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Commitf896818

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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 parentbe7b4d9 commitf896818

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3 files changed

+49
-33
lines changed

3 files changed

+49
-33
lines changed

‎src/interfaces/libpq/fe-exec.c‎

Lines changed: 41 additions & 22 deletions
Original file line numberDiff line numberDiff line change
@@ -2078,30 +2078,22 @@ PQgetResult(PGconn *conn)
20782078

20792079
/*
20802080
* We're about to return the NULL that terminates the round of
2081-
* results from the current query; prepare to send the results
2082-
* of the next query, if any, when we're called next. If there's
2083-
* no next element in the command queue, this gets us in IDLE
2084-
* state.
2081+
* results from the current query; prepare to send the results of
2082+
* the next query, if any, when we're called next. If there's no
2083+
* next element in the command queue, this gets us in IDLE state.
20852084
*/
20862085
resetPQExpBuffer(&conn->errorMessage);
20872086
pqPipelineProcessQueue(conn);
20882087
res=NULL;/* query is complete */
20892088
break;
20902089

20912090
casePGASYNC_READY:
2092-
2093-
/*
2094-
* For any query type other than simple query protocol, we advance
2095-
* the command queue here. This is because for simple query
2096-
* protocol we can get the READY state multiple times before the
2097-
* command is actually complete, since the command string can
2098-
* contain many queries. In simple query protocol, the queue
2099-
* advance is done by fe-protocol3 when it receives ReadyForQuery.
2100-
*/
2101-
if (conn->cmd_queue_head&&
2102-
conn->cmd_queue_head->queryclass!=PGQUERY_SIMPLE)
2103-
pqCommandQueueAdvance(conn);
21042091
res=pqPrepareAsyncResult(conn);
2092+
2093+
/* Advance the queue as appropriate */
2094+
pqCommandQueueAdvance(conn, false,
2095+
res->resultStatus==PGRES_PIPELINE_SYNC);
2096+
21052097
if (conn->pipelineStatus!=PQ_PIPELINE_OFF)
21062098
{
21072099
/*
@@ -2165,7 +2157,7 @@ PQgetResult(PGconn *conn)
21652157
}
21662158
else
21672159
/* we won't ever see the Close */
2168-
pqCommandQueueAdvance(conn);
2160+
pqCommandQueueAdvance(conn, false, false);
21692161
}
21702162

21712163
if (res)
@@ -3027,26 +3019,52 @@ PQexitPipelineMode(PGconn *conn)
30273019

30283020
/*
30293021
* pqCommandQueueAdvance
3030-
*Remove one query from the command queue, when we receive
3031-
*all results from the server that pertain to it.
3022+
*Remove one query from the command queue, if appropriate.
3023+
*
3024+
* If we have received all results corresponding to the head element
3025+
* in the command queue, remove it.
3026+
*
3027+
* In simple query protocol we must not advance the command queue until the
3028+
* ReadyForQuery message has been received. This is because in simple mode a
3029+
* command can have multiple queries, and we must process result for all of
3030+
* them before moving on to the next command.
3031+
*
3032+
* Another consideration is synchronization during error processing in
3033+
* extended query protocol: we refuse to advance the queue past a SYNC queue
3034+
* element, unless the result we've received is also a SYNC. In particular
3035+
* this protects us from advancing when an error is received at an
3036+
* inappropriate moment.
30323037
*/
30333038
void
3034-
pqCommandQueueAdvance(PGconn*conn)
3039+
pqCommandQueueAdvance(PGconn*conn,boolisReadyForQuery,boolgotSync)
30353040
{
30363041
PGcmdQueueEntry*prevquery;
30373042

30383043
if (conn->cmd_queue_head==NULL)
30393044
return;
30403045

3041-
/* delink from queue */
3046+
/*
3047+
* If processing a query of simple query protocol, we only advance the
3048+
* queue when we receive the ReadyForQuery message for it.
3049+
*/
3050+
if (conn->cmd_queue_head->queryclass==PGQUERY_SIMPLE&& !isReadyForQuery)
3051+
return;
3052+
3053+
/*
3054+
* If we're waiting for a SYNC, don't advance the queue until we get one.
3055+
*/
3056+
if (conn->cmd_queue_head->queryclass==PGQUERY_SYNC&& !gotSync)
3057+
return;
3058+
3059+
/* delink element from queue */
30423060
prevquery=conn->cmd_queue_head;
30433061
conn->cmd_queue_head=conn->cmd_queue_head->next;
30443062

30453063
/* If the queue is now empty, reset the tail too */
30463064
if (conn->cmd_queue_head==NULL)
30473065
conn->cmd_queue_tail=NULL;
30483066

3049-
/* and makeit recyclable */
3067+
/* and makethe queue element recyclable */
30503068
prevquery->next=NULL;
30513069
pqRecycleCmdQueueEntry(conn,prevquery);
30523070
}
@@ -3070,6 +3088,7 @@ pqPipelineProcessQueue(PGconn *conn)
30703088
return;
30713089

30723090
casePGASYNC_IDLE:
3091+
30733092
/*
30743093
* If we're in IDLE mode and there's some command in the queue,
30753094
* get us into PIPELINE_IDLE mode and process normally. Otherwise

‎src/interfaces/libpq/fe-protocol3.c‎

Lines changed: 6 additions & 10 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;
@@ -287,19 +282,20 @@ pqParseInput3(PGconn *conn)
287282
/* Nothing to do for this message type */
288283
break;
289284
case'3':/* Close Complete */
285+
290286
/*
291287
* If we get CloseComplete when waiting for it, consume
292288
* the queue element and keep going. A result is not
293-
* expected from this message; it is just there so that
294-
*weknow to wait for it when PQsendQuery is used in
289+
* expected from this message; it is just there so that we
290+
* know to wait for it when PQsendQuery is used in
295291
* pipeline mode, before going in IDLE state. Failing to
296292
* do this makes us receive CloseComplete when IDLE, which
297293
* creates problems.
298294
*/
299295
if (conn->cmd_queue_head&&
300296
conn->cmd_queue_head->queryclass==PGQUERY_CLOSE)
301297
{
302-
pqCommandQueueAdvance(conn);
298+
pqCommandQueueAdvance(conn, false, false);
303299
}
304300

305301
break;

‎src/interfaces/libpq/libpq-int.h‎

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -656,7 +656,8 @@ extern void pqSaveMessageField(PGresult *res, char code,
656656
externvoidpqSaveParameterStatus(PGconn*conn,constchar*name,
657657
constchar*value);
658658
externintpqRowProcessor(PGconn*conn,constchar**errmsgp);
659-
externvoidpqCommandQueueAdvance(PGconn*conn);
659+
externvoidpqCommandQueueAdvance(PGconn*conn,boolisReadyForQuery,
660+
boolgotSync);
660661
externintPQsendQueryContinue(PGconn*conn,constchar*query);
661662

662663
/* === in fe-protocol3.c === */

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