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Commit90e61df

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Fix treatment of *lpNumberOfBytesRecvd == 0: that's a completion condition.
pgwin32_recv() has treated a non-error return of zero bytes from WSARecv()as being a reason to block ever since the current implementation wasintroduced in commita4c40f1. However, so far as one can tellfrom Microsoft's documentation, that is just wrong: what it means isgraceful connection closure (in stream protocols) or receipt of azero-length message (in message protocols), and neither case should resultin blocking here. The only reason the code worked at all was that controlthen fell into the retry loop, which did *not* treat zero bytes specially,so we'd get out after only wasting some cycles. But as of 9.5 we do notnormally reach the retry loop and so the bug is exposed, as reported byShay Rojansky and diagnosed by Andres Freund.Remove the unnecessary test on the byte count, and rearrange the codein the retry loop so that it looks identical to the initial sequence.Back-patch to 9.5. The code is wrong all the way back, AFAICS, butsince it's relatively harmless in earlier branches we'll leave it alone.
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‎src/backend/port/win32/socket.c

Lines changed: 16 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -323,12 +323,10 @@ pgwin32_recv(SOCKET s, char *buf, int len, int f)
323323
wbuf.buf=buf;
324324

325325
r=WSARecv(s,&wbuf,1,&b,&flags,NULL,NULL);
326-
if (r!=SOCKET_ERROR&&b>0)
327-
/* Read succeeded right away */
328-
returnb;
326+
if (r!=SOCKET_ERROR)
327+
returnb;/* success */
329328

330-
if (r==SOCKET_ERROR&&
331-
WSAGetLastError()!=WSAEWOULDBLOCK)
329+
if (WSAGetLastError()!=WSAEWOULDBLOCK)
332330
{
333331
TranslateSocketError();
334332
return-1;
@@ -344,7 +342,7 @@ pgwin32_recv(SOCKET s, char *buf, int len, int f)
344342
return-1;
345343
}
346344

347-
/*No error, zero bytes (win2000+) or error+WSAEWOULDBLOCK (<=nt4) */
345+
/*We're in blocking mode, so wait for data */
348346

349347
for (n=0;n<5;n++)
350348
{
@@ -353,25 +351,22 @@ pgwin32_recv(SOCKET s, char *buf, int len, int f)
353351
return-1;/* errno already set */
354352

355353
r=WSARecv(s,&wbuf,1,&b,&flags,NULL,NULL);
356-
if (r==SOCKET_ERROR)
354+
if (r!=SOCKET_ERROR)
355+
returnb;/* success */
356+
if (WSAGetLastError()!=WSAEWOULDBLOCK)
357357
{
358-
if (WSAGetLastError()==WSAEWOULDBLOCK)
359-
{
360-
/*
361-
* There seem to be cases on win2k (at least) where WSARecv
362-
* can return WSAEWOULDBLOCK even when
363-
* pgwin32_waitforsinglesocket claims the socket is readable.
364-
* In this case, just sleep for a moment and try again. We try
365-
* up to 5 times - if it fails more than that it's not likely
366-
* to ever come back.
367-
*/
368-
pg_usleep(10000);
369-
continue;
370-
}
371358
TranslateSocketError();
372359
return-1;
373360
}
374-
returnb;
361+
362+
/*
363+
* There seem to be cases on win2k (at least) where WSARecv can return
364+
* WSAEWOULDBLOCK even when pgwin32_waitforsinglesocket claims the
365+
* socket is readable. In this case, just sleep for a moment and try
366+
* again. We try up to 5 times - if it fails more than that it's not
367+
* likely to ever come back.
368+
*/
369+
pg_usleep(10000);
375370
}
376371
ereport(NOTICE,
377372
(errmsg_internal("could not read from ready socket (after retries)")));

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