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Commit1171c6e

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Fix handling of errors in libpq pipelines
The logic to keep the libpq command queue in sync with queries that havebeen processed had a bug when errors were returned for reasons otherthan problems in queries -- for example, when a connection is lost. Weincorrectly consumed an element from the command queue every time, butthis is wrong and can lead to the queue becoming empty ahead of time,leading to later malfunction: PQgetResult would return nothing,potentially causing the calling application to enter a busy loop.Fix by making the SYNC queue element a barrier that can only be consumedwhen a SYNC message is received.Backpatch to 14.Reported by: Иван Трофимов (Ivan Trofimov) <i.trofimow@yandex.ru>Discussion:https://postgr.es/m/17948-fcace7557e449957@postgresql.org
1 parent25f2a43 commit1171c6e

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3 files changed

+44
-29
lines changed

3 files changed

+44
-29
lines changed

‎src/interfaces/libpq/fe-exec.c‎

Lines changed: 40 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -2119,29 +2119,21 @@ PQgetResult(PGconn *conn)
21192119

21202120
/*
21212121
* We're about to return the NULL that terminates the round of
2122-
* results from the current query; prepare to send the results
2123-
* of the next query, if any, when we're called next. If there's
2124-
* no next element in the command queue, this gets us in IDLE
2125-
* state.
2122+
* results from the current query; prepare to send the results of
2123+
* the next query, if any, when we're called next. If there's no
2124+
* next element in the command queue, this gets us in IDLE state.
21262125
*/
21272126
pqPipelineProcessQueue(conn);
21282127
res=NULL;/* query is complete */
21292128
break;
21302129

21312130
casePGASYNC_READY:
2132-
2133-
/*
2134-
* For any query type other than simple query protocol, we advance
2135-
* the command queue here. This is because for simple query
2136-
* protocol we can get the READY state multiple times before the
2137-
* command is actually complete, since the command string can
2138-
* contain many queries. In simple query protocol, the queue
2139-
* advance is done by fe-protocol3 when it receives ReadyForQuery.
2140-
*/
2141-
if (conn->cmd_queue_head&&
2142-
conn->cmd_queue_head->queryclass!=PGQUERY_SIMPLE)
2143-
pqCommandQueueAdvance(conn);
21442131
res=pqPrepareAsyncResult(conn);
2132+
2133+
/* Advance the queue as appropriate */
2134+
pqCommandQueueAdvance(conn, false,
2135+
res->resultStatus==PGRES_PIPELINE_SYNC);
2136+
21452137
if (conn->pipelineStatus!=PQ_PIPELINE_OFF)
21462138
{
21472139
/*
@@ -3040,26 +3032,52 @@ PQexitPipelineMode(PGconn *conn)
30403032

30413033
/*
30423034
* pqCommandQueueAdvance
3043-
*Remove one query from the command queue, when we receive
3044-
*all results from the server that pertain to it.
3035+
*Remove one query from the command queue, if appropriate.
3036+
*
3037+
* If we have received all results corresponding to the head element
3038+
* in the command queue, remove it.
3039+
*
3040+
* In simple query protocol we must not advance the command queue until the
3041+
* ReadyForQuery message has been received. This is because in simple mode a
3042+
* command can have multiple queries, and we must process result for all of
3043+
* them before moving on to the next command.
3044+
*
3045+
* Another consideration is synchronization during error processing in
3046+
* extended query protocol: we refuse to advance the queue past a SYNC queue
3047+
* element, unless the result we've received is also a SYNC. In particular
3048+
* this protects us from advancing when an error is received at an
3049+
* inappropriate moment.
30453050
*/
30463051
void
3047-
pqCommandQueueAdvance(PGconn*conn)
3052+
pqCommandQueueAdvance(PGconn*conn,boolisReadyForQuery,boolgotSync)
30483053
{
30493054
PGcmdQueueEntry*prevquery;
30503055

30513056
if (conn->cmd_queue_head==NULL)
30523057
return;
30533058

3054-
/* delink from queue */
3059+
/*
3060+
* If processing a query of simple query protocol, we only advance the
3061+
* queue when we receive the ReadyForQuery message for it.
3062+
*/
3063+
if (conn->cmd_queue_head->queryclass==PGQUERY_SIMPLE&& !isReadyForQuery)
3064+
return;
3065+
3066+
/*
3067+
* If we're waiting for a SYNC, don't advance the queue until we get one.
3068+
*/
3069+
if (conn->cmd_queue_head->queryclass==PGQUERY_SYNC&& !gotSync)
3070+
return;
3071+
3072+
/* delink element from queue */
30553073
prevquery=conn->cmd_queue_head;
30563074
conn->cmd_queue_head=conn->cmd_queue_head->next;
30573075

30583076
/* If the queue is now empty, reset the tail too */
30593077
if (conn->cmd_queue_head==NULL)
30603078
conn->cmd_queue_tail=NULL;
30613079

3062-
/* and makeit recyclable */
3080+
/* and makethe queue element recyclable */
30633081
prevquery->next=NULL;
30643082
pqRecycleCmdQueueEntry(conn,prevquery);
30653083
}
@@ -3083,6 +3101,7 @@ pqPipelineProcessQueue(PGconn *conn)
30833101
return;
30843102

30853103
casePGASYNC_IDLE:
3104+
30863105
/*
30873106
* If we're in IDLE mode and there's some command in the queue,
30883107
* get us into PIPELINE_IDLE mode and process normally. Otherwise

‎src/interfaces/libpq/fe-protocol3.c‎

Lines changed: 2 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -240,13 +240,8 @@ pqParseInput3(PGconn *conn)
240240
}
241241
else
242242
{
243-
/*
244-
* In simple query protocol, advance the command queue
245-
* (see PQgetResult).
246-
*/
247-
if (conn->cmd_queue_head&&
248-
conn->cmd_queue_head->queryclass==PGQUERY_SIMPLE)
249-
pqCommandQueueAdvance(conn);
243+
/* Advance the command queue and set us idle */
244+
pqCommandQueueAdvance(conn, true, false);
250245
conn->asyncStatus=PGASYNC_IDLE;
251246
}
252247
break;

‎src/interfaces/libpq/libpq-int.h‎

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -673,7 +673,8 @@ extern void pqSaveMessageField(PGresult *res, char code,
673673
externvoidpqSaveParameterStatus(PGconn*conn,constchar*name,
674674
constchar*value);
675675
externintpqRowProcessor(PGconn*conn,constchar**errmsgp);
676-
externvoidpqCommandQueueAdvance(PGconn*conn);
676+
externvoidpqCommandQueueAdvance(PGconn*conn,boolisReadyForQuery,
677+
boolgotSync);
677678
externintPQsendQueryContinue(PGconn*conn,constchar*query);
678679

679680
/* === in fe-protocol3.c === */

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