regrtest: run tests in subprocesses with -j1 on buildbots

[vstinner]

BPO	25285
Nosy	@vstinner
Files	regrtest_run_in_subprocess.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2016-05-20.11:42:12.402>
created_at = <Date 2015-09-30.20:36:02.789>
labels = ['tests']
title = 'regrtest: run tests in subprocesses with -j1 on buildbots'
updated_at = <Date 2016-05-20.11:42:12.401>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2016-05-20.11:42:12.401>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2016-05-20.11:42:12.402>
closer = 'vstinner'
components = ['Tests']
creation = <Date 2015-09-30.20:36:02.789>
creator = 'vstinner'
dependencies = []
files = ['40632']
hgrepos = []
issue_num = 25285
keywords = ['patch']
message_count = 2.0
messages = ['251962', '265936']
nosy_count = 2.0
nosy_names = ['vstinner', 'Arfrever']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue25285'
versions = ['Python 3.6']

[vstinner]

Currently, regrtest ignores the -j1 parameter. For -jN, the multiprocess mode is only enabled for N != 1. I propose to modify regrtest to run tests in subprocesses when -j1 is used. It would solve the issue bpo-18906.

I also want to run tests in subprocesses in buildbot to better isolate tests. Running all tests in the same process, sequentially, cause various issues:

• various kinds of side effects
• memory leaks
• random failures (unreliable tests)

"make buildbot" already runs regrtest with -j1. I propose to also modify Tools/buildbot/test.bat to use run tests with -j1 by default on Windows.

I fixed regrtest to setup tests using the same code for classic mode and multiprocess mode: slaves (child processes running tests for regrtest -jN) now inherit
--memlimit/-M, --threshold/-t and --nowindows/-n options.

Only 3 remaining functions are incompatible with -jN: --coverage/-T and --findleaks/-l.

Finally, the multiprocess mode of regrtest now displays the duration of tests which took longer than 30 seconds, and every minute, it displays the tests running since longer than 30 seconds. Example with multiple workers (-j4):
---
...
[395/399/1] test_mimetypes -- running: test_lzma (111 sec), test_multiprocessing_forkserver (224 sec)
[396/399/1] test_poplib -- running: test_lzma (111 sec), test_multiprocessing_forkserver (225 sec)
[397/399/1] test_lzma (111 sec) -- running: test_multiprocessing_forkserver (225 sec)
[398/399/1] test_range -- running: test_multiprocessing_forkserver (227 sec)
running: test_multiprocessing_forkserver (287 sec)
[399/399/1] test_multiprocessing_forkserver (340 sec)
378 tests OK.
---

Note: "python -m test -j1 -j4" uses 4 workers, only the last -jN parameter is used. Some buildbots add -jN to use more workers.

[vstinner]

changeset: 101449:6f7e5b51c8fb
user: Victor Stinner <victor.stinner@gmail.com>
date: Fri May 20 13:15:55 2016 +0200
files: Lib/test/libregrtest/cmdline.py Misc/NEWS Tools/buildbot/test.bat
description:
regrtest doesn't ignore -j1 anymore

• regrtest now uses subprocesses when the -j1 command line option
  is used: each test file runs in a fresh child process. Before, the -j1 option
  was ignored.
• Tools/buildbot/test.bat script now uses -j1 by default to run
  each test file in fresh child process.