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Commit0c39c29

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Support "postgres -C" with runtime-computed GUCs
Until now, the -C option of postgres was handled before a small subsetof GUCs computed at runtime are initialized, leading to incorrectresults as GUC machinery would fall back to default values for suchparameters.For example, data_checksums could report "off" for a cluster as thecontrol file is not loaded yet. Or wal_segment_size would show asegment size at 16MB even if initdb --wal-segsize used something else.Worse, the command would fail to properly report the recently-introducedshared_memory, that requires to load shared_preload_libraries as thesecould ask for a chunk of shared memory.Support for runtime GUCs comes with a limitation, as the operation isnow allowed on a running server. One notable reason for this is that_PG_init() functions of loadable libraries are called before allruntime-computed GUCs are initialized, and this is not guaranteed to besafe to do on running servers. For the case of shared_memory_size,where we want to know how much memory would be used without allocatingit, this limitation is fine. Another case where this will help is forhuge pages, with the introduction of a different GUC to evaluate theamount of huge pages required for a server before starting it, withouthaving to allocate large chunks of memory.This feature is controlled with a new GUC flag, and four parameters areclassified as runtime-computed as of this change:- data_checksums- shared_memory_size- data_directory_mode- wal_segment_sizeSome TAP tests are added to provide some coverage here, usingdata_checksums in the tests of pg_checksums.Per discussion with Andres Freund, Justin Pryzby, Magnus Hagander andmore.Author: Nathan BossartDiscussion:https://postgr.es/m/F2772387-CE0F-46BF-B5F1-CC55516EB885@amazon.com
1 parent2c7615f commit0c39c29

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5 files changed

+83
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lines changed

5 files changed

+83
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lines changed

‎doc/src/sgml/ref/postgres-ref.sgml

Lines changed: 9 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -133,13 +133,20 @@ PostgreSQL documentation
133133
<listitem>
134134
<para>
135135
Prints the value of the named run-time parameter, and exits.
136-
(See the <option>-c</option> option above for details.) This can
137-
be used on a running server, andreturns values from
136+
(See the <option>-c</option> option above for details.) This
137+
returns values from
138138
<filename>postgresql.conf</filename>, modified by any parameters
139139
supplied in this invocation. It does not reflect parameters
140140
supplied when the cluster was started.
141141
</para>
142142

143+
<para>
144+
This can be used on a running server for most parameters. However,
145+
the server must be shut down for some runtime-computed parameters
146+
(e.g., <xref linkend="guc-shared-memory-size"/> and
147+
<xref linkend="guc-wal-segment-size"/>).
148+
</para>
149+
143150
<para>
144151
This option is meant for other programs that interact with a server
145152
instance, such as <xref linkend="app-pg-ctl"/>, to query configuration

‎src/backend/postmaster/postmaster.c

Lines changed: 43 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -896,15 +896,31 @@ PostmasterMain(int argc, char *argv[])
896896
if (output_config_variable!=NULL)
897897
{
898898
/*
899-
* "-C guc" was specified, so print GUC's value and exit. No extra
900-
* permission check is needed because the user is reading inside the
901-
* data dir.
899+
* If this is a runtime-computed GUC, it hasn't yet been initialized,
900+
* and the present value is not useful. However, this is a convenient
901+
* place to print the value for most GUCs because it is safe to run
902+
* postmaster startup to this point even if the server is already
903+
* running. For the handful of runtime-computed GUCs that we cannot
904+
* provide meaningful values for yet, we wait until later in
905+
* postmaster startup to print the value. We won't be able to use -C
906+
* on running servers for those GUCs, but using this option now would
907+
* lead to incorrect results for them.
902908
*/
903-
constchar*config_val=GetConfigOption(output_config_variable,
904-
false, false);
909+
intflags=GetConfigOptionFlags(output_config_variable, true);
905910

906-
puts(config_val ?config_val :"");
907-
ExitPostmaster(0);
911+
if ((flags&GUC_RUNTIME_COMPUTED)==0)
912+
{
913+
/*
914+
* "-C guc" was specified, so print GUC's value and exit. No
915+
* extra permission check is needed because the user is reading
916+
* inside the data dir.
917+
*/
918+
constchar*config_val=GetConfigOption(output_config_variable,
919+
false, false);
920+
921+
puts(config_val ?config_val :"");
922+
ExitPostmaster(0);
923+
}
908924
}
909925

910926
/* Verify that DataDir looks reasonable */
@@ -1033,6 +1049,26 @@ PostmasterMain(int argc, char *argv[])
10331049
*/
10341050
InitializeShmemGUCs();
10351051

1052+
/*
1053+
* If -C was specified with a runtime-computed GUC, we held off printing
1054+
* the value earlier, as the GUC was not yet initialized. We handle -C
1055+
* for most GUCs before we lock the data directory so that the option may
1056+
* be used on a running server. However, a handful of GUCs are runtime-
1057+
* computed and do not have meaningful values until after locking the data
1058+
* directory, and we cannot safely calculate their values earlier on a
1059+
* running server. At this point, such GUCs should be properly
1060+
* initialized, and we haven't yet set up shared memory, so this is a good
1061+
* time to handle the -C option for these special GUCs.
1062+
*/
1063+
if (output_config_variable!=NULL)
1064+
{
1065+
constchar*config_val=GetConfigOption(output_config_variable,
1066+
false, false);
1067+
1068+
puts(config_val ?config_val :"");
1069+
ExitPostmaster(0);
1070+
}
1071+
10361072
/*
10371073
* Set up shared memory and semaphores.
10381074
*/

‎src/backend/utils/misc/guc.c

Lines changed: 4 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -1983,7 +1983,7 @@ static struct config_bool ConfigureNamesBool[] =
19831983
{"data_checksums",PGC_INTERNAL,PRESET_OPTIONS,
19841984
gettext_noop("Shows whether data checksums are turned on for this cluster."),
19851985
NULL,
1986-
GUC_NOT_IN_SAMPLE |GUC_DISALLOW_IN_FILE
1986+
GUC_NOT_IN_SAMPLE |GUC_DISALLOW_IN_FILE |GUC_RUNTIME_COMPUTED
19871987
},
19881988
&data_checksums,
19891989
false,
@@ -2342,7 +2342,7 @@ static struct config_int ConfigureNamesInt[] =
23422342
{"shared_memory_size",PGC_INTERNAL,PRESET_OPTIONS,
23432343
gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
23442344
NULL,
2345-
GUC_NOT_IN_SAMPLE |GUC_DISALLOW_IN_FILE |GUC_UNIT_MB
2345+
GUC_NOT_IN_SAMPLE |GUC_DISALLOW_IN_FILE |GUC_UNIT_MB |GUC_RUNTIME_COMPUTED
23462346
},
23472347
&shared_memory_size_mb,
23482348
0,0,INT_MAX,
@@ -2407,7 +2407,7 @@ static struct config_int ConfigureNamesInt[] =
24072407
"in the form accepted by the chmod and umask system "
24082408
"calls. (To use the customary octal format the number "
24092409
"must start with a 0 (zero).)"),
2410-
GUC_NOT_IN_SAMPLE |GUC_DISALLOW_IN_FILE
2410+
GUC_NOT_IN_SAMPLE |GUC_DISALLOW_IN_FILE |GUC_RUNTIME_COMPUTED
24112411
},
24122412
&data_directory_mode,
24132413
0700,0000,0777,
@@ -3231,7 +3231,7 @@ static struct config_int ConfigureNamesInt[] =
32313231
{"wal_segment_size",PGC_INTERNAL,PRESET_OPTIONS,
32323232
gettext_noop("Shows the size of write ahead log segments."),
32333233
NULL,
3234-
GUC_UNIT_BYTE |GUC_NOT_IN_SAMPLE |GUC_DISALLOW_IN_FILE
3234+
GUC_UNIT_BYTE |GUC_NOT_IN_SAMPLE |GUC_DISALLOW_IN_FILE |GUC_RUNTIME_COMPUTED
32353235
},
32363236
&wal_segment_size,
32373237
DEFAULT_XLOG_SEG_SIZE,

‎src/bin/pg_checksums/t/002_actions.pl

Lines changed: 21 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -10,7 +10,7 @@
1010
use TestLib;
1111

1212
use Fcntlqw(:seek);
13-
use Test::Moretests=>63;
13+
use Test::Moretests=>69;
1414

1515

1616
# Utility routine to create and check a table with corrupted checksums
@@ -178,11 +178,31 @@ sub check_relation_corruption
178178
['pg_checksums','--enable','--filenode','1234','-D',$pgdata ],
179179
"fails when relfilenodes are requested and action is --enable");
180180

181+
# Test postgres -C for an offline cluster.
182+
# Run-time GUCs are safe to query here. Note that a lock file is created,
183+
# then unlinked, leading to an extra LOG entry showing in stderr.
184+
command_checks_all(
185+
['postgres','-D',$pgdata,'-C','data_checksums' ],
186+
0,
187+
[qr/^on$/],
188+
# LOG entry when unlinking lock file.
189+
[qr/database system is shut down/],
190+
'data_checksums=on is reported on an offline cluster');
191+
181192
# Checks cannot happen with an online cluster
182193
$node->start;
183194
command_fails(['pg_checksums','--check','-D',$pgdata ],
184195
"fails with online cluster");
185196

197+
# Test postgres -C on an online cluster.
198+
command_fails_like(
199+
['postgres','-D',$pgdata,'-C','data_checksums' ],
200+
qr/lock file .* already exists/,
201+
'data_checksums is not reported on an online cluster');
202+
command_ok(
203+
['postgres','-D',$pgdata,'-C','work_mem' ],
204+
'non-runtime parameter is reported on an online cluster');
205+
186206
# Check corruption of table on default tablespace.
187207
check_relation_corruption($node,'corrupt1','pg_default');
188208

‎src/include/utils/guc.h

Lines changed: 6 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -229,6 +229,12 @@ typedef enum
229229

230230
#defineGUC_EXPLAIN 0x100000/* include in explain */
231231

232+
/*
233+
* GUC_RUNTIME_COMPUTED is intended for runtime-computed GUCs that are only
234+
* available via 'postgres -C' if the server is not running.
235+
*/
236+
#defineGUC_RUNTIME_COMPUTED 0x200000
237+
232238
#defineGUC_UNIT(GUC_UNIT_MEMORY | GUC_UNIT_TIME)
233239

234240

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