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Commitb8ba734

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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 parent63c5df1 commitb8ba734

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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
/*
@@ -3088,26 +3081,52 @@ PQexitPipelineMode(PGconn *conn)
30883081

30893082
/*
30903083
* pqCommandQueueAdvance
3091-
*Remove one query from the command queue, when we receive
3092-
*all results from the server that pertain to it.
3084+
*Remove one query from the command queue, if appropriate.
3085+
*
3086+
* If we have received all results corresponding to the head element
3087+
* in the command queue, remove it.
3088+
*
3089+
* In simple query protocol we must not advance the command queue until the
3090+
* ReadyForQuery message has been received. This is because in simple mode a
3091+
* command can have multiple queries, and we must process result for all of
3092+
* them before moving on to the next command.
3093+
*
3094+
* Another consideration is synchronization during error processing in
3095+
* extended query protocol: we refuse to advance the queue past a SYNC queue
3096+
* element, unless the result we've received is also a SYNC. In particular
3097+
* this protects us from advancing when an error is received at an
3098+
* inappropriate moment.
30933099
*/
30943100
void
3095-
pqCommandQueueAdvance(PGconn*conn)
3101+
pqCommandQueueAdvance(PGconn*conn,boolisReadyForQuery,boolgotSync)
30963102
{
30973103
PGcmdQueueEntry*prevquery;
30983104

30993105
if (conn->cmd_queue_head==NULL)
31003106
return;
31013107

3102-
/* delink from queue */
3108+
/*
3109+
* If processing a query of simple query protocol, we only advance the
3110+
* queue when we receive the ReadyForQuery message for it.
3111+
*/
3112+
if (conn->cmd_queue_head->queryclass==PGQUERY_SIMPLE&& !isReadyForQuery)
3113+
return;
3114+
3115+
/*
3116+
* If we're waiting for a SYNC, don't advance the queue until we get one.
3117+
*/
3118+
if (conn->cmd_queue_head->queryclass==PGQUERY_SYNC&& !gotSync)
3119+
return;
3120+
3121+
/* delink element from queue */
31033122
prevquery=conn->cmd_queue_head;
31043123
conn->cmd_queue_head=conn->cmd_queue_head->next;
31053124

31063125
/* If the queue is now empty, reset the tail too */
31073126
if (conn->cmd_queue_head==NULL)
31083127
conn->cmd_queue_tail=NULL;
31093128

3110-
/* and makeit recyclable */
3129+
/* and makethe queue element recyclable */
31113130
prevquery->next=NULL;
31123131
pqRecycleCmdQueueEntry(conn,prevquery);
31133132
}

‎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
@@ -698,7 +698,8 @@ extern void pqSaveMessageField(PGresult *res, char code,
698698
externvoidpqSaveParameterStatus(PGconn*conn,constchar*name,
699699
constchar*value);
700700
externintpqRowProcessor(PGconn*conn,constchar**errmsgp);
701-
externvoidpqCommandQueueAdvance(PGconn*conn);
701+
externvoidpqCommandQueueAdvance(PGconn*conn,boolisReadyForQuery,
702+
boolgotSync);
702703
externintPQsendQueryContinue(PGconn*conn,constchar*query);
703704

704705
/* === in fe-protocol3.c === */

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