Add a test for the real issue: error overriding

The underlying issue is actually that a nested `Context` which was
cancelled was overriding the local error that triggered that secondary's
context's cancellation in the first place XD. This test catches that
case.

Relates to https://github.com/pikers/piker/issues/244

tests/test_context_stream_semantics.py

@@ -5,6 +5,7 @@ Verify the we raise errors when streams are opened prior to sync-opening
 a ``tractor.Context`` beforehand.
 
 '''
+from contextlib import asynccontextmanager as acm
 from itertools import count
 import platform
 from typing import Optional
@@ -58,33 +59,6 @@ from conftest import tractor_test
 _state: bool = False
 
 
-@tractor.context
-async def error_before_started(
-    ctx: tractor.Context,
-) -> None:
-    # send an unserializable type
-    await ctx.started(object())
-
-
-def test_error_before_started():
-    async def main():
-        async with tractor.open_nursery() as n:
-            portal = await n.start_actor(
-                'errorer',
-                enable_modules=[__name__],
-            )
-
-            async with portal.open_context(
-                error_before_started
-            ) as (ctx, sent):
-                await trio.sleep(1)
-
-    with pytest.raises(tractor.RemoteActorError) as excinfo:
-        trio.run(main)
-
-    assert excinfo.value.type == TypeError
-
-
 @tractor.context
 async def too_many_starteds(
     ctx: tractor.Context,
@@ -494,7 +468,10 @@ async def cancel_self(
         async with ctx.open_stream():
             pass
     except tractor.ContextCancelled:
+        # suppress for now so we can do checkpoint tests below
         pass
+    else:
+        raise RuntimeError('Context didnt cancel itself?!')
 
     # check a real ``trio.Cancelled`` is raised on a checkpoint
     try:
@@ -534,6 +511,9 @@ async def test_callee_cancels_before_started():
         except tractor.ContextCancelled as ce:
             ce.type == trio.Cancelled
 
+            # the traceback should be informative
+            assert 'cancelled itself' in ce.msgdata['tb_str']
+
         # teardown the actor
         await portal.cancel_actor()
 
@@ -739,3 +719,92 @@ def test_stream_backpressure():
             await portal.cancel_actor()
 
     trio.run(main)
+
+
+@tractor.context
+async def sleep_forever(
+    ctx: tractor.Context,
+) -> None:
+    await ctx.started()
+    async with ctx.open_stream():
+        await trio.sleep_forever()
+
+
+@acm
+async def attach_to_sleep_forever():
+    '''
+    Cancel a context **before** any underlying error is raised in order
+    to trigger a local reception of a ``ContextCancelled`` which **should not**
+    be re-raised in the local surrounding ``Context`` *iff* the cancel was
+    requested by **this** side of the context.
+
+    '''
+    async with tractor.wait_for_actor('sleeper') as p2:
+        async with (
+            p2.open_context(sleep_forever) as (peer_ctx, first),
+            peer_ctx.open_stream(),
+        ):
+            try:
+                yield
+            finally:
+                # XXX: previously this would trigger local
+                # ``ContextCancelled`` to be received and raised in the
+                # local context overriding any local error due to
+                # logic inside ``_invoke()`` which checked for
+                # an error set on ``Context._error`` and raised it in
+                # under a cancellation scenario.
+
+                # The problem is you can have a remote cancellation
+                # that is part of a local error and we shouldn't raise
+                # ``ContextCancelled`` **iff** we weren't the side of
+                # the context to initiate it, i.e.
+                # ``Context._cancel_called`` should **NOT** have been
+                # set. The special logic to handle this case is now
+                # inside ``Context._may_raise_from_remote_msg()`` XD
+                await peer_ctx.cancel()
+
+
+@tractor.context
+async def error_before_started(
+    ctx: tractor.Context,
+) -> None:
+    '''
+    This simulates exactly an original bug discovered in:
+    https://github.com/pikers/piker/issues/244
+
+    '''
+    async with attach_to_sleep_forever():
+        # send an unserializable type which should raise a type error
+        # here and **NOT BE SWALLOWED** by the surrounding acm!!?!
+        await ctx.started(object())
+
+
+def test_do_not_swallow_error_before_started_by_remote_contextcancelled():
+    '''
+    Verify that an error raised in a remote context which itself opens another
+    remote context, which it cancels, does not ovverride the original error that
+    caused the cancellation of the secondardy context.
+
+    '''
+    async def main():
+        async with tractor.open_nursery() as n:
+            portal = await n.start_actor(
+                'errorer',
+                enable_modules=[__name__],
+            )
+            await n.start_actor(
+                'sleeper',
+                enable_modules=[__name__],
+            )
+
+            async with (
+                portal.open_context(
+                    error_before_started
+                ) as (ctx, sent),
+            ):
+                await trio.sleep_forever()
+
+    with pytest.raises(tractor.RemoteActorError) as excinfo:
+        trio.run(main)
+
+    assert excinfo.value.type == TypeError