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Commit878aa41

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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 parent419cac0 commit878aa41

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3 files changed

+40
-25
lines changed

3 files changed

+40
-25
lines changed

‎src/interfaces/libpq/fe-exec.c

Lines changed: 36 additions & 17 deletions
Original file line numberDiff line numberDiff line change
@@ -2112,19 +2112,12 @@ PQgetResult(PGconn *conn)
21122112
break;
21132113

21142114
casePGASYNC_READY:
2115-
2116-
/*
2117-
* For any query type other than simple query protocol, we advance
2118-
* the command queue here. This is because for simple query
2119-
* protocol we can get the READY state multiple times before the
2120-
* command is actually complete, since the command string can
2121-
* contain many queries. In simple query protocol, the queue
2122-
* advance is done by fe-protocol3 when it receives ReadyForQuery.
2123-
*/
2124-
if (conn->cmd_queue_head&&
2125-
conn->cmd_queue_head->queryclass!=PGQUERY_SIMPLE)
2126-
pqCommandQueueAdvance(conn);
21272115
res=pqPrepareAsyncResult(conn);
2116+
2117+
/* Advance the queue as appropriate */
2118+
pqCommandQueueAdvance(conn, false,
2119+
res->resultStatus==PGRES_PIPELINE_SYNC);
2120+
21282121
if (conn->pipelineStatus!=PQ_PIPELINE_OFF)
21292122
{
21302123
/*
@@ -3007,26 +3000,52 @@ PQexitPipelineMode(PGconn *conn)
30073000

30083001
/*
30093002
* pqCommandQueueAdvance
3010-
*Remove one query from the command queue, when we receive
3011-
*all results from the server that pertain to it.
3003+
*Remove one query from the command queue, if appropriate.
3004+
*
3005+
* If we have received all results corresponding to the head element
3006+
* in the command queue, remove it.
3007+
*
3008+
* In simple query protocol we must not advance the command queue until the
3009+
* ReadyForQuery message has been received. This is because in simple mode a
3010+
* command can have multiple queries, and we must process result for all of
3011+
* them before moving on to the next command.
3012+
*
3013+
* Another consideration is synchronization during error processing in
3014+
* extended query protocol: we refuse to advance the queue past a SYNC queue
3015+
* element, unless the result we've received is also a SYNC. In particular
3016+
* this protects us from advancing when an error is received at an
3017+
* inappropriate moment.
30123018
*/
30133019
void
3014-
pqCommandQueueAdvance(PGconn*conn)
3020+
pqCommandQueueAdvance(PGconn*conn,boolisReadyForQuery,boolgotSync)
30153021
{
30163022
PGcmdQueueEntry*prevquery;
30173023

30183024
if (conn->cmd_queue_head==NULL)
30193025
return;
30203026

3021-
/* delink from queue */
3027+
/*
3028+
* If processing a query of simple query protocol, we only advance the
3029+
* queue when we receive the ReadyForQuery message for it.
3030+
*/
3031+
if (conn->cmd_queue_head->queryclass==PGQUERY_SIMPLE&& !isReadyForQuery)
3032+
return;
3033+
3034+
/*
3035+
* If we're waiting for a SYNC, don't advance the queue until we get one.
3036+
*/
3037+
if (conn->cmd_queue_head->queryclass==PGQUERY_SYNC&& !gotSync)
3038+
return;
3039+
3040+
/* delink element from queue */
30223041
prevquery=conn->cmd_queue_head;
30233042
conn->cmd_queue_head=conn->cmd_queue_head->next;
30243043

30253044
/* If the queue is now empty, reset the tail too */
30263045
if (conn->cmd_queue_head==NULL)
30273046
conn->cmd_queue_tail=NULL;
30283047

3029-
/* and makeit recyclable */
3048+
/* and makethe queue element recyclable */
30303049
prevquery->next=NULL;
30313050
pqRecycleCmdQueueEntry(conn,prevquery);
30323051
}

‎src/interfaces/libpq/fe-protocol3.c

Lines changed: 2 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -236,13 +236,8 @@ pqParseInput3(PGconn *conn)
236236
}
237237
else
238238
{
239-
/*
240-
* In simple query protocol, advance the command queue
241-
* (see PQgetResult).
242-
*/
243-
if (conn->cmd_queue_head&&
244-
conn->cmd_queue_head->queryclass==PGQUERY_SIMPLE)
245-
pqCommandQueueAdvance(conn);
239+
/* Advance the command queue and set us idle */
240+
pqCommandQueueAdvance(conn, true, false);
246241
conn->asyncStatus=PGASYNC_IDLE;
247242
}
248243
break;

‎src/interfaces/libpq/libpq-int.h

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -705,7 +705,8 @@ extern void pqSaveMessageField(PGresult *res, char code,
705705
externvoidpqSaveParameterStatus(PGconn*conn,constchar*name,
706706
constchar*value);
707707
externintpqRowProcessor(PGconn*conn,constchar**errmsgp);
708-
externvoidpqCommandQueueAdvance(PGconn*conn);
708+
externvoidpqCommandQueueAdvance(PGconn*conn,boolisReadyForQuery,
709+
boolgotSync);
709710
externintPQsendQueryContinue(PGconn*conn,constchar*query);
710711

711712
/* === in fe-protocol3.c === */

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