Test for scheduler determinism

pytest_trio/_tests/test_hypothesis_interaction.py

@@ -1,6 +1,9 @@
 import pytest
+import trio
 from hypothesis import given, settings, strategies as st
 
+from pytest_trio.plugin import _trio_test_runner_factory
+
 # deadline=None avoids unpredictable warnings/errors when CI happens to be
 # slow (example: https://travis-ci.org/python-trio/pytest-trio/jobs/406738296)
 # max_examples=5 speeds things up a bit
@@ -28,3 +31,47 @@ async def test_mark_outer(n):
 async def test_mark_and_parametrize(x, y):
     assert x is None
     assert y in (1, 2)
+
+
+async def scheduler_trace():
+    """Returns a scheduler-dependent value we can use to check determinism."""
+    trace = []
+
+    async def tracer(name):
+        for i in range(10):
+            trace.append((name, i))
+            await trio.sleep(0)
+
+    async with trio.open_nursery() as nursery:
+        for i in range(5):
+            nursery.start_soon(tracer, i)
+
+    return tuple(trace)
+
+
+def test_the_trio_scheduler_is_not_deterministic():
+    # At least, not yet.  See https://github.com/python-trio/trio/issues/32
+    traces = []
+    for _ in range(10):
+        traces.append(trio.run(scheduler_trace))
+    assert len(set(traces)) == len(traces)
+
+
+def test_the_trio_scheduler_is_deterministic_under_hypothesis():
+    traces = []
+
+    @our_settings
+    @given(st.integers())
+    @pytest.mark.trio
+    async def inner(_):
+        traces.append(await scheduler_trace())
+
+    # The pytest.mark.trio doesn't do it's magic thing to
+    # inner functions, so we invoke it explicitly here.
+    inner.hypothesis.inner_test = _trio_test_runner_factory(
+        None, inner.hypothesis.inner_test
+    )
+    inner()  # Tada, now it's a sync function!
+
+    assert len(traces) >= 5
+    assert len(set(traces)) == 1