Comment 1 for bug 1584744

buildd-manager is getting EMFILE, caused by it just firing off all the downloads at once. The exact behaviour will depend on what else is going on in the build farm at the time.

We could just bump the resource limit, but I think the proper fix is probably to migrate to twisted.web.client.Agent, which uses a connection pool. We would still have a problem if the number of builders multiplied by HTTPConnectionPool.maxPersistentPerHost (2 by default) approached or exceeded the resource limit, but we're quite some way off that being a problem.

Compare https://stackoverflow.com/questions/28162301/twisted-getpage-exceptions-oserror-errno-24-too-many-open-files, where Glyph says we shouldn't be using the interface we're currently using.