Update thread_util.py

[seewindcn]

Pool object call GreenletIdent.watch(on_thread_died) and not unlink. if the greenlet is resident, greenlet._links is growth

[ajdavis]

Merged, thanks!

[ajdavis]

Summary: When a Gevent Greenlet calls MongoClient.start_request(), it reserves for itself a socket to use until it calls end_request() or dies. We use Greenlet.link(callback) to know if a Greenlet dies without calling end_request(). We had a bug that failed to clear the callback if the Greenlet calls end_request() before it dies, so the callback would fire even though there was no work for it to do.

This wasn't a big problem in PyMongo since the callback sees that it has no work to do, and exits normally. But if a Greenlet calls start_request and end_request repeatedly, we attached a growing list of useless callbacks to it. Additionally, this unexpected behavior was a likely source of future bugs if we ever changed the callback code.