Support deterministic scheduling

Very important for Hypothesis, and arguably for debugging in general.

newsfragments/890.feature.rst

@@ -0,0 +1,4 @@
+The Trio scheduler can now be made deterministic by seeding the internal
+random.Random instance that is used to shuffle available tasks.
+This is not public API, but is important for some tools such as
+pytest-trio's `Hypothesis <https://hypothesis.readthedocs.io>`_ integration.

trio/_core/_run.py

@@ -1,4 +1,5 @@
 import functools
+import itertools
 import logging
 import os
 import random
@@ -682,6 +683,7 @@ class Task:
     name = attr.ib()
     # PEP 567 contextvars context
     context = attr.ib()
+    _counter = attr.ib(init=False, factory=itertools.count().__next__)
 
     # Invariant:
     # - for unscheduled tasks, _next_send is None
@@ -1556,7 +1558,12 @@ def run_impl(runner, async_fn, args):
         # scheduling, then there are other things that will probably need to
         # change too, like the deadlines tie-breaker and the non-deterministic
         # ordering of task._notify_queues.)
-        batch = list(runner.runq)
+        #
+        # We sort tasks before shuffling them so that seeding the Random
+        # instance can make the scheduler deterministic, which is important
+        # for testing and debugging, especially with tools such as Hypothesis,
+        # without giving up the advantages of sets everywhere else.
+        batch = sorted(runner.runq, key=lambda t: t._counter)
         runner.runq.clear()
         _r.shuffle(batch)
         while batch:

trio/tests/test_scheduler_determinism.py

@@ -0,0 +1,39 @@
+import trio
+
+
+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_if_seeded():
+    traces = []
+    for _ in range(10):
+        state = trio._core._run._r.getstate()
+        try:
+            trio._core._run._r.seed(0)
+            traces.append(trio.run(scheduler_trace))
+        finally:
+            trio._core._run._r.setstate(state)
+
+    assert len(traces) == 10
+    assert len(set(traces)) == 1