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michaelpqmichaelpq
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Handle better implicit transaction state of pipeline mode
When using a pipeline, a transaction starts from the first command and is committed with a Sync message or when the pipeline ends. Functions like IsInTransactionBlock() or PreventInTransactionBlock() were already able to understand a pipeline as being in a transaction block, but it was not the case of CheckTransactionBlock(). This function is called for example to generate a WARNING for SET LOCAL, complaining that it is used outside of a transaction block. The current state of the code caused multiple problems, like: - SET LOCAL executed at any stage of a pipeline issued a WARNING, even if the command was at least second in line where the pipeline is in a transaction state. - LOCK TABLE failed when invoked at any step of a pipeline, even if it should be able to work within a transaction block. The pipeline protocol assumes that the first command of a pipeline is not part of a transaction block, and that any follow-up commands is considered as within a transaction block. This commit changes the backend so as an implicit transaction block is started each time the first Execute message of a pipeline has finished processing, with this implicit transaction block ended once a sync is processed. The checks based on XACT_FLAGS_PIPELINING in the routines checking if we are in a transaction block are not necessary: it is enough to rely on the existing ones. Some tests are added to pgbench, that can be backpatched down to v17 when \syncpipeline is involved and down to v14 where \startpipeline and \endpipeline are available. This is unfortunately limited regarding the error patterns that can be checked, but it provides coverage for various pipeline combinations to check if these succeed or fail. These tests are able to capture the case of SET LOCAL's WARNING. The author has proposed a different feature to improve the coverage by adding similar meta-commands to psql where error messages could be checked, something more useful for the cases where commands cannot be used in transaction blocks, like REINDEX CONCURRENTLY or VACUUM. This is considered as future work for v18~. Author: Anthonin Bonnefoy Reviewed-by: Jelte Fennema-Nio, Michael Paquier Discussion: https://postgr.es/m/CAO6_XqrWO8uNBQrSu5r6jh+vTGi5Oiyk4y8yXDORdE2jbzw8xw@mail.gmail.com Backpatch-through: 13
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4 files changed

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‎doc/src/sgml/protocol.sgml

Lines changed: 11 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -1070,16 +1070,17 @@ SELCT 1/0;<!-- this typo is intentional -->
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10711071
<para>
10721072
If the client has not issued an explicit <command>BEGIN</command>,
1073-
then each Sync ordinarily causes an implicit <command>COMMIT</command>
1074-
if the preceding step(s) succeeded, or an
1075-
implicit <command>ROLLBACK</command> if they failed. However, there
1076-
are a few DDL commands (such as <command>CREATE DATABASE</command>)
1077-
that cannot be executed inside a transaction block. If one of
1078-
these is executed in a pipeline, it will fail unless it is the first
1079-
command in the pipeline. Furthermore, upon success it will force an
1080-
immediate commit to preserve database consistency. Thus a Sync
1081-
immediately following one of these commands has no effect except to
1082-
respond with ReadyForQuery.
1073+
then an implicit transaction block is started and each Sync ordinarily
1074+
causes an implicit <command>COMMIT</command> if the preceding step(s)
1075+
succeeded, or an implicit <command>ROLLBACK</command> if they failed.
1076+
This implicit transaction block will only be detected by the server
1077+
when the first command ends without a sync. There are a few DDL
1078+
commands (such as <command>CREATE DATABASE</command>) that cannot be
1079+
executed inside a transaction block. If one of these is executed in a
1080+
pipeline, it will fail unless it is the first command after a Sync.
1081+
Furthermore, upon success it will force an immediate commit to preserve
1082+
database consistency. Thus a Sync immediately following one of these
1083+
commands has no effect except to respond with ReadyForQuery.
10831084
</para>
10841085

10851086
<para>

‎src/backend/access/transam/xact.c

Lines changed: 0 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -3659,16 +3659,6 @@ PreventInTransactionBlock(bool isTopLevel, const char *stmtType)
36593659
errmsg("%s cannot run inside a subtransaction",
36603660
stmtType)));
36613661

3662-
/*
3663-
* inside a pipeline that has started an implicit transaction?
3664-
*/
3665-
if (MyXactFlags & XACT_FLAGS_PIPELINING)
3666-
ereport(ERROR,
3667-
(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
3668-
/* translator: %s represents an SQL statement name */
3669-
errmsg("%s cannot be executed within a pipeline",
3670-
stmtType)));
3671-
36723662
/*
36733663
* inside a function call?
36743664
*/
@@ -3780,9 +3770,6 @@ IsInTransactionBlock(bool isTopLevel)
37803770
if (IsSubTransaction())
37813771
return true;
37823772

3783-
if (MyXactFlags & XACT_FLAGS_PIPELINING)
3784-
return true;
3785-
37863773
if (!isTopLevel)
37873774
return true;
37883775

‎src/backend/tcop/postgres.c

Lines changed: 18 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2780,6 +2780,17 @@ start_xact_command(void)
27802780

27812781
xact_started = true;
27822782
}
2783+
else if (MyXactFlags & XACT_FLAGS_PIPELINING)
2784+
{
2785+
/*
2786+
* When the first Execute message is completed, following commands
2787+
* will be done in an implicit transaction block created via
2788+
* pipelining. The transaction state needs to be updated to an
2789+
* implicit block if we're not already in a transaction block (like
2790+
* one started by an explicit BEGIN).
2791+
*/
2792+
BeginImplicitTransactionBlock();
2793+
}
27832794

27842795
/*
27852796
* Start statement timeout if necessary. Note that this'll intentionally
@@ -4991,6 +5002,13 @@ PostgresMain(const char *dbname, const char *username)
49915002

49925003
case PqMsg_Sync:
49935004
pq_getmsgend(&input_message);
5005+
5006+
/*
5007+
* If pipelining was used, we may be in an implicit
5008+
* transaction block. Close it before calling
5009+
* finish_xact_command.
5010+
*/
5011+
EndImplicitTransactionBlock();
49945012
finish_xact_command();
49955013
valgrind_report_error_query("SYNC message");
49965014
send_ready_for_query = true;

‎src/bin/pgbench/t/001_pgbench_with_server.pl

Lines changed: 174 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -968,6 +968,180 @@ sub check_data_state
968968
}
969969
});
970970

971+
# Try SET LOCAL as first pipeline command. This succeeds and the first
972+
# command is not executed inside an implicit transaction block, causing
973+
# a WARNING.
974+
$node->pgbench(
975+
'-t 1 -n -M extended',
976+
0,
977+
[],
978+
[qr{WARNING: SET LOCAL can only be used in transaction blocks}],
979+
'SET LOCAL outside implicit transaction block of pipeline',
980+
{
981+
'001_pgbench_pipeline_set_local_1' => q{
982+
\startpipeline
983+
SET LOCAL statement_timeout='1h';
984+
\endpipeline
985+
}
986+
});
987+
988+
# Try SET LOCAL as second pipeline command. This succeeds and the second
989+
# command does not cause a WARNING to be generated.
990+
$node->pgbench(
991+
'-t 1 -n -M extended',
992+
0,
993+
[],
994+
[qr{^$}],
995+
'SET LOCAL inside implicit transaction block of pipeline',
996+
{
997+
'001_pgbench_pipeline_set_local_2' => q{
998+
\startpipeline
999+
SELECT 1;
1000+
SET LOCAL statement_timeout='1h';
1001+
\endpipeline
1002+
}
1003+
});
1004+
1005+
# Try SET LOCAL with \syncpipeline. This succeeds and the command
1006+
# launched after the sync is outside the implicit transaction block
1007+
# of the pipeline, causing a WARNING.
1008+
$node->pgbench(
1009+
'-t 1 -n -M extended',
1010+
0,
1011+
[],
1012+
[qr{WARNING: SET LOCAL can only be used in transaction blocks}],
1013+
'SET LOCAL and \syncpipeline',
1014+
{
1015+
'001_pgbench_pipeline_set_local_3' => q{
1016+
\startpipeline
1017+
SELECT 1;
1018+
\syncpipeline
1019+
SET LOCAL statement_timeout='1h';
1020+
\endpipeline
1021+
}
1022+
});
1023+
1024+
# Try REINDEX CONCURRENTLY as first pipeline command. This succeeds
1025+
# as the first command is outside the implicit transaction block of
1026+
# a pipeline.
1027+
$node->pgbench(
1028+
'-t 1 -n -M extended',
1029+
0,
1030+
[],
1031+
[],
1032+
'REINDEX CONCURRENTLY outside implicit transaction block of pipeline',
1033+
{
1034+
'001_pgbench_pipeline_reindex_1' => q{
1035+
\startpipeline
1036+
REINDEX TABLE CONCURRENTLY pgbench_accounts;
1037+
SELECT 1;
1038+
\endpipeline
1039+
}
1040+
});
1041+
1042+
# Try REINDEX CONCURRENTLY as second pipeline command. This fails
1043+
# as the second command is inside an implicit transaction block.
1044+
$node->pgbench(
1045+
'-t 1 -n -M extended',
1046+
2,
1047+
[],
1048+
[],
1049+
'error: REINDEX CONCURRENTLY inside implicit transaction block of pipeline',
1050+
{
1051+
'001_pgbench_pipeline_reindex_2' => q{
1052+
\startpipeline
1053+
SELECT 1;
1054+
REINDEX TABLE CONCURRENTLY pgbench_accounts;
1055+
\endpipeline
1056+
}
1057+
});
1058+
1059+
# Try VACUUM as first pipeline command. Like REINDEX CONCURRENTLY, this
1060+
# succeeds as this is outside the implicit transaction block of a pipeline.
1061+
$node->pgbench(
1062+
'-t 1 -n -M extended',
1063+
0,
1064+
[],
1065+
[],
1066+
'VACUUM outside implicit transaction block of pipeline',
1067+
{
1068+
'001_pgbench_pipeline_vacuum_1' => q{
1069+
\startpipeline
1070+
VACUUM pgbench_accounts;
1071+
\endpipeline
1072+
}
1073+
});
1074+
1075+
# Try VACUUM as second pipeline command. This fails, as the second command
1076+
# of a pipeline is inside an implicit transaction block.
1077+
$node->pgbench(
1078+
'-t 1 -n -M extended',
1079+
2,
1080+
[],
1081+
[],
1082+
'error: VACUUM inside implicit transaction block of pipeline',
1083+
{
1084+
'001_pgbench_pipeline_vacuum_2' => q{
1085+
\startpipeline
1086+
SELECT 1;
1087+
VACUUM pgbench_accounts;
1088+
\endpipeline
1089+
}
1090+
});
1091+
1092+
# Try subtransactions in a pipeline. These are forbidden in implicit
1093+
# transaction blocks.
1094+
$node->pgbench(
1095+
'-t 1 -n -M extended',
1096+
2,
1097+
[],
1098+
[],
1099+
'error: subtransactions not allowed in pipeline',
1100+
{
1101+
'001_pgbench_pipeline_subtrans' => q{
1102+
\startpipeline
1103+
SAVEPOINT a;
1104+
SELECT 1;
1105+
ROLLBACK TO SAVEPOINT a;
1106+
SELECT 2;
1107+
\endpipeline
1108+
}
1109+
});
1110+
1111+
# Try LOCK TABLE as first pipeline command. This fails as LOCK is outside
1112+
# an implicit transaction block.
1113+
$node->pgbench(
1114+
'-t 1 -n -M extended',
1115+
2,
1116+
[],
1117+
[],
1118+
'error: LOCK TABLE outside implicit transaction block of pipeline',
1119+
{
1120+
'001_pgbench_pipeline_lock_1' => q{
1121+
\startpipeline
1122+
LOCK pgbench_accounts;
1123+
SELECT 1;
1124+
\endpipeline
1125+
}
1126+
});
1127+
1128+
# Try LOCK TABLE as second pipeline command. This succeeds as LOCK is inside
1129+
# an implicit transaction block.
1130+
$node->pgbench(
1131+
'-t 1 -n -M extended',
1132+
0,
1133+
[],
1134+
[],
1135+
'LOCK TABLE inside implicit transaction block of pipeline',
1136+
{
1137+
'001_pgbench_pipeline_lock_2' => q{
1138+
\startpipeline
1139+
SELECT 1;
1140+
LOCK pgbench_accounts;
1141+
\endpipeline
1142+
}
1143+
});
1144+
9711145
# Working \startpipeline in prepared query mode with serializable
9721146
$node->pgbench(
9731147
'-c4 -t 10 -n -M prepared',

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