From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alexander Lakhin <exclusion@gmail.com>
cc: Victor Spirin <v.spirin@postgrespro.ru>,
        pgsql-hackers@lists.postgresql.org
Subject: Re: Sometimes the output to the stdout in Windows disappears
In-reply-to: <ee02eaa2-03f7-74ea-bbdf-3196e506bae3@gmail.com>
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Comments: In-reply-to Alexander Lakhin <exclusion@gmail.com>
	message dated "Fri, 16 Oct 2020 19:00:00 +0300"
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Date: Fri, 16 Oct 2020 12:18:32 -0400
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Alexander Lakhin <exclusion@gmail.com> writes:
> I've managed to make a simple reproducer. Please look at the patch attac=
hed.
> There are two things crucial for reproducing the bug:
> =C2=A0=C2=A0=C2=A0 ioctlsocket(sock, FIONBIO, &ioctlsocket_ret); // from=
 pgwin32_socket()
> and
> =C2=A0=C2=A0=C2=A0 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