Re: Sometimes the output to the stdout in Windows disappears

Alexander Lakhin <exclusion(at)gmail(dot)com> writes:
> I've managed to make a simple reproducer. Please look at the patch attached.
> There are two things crucial for reproducing the bug:
>     ioctlsocket(sock, FIONBIO, &ioctlsocket_ret); // from pgwin32_socket()
> and
>     WSACleanup();

Oh, very interesting.

Now that you have it somewhat in captivity, maybe you could determine
some things:

1. Is it only stdout that's affected? What of other stdio streams?
(Note that testing stderr might be tricky because it's probably
line-buffered.)

2. Does an fflush() just before, or just after, WSACleanup() fix it?

> I see no solution for this on the postgres side for now, but this
> information about Windows quirks could be useful in case someone
> stumbled upon it too.

Depending on your answers to the above, maybe some hack like this
would be acceptable:

free(conn);

#ifdef WIN32
+ fflush(NULL);
WSACleanup();
#endif
}

It's not very nice for a library to be doing global things like that,
but if the alternative is loss of output, maybe we should.

But wait a minute: I just looked at Microsoft's docs [1] and found

In a multithreaded environment, WSACleanup terminates Windows Sockets
operations for all threads.

This makes me (a) wonder if that explains the side-effects on stdio,
and (b) question why libpq is calling WSACleanup at all.
What if we arranged to call WSAStartup just once, during the first
libpq connection-open in a process, and then never did WSACleanup?
Surely the OS can cope with that, and it eliminates any risk that
WSACleanup breaks something.

regards, tom lane

[1] https://docs.microsoft.com/en-us/windows/win32/api/winsock/nf-winsock-wsacleanup