Re: [Merge] ~dirk.zimoch/epics-base:iocLogClientFixes into epics-base:7.0

[Dirk Zimoch via Core-talk <core-talk@aps.anl.gov>]

Unfortunately send() does not return an error immediately. It simply puts the message into the TCP send queue, even when the peer has already shut down the socket. Only one or more messages later send() returns an error. But by then the messages are lost. The logClient has already "successfully" sent them ti the TCP stack but TCP failed to deliver them and there is no way to recover the messages now.

>From the send man page: "No indication of failure to deliver is implicit in a send()."
It only tells you that queuing the message was successful.

When the peer closes the socket, select() will show the socket readable and read() will return 0, but only after everything the peer may have actually send has been read (and discarded).
That is the only reliable way to detect a closed connection.
However this method only works if the receiving side had not been shut down.

I had to google a lot until I found this out. It seems many people get this wrong assuming send() should fail immediately. It doesn't.

I can't find the page that helped me in the end any more, but this one comes close:
https://www.softlab.ntua.gr/facilities/documentation/unix/unix-socket-faq/unix-socket-faq-2.html

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