Implement loop.shutdown_default_executor()

* Adds loop.shutdown_default_executor(), a new coroutine method added to
  asyncio in 3.9 for safe finalization of the event loop's ThreadPoolExecutor.

tests/test_base.py

@@ -689,6 +689,17 @@ async def foo():
             asyncio.wait_for(foo(), timeout=float('inf')))
         self.assertEqual(res, 123)
 
+    def test_shutdown_default_executor(self):
+        if not hasattr(self.loop, "shutdown_default_executor"):
+            raise unittest.SkipTest()
+
+        async def foo():
+            await self.loop.run_in_executor(None, time.sleep, .1)
+
+        self.loop.run_until_complete(foo())
+        self.loop.run_until_complete(
+            self.loop.shutdown_default_executor())
+
 
 class TestBaseUV(_TestBase, UVTestCase):
 

uvloop/includes/stdlib.pxi

@@ -134,6 +134,7 @@ cdef int ssl_SSL_ERROR_WANT_WRITE = ssl.SSL_ERROR_WANT_WRITE
 cdef int ssl_SSL_ERROR_SYSCALL = ssl.SSL_ERROR_SYSCALL
 
 cdef uint64_t MAIN_THREAD_ID = <uint64_t><int64_t>threading.main_thread().ident
+cdef threading_Thread = threading.Thread
 
 cdef int subprocess_PIPE = subprocess.PIPE
 cdef int subprocess_STDOUT = subprocess.STDOUT

uvloop/loop.pxd

@@ -83,6 +83,8 @@ cdef class Loop:
         object _asyncgens
         bint _asyncgens_shutdown_called
 
+        bint _executor_shutdown_called
+
         char _recv_buffer[UV_STREAM_RECV_BUF_SIZE]
         bint _recv_buffer_in_use
 

uvloop/loop.pyx

@@ -186,6 +186,8 @@ cdef class Loop:
 
         # Set to True when `loop.shutdown_asyncgens` is called.
         self._asyncgens_shutdown_called = False
+        # Set to True when `loop.shutdown_default_executor` is called.
+        self._executor_shutdown_called = False
 
         self._servers = set()
 
@@ -591,6 +593,7 @@ cdef class Loop:
         self.handler_idle = None
         self.handler_check__exec_writes = None
 
+        self._executor_shutdown_called = True
         executor = self._default_executor
         if executor is not None:
             self._default_executor = None
@@ -2669,6 +2672,8 @@ cdef class Loop:
 
         if executor is None:
             executor = self._default_executor
+            # Only check when the default executor is being used
+            self._check_default_executor()
             if executor is None:
                 executor = cc_ThreadPoolExecutor()
                 self._default_executor = executor
@@ -3090,6 +3095,10 @@ cdef class Loop:
         await waiter
         return udp, protocol
 
+    def _check_default_executor(self):
+        if self._executor_shutdown_called:
+            raise RuntimeError('Executor shutdown has been called')
+
     def _asyncgen_finalizer_hook(self, agen):
         self._asyncgens.discard(agen)
         if not self.is_closed():
@@ -3131,6 +3140,27 @@ cdef class Loop:
                     'asyncgen': agen
                 })
 
+    @cython.iterable_coroutine
+    async def shutdown_default_executor(self):
+        """Schedule the shutdown of the default executor."""
+        self._executor_shutdown_called = True
+        if self._default_executor is None:
+            return
+        future = self.create_future()
+        thread = threading_Thread(target=self._do_shutdown, args=(future,))
+        thread.start()
+        try:
+            await future
+        finally:
+            thread.join()
+
+    def _do_shutdown(self, future):
+        try:
+            self._default_executor.shutdown(wait=True)
+            self.call_soon_threadsafe(future.set_result, None)
+        except Exception as ex:
+            self.call_soon_threadsafe(future.set_exception, ex)
+
 
 cdef void __loop_alloc_buffer(uv.uv_handle_t* uvhandle,
                               size_t suggested_size,