t.p.threadpool leaks threads

New submission from dustin <None>:

My project is using sqlite3 via adbapi, so the unit tests create a lot of "test" databases that are immediately destroyed.

These tests started failing with resource-exhaustion errors -- !MemoryError or "can't start new thread" -- so I went hunting for a thread leak.  I narrowed the search down to adbapi, and then to the threadpool.  This is in Twisted-9.0.0.  The failing test is

 http://gist.github.com/310039
{{{
import threading
 
from twisted.python import threadpool
from twisted.internet import threads
from twisted.trial import unittest
 
class t(unittest.TestCase):
    def setUp(self):
        self.pool = threadpool.ThreadPool(3, 5)
        self.pool.start()
 
    def tearDown(self):
        print list(threading.enumerate())
 
    def testit(self):
        from twisted.internet import reactor
        return threads.deferToThreadPool(reactor, self.pool, lambda : True)
 
    def testit2(self):
        return self.testit()
 
    def testit3(self):
        return self.testit()
    def testit4(self):
        return self.testit()
    def testit5(self):
        return self.testit()
    def testit6(self):
        return self.testit()
    def testit7(self):
        return self.testit()
    def testit8(self):
        return self.testit()
}}}

This shows that the number of extant threads is increasing by 3 for each test.

I've seen this under Python 2.4-2.6.  I've only tried Twisted-9.0.0, so I don't know if this is a regression or not.

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Type     : defect
Component: core
Keywords : 
Priority : normal
Nosy     : 
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http://twistedmatrix.com/trac/ticket/4284