Allow --zerocopy and --skip-rx-copy when only server supports them

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master 3.21+

• Issues fixed (if any): zerocopy os check should wait for sever response when in reverse mode #2044

• Brief description of code changes (suitable for use as a commit message):

Allow --zerocopy and --skip-rx-copy even when one of the sides OS, client or server, not support them, as they may be supported by the other side. When not supported, allow zerocopy only when not sending and skip-rx-copy only when not receiving.

(UPDATE note: deleted the -P related changes and created a separate PR for them.)

[bmah888]

Thanks for the pull request. I'm trying to test it, but it looks like this is a fix (with respect to --zerocopy and --skip-rx-copy for a problem that does not occur on Linux, FreeBSD, or macOS (the three main development problems). If this is only an issues (a platfom that we don't officially support) this is going to be a little hard for our team to test. Haven't quiet figured out how to deal with this yet.

[davidBar-On]

Haven't quiet figured out how to deal with this yet.

I tested it by creating a version with HAVE_MSG_TRUNC set to zero, and then run it once as a client and once as a server (when the other side is the version with HAVE_MSG_TRUNC set to 1). I believe this is sufficient as each of these options affect either sending or sides but not both.

[bmah888]

Thanks for the PR. I have a concern that this adds argument parsing complexity basically to cater to the case of a Windows client connecting to a Linux (or other UNIX-like) host, and Windows isn't even an officially supported platform. This isn't even applicable to any of the supported platforms (Linux, FreeBSD, macOS), but we have to pay the cost of more complicated code that's harder to test and maintain.

[bmah888]

Suggested change

	i_errno = IENOSENDFILE; // Zerocopy required for the Client but is not supported
	i_errno = IENOSENDFILE; // Zerocopy required for the Server but is not supported

[davidBar-On]

Changed

[bmah888]

Suggested change

	snprintf(errstr, len, "this OS does not support Skip RX Copy");
	snprintf(errstr, len, "this OS does not support --skip-rx-copy");

[davidBar-On]

Changed

[bmah888]

I'm opposed to making the help text more complicated in this way. I'd almost rather just emit the existing --skip-rx-copy text all the time.

[davidBar-On]

Modified the change - removed the #define IPERF_HELP_MSG_TRUNK_SUPPORTED_BY_THIS_OS but still added a specific --skip-rx-copy help version for when not supported by the local iperf3 but may be supported by the remote end.