Make monkey patch work in Python 3.6 (#7848)

* Make monkey patch work in Python 3.6

* Update dockerfiles back to 3.6

* Lint

* Do not set env variable for dockerfile

* Lint

homeassistant/monkey_patch.py

@@ -0,0 +1,75 @@
+"""Monkey patch Python to work around issues causing segfaults.
+
+Under heavy threading operations that schedule calls into
+the asyncio event loop, Task objects are created. Due to
+a bug in Python, GC may have an issue when switching between
+the threads and objects with __del__ (which various components
+in HASS have).
+
+This monkey-patch removes the weakref.Weakset, and replaces it
+with an object that ignores the only call utilizing it (the
+Task.__init__ which calls _all_tasks.add(self)). It also removes
+the __del__ which could trigger the future objects __del__ at
+unpredictable times.
+
+The side-effect of this manipulation of the Task is that
+Task.all_tasks() is no longer accurate, and there will be no
+warning emitted if a Task is GC'd while in use.
+
+Related Python bugs:
+ - https://bugs.python.org/issue26617
+"""
+import sys
+
+
+def patch_weakref_tasks():
+    """Replace weakref.WeakSet to address Python 3 bug."""
+    # pylint: disable=no-self-use, protected-access, bare-except
+    import asyncio.tasks
+
+    class IgnoreCalls:
+        """Ignore add calls."""
+
+        def add(self, other):
+            """No-op add."""
+            return
+
+    asyncio.tasks.Task._all_tasks = IgnoreCalls()
+    try:
+        del asyncio.tasks.Task.__del__
+    except:
+        pass
+
+
+def disable_c_asyncio():
+    """Disable using C implementation of asyncio.
+
+    Required to be able to apply the weakref monkey patch.
+
+    Requires Python 3.6+.
+    """
+    class AsyncioImportFinder:
+        """Finder that blocks C version of asyncio being loaded."""
+
+        PATH_TRIGGER = '_asyncio'
+
+        def __init__(self, path_entry):
+            if path_entry != self.PATH_TRIGGER:
+                raise ImportError()
+            return
+
+        def find_module(self, fullname, path=None):
+            """Find a module."""
+            if fullname == self.PATH_TRIGGER:
+                # We lint in Py34, exception is introduced in Py36
+                # pylint: disable=undefined-variable
+                raise ModuleNotFoundError()  # noqa
+            return None
+
+    sys.path_hooks.append(AsyncioImportFinder)
+    sys.path.insert(0, AsyncioImportFinder.PATH_TRIGGER)
+
+    try:
+        import _asyncio  # noqa
+    except ImportError:
+        pass