[Windows][melodic-devel] workaround WSAPoll doesn't report failed connections.

[seanyen]

This fixes ms-iot/ROSOnWindows#151

The original problem manifests as ros::master::check() never return back when the master is not running. It turns out that it is waiting for WSAPoll to be returned, which is at this below line:

ros_comm/utilities/xmlrpcpp/src/XmlRpcDispatch.cpp

Line 125 in d487d58

int nEvents = poll(&fds[0], source_cnt, (timeout_ms < 0) ? -1 : timeout_ms);

However, for a non-blocking socket which has a failed connection, WSAPoll exhibits a behavior which is not to signal this error for the caller, and with the infinite timeout, it just waits forever. There are many discussions about this behavior.

To workaround this issue, I am proposing for Windows platform adding a connection readiness pre-checks for writable sockets, which can be done by calling select and it will come back with a result immediately whether it is an connection-error socket or writable-ready socket. After we have the result, we can decide to error out or go ahead with the original WSAPoll to finish its job.

[seanyen]

@mikepurvis This is ready for review and merge. Thanks!

[mikepurvis]

I know it's not directly related here, but in the process of reviewing this, I noticed some if/endif mismatching going on. The #endif at the bottom of the file is labeled for USE_FTIME but I believe actually corresponds to WINDOWS.

@seanyen Do you mind looking into this and correcting the comments if applicable?

[seanyen]

By checking the commit history, it seems to me that USE_FTIME was first introduced because Windows doesn't have gettimeofday so it ended up with diverged implementations. However, two years ago, a PR was merged and now non-Windows implementation is in favor of ros::ros_steadytime.

I would say perhaps we should see if there is any opportunity to converge the implementations now, since ros::ros_steadytime is a portable thing and theoretically it should behave the same across the platforms.

@mikepurvis, what do you think?

[dirk-thomas]

Since this is orthogonal please follow up on this separately form this PR.

[dirk-thomas]

Thanks for the patch.