test_close_backend_fd not relevant for libuv.

src/gevent/libuv/loop.py

@@ -235,7 +235,9 @@ class loop(AbstractLoop):
 
     def fileno(self):
         if self._ptr:
-            return libuv.uv_backend_fd(self._ptr)
+            fd = libuv.uv_backend_fd(self._ptr)
+            if fd >= 0:
+                return fd
 
     _sigchld_watcher = None
     _sigchld_callback_ffi = None

src/greentest/test_close_backend_fd.py

@@ -4,9 +4,12 @@ import gevent
 from gevent import core
 
 
-if core.LIBEV_EMBED:
+if getattr(core, 'LIBEV_EMBED', False):
     # hub.loop.fileno is only defined when
-    # we embed libev for some reason
+    # we embed libev for some reason.
+    # Choosing specific backends is also only supported by libev
+    # (not libuv), and besides, libuv has a nasty tendency to
+    # abort() the process if its FD gets closed.
 
     for count in range(2):
         for backend in core.supported_backends():