Buildbot master's http.log error - twisted

Question

Recently when I tried to restart my buildmaster it failed because of some issues with http.log. I get the following error:
2014-07-17 11:21:55+0200 [-] RotateLogSite starting on 8000
2014-07-17 11:21:55+0200 [-] Starting factory <buildbot.status.web.baseweb.RotateLogSite instance at 0x0295F300>
2014-07-17 11:21:55+0200 [-] Setting up http.log rotating 10 files of 10000000 bytes each
2014-07-17 11:21:55+0200 [-] while starting BuildMaster
Traceback (most recent call last):
File "C:\Python27\lib\site-packages\twisted\application\service.py", line 308, in addService
service.privilegedStartService()
File "C:\Python27\lib\site-packages\twisted\application\service.py", line 277, in privilegedStartService
service.privilegedStartService()
File "C:\Python27\lib\site-packages\twisted\application\internet.py", line 358, in privilegedStartService
self._waitingForPort = self.endpoint.listen(self.factory)
File "C:\Python27\lib\site-packages\twisted\internet\endpoints.py", line 461, in listen
interface=self._interface)
--- <exception caught here> ---
File "C:\Python27\lib\site-packages\twisted\internet\defer.py", line 108, in execute
result = callable(*args, **kw)
File "C:\Python27\lib\site-packages\twisted\internet\posixbase.py", line 482, in listenTCP
p.startListening()
File "C:\Python27\lib\site-packages\twisted\internet\tcp.py", line 998, in startListening
self.factory.doStart()
File "C:\Python27\lib\site-packages\twisted\internet\protocol.py", line 72, in doStart
self.startFactory()
File "C:\Python27\lib\site-packages\twisted\web\http.py", line 1844, in startFactory
self.logFile = self._openLogFile(self.logPath)
File "C:\Python27\lib\site-packages\buildbot-0.8.8-py2.7.egg\buildbot\status\web\baseweb.py", line 477, in _openLogFile
return LogFile.fromFullPath(path, rotateLength=rotateLength, maxRotatedFiles=maxRotatedFiles)
File "C:\Python27\lib\site-packages\twisted\python\logfile.py", line 48, in fromFullPath
os.path.dirname(logPath), *args, **kwargs)
File "C:\Python27\lib\site-packages\twisted\python\logfile.py", line 161, in __init__
BaseLogFile.__init__(self, name, directory, defaultMode)
File "C:\Python27\lib\site-packages\twisted\python\logfile.py", line 40, in __init__
self._openFile()
File "C:\Python27\lib\site-packages\twisted\python\logfile.py", line 166, in _openFile
BaseLogFile._openFile(self)
File "C:\Python27\lib\site-packages\twisted\python\logfile.py", line 64, in _openFile
self._file = file(self.path, "r+", 1)
exceptions.IOError: [Errno 22] invalid mode ('r+') or filename: 'E:\\master_dir\\http.log'
I found that my http.log has reached a size of 4GB although it should rotate after 10MB. Why the buildbot was not able to rotate the http.log after 10MB? Why I am getting the error Invalid mode (r+)? Is there a build config parameter to set the rotate size for http.log?
Deleting the file atleast solved the problem of starting buildmaster but I am still clueless why I got this error at the very first place.
I am running buildbot version 0.8.8
Thanks.
EDIT
As asked by djmitche, I tried with twisted logging module. It was logging fine, irrespective of the order of slashes.
######## TWISTED ##########
from twisted.python.logfile import LogFile
LOG_FILENAME_TWISTED = r'C:\wp\log_rotate_twisted.log'
lf = LogFile.fromFullPath(LOG_FILENAME_TWISTED, rotateLength=20)
# Log some messages
for i in range(20):
lf.write('i = %d' % i)

Answer 1

This may be an issue with Twisted Python (and I've proposed adding the corresponding tag).
Can you verify that the log rotation parameters are set correctly, perhaps by patching C:\Python27\lib\site-packages\buildbot-0.8.8-py2.7.egg\buildbot\status\web\baseweb.py in _openLogFile to print rotateLength? The printed output will appear in twistd.log when you restart the master.