I'm using Medusa by way of Zope's ZServer. I have ZServer sitting behind an Apache proxy and am bombing it from a threaded client script that reads a web server log file as input and throws requests at the Apache server. I can vary the number of threads to adjust the load. I normally run it with five simultaneous threads. By default, the system runs briefly then (as they say), "Whammo! Blammo!", I get the following error: Unhandled exception in thread: Traceback (innermost last): File "/home/dolphin/skip/src/Zope/ZServer/PubCore/ZServerPublisher.py", line 97, in __init__ response._finish() File "/home/dolphin/skip/src/Zope/ZServer/HTTPResponse.py", line 209, in _finish self.stdout.close() File "/home/dolphin/skip/src/Zope/ZServer/HTTPResponse.py", line 235, in close self._channel.push(CallbackProducer(self._channel.done)) File "/home/dolphin/skip/src/Zope/ZServer/HTTPServer.py", line 307, in push if send: self.initiate_send() File "/home/dolphin/skip/src/Zope/ZServer/medusa/asynchat.py", line 200, in initiate_send self.refill_buffer() File "/home/dolphin/skip/src/Zope/ZServer/medusa/asynchat.py", line 184, in refill_buffer self.producer_fifo.pop() File "/home/dolphin/skip/src/Zope/ZServer/medusa/asynchat.py", line 255, in pop del self.list[0] IndexError: list assignment index out of range If you look at the code in asynchat.fifo.pop, the error can only be explained by the fact that two threads are modifying the fifo simultaneously, since access to self.list[0] succeeded in the statement before the statement that throws the IndexError: def pop (self): if self.list: result = (1, self.list[0]) # non-null list here del self.list[0] # empty list here else: result = (0, None) return result I modified the fifo class to lock access to its list: class fifo: def __init__ (self, list=None): self.lock = thread.allocate_lock() self.lock.acquire() if not list: self.list = [] else: self.list = list self.lock.release() def __len__ (self): self.lock.acquire() l = len(self.list) self.lock.release() return l def first (self): self.lock.acquire() item = self.list[0] self.lock.release() return item def push (self, data): self.lock.acquire() self.list.append (data) self.lock.release() def pop (self): self.lock.acquire() if self.list: result = (1, self.list[0]) del self.list[0] else: result = (0, None) self.lock.release() return result but this didn't help: Unhandled exception in thread: Traceback (innermost last): File "/home/dolphin/skip/src/Zope/ZServer/PubCore/ZServerPublisher.py", line 97, in __init__ response._finish() File "/home/dolphin/skip/src/Zope/ZServer/HTTPResponse.py", line 209, in _finish self.stdout.close() File "/home/dolphin/skip/src/Zope/ZServer/HTTPResponse.py", line 235, in close self._channel.push(CallbackProducer(self._channel.done)) File "/home/dolphin/skip/src/Zope/ZServer/HTTPServer.py", line 307, in push if send: self.initiate_send() File "/home/dolphin/skip/src/Zope/ZServer/medusa/asynchat.py", line 200, in initiate_send self.refill_buffer() File "/home/dolphin/skip/src/Zope/ZServer/medusa/asynchat.py", line 175, in refill_buffer p = self.producer_fifo.first() File "/home/dolphin/skip/src/Zope/ZServer/medusa/asynchat.py", line 254, in first item = self.list[0] IndexError: list index out of range This error also looks like a threading problem. When refill_buffer tests self.producer.fifo, it sees something, but by the time it calls fifo.first, the list has evaporated: while 1: if len(self.producer_fifo): # non-null list here p = self.producer_fifo.first() # null list here Any ideas? Is there a way to easily turn off threading? It seems that the data structures are not properly protected. Skip Montanaro | Mojam: "Uniting the World of Music" http://www.mojam.com/ skip@mojam.com | Musi-Cal: http://www.musi-cal.com/ 518-372-5583