bpo-34521: Fix tests in test_socket to use correctly CMSG_LEN

[pablogsal]

After some failures in AMD64 FreeBSD CURRENT Debug 3.x buildbots
regarding tests in test_socket that are using
testFDPassSeparateMinSpace(), FreeBDS revision 337423 was pointed
out to be the reason the test started to fail.

A close examination of the manpage for cmsg_space(3) reveals that
the number of file descriptors needs to be taken into account when
using CMSG_LEN().

https://linux.die.net/man/3/cmsg_space

Example from the manpage. The code below passes an array of file descriptors over a UNIX domain socket using SCM_RIGHTS:

struct msghdr msg = {0};
struct cmsghdr *cmsg;
int myfds[NUM_FD]; /* Contains the file descriptors to pass. */
char buf[CMSG_SPACE(sizeof myfds)];  /* ancillary data buffer */
int *fdptr;

msg.msg_control = buf;
msg.msg_controllen = sizeof buf;
cmsg = CMSG_FIRSTHDR(&msg);
cmsg->cmsg_level = SOL_SOCKET;
cmsg->cmsg_type = SCM_RIGHTS;
cmsg->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);   <----------
/* Initialize the payload: */
fdptr = (int *) CMSG_DATA(cmsg);
memcpy(fdptr, myfds, NUM_FD * sizeof(int));
/* Sum of the length of all control messages in the buffer: */
msg.msg_controllen = cmsg->cmsg_len;

https://bugs.python.org/issue34521

[pablogsal]

Manually checking on the buildbot itself reveals that all test for test_socket now pass:

CURRENT-amd64% uname -a
FreeBSD CURRENT-amd64 12.0-ALPHA6 FreeBSD 12.0-ALPHA6  r338678  amd64

CURRENT-amd64% ./python -m test test_socket
Run tests sequentially
0:00:00 load avg: 1.69 [1/1] test_socket

== Tests result: SUCCESS ==

1 test OK.

Total duration: 28 sec 611 ms
Tests result: SUCCESS

[vstinner]

LGTM.

[pablogsal]

I will wait for the buildbots before evaluating if we should backport this or not.

[miss-islington]

Thanks @pablogsal for the PR 🌮🎉.. I'm working now to backport this PR to: 3.6.
🐍🍒⛏🤖

[miss-islington]

Thanks @pablogsal for the PR 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[miss-islington]

Thanks @pablogsal for the PR 🌮🎉.. I'm working now to backport this PR to: 2.7.
🐍🍒⛏🤖

[bedevere-bot]

GH-9597 is a backport of this pull request to the 3.7 branch.

[bedevere-bot]

GH-9598 is a backport of this pull request to the 3.6 branch.

[miss-islington]

Sorry, @pablogsal, I could not cleanly backport this to 2.7 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 7291108d88ea31d205da4db19d202d6cbffc6d93 2.7

[pablogsal]

Good job, @miss-islington!

[miss-islington]

Thanks @pablogsal for the PR 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[vstinner]

@pablogsal: It seems like the merged commit message lacks "bpo-34521".