select: Need to keep parallel mapping of fd -> callback obj on uPy side.

Even despite epoll allows us to map fd onto any object, we need to keep
parallel bookkeeping on Python side, to: 1) keep associated object from
being garbage collection; 2) to be able to release associated object back
to GC when we finished watching fd.

select/select.py

@@ -31,6 +31,7 @@ class Epoll:
     def __init__(self, epfd):
         self.epfd = epfd
         self.evbuf = struct.pack(self.epoll_event, 0, 0)
+        self.registry = {}
 
     def register(self, fd, eventmask=EPOLLIN|EPOLLPRI|EPOLLOUT, retval=None):
         "retval is extension to stdlib, value to use in results from .poll()."
@@ -41,11 +42,15 @@ class Epoll:
         if r == -1 and os.errno.get() == errno.EEXIST:
             r = epoll_ctl(self.epfd, EPOLL_CTL_MOD, fd, s)
         os.check_error(r)
+        # We must keep reference to retval, or it may be GCed. And we must
+        # keep mapping from fd to retval to be able to get rid of retval reference.
+        self.registry[fd] = retval
 
     def unregister(self, fd):
         # Pass dummy event structure, to workaround kernel bug
         r = epoll_ctl(self.epfd, EPOLL_CTL_DEL, fd, self.evbuf)
         os.check_error(r)
+        del self.registry[fd]
 
     def poll(self, timeout=-1):
         s = bytearray(self.evbuf)

select/setup.py

@@ -5,7 +5,7 @@ sys.path.pop(0)
 from setuptools import setup
 
 setup(name='micropython-select',
-      version='0.0.4',
+      version='0.0.5',
       description='select module to MicroPython',
       url='https://github.com/micropython/micropython/issues/405',
       author='Paul Sokolovsky',