CONTRIBUTING

thirdparty: vendor futures 3.2.0

Python 3 has a concurrent.futures package in the standard library
for representing futures. The "futures" package on PyPI is a backport
of this package to work with Python 2.

The wire protocol code today has its own future concept for handling
of "batch" requests. The frame-based protocol will also want to
use futures.

I've heavily used the "futures" package on Python 2 in other projects
and it is pretty nice. It even has a built-in thread and process pool
for running functions in parallel. I've used this heavily for concurrent
I/O and other GIL-less activities.

The existing futures API in the wire protocol code is not as nice as
concurrent.futures. Since concurrent.futures is in the Python standard
library and will presumably be the long-term future for futures in our
code base, let's vendor the backport so we can use proper futures today.

# no-check-commit because of style violations

Differential Revision: https://phab.mercurial-scm.org/D3261

Our full contribution guidelines are in our wiki, please see:

https://www.mercurial-scm.org/wiki/ContributingChanges

If you just want a checklist to follow, you can go straight to

https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).