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This bug occurs because, in the function bridgedb.Bridges.getConfigLine(), the following lines determine ― when a request comes in ― the client's position in the hashring:

        if not request: request = 'default'
        digest = get_hmac_fn('Order-Or-Addresses')(request)
        pos = long(digest[:8], 16) # lower 8 bytes -> long

Later on, in the same function, pos modulo the number of addresses this Bridge has, is used to index the collected/filtered addresses. Similarly, pos modulo the number of ports in the portlist is used to index the portlist:

        # default ip, orport should get a chance at being selected
        if isinstance(self.ip, addressClass):
            addresses.insert(0,(self.ip, addr.PortList(self.orport)))

        if addresses:
            address,portlist = addresses[pos % len(addresses)]
            if isinstance(address, ipaddr.IPv6Address): ip = "[%s]"%address
            else: ip = "%s"%address
            orport = portlist[pos % len(portlist)]

Meaning that the fewer number of addresses/ports there are, the higher the chance that the pos will be modulo some small number which is a factor. For example, if there is only one address, this would be something like:

>>>from bridgedb import Bridges
>>> digest = Bridges.get_hmac_fn('Order-Or-Addresses')('default')
>>> pos = long(digest[:8], 16)
>>> pos
3790090317L
>>> pos % 1
0L

Which causes the following bug. What is extra strange is that the ValueError from trying to index these list-like things with a 0L doesn't seem to occur on the addresses, only on the orport = portlist[… line. Though, it's also worth noting that the 'default' value used in the hmac generation is usually a client's IP address, truncated to the /24. This means that this indexing-by-0L bug will occur rather indeterminately.

(bridgedb)∃!isisⒶwintermute:(fix/9462-refactor-netstatus-parsers_r9462C *+$=)~/code/torproject/bridgedb ∴ bridgedb
Unhandled Error
Traceback (most recent call last):
  File "/home/isis/.virtualenvs/bridgedb/local/lib/python2.7/site-packages/twisted/protocols/basic.py", line 571, in dataReceived
    why = self.lineReceived(line)
  File "/home/isis/.virtualenvs/bridgedb/local/lib/python2.7/site-packages/twisted/web/http.py", line 1619, in lineReceived
    self.allContentReceived()
  File "/home/isis/.virtualenvs/bridgedb/local/lib/python2.7/site-packages/twisted/web/http.py", line 1694, in allContentReceived
    req.requestReceived(command, path, version)
  File "/home/isis/.virtualenvs/bridgedb/local/lib/python2.7/site-packages/twisted/web/http.py", line 790, in requestReceived
    self.process()
--- <exception caught here> ---
  File "/home/isis/.virtualenvs/bridgedb/local/lib/python2.7/site-packages/twisted/web/server.py", line 189, in process
    self.render(resrc)
  File "/home/isis/.virtualenvs/bridgedb/local/lib/python2.7/site-packages/twisted/web/server.py", line 238, in render
    body = resrc.render(self)
  File "/home/isis/.virtualenvs/bridgedb/local/lib/python2.7/site-packages/bridgedb-0.0.1_274_g594a221_dirty-py2.7.egg/bridgedb/HTTPServer.py", line 140, in render
    return self.getBridgeRequestAnswer(request)
  File "/home/isis/.virtualenvs/bridgedb/local/lib/python2.7/site-packages/bridgedb-0.0.1_274_g594a221_dirty-py2.7.egg/bridgedb/HTTPServer.py", line 218, in getBridgeRequestAnswer
    ) for b in bridges)
  File "/home/isis/.virtualenvs/bridgedb/local/lib/python2.7/site-packages/bridgedb-0.0.1_274_g594a221_dirty-py2.7.egg/bridgedb/HTTPServer.py", line 218, in <genexpr>
    ) for b in bridges)
  File "/home/isis/.virtualenvs/bridgedb/local/lib/python2.7/site-packages/bridgedb-0.0.1_274_g594a221_dirty-py2.7.egg/bridgedb/Bridges.py", line 192, in getConfigLine
    orport = portlist[pos % len(portlist)]
  File "/home/isis/.virtualenvs/bridgedb/local/lib/python2.7/site-packages/bridgedb-0.0.1_274_g594a221_dirty-py2.7.egg/bridgedb/parse/addr.py", line 372, in __getitem__
    return portlist[portlist.index(port)]
exceptions.ValueError: 0L is not in list
^C