On 17/03/17 16:07, Tom Lane wrote:
> Andrew Dunstan <andrew.duns...@2ndquadrant.com> writes:
>> I have confirmed on jacana Petr's observation that adding a timeout to
>> the WaitLatchOrSocket cures the problem.
>
> Does anyone have a theory as to why that cures the problem?
>
I now have theory and even PoC patch for it.
The long wait of WaitLatchOrSocket happens after connection state
switches from CONNECTION_STARTED to CONNECTION_MADE in both cases the
return value of PQconnectPoll is PGRES_POLLING_WRITING so we wait for
FD_WRITE.
Now the documentation for WSAEventSelect says "The FD_WRITE network
event is handled slightly differently. An FD_WRITE network event is
recorded when a socket is first connected with a call to the connect,
ConnectEx, WSAConnect, WSAConnectByList, or WSAConnectByName function or
when a socket is accepted with accept, AcceptEx, or WSAAccept function
and then after a send fails with WSAEWOULDBLOCK and buffer space becomes
available. Therefore, an application can assume that sends are possible
starting from the first FD_WRITE network event setting and lasting until
a send returns WSAEWOULDBLOCK. After such a failure the application will
find out that sends are again possible when an FD_WRITE network event is
recorded and the associated event object is set."
But while PQconnectPoll does connect() before setting connection status
to CONNECTION_STARTED and returns PGRES_POLLING_WRITING so the FD_WRITE
eventually happens, it does not do any writes in the code block that
switches to CONNECTION_MADE and PGRES_POLLING_WRITING. That means
FD_WRITE event is never recorded as per quoted documentation.
Then what remains is why it works in libpq. If you look at
pgwin32_select which is eventually called by libpq code, it actually
handles the situation by trying empty WSASend on any socket that is
supposed to wait for FD_WRITE and only calling the
WaitForMultipleObjectsEx when WSASend failed with WSAEWOULDBLOCK, when
the WSASend succeeds it immediately returns ok.
So I did something similar in attached for WaitEventSetWaitBlock() and
it indeed solves the issue my windows test machine. Now the
coding/placement probably isn't the best (and there are no comments),
but maybe somebody will find proper place for this now that we know the
cause.
--
Petr Jelinek http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
diff --git a/src/backend/storage/ipc/latch.c b/src/backend/storage/ipc/latch.c
index ea7f930..300b866 100644
--- a/src/backend/storage/ipc/latch.c
+++ b/src/backend/storage/ipc/latch.c
@@ -1401,6 +1401,37 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
WaitEventAdjustWin32(set, cur_event);
cur_event->reset = false;
}
+
+ if (cur_event->events & WL_SOCKET_WRITEABLE)
+ {
+ char c;
+ WSABUF buf;
+ DWORD sent;
+ int r;
+
+ buf.buf = &c;
+ buf.len = 0;
+
+ r = WSASend(cur_event->fd, &buf, 1, &sent, 0, NULL, NULL);
+ if (r == 0)
+ {
+ occurred_events->pos = cur_event->pos;
+ occurred_events->user_data = cur_event->user_data;
+ occurred_events->events = WL_SOCKET_WRITEABLE;
+ occurred_events->fd = cur_event->fd;
+ return 1;
+ }
+ else
+ {
+ if (WSAGetLastError() != WSAEWOULDBLOCK)
+ /*
+ * Not completed, and not just "would block", so an error
+ * occurred
+ */
+ elog(ERROR, "failed writability check on socket: error code %u",
+ WSAGetLastError());
+ }
+ }
}
/*
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