ioloop,concurrent: Fix reference cycles #3315
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This script was only ever run irregularly on its own; bring it in to the test suite so it can be run automatically.
In a few places we were referring to a future via a closure instead of using the reference passed as an argument to the callback. This sometimes causes a reference cycle that can slow GC. This commit adds a test which covers two of the cases (chain_future and the concurrent.future branch of add_future) while the third was found by inspecting other calls to add_done_callback for obvious instances of this pattern. Fixes tornadoweb#2620
Local/attribute dicts are reported a bit differently here.
Pypy doesn't have the same refcount fast-path as cpython so the gc behavior is different and this test is irrelevant.
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Convert the circlerefs script into a regular unit test and expand it with new cases. Testing
tornado.concurrent.run_on_executoras described in #2620 reveals several reference cycles in Future done callbacks.Fixes #2620