NET_SOFTERROR on UDP send EAGAIN/EINTR errno if no data was sent

[davidBar-On]

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

• Issues fixed (if any):
  None

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

Change Nwrite() to return NET_FOFERROR on errno EINTR or EAGAIN if no data was sent yet, similar to what is already implemented in Nsendfile().

In addition, add debug message in case UDP send failed with NET_SOFTERROR. This is to help evaluating issues such as #1367. The message is printed on --debug=3 (or higher level), so it will be possible to see it without seeing a message per UDP packet sent (which is the case when -d or --debug=4 is used).

[swlars]

Hi David! Thanks for the merge request! It looks like part of this was merged already, but we're not quite sure how to test the last few lines.

[davidBar-On]

Hi Sarah, I assume you are asking about the indentation changes in lines 507-510 in net.c. I don't remember what they are, but I suspect these changes are because I copied the relevant lines from Nsendfile(). By the way, there are indentations problems in both Nwrite() and nSendfile() - the code following the case/default is not indented. I didn't make these changes to minimuze the changes in the code.

Just to make sure I fully answer the question, regarding the following that was added in Nwrite():

    if (count == nleft)
        return NET_SOFTERROR;

This is copied from Nsendfile() and is the suggested fix to #1367 - when a UDP packet was not sent, do not count it as sent and do not increase the UDP packets number. See this comment.