gh-109015: Add test.support.socket_helper.tcp_blackhole()

[vstinner]

• Issue: test_asyncio, test_imaplib and test_socket fail with FreeBSD TCP blackhole #109015

[vstinner]

My previous DEBUG PR: PR #108972.

See also issue #91960 "Add FreeBSD build and test using Cirrus-CI" and PR #91961 which the TCP blackhole was discussed.

[vstinner]

I validated my fix on Cirrus-CI with this patch:

diff --git a/.cirrus.yml b/.cirrus.yml
index 823b1f921d66d..2e087f1f57706 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -7,9 +7,9 @@ freebsd_task:
   # local local connections to fail with ETIMEDOUT instead of ECONNREFUSED.
   # For more information see https://reviews.freebsd.org/D41751 and
   # https://github.com/cirruslabs/cirrus-ci-docs/issues/483.
-  sysctl_script:
-    - sysctl net.inet.tcp.blackhole=0
-    - sysctl net.inet.udp.blackhole=0
+#  sysctl_script:
+#    - sysctl net.inet.tcp.blackhole=0
+#    - sysctl net.inet.udp.blackhole=0
   build_script:
     - mkdir build
     - cd build

test_asyncio failed at the first version of my PR, which proves that the Cirrus-CI did its job: test my fix :-) I had to skip 2 more tests in test_asyncio.

At the second version, it passed: Total test files: run=461/461 skipped=25 resource_denied=1 rerun=1: https://cirrus-ci.com/task/4512145228955648 (test.test_asyncio.test_subprocess had to be re-run, it's unrelated to this fix.)

cc @emaste

[vstinner]

My second patch adds skips to Lib/test/test_asyncio/test_events.py:

• test_create_connection_local_addr_skip_different_family()
• test_create_connection_local_addr_nomatch_family()

[vstinner]

I reverted the Cirrus CI change.

[emaste]

sysctl provides a -n arg to suppress the name:

$ sysctl -n net.inet.tcp.blackhole
0

[vstinner]

Interesting, I didn't know it. It's also supported by Linux (even if right now, this function is only called on FreeBSD ;-)). I updated my PR.

[vstinner]

I ran a quick test on my latest PR:

vstinner@freebsd$ sudo sysctl net.inet.tcp.blackhole=0
net.inet.tcp.blackhole: 0 -> 0
vstinner@freebsd$ ./python -m test -v test_socket -m test_connect -v
(...)
test_connect (test.test_socket.NetworkConnectionNoServer.test_connect) ... ok
(...)
Result: SUCCESS

and:

vstinner@freebsd$ sudo sysctl net.inet.tcp.blackhole=2
net.inet.tcp.blackhole: 0 -> 2

vstinner@freebsd$ ./python -m test -v test_socket -m test_connect -v
(...)
test_connect (test.test_socket.NetworkConnectionNoServer.test_connect) ... skipped 'TCP blackhole is enabled (sysctl 
(...)
Result: SUCCESS

[vstinner]

@emaste: Would you mind to review the updated PR?

[emaste]

I think this comment is leftover

[miss-islington]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[miss-islington]

Sorry, @vstinner, I could not cleanly backport this to 3.11 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker a52a3509770f29f940cda9307704908949912276 3.11

[bedevere-bot]

GH-109041 is a backport of this pull request to the 3.12 branch.

[bedevere-bot]

GH-109042 is a backport of this pull request to the 3.11 branch.