Added multiple-reader prevention code because it seems to be a fairly common pitfall. Also added defaults to the debug methods so you can call them no-args to reset them.

eventlet/debug.py

@@ -93,7 +93,7 @@ def format_hub_timers():
         result.append(repr(l))
     return os.linesep.join(result)
     
-def hub_listener_stacks(state):
+def hub_listener_stacks(state = False):
     """Toggles whether or not the hub records the stack when clients register 
     listeners on file descriptors.  This can be useful when trying to figure 
     out what the hub is up to at any given moment.  To inspect the stacks
@@ -103,7 +103,7 @@ def hub_listener_stacks(state):
     from eventlet import hubs
     hubs.get_hub().set_debug_listeners(state)
     
-def hub_timer_stacks(state):
+def hub_timer_stacks(state = False):
     """Toggles whether or not the hub records the stack when timers are set.  
     To inspect the stacks of the current timers, call :func:`format_hub_timers` 
     at critical junctures in the application logic.
@@ -111,7 +111,11 @@ def hub_timer_stacks(state):
     from eventlet.hubs import timer
     timer._g_debug = state
 
-def hub_exceptions(state):
+def hub_prevent_multiple_readers(state = True):
+    from eventlet.hubs import hub
+    hub.g_prevent_multiple_readers = state
+    
+def hub_exceptions(state = True):
     """Toggles whether the hub prints exceptions that are raised from its
     timers.  This can be useful to see how greenthreads are terminating.
     """
@@ -120,7 +124,7 @@ def hub_exceptions(state):
     from eventlet import greenpool
     greenpool.DEBUG = state
     
-def tpool_exceptions(state):
+def tpool_exceptions(state = False):
     """Toggles whether tpool itself prints exceptions that are raised from 
     functions that are executed in it, in addition to raising them like
     it normally does."""

eventlet/hubs/hub.py

@@ -1,12 +1,15 @@
 import heapq
 import sys
 import traceback
+import warnings
 
 from eventlet.support import greenlets as greenlet, clear_sys_exc_info
 from eventlet.hubs import timer
 from eventlet import patcher
 time = patcher.original('time')
 
+g_prevent_multiple_readers = True
+
 READ="read"
 WRITE="write"
 
@@ -74,6 +77,15 @@ class BaseHub(object):
         listener = self.lclass(evtype, fileno, cb)
         bucket = self.listeners[evtype]
         if fileno in bucket:
+            if g_prevent_multiple_readers:
+               raise RuntimeError("Second simultaneous %s on fileno %s "\
+                     "detected.  Unless you really know what you're doing, "\
+                     "make sure that only one greenthread can %s any "\
+                     "particular socket.  Consider using a pools.Pool. "\
+                     "If you do know what you're doing and want to disable "\
+                     "this error, call "\
+                     "eventlet.debug.hub_multiple_reader_prevention(False)" % (
+                     evtype, fileno, evtype))
             # store off the second listener in another structure
             self.secondaries[evtype].setdefault(fileno, []).append(listener)
         else:

tests/greenio_test.py

@@ -483,11 +483,34 @@ class TestGreenIo(LimitedTestCase):
 
         gt.wait()
 
+    @skip_with_pyevent
+    def test_raised_multiple_readers(self):
+        debug.hub_prevent_multiple_readers(True)
+
+        def handle(sock, addr):
+            sock.recv(1)
+            sock.sendall("a")
+            raise eventlet.StopServe()
+        listener = eventlet.listen(('127.0.0.1', 0))
+        server = eventlet.spawn(eventlet.serve, 
+                                listener,
+                                handle)
+        def reader(s):
+            s.recv(1)
+
+        s = eventlet.connect(('127.0.0.1', listener.getsockname()[1]))
+        a = eventlet.spawn(reader, s)
+        eventlet.sleep(0)
+        self.assertRaises(RuntimeError, s.recv, 1)
+        s.sendall('b')
+        a.wait()
+        
 
 class TestGreenIoLong(LimitedTestCase):
     TEST_TIMEOUT=10  # the test here might take a while depending on the OS
     @skip_with_pyevent
     def test_multiple_readers(self, clibufsize=False):
+        debug.hub_prevent_multiple_readers(False)
         recvsize = 2 * min_buf_size()
         sendsize = 10 * recvsize
         # test that we can have multiple coroutines reading
@@ -534,6 +557,7 @@ class TestGreenIoLong(LimitedTestCase):
         listener.close()
         self.assert_(len(results1) > 0)
         self.assert_(len(results2) > 0)
+        debug.hub_prevent_multiple_readers()
 
     @skipped  # by rdw because it fails but it's not clear how to make it pass
     @skip_with_pyevent

tests/stdlib/all.py

@@ -3,6 +3,8 @@
 Many of these tests make connections to external servers, and all.py tries to skip these tests rather than failing them, so you can get some work done on a plane.
 """
 
+from eventlet import debug
+debug.hub_prevent_multiple_readers(False)
 
 def restart_hub():
     from eventlet import hubs