Add a basic no-errors-when-backpressure stream test

tests/test_context_streams.py

@@ -6,6 +6,7 @@ a ``tractor.Context`` beforehand.
 
 '''
 from itertools import count
+from typing import Optional
 
 import pytest
 import trio
@@ -59,6 +60,7 @@ async def never_open_stream(
 async def keep_sending_from_callee(
 
     ctx:  tractor.Context,
+    msg_buffer_size: Optional[int] = None,
 
 ) -> None:
     '''
@@ -66,8 +68,11 @@ async def keep_sending_from_callee(
 
     '''
     await ctx.started()
-    async with ctx.open_stream() as stream:
+    async with ctx.open_stream(
+        msg_buffer_size=msg_buffer_size,
+    ) as stream:
         for msg in count():
+            print(f'callee sending {msg}')
             await stream.send(msg)
             await trio.sleep(0.01)
 
@@ -94,7 +99,7 @@ def test_one_end_stream_not_opened(overrun_by):
     async def main():
         async with tractor.open_nursery() as n:
             portal = await n.start_actor(
-                'starts_no_stream',
+                entrypoint.__name__,
                 enable_modules=[__name__],
             )
 
@@ -158,3 +163,81 @@ def test_one_end_stream_not_opened(overrun_by):
 
     else:
         trio.run(main)
+
+@tractor.context
+async def echo_back_sequence(
+
+    ctx:  tractor.Context,
+    seq: list[int],
+    msg_buffer_size: Optional[int] = None,
+
+) -> None:
+    '''
+    Send endlessly on the calleee stream.
+
+    '''
+    await ctx.started()
+    async with ctx.open_stream(
+        msg_buffer_size=msg_buffer_size,
+    ) as stream:
+
+        count = 0
+        while count < 3:
+            batch = []
+            async for msg in stream:
+                batch.append(msg)
+                if batch == seq:
+                    break
+
+            for msg in batch:
+                print(f'callee sending {msg}')
+                await stream.send(msg)
+
+            count += 1
+
+        return 'yo'
+
+
+def test_stream_backpressure():
+    '''
+    Demonstrate small overruns of each task back and forth
+    on a stream not raising any errors by default.
+
+    '''
+    async def main():
+        async with tractor.open_nursery() as n:
+            portal = await n.start_actor(
+                'callee_sends_forever',
+                enable_modules=[__name__],
+            )
+            seq = list(range(3))
+            async with portal.open_context(
+                echo_back_sequence,
+                seq=seq,
+                msg_buffer_size=1,
+            ) as (ctx, sent):
+                assert sent is None
+
+                async with ctx.open_stream(msg_buffer_size=1) as stream:
+                    count = 0
+                    while count < 3:
+                        for msg in seq:
+                            print(f'caller sending {msg}')
+                            await stream.send(msg)
+                            await trio.sleep(0.1)
+
+                        batch = []
+                        async for msg in stream:
+                            batch.append(msg)
+                            if batch == seq:
+                                break
+
+                        count += 1
+
+            # here the context should return
+            assert await ctx.result() == 'yo'
+
+            # cancel the daemon
+            await portal.cancel_actor()
+
+    trio.run(main)