Don't crash on C coroutine objects

newsfragments/1191.bugfix.rst

@@ -0,0 +1,3 @@
+Trio no longer crashes when an async function is implemented in C or
+Cython and then passed directly to `trio.run` or
+``nursery.start_soon``.

newsfragments/550.bugfix.rst

@@ -0,0 +1,3 @@
+Trio no longer crashes when an async function is implemented in C or
+Cython and then passed directly to `trio.run` or
+``nursery.start_soon``.

trio/_core/_run.py

@@ -1348,18 +1348,25 @@ def _return_value_looks_like_wrong_library(value):
         else:
             context = copy_context()
 
+        if not hasattr(coro, "cr_frame"):
+            # This async function is implemented in C or Cython
+            async def python_wrapper(orig_coro):
+                return await orig_coro
+
+            coro = python_wrapper(coro)
+        coro.cr_frame.f_locals.setdefault(
+            LOCALS_KEY_KI_PROTECTION_ENABLED, system_task
+        )
+
         task = Task(
             coro=coro,
             parent_nursery=nursery,
             runner=self,
             name=name,
             context=context,
         )
-        self.tasks.add(task)
-        coro.cr_frame.f_locals.setdefault(
-            LOCALS_KEY_KI_PROTECTION_ENABLED, system_task
-        )
 
+        self.tasks.add(task)
         if nursery is not None:
             nursery._children.add(task)
             task._activate_cancel_status(nursery._cancel_status)

trio/_core/tests/test_run.py

@@ -8,6 +8,7 @@
 import warnings
 from contextlib import contextmanager, ExitStack
 from math import inf
+from textwrap import dedent
 
 import attr
 import outcome
@@ -2389,3 +2390,40 @@ def abort_fn(_):
         task.coro.send(None)
 
     assert abort_fn_called
+
+
+def test_async_function_implemented_in_C():
+    # These used to crash because we'd try to mutate the coroutine object's
+    # cr_frame, but C functions don't have Python frames.
+
+    ns = {"_core": _core}
+    try:
+        exec(
+            dedent(
+                """
+                async def agen_fn(record):
+                    assert not _core.currently_ki_protected()
+                    record.append("the generator ran")
+                    yield
+                """
+            ),
+            ns,
+        )
+    except SyntaxError:
+        pytest.skip("Requires Python 3.6+")
+    else:
+        agen_fn = ns["agen_fn"]
+
+    run_record = []
+    agen = agen_fn(run_record)
+    _core.run(agen.__anext__)
+    assert run_record == ["the generator ran"]
+
+    async def main():
+        start_soon_record = []
+        agen = agen_fn(start_soon_record)
+        async with _core.open_nursery() as nursery:
+            nursery.start_soon(agen.__anext__)
+        assert start_soon_record == ["the generator ran"]
+
+    _core.run(main)