Reference leak if function called by deferToThread() raises an error

New submission from haypo <None>:

In the following example, the DestroyMe object is not destroyed at inthread() exit.

It looks like a Failure() stores the Python frame and so stores the 'context' argument of inthread(). But the context object is not destroyed after displayError().

I think that the context object is still stored (reference leak)... somewhere. It's maybe referenced by the failure object, which is not destroyed directly? Another idea: the reference is maybe stored in the thread pool?

{{{
#!python
from twisted.internet.threads import deferToThread
from twisted.internet import reactor
from twisted.internet.defer import succeed

class DestroyMe:
    def __init__(self):
        print "CREATE DESTROY ME"

    def __del__(self):
        print "DELETE DESTROY ME"

def inthread(x, context):
    raise ValueError("error!")
    return x * 10

def func1(x, context):
    x = x + 1
    d = deferToThread(inthread, x, context)
    d.addErrback(displayError, "thread")
    return d

def printResult(x):
    print "Result = %s" % x
    print "test done"

def displayError(failure, where):
    print "FAILURE in %s: %s" % (where, failure)

def test1():
    print "test1"
    d = succeed(10)
    context = DestroyMe()
    d.addErrback(displayError, "test1")
    d.addCallback(func1, context)
    d.addCallback(printResult)
    return d

def stop():
    print "Stop!"
    reactor.stop()

reactor.callLater(0, test1)
reactor.callLater(4, stop)

reactor.run()
}}}

I tested Twisted core 8.1.0 (Ubuntu Ibex).

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Type     : defect
Component: core
Keywords : 
Priority : normal
Nosy     : 
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http://twistedmatrix.com/trac/ticket/3853