def install_default_pool(maxthreads=max_threads_for_default_pool):
    """Install a custom pool as Twisted's global/reactor thread-pool.

    Disallow all database activity in the reactor thread-pool. Why such a
    strict policy? We've been following Django's model, where threads and
    database connections are wedded together. In MAAS this limits concurrency,
    contributes to crashes and deadlocks, and has spawned workarounds like
    post-commit hooks. From here on, using a database connection requires the
    use of a specific, separate, carefully-sized, thread-pool.
    """
    if reactor.threadpool is None:
        # Start with ZERO threads to avoid pulling in all of Django's
        # configuration straight away; it may not be ready yet.
        reactor.threadpool = make_default_pool(maxthreads)
        reactor.callWhenRunning(reactor.threadpool.start)
        reactor.addSystemEventTrigger(
            "during", "shutdown", reactor.threadpool.stop
        )
    else:
        raise AssertionError(
            "Too late; global/reactor thread-pool has "
            "already been configured and installed."
        ) 

def postClassCleanup(self):
        """
        Called by L{unittest.TestCase} after the last test in a C{TestCase}
        subclass. Ensures the reactor is clean by murdering the threadpool,
        catching any pending
        L{DelayedCall<twisted.internet.base.DelayedCall>}s, open sockets etc.
        """
        selectables = self._cleanReactor()
        calls = self._cleanPending()
        if selectables or calls:
            aggregate = DirtyReactorAggregateError(calls, selectables)
            self.result.addError(self.test, Failure(aggregate))
        self._cleanThreads() 

def _cleanThreads(self):
        reactor = self._getReactor()
        if interfaces.IReactorThreads.providedBy(reactor):
            if reactor.threadpool is not None:
                # Stop the threadpool now so that a new one is created.
                # This improves test isolation somewhat (although this is a
                # post class cleanup hook, so it's only isolating classes
                # from each other, not methods from each other).
                reactor._stopThreadPool() 

def postClassCleanup(self):
        """
        Called by L{unittest.TestCase} after the last test in a C{TestCase}
        subclass. Ensures the reactor is clean by murdering the threadpool,
        catching any pending
        L{DelayedCall<twisted.internet.base.DelayedCall>}s, open sockets etc.
        """
        selectables = self._cleanReactor()
        calls = self._cleanPending()
        if selectables or calls:
            aggregate = DirtyReactorAggregateError(calls, selectables)
            self.result.addError(self.test, Failure(aggregate))
        self._cleanThreads() 

def _cleanThreads(self):
        reactor = self._getReactor()
        if interfaces.IReactorThreads.providedBy(reactor):
            if reactor.threadpool is not None:
                # Stop the threadpool now so that a new one is created.
                # This improves test isolation somewhat (although this is a
                # post class cleanup hook, so it's only isolating classes
                # from each other, not methods from each other).
                reactor._stopThreadPool() 

def postClassCleanup(self):
        """
        Called by L{unittest.TestCase} after the last test in a C{TestCase}
        subclass. Ensures the reactor is clean by murdering the threadpool,
        catching any pending L{DelayedCall}s, open sockets etc.
        """
        selectables = self._cleanReactor()
        calls = self._cleanPending()
        if selectables or calls:
            aggregate = DirtyReactorAggregateError(calls, selectables)
            self.result.addError(self.test, Failure(aggregate))
        self._cleanThreads() 

def _cleanThreads(self):
        reactor = self._getReactor()
        if interfaces.IReactorThreads.providedBy(reactor):
            if reactor.threadpool is not None:
                # Stop the threadpool now so that a new one is created. 
                # This improves test isolation somewhat (although this is a
                # post class cleanup hook, so it's only isolating classes
                # from each other, not methods from each other).
                reactor._stopThreadPool() 

def do_cleanThreads(cls):
        from twisted.internet import reactor
        if interfaces.IReactorThreads.providedBy(reactor):
            reactor.suggestThreadPoolSize(0)
            if hasattr(reactor, 'threadpool') and reactor.threadpool:
                reactor.threadpool.stop()
                reactor.threadpool = None
                # *Put it back* and *start it up again*.  The
                # reactor's threadpool is *private*: we cannot just
                # rape it and walk away.
                reactor.threadpool = threadpool.ThreadPool(0, 10)
                reactor.threadpool.start() 

def test_default_pool_is_disconnected_pool(self):
        pool = reactor.threadpool
        self.assertThat(pool, IsInstance(ThreadPool))
        self.assertThat(
            pool.context.contextFactory, Is(orm.TotallyDisconnected)
        )
        self.assertThat(pool.min, Equals(0))