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Commitf7819ba

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Fix WaitEventSetWait() to handle write-ready waits properly on Windows.
Windows apparently will not detect socket write-ready events unless apreceding send attempt returned WSAEWOULDBLOCK. In many usage patternsthat's satisfied by the caller of WaitEvenSetWait(), but not always.Apply the same solution that we already had in pgwin32_select(), namely toperform a dummy WSASend() call with len=0. This will return WSAEWOULDBLOCKif there's no buffer space (even though it could legitimately do nothingand report success, which makes me a bit nervous about this solution;but since it's been working fine in libpq, let's roll with it).In passing, improve the comments about this in pgwin32_select(), and removeduplicated code there.Back-patch to 9.6 where WaitEventSetWait() was introduced. We might needto back-patch something similar into predecessor code. But given the lackof complaints so far, it's not clear that the case ever gets exercisedin the back branches, so I'm not going to expend effort on it right now.This should resolve recurring failures on buildfarm member bowerbird,which has been failing since1e8a850 went in.Diagnosis and patch by Petr Jelinek, cosmetic adjustments by me.Discussion:https://postgr.es/m/5b6a6d6d-fb45-0afb-2e95-5600063c3dbd@2ndquadrant.com
1 parentfef2bcd commitf7819ba

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2 files changed

+44
-16
lines changed

2 files changed

+44
-16
lines changed

‎src/backend/port/win32/socket.c

Lines changed: 12 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -523,11 +523,16 @@ pgwin32_select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, c
523523
FD_ZERO(&outwritefds);
524524

525525
/*
526-
* Write FDs are different in the way that it is only flagged by
527-
* WSASelectEvent() if we have tried to write to them first. So try an
528-
* empty write
526+
* Windows does not guarantee to log an FD_WRITE network event indicating
527+
* that more data can be sent unless the previous send() failed with
528+
* WSAEWOULDBLOCK. While our caller might well have made such a call, we
529+
* cannot assume that here. Therefore, if waiting for write-ready, force
530+
* the issue by doing a dummy send(). If the dummy send() succeeds,
531+
* assume that the socket is in fact write-ready, and return immediately.
532+
* Also, if it fails with something other than WSAEWOULDBLOCK, return a
533+
* write-ready indication to let our caller deal with the error condition.
529534
*/
530-
if (writefds)
535+
if (writefds!=NULL)
531536
{
532537
for (i=0;i<writefds->fd_count;i++)
533538
{
@@ -539,20 +544,11 @@ pgwin32_select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, c
539544
buf.len=0;
540545

541546
r=WSASend(writefds->fd_array[i],&buf,1,&sent,0,NULL,NULL);
542-
if (r==0)/* Completed - means things are fine! */
547+
if (r==0||WSAGetLastError()!=WSAEWOULDBLOCK)
543548
FD_SET(writefds->fd_array[i],&outwritefds);
544-
545-
else
546-
{/* Not completed */
547-
if (WSAGetLastError()!=WSAEWOULDBLOCK)
548-
549-
/*
550-
* Not completed, and not just "would block", so an error
551-
* occurred
552-
*/
553-
FD_SET(writefds->fd_array[i],&outwritefds);
554-
}
555549
}
550+
551+
/* If we found any write-ready sockets, just return them immediately */
556552
if (outwritefds.fd_count>0)
557553
{
558554
memcpy(writefds,&outwritefds,sizeof(fd_set));

‎src/backend/storage/ipc/latch.c

Lines changed: 32 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1401,6 +1401,38 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
14011401
WaitEventAdjustWin32(set,cur_event);
14021402
cur_event->reset= false;
14031403
}
1404+
1405+
/*
1406+
* Windows does not guarantee to log an FD_WRITE network event
1407+
* indicating that more data can be sent unless the previous send()
1408+
* failed with WSAEWOULDBLOCK. While our caller might well have made
1409+
* such a call, we cannot assume that here. Therefore, if waiting for
1410+
* write-ready, force the issue by doing a dummy send(). If the dummy
1411+
* send() succeeds, assume that the socket is in fact write-ready, and
1412+
* return immediately. Also, if it fails with something other than
1413+
* WSAEWOULDBLOCK, return a write-ready indication to let our caller
1414+
* deal with the error condition.
1415+
*/
1416+
if (cur_event->events&WL_SOCKET_WRITEABLE)
1417+
{
1418+
charc;
1419+
WSABUFbuf;
1420+
DWORDsent;
1421+
intr;
1422+
1423+
buf.buf=&c;
1424+
buf.len=0;
1425+
1426+
r=WSASend(cur_event->fd,&buf,1,&sent,0,NULL,NULL);
1427+
if (r==0||WSAGetLastError()!=WSAEWOULDBLOCK)
1428+
{
1429+
occurred_events->pos=cur_event->pos;
1430+
occurred_events->user_data=cur_event->user_data;
1431+
occurred_events->events=WL_SOCKET_WRITEABLE;
1432+
occurred_events->fd=cur_event->fd;
1433+
return1;
1434+
}
1435+
}
14041436
}
14051437

14061438
/*

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