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$ ./python -m test --match=test.test_multiprocessing_forkserver.WithProcessesTestBarrier.test_barrier --match=test.test_multiprocessing_forkserver.WithProcessesTestFinalize.test_finalize -R 3:3 test_multiprocessing_forkserver 
test_multiprocessing_forkserver leaked [222, 222, 225] memory blocks, sum=669
test_multiprocessing_forkserver failed
Total duration: 5 sec
Tests result: FAILURE

test_multiprocessing_spawn doesn't leak:
haypo@selma$ ./python -m test --match=test.test_multiprocessing_spawn.WithProcessesTestBarrier.test_barrier --match=test.test_multiprocessing_spawn.WithProcessesTestFinalize.test_finalize -R 3:3 test_multiprocessing_spawn
Run tests sequentially
0:00:00 load avg: 0.11 [1/1] test_multiprocessing_spawn
beginning 6 repetitions
123456
......
1 test OK.
Total duration: 10 sec
Tests result: SUCCESS

The first message was on Python 3.6. On master, regrtest sees even more kinds of leaks: references, memory blocks and file descriptors.
haypo@selma$ ./python -m test --match=test.test_multiprocessing_forkserver.WithProcessesTestBarrier.test_barrier --match=test.test_multiprocessing_forkserver.WithProcessesTestFinalize.test_finalize -R 3:3 test_multiprocessing_forkserver 
Run tests sequentially
0:00:00 load avg: 0.25 [1/1] test_multiprocessing_forkserver
beginning 6 repetitions
123456
......
test_multiprocessing_forkserver leaked [492, 492, 492] references, sum=1476
test_multiprocessing_forkserver leaked [221, 222, 225] memory blocks, sum=668
test_multiprocessing_forkserver leaked [4, 4, 4] file descriptors, sum=12
test_multiprocessing_forkserver failed
1 test failed:
 test_multiprocessing_forkserver
Total duration: 5 sec
Tests result: FAILURE

Is it test_finalize or test_barrier?

> Is it test_finalize or test_barrier?
Try yourself: it only leaks when you run both :-)

New changeset a79f8faccf5e26f55e8b9496ad49d2071b5e299c by Victor Stinner (Antoine Pitrou) in branch 'master':
bpo-30775: Fix refleaks in test_multiprocessing (#2467)
https://github.com/python/cpython/commit/a79f8faccf5e26f55e8b9496ad49d2071b5e299c

Hey, thank you Antoine for fixing the last known reference leak!

New changeset e022aad73a4151b5628e2476a8465ce6c0d18b8c by Antoine Pitrou in branch '3.6':
[3.6] bpo-30775: Fix refleaks in test_multiprocessing (GH-2467) (#2468)
https://github.com/python/cpython/commit/e022aad73a4151b5628e2476a8465ce6c0d18b8c

New changeset f15bf1f3f3104f6ab2229e4b359984489a74685b by Victor Stinner (Antoine Pitrou) in branch '3.5':
[3.5] bpo-30775: Fix refleaks in test_multiprocessing (GH-2467) (#2469)
https://github.com/python/cpython/commit/f15bf1f3f3104f6ab2229e4b359984489a74685b

New changeset 79d37ae979a65ada0b2ac820279ccc3b1cd41ba6 by Victor Stinner (Antoine Pitrou) in branch 'master':
Clear potential ref cycle between Process and Process target (#2470)
https://github.com/python/cpython/commit/79d37ae979a65ada0b2ac820279ccc3b1cd41ba6

New changeset 38d6a40898940d2c62c637fcc8acce05e75de5c2 by Antoine Pitrou in branch '3.6':
[3.6] Clear potential ref cycle between Process and Process target (GH-2470) (#2471)
https://github.com/python/cpython/commit/38d6a40898940d2c62c637fcc8acce05e75de5c2

New changeset a9705b777859f555d50eb5dcd5fc4260c42a0188 by Antoine Pitrou in branch '3.5':
[3.5] Clear potential ref cycle between Process and Process target (GH-2470) (#2472)
https://github.com/python/cpython/commit/a9705b777859f555d50eb5dcd5fc4260c42a0188

New changeset 12536bd261ba95cd2748f3d7d47768742a6ffa7a by Victor Stinner (Antoine Pitrou) in branch '2.7':
[2.7] Clear potential ref cycle between Process and Process target (GH-2470) (#2473)
https://github.com/python/cpython/commit/12536bd261ba95cd2748f3d7d47768742a6ffa7a

The Python 2.7 change caused a regression in Sphinx: bpo-31016.