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Commit34aad21

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Client-side fixes for delayed NOTIFY receipt.
PQnotifies() is defined to just process already-read data, not try to readany more from the socket. (This is a debatable decision, perhaps, but I'mhesitant to change longstanding library behavior.) The documentation haslong recommended calling PQconsumeInput() before PQnotifies() to ensurethat any already-arrived message would get absorbed and processed.However, psql did not get that memo, which explains why it's not veryreliable about reporting notifications promptly.Also, most (not quite all) callers called PQconsumeInput() just once beforea PQnotifies() loop. Taking this recommendation seriously implies that weshould do PQconsumeInput() before each call. This is more important nowthat we have "payload" strings in notification messages than it was before;that increases the probability of having more than one packet's worthof notify messages. Hence, adjust code as well as documentation examplesto do it like that.Back-patch to 9.5 to match related server fixes. In principle we couldprobably go back further with these changes, but given lack of fieldcomplaints I doubt it's worthwhile.Discussion:https://postgr.es/m/CAOYf6ec-TmRYjKBXLLaGaB-jrd=mjG1Hzn1a1wufUAR39PQYhw@mail.gmail.com
1 parentcbab940 commit34aad21

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5 files changed

+12
-4
lines changed

5 files changed

+12
-4
lines changed

‎doc/src/sgml/libpq.sgml‎

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -5000,7 +5000,7 @@ typedef struct pgNotify
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<para>
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<function>PQnotifies</function> does not actually read data from the
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server; it just returns messages previously absorbed by another
5003-
<application>libpq</application> function. Inprior releases of
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<application>libpq</application> function. Inancient releases of
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<application>libpq</application>, the only way to ensure timely receipt
50055005
of <command>NOTIFY</> messages was to constantly submit commands, even
50065006
empty ones, and then check <function>PQnotifies</function> after each
@@ -8313,6 +8313,7 @@ main(int argc, char **argv)
83138313
notify->relname, notify->be_pid);
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PQfreemem(notify);
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nnotifies++;
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PQconsumeInput(conn);
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}
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}
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‎src/bin/psql/common.c‎

Lines changed: 3 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -693,7 +693,8 @@ PrintNotifications(void)
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{
694694
PGnotify*notify;
695695

696-
while ((notify=PQnotifies(pset.db)))
696+
PQconsumeInput(pset.db);
697+
while ((notify=PQnotifies(pset.db))!=NULL)
697698
{
698699
/* for backward compatibility, only show payload if nonempty */
699700
if (notify->extra[0])
@@ -704,6 +705,7 @@ PrintNotifications(void)
704705
notify->relname,notify->be_pid);
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fflush(pset.queryFout);
706707
PQfreemem(notify);
708+
PQconsumeInput(pset.db);
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}
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}
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‎src/interfaces/ecpg/ecpglib/execute.c‎

Lines changed: 3 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1730,12 +1730,13 @@ ecpg_process_output(struct statement * stmt, bool clear_result)
17301730
}
17311731

17321732
/* check for asynchronous returns */
1733-
notify=PQnotifies(stmt->connection->connection);
1734-
if (notify)
1733+
PQconsumeInput(stmt->connection->connection);
1734+
while ((notify=PQnotifies(stmt->connection->connection))!=NULL)
17351735
{
17361736
ecpg_log("ecpg_process_output on line %d: asynchronous notification of \"%s\" from backend PID %d received\n",
17371737
stmt->lineno,notify->relname,notify->be_pid);
17381738
PQfreemem(notify);
1739+
PQconsumeInput(stmt->connection->connection);
17391740
}
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returnstatus;

‎src/interfaces/libpq/fe-exec.c‎

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2239,6 +2239,9 @@ PQsendDescribe(PGconn *conn, char desc_type, const char *desc_target)
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* no unhandled async notification from the backend
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*
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* the CALLER is responsible for FREE'ing the structure returned
2242+
*
2243+
* Note that this function does not read any new data from the socket;
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* so usually, caller should call PQconsumeInput() first.
22422245
*/
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PGnotify*
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PQnotifies(PGconn*conn)

‎src/test/examples/testlibpq2.c‎

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -140,6 +140,7 @@ main(int argc, char **argv)
140140
notify->relname,notify->be_pid);
141141
PQfreemem(notify);
142142
nnotifies++;
143+
PQconsumeInput(conn);
143144
}
144145
}
145146

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