8000 Fix unwanted flushing of libpq's input buffer when socket EOF is seen. · postgres/postgres@40879a9 · GitHub
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Fix unwanted flushing of libpq's input buffer when socket EOF is seen.
In commit 210eb9b I centralized libpq's logic for closing down the backend communication socket, and made the new pqDropConnection routine always reset the I/O buffers to empty. Many of the call sites previously had not had such code, and while that amounted to an oversight in some cases, there was one place where it was intentional and necessary *not* to flush the input buffer: pqReadData should never cause that to happen, since we probably still want to process whatever data we read. This is the true cause of the problem Robert was attempting to fix in c3e7c24, namely that libpq no longer reported the backend's final ERROR message before reporting "server closed the connection unexpectedly". But that only accidentally fixed it, by invoking parseInput before the input buffer got flushed; and very likely there are timing scenarios where we'd still lose the message before processing it. To fix, pass a flag to pqDropConnection to tell it whether to flush the input buffer or not. On review I think flushing is actually correct for every other call site. Back-patch to 9.3 where the problem was introduced. In HEAD, also improve the comments added by c3e7c24.
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4 files changed

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‎src/interfaces/libpq/fe-connect.c

Lines changed: 22 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -391,18 +391,24 @@ pgthreadlock_t pg_g_threadlock = default_threadlock;
391391
* Close any physical connection to the server, and reset associated
392392
* state inside the connection object. We don't release state that
393393
* would be needed to reconnect, though.
394+
*
395+
* We can always flush the output buffer, since there's no longer any hope
396+
* of sending that data. However, unprocessed input data might still be
397+
* valuable, so the caller must tell us whether to flush that or not.
394398
*/
395399
void
396-
pqDropConnection(PGconn *conn)
400+
pqDropConnection(PGconn *conn, bool flushInput)
397401
{
398402
/* Drop any SSL state */
399403
pqsecure_close(conn);
400404
/* Close the socket itself */
401405
if (conn->sock != PGINVALID_SOCKET)
402406
closesocket(conn->sock);
403407
conn->sock = PGINVALID_SOCKET;
404-
/* Discard any unread/unsent data */
405-
conn->inStart = conn->inCursor = conn->inEnd = 0;
408+
/* Optionally discard any unread data */
409+
if (flushInput)
410+
conn->inStart = conn->inCursor = conn->inEnd = 0;
411+
/* Always discard any unsent data */
406412
conn->outCount = 0;
407413
}
408414

@@ -1510,7 +1516,7 @@ connectDBStart(PGconn *conn)
15101516
return 1;
15111517

15121518
connect_errReturn:
1513-
pqDropConnection(conn);
1519+
pqDropConnection(conn, true);
15141520
conn->status = CONNECTION_BAD;
15151521
return 0;
15161522
}
@@ -1732,7 +1738,7 @@ PQconnectPoll(PGconn *conn)
17321738
{
17331739
if (!connectNoDelay(conn))
17341740
{
1735-
pqDropConnection(conn);
1741+
pqDropConnection(conn, true);
17361742
conn->addr_cur = addr_cur->ai_next;
17371743
continue;
17381744
}
@@ -1742,7 +1748,7 @@ PQconnectPoll(PGconn *conn)
17421748
appendPQExpBuffer(&conn->errorMessage,
17431749
libpq_gettext("could not set socket to nonblocking mode: %s\n"),
17441750
SOCK_STRERROR(SOCK_ERRNO, sebuf, sizeof(sebuf)));
1745-
pqDropConnection(conn);
1751+
pqDropConnection(conn, true);
17461752
conn->addr_cur = addr_cur->ai_next;
17471753
continue;
17481754
}
@@ -1753,7 +1759,7 @@ PQconnectPoll(PGconn *conn)
17531759
appendPQExpBuffer(&conn->errorMessage,
17541760
libpq_gettext("could not set socket to close-on-exec mode: %s\n"),
17551761
SOCK_STRERROR(SOCK_ERRNO, sebuf, sizeof(sebuf)));
1756-
pqDropConnection(conn);
1762+
pqDropConnection(conn, true);
17571763
conn->addr_cur = addr_cur->ai_next;
17581764
continue;
17591765
}
@@ -1800,7 +1806,7 @@ PQconnectPoll(PGconn *conn)
18001806

18011807
if (err)
18021808
{
1803-
pqDropConnection(conn);
1809+
pqDropConnection(conn, true);
18041810
conn->addr_cur = addr_cur->ai_next;
18051811
continue;
18061812
}
@@ -1887,7 +1893,7 @@ PQconnectPoll(PGconn *conn)
18871893
* failure and keep going if there are more addresses.
18881894
*/
18891895
connectFailureMessage(conn, SOCK_ERRNO);
1890-
pqDropConnection(conn);
1896+
pqDropConnection(conn, true);
18911897

18921898
/*
18931899
* Try the next address, if any.
@@ -1932,7 +1938,7 @@ PQconnectPoll(PGconn *conn)
19321938
* error message.
19331939
*/
19341940
connectFailureMessage(conn, optval);
1935-
pqDropConnection(conn);
1941+
pqDropConnection(conn, true);
19361942

19371943
/*
19381944
* If more addresses remain, keep trying, just as in the
@@ -2220,7 +2226,7 @@ PQconnectPoll(PGconn *conn)
22202226
/* only retry once */
22212227
conn->allow_ssl_try = false;
22222228
/* Must drop the old connection */
2223-
pqDropConnection(conn);
2229+
pqDropConnection(conn, true);
22242230
conn->status = CONNECTION_NEEDED;
22252231
goto keep_going;
22262232
}
@@ -2331,7 +2337,7 @@ PQconnectPoll(PGconn *conn)
23312337
{
23322338
conn->pversion = PG_PROTOCOL(2, 0);
23332339
/* Must drop the old connection */
2334-
pqDropConnection(conn);
2340+
pqDropConnection(conn, true);
23352341
conn->status = CONNECTION_NEEDED;
23362342
goto keep_going;
23372343
}
@@ -2397,7 +2403,7 @@ PQconnectPoll(PGconn *conn)
23972403
/* only retry once */
23982404
conn->wait_ssl_try = false;
23992405
/* Must drop the old connection */
2400-
pqDropConnection(conn);
2406+
pqDropConnection(conn, true);
24012407
conn->status = CONNECTION_NEEDED;
24022408
goto keep_going;
24032409
}
@@ -2413,7 +2419,7 @@ PQconnectPoll(PGconn *conn)
24132419
/* only retry once */
24142420
conn->allow_ssl_try = false;
24152421
/* Must drop the old connection */
2416-
pqDropConnection(conn);
2422+
pqDropConnection(conn, true);
24172423
conn->status = CONNECTION_NEEDED;
24182424
goto keep_going;
24192425
}
@@ -2574,7 +2580,7 @@ PQconnectPoll(PGconn *conn)
25742580
PQclear(res);
25752581
conn->send_appname = false;
25762582
/* Must drop the old connection */
2577-
pqDropConnection(conn);
2583+
pqDropConnection(conn, true);
25782584
conn->status = CONNECTION_NEEDED;
25792585
goto keep_going;
25802586
}
@@ -2969,7 +2975,7 @@ closePGconn(PGconn *conn)
29692975
/*
29702976
* Close the connection, reset all transient state, flush I/O buffers.
29712977
*/
2972-
pqDropConnection(conn);
2978+
pqDropConnection(conn, true);
29732979
conn->status = CONNECTION_BAD; /* Well, not really _bad_ - just
29742980
* absent */
29752981
conn->asyncStatus = PGASYNC_IDLE;

‎src/interfaces/libpq/fe-misc.c

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -814,7 +814,8 @@ pqReadData(PGconn *conn)
814814

815815
/* Come here if lower-level code already set a suitable errorMessage */
816816
definitelyFailed:
817-
pqDropConnection(conn);
817+
/* Do *not* drop any already-read data; caller still wants it */
818+
pqDropConnection(conn, false);
818819
conn->status = CONNECTION_BAD; /* No more connection to backend */
819820
return -1;
820821
}

‎src/interfaces/libpq/fe-protocol3.c

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -446,8 +446,8 @@ handleSyncLoss(PGconn *conn, char id, int msgLength)
446446
/* build an error result holding the error message */
447447
pqSaveErrorResult(conn);
448448
conn->asyncStatus = PGASYNC_READY; /* drop out of GetResult wait loop */
449-
450-
pqDropConnection(conn);
449+
/* flush input data since we're giving up on processing it */
450+
pqDropConnection(conn, true);
451451
conn->status = CONNECTION_BAD; /* No more connection to backend */
452452
}
453453

‎src/interfaces/libpq/libpq-int.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -493,7 +493,7 @@ extern char *const pgresStatus[];
493493

494494
/* === in fe-connect.c === */
495495

496-
extern void pqDropConnection(PGconn *conn);
496+
extern void pqDropConnection(PGconn *conn, bool flushInput);
497497
extern int pqPacketSend(PGconn *conn, char pack_type,
498498
const void *buf, size_t buf_len);
499499
extern bool pqGetHomeDirectory(char *buf, int bufsize);

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