Don't raise ignored exception return nullptr instead

Summary:
Sometimes get_executor gets called from threads where there is no asyncio evenloop.
In those cases return NULL.

The caller is responsible for this situation.

Reviewed By: yfeldblum

Differential Revision: D8492709

fbshipit-source-id: 325689ecd4541b9ba097cfe1cb4d6d61ad81cb43

folly/python/executor.pyx

 import asyncio
+
 from folly.executor cimport cAsyncioExecutor
 from libcpp.memory cimport make_unique, unique_ptr
 from cython.operator cimport dereference as deref
 from weakref import WeakKeyDictionary
 
-#asynico Loops to AsyncioExecutor
+# asyncio Loops to AsyncioExecutor
 loop_to_q = WeakKeyDictionary()
 
 
 cdef class AsyncioExecutor:
-   def __cinit__(self):
-       self.cQ = make_unique[cAsyncioExecutor]();
+    def __cinit__(self):
+        self.cQ = make_unique[cAsyncioExecutor]()
 
-   def fileno(AsyncioExecutor self):
-       return deref(self.cQ).fileno()
+    def fileno(AsyncioExecutor self):
+        return deref(self.cQ).fileno()
 
-   def drive(AsyncioExecutor self):
-       deref(self.cQ).drive()
+    def drive(AsyncioExecutor self):
+        deref(self.cQ).drive()
 
-   def __dealloc__(AsyncioExecutor self):
-       # We drive it one last time
-       deref(self.cQ).drive()
+    def __dealloc__(AsyncioExecutor self):
+        # We drive it one last time
+        deref(self.cQ).drive()
 
 
 cdef cAsyncioExecutor* get_executor():
-   loop = asyncio.get_event_loop()
-   try:
-       Q = <AsyncioExecutor>(loop_to_q[loop])
-   except KeyError:
-       Q = AsyncioExecutor()
-       loop.add_reader(Q.fileno(), Q.drive)
-       loop_to_q[loop] = Q
-   return Q.cQ.get()
+    try:
+        loop = asyncio.get_event_loop()
+    except RuntimeError:
+        return NULL
+    try:
+        Q = <AsyncioExecutor>(loop_to_q[loop])
+    except KeyError:
+        Q = AsyncioExecutor()
+        loop.add_reader(Q.fileno(), Q.drive)
+        loop_to_q[loop] = Q
+    return Q.cQ.get()