-    ) as an:
-        async with trio.open_nursery() as n:
+    async with (
+        # tractor.trionics.collapse_eg(),
+        tractor.open_nursery(
+            **runtime_kwargs,
+        ) as an
+    ):
+        async with (
+            tractor.trionics.collapse_eg(),
+            trio.open_nursery() as tn,
+            tractor.trionics.maybe_raise_from_masking_exc()
+        ):
             uid = tractor.current_actor().uid
 
             async def _start(name: str) -> None:
@@ -69,9 +76,8 @@ async def open_actor_cluster(
                 )
 
             for name in names:
-                n.start_soon(_start, name)
+                tn.start_soon(_start, name)
 
         assert len(portals) == count
         yield portals
-
         await an.cancel(hard_kill=hard_kill)