docs: Add notice about WindowsSelectorEventLoop on py38

Fixes #2608

diff --git a/docs/index.rst b/docs/index.rst
index ac5ac61966a89ea4b728cceadd2a32aad7ad0e73..0239606ffdeb016e304d6446f9c38b1ac5be4c6f 100644 (file)
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -120,6 +120,13 @@ development use. Without reworking Tornado IOLoop interface, it's not
 possible to add a native Tornado Windows IOLoop implementation or
 leverage Windows' IOCP support from frameworks like AsyncIO or Twisted.
 
+On Windows, Tornado requires the ``WindowsSelectorEventLoop``. This is
+the default in Python 3.7 and older, but Python 3.8 defaults to an
+event loop that is not compatible with Tornado. Applications that use
+Tornado on Windows with Python 3.8 must call
+``asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())``
+at the beginning of their ``main`` file/function.
+
 Documentation
 -------------