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robertmhaasrobertmhaas
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Add a new shmem_request_hook hook.
Currently, preloaded libraries are expected to request additional shared memory and LWLocks in _PG_init(). However, it is not unusal for such requests to depend on MaxBackends, which won't be initialized at that time. Such requests could also depend on GUCs that other modules might change. This introduces a new hook where modules can safely use MaxBackends and GUCs to request additional shared memory and LWLocks. Furthermore, this change restricts requests for shared memory and LWLocks to this hook. Previously, libraries could make requests until the size of the main shared memory segment was calculated. Unlike before, we no longer silently ignore requests received at invalid times. Instead, we FATAL if someone tries to request additional shared memory or LWLocks outside of the hook. Nathan Bossart and Julien Rouhaud Discussion: https://postgr.es/m/20220412210112.GA2065815%40nathanxps13 Discussion: https://postgr.es/m/Yn2jE/lmDhKtkUdr@paquier.xyz
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9 files changed

+81
-38
lines changed

9 files changed

+81
-38
lines changed

‎contrib/pg_prewarm/autoprewarm.c

Lines changed: 16 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -96,6 +96,8 @@ static void apw_start_database_worker(void);
9696
static bool apw_init_shmem(void);
9797
static void apw_detach_shmem(int code, Datum arg);
9898
static int apw_compare_blockinfo(const void *p, const void *q);
99+
static void autoprewarm_shmem_request(void);
100+
static shmem_request_hook_type prev_shmem_request_hook = NULL;
99101

100102
/* Pointer to shared-memory state. */
101103
static AutoPrewarmSharedState *apw_state = NULL;
@@ -139,13 +141,26 @@ _PG_init(void)
139141

140142
MarkGUCPrefixReserved("pg_prewarm");
141143

142-
RequestAddinShmemSpace(MAXALIGN(sizeof(AutoPrewarmSharedState)));
144+
prev_shmem_request_hook = shmem_request_hook;
145+
shmem_request_hook = autoprewarm_shmem_request;
143146

144147
/* Register autoprewarm worker, if enabled. */
145148
if (autoprewarm)
146149
apw_start_leader_worker();
147150
}
148151

152+
/*
153+
* Requests any additional shared memory required for autoprewarm.
154+
*/
155+
static void
156+
autoprewarm_shmem_request(void)
157+
{
158+
if (prev_shmem_request_hook)
159+
prev_shmem_request_hook();
160+
161+
RequestAddinShmemSpace(MAXALIGN(sizeof(AutoPrewarmSharedState)));
162+
}
163+
149164
/*
150165
* Main entry point for the leader autoprewarm process. Per-database workers
151166
* have a separate entry point.

‎contrib/pg_stat_statements/pg_stat_statements.c

Lines changed: 18 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -252,6 +252,7 @@ static int exec_nested_level = 0;
252252
static int plan_nested_level = 0;
253253

254254
/* Saved hook values in case of unload */
255+
static shmem_request_hook_type prev_shmem_request_hook = NULL;
255256
static shmem_startup_hook_type prev_shmem_startup_hook = NULL;
256257
static post_parse_analyze_hook_type prev_post_parse_analyze_hook = NULL;
257258
static planner_hook_type prev_planner_hook = NULL;
@@ -316,6 +317,7 @@ PG_FUNCTION_INFO_V1(pg_stat_statements_1_10);
316317
PG_FUNCTION_INFO_V1(pg_stat_statements);
317318
PG_FUNCTION_INFO_V1(pg_stat_statements_info);
318319

320+
static void pgss_shmem_request(void);
319321
static void pgss_shmem_startup(void);
320322
static void pgss_shmem_shutdown(int code, Datum arg);
321323
static void pgss_post_parse_analyze(ParseState *pstate, Query *query,
@@ -451,17 +453,11 @@ _PG_init(void)
451453

452454
MarkGUCPrefixReserved("pg_stat_statements");
453455

454-
/*
455-
* Request additional shared resources. (These are no-ops if we're not in
456-
* the postmaster process.) We'll allocate or attach to the shared
457-
* resources in pgss_shmem_startup().
458-
*/
459-
RequestAddinShmemSpace(pgss_memsize());
460-
RequestNamedLWLockTranche("pg_stat_statements", 1);
461-
462456
/*
463457
* Install hooks.
464458
*/
459+
prev_shmem_request_hook = shmem_request_hook;
460+
shmem_request_hook = pgss_shmem_request;
465461
prev_shmem_startup_hook = shmem_startup_hook;
466462
shmem_startup_hook = pgss_shmem_startup;
467463
prev_post_parse_analyze_hook = post_parse_analyze_hook;
@@ -480,6 +476,20 @@ _PG_init(void)
480476
ProcessUtility_hook = pgss_ProcessUtility;
481477
}
482478

479+
/*
480+
* shmem_request hook: request additional shared resources. We'll allocate or
481+
* attach to the shared resources in pgss_shmem_startup().
482+
*/
483+
static void
484+
pgss_shmem_request(void)
485+
{
486+
if (prev_shmem_request_hook)
487+
prev_shmem_request_hook();
488+
489+
RequestAddinShmemSpace(pgss_memsize());
490+
RequestNamedLWLockTranche("pg_stat_statements", 1);
491+
}
492+
483493
/*
484494
* shmem_startup hook: allocate or attach to shared memory,
485495
* then load any pre-existing statistics from file.

‎doc/src/sgml/xfunc.sgml

Lines changed: 10 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -3402,22 +3402,30 @@ CREATE FUNCTION make_array(anyelement) RETURNS anyarray
34023402
startup. The add-in's shared library must be preloaded by specifying
34033403
it in
34043404
<xref linkend="guc-shared-preload-libraries"/><indexterm><primary>shared_preload_libraries</primary></indexterm>.
3405+
The shared library should register a <literal>shmem_request_hook</literal>
3406+
in its <function>_PG_init</function> function. This
3407+
<literal>shmem_request_hook</literal> can reserve LWLocks or shared memory.
34053408
Shared memory is reserved by calling:
34063409
<programlisting>
34073410
void RequestAddinShmemSpace(int size)
34083411
</programlisting>
3409-
from your <function>_PG_init</function> function.
3412+
from your <literal>shmem_request_hook</literal>.
34103413
</para>
34113414
<para>
34123415
LWLocks are reserved by calling:
34133416
<programlisting>
34143417
void RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
34153418
</programlisting>
3416-
from <function>_PG_init</function>. This will ensure that an array of
3419+
from your <literal>shmem_request_hook</literal>. This will ensure that an array of
34173420
<literal>num_lwlocks</literal> LWLocks is available under the name
34183421
<literal>tranche_name</literal>. Use <function>GetNamedLWLockTranche</function>
34193422
to get a pointer to this array.
34203423
</para>
3424+
<para>
3425+
An example of a <literal>shmem_request_hook</literal> can be found in
3426+
<filename>contrib/pg_stat_statements/pg_stat_statements.c</filename> in the
3427+
<productname>PostgreSQL</productname> source tree.
3428+
</para>
34213429
<para>
34223430
To avoid possible race-conditions, each backend should use the LWLock
34233431
<function>AddinShmemInitLock</function> when connecting to and initializing

‎src/backend/postmaster/postmaster.c

Lines changed: 5 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1042,6 +1042,11 @@ PostmasterMain(int argc, char *argv[])
10421042
*/
10431043
InitializeMaxBackends();
10441044

1045+
/*
1046+
* Give preloaded libraries a chance to request additional shared memory.
1047+
*/
1048+
process_shmem_requests();
1049+
10451050
/*
10461051
* Now that loadable modules have had their chance to request additional
10471052
* shared memory, determine the value of any runtime-computed GUCs that

‎src/backend/storage/ipc/ipci.c

Lines changed: 6 additions & 14 deletions
Original file line numberDiff line numberDiff line change
@@ -55,25 +55,21 @@ int shared_memory_type = DEFAULT_SHARED_MEMORY_TYPE;
5555
shmem_startup_hook_type shmem_startup_hook = NULL;
5656

5757
static Size total_addin_request = 0;
58-
static bool addin_request_allowed = true;
59-
6058

6159
/*
6260
* RequestAddinShmemSpace
6361
* Request that extra shmem space be allocated for use by
6462
* a loadable module.
6563
*
66-
* This is only useful if called from the _PG_init hook of a library that
67-
* is loaded into the postmaster via shared_preload_libraries. Once
68-
* shared memory has been allocated, calls will be ignored. (We could
69-
* raise an error, but it seems better to make it a no-op, so that
70-
* libraries containing such calls can be reloaded if needed.)
64+
* This may only be called via the shmem_request_hook of a library that is
65+
* loaded into the postmaster via shared_preload_libraries. Calls from
66+
* elsewhere will fail.
7167
*/
7268
void
7369
RequestAddinShmemSpace(Size size)
7470
{
75-
if (IsUnderPostmaster || !addin_request_allowed)
76-
return; /* too late */
71+
if (!process_shmem_requests_in_progress)
72+
elog(FATAL, "cannot request additional shared memory outside shmem_request_hook");
7773
total_addin_request = add_size(total_addin_request, size);
7874
}
7975

@@ -83,9 +79,6 @@ RequestAddinShmemSpace(Size size)
8379
*
8480
* If num_semaphores is not NULL, it will be set to the number of semaphores
8581
* required.
86-
*
87-
* Note that this function freezes the additional shared memory request size
88-
* from loadable modules.
8982
*/
9083
Size
9184
CalculateShmemSize(int *num_semaphores)
@@ -152,8 +145,7 @@ CalculateShmemSize(int *num_semaphores)
152145
size = add_size(size, ShmemBackendArraySize());
153146
#endif
154147

155-
/* freeze the addin request size and include it */
156-
addin_request_allowed = false;
148+
/* include additional requested shmem from preload libraries */
157149
size = add_size(size, total_addin_request);
158150

159151
/* might as well round it off to a multiple of a typical page size */

‎src/backend/storage/lmgr/lwlock.c

Lines changed: 5 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -243,8 +243,6 @@ int NamedLWLockTrancheRequests = 0;
243243
/* points to data in shared memory: */
244244
NamedLWLockTranche *NamedLWLockTrancheArray = NULL;
245245

246-
static bool lock_named_request_allowed = true;
247-
248246
static void InitializeLWLocks(void);
249247
static inline void LWLockReportWaitStart(LWLock *lock);
250248
static inline void LWLockReportWaitEnd(void);
@@ -458,9 +456,6 @@ LWLockShmemSize(void)
458456
for (i = 0; i < NamedLWLockTrancheRequests; i++)
459457
size = add_size(size, strlen(NamedLWLockTrancheRequestArray[i].tranche_name) + 1);
460458

461-
/* Disallow adding any more named tranches. */
462-
lock_named_request_allowed = false;
463-
464459
return size;
465460
}
466461

@@ -691,12 +686,9 @@ LWLockRegisterTranche(int tranche_id, const char *tranche_name)
691686
* Request that extra LWLocks be allocated during postmaster
692687
* startup.
693688
*
694-
* This is only useful for extensions if called from the _PG_init hook
695-
* of a library that is loaded into the postmaster via
696-
* shared_preload_libraries. Once shared memory has been allocated, calls
697-
* will be ignored. (We could raise an error, but it seems better to make
698-
* it a no-op, so that libraries containing such calls can be reloaded if
699-
* needed.)
689+
* This may only be called via the shmem_request_hook of a library that is
690+
* loaded into the postmaster via shared_preload_libraries. Calls from
691+
* elsewhere will fail.
700692
*
701693
* The tranche name will be user-visible as a wait event name, so try to
702694
* use a name that fits the style for those.
@@ -706,8 +698,8 @@ RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
706698
{
707699
NamedLWLockTrancheRequest *request;
708700

709-
if (IsUnderPostmaster || !lock_named_request_allowed)
710-
return; /* too late */
701+
if (!process_shmem_requests_in_progress)
702+
elog(FATAL, "cannot request additional LWLocks outside shmem_request_hook");
711703

712704
if (NamedLWLockTrancheRequestArray == NULL)
713705
{

‎src/backend/utils/init/miscinit.c

Lines changed: 15 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1618,6 +1618,9 @@ char *local_preload_libraries_string = NULL;
16181618
bool process_shared_preload_libraries_in_progress = false;
16191619
bool process_shared_preload_libraries_done = false;
16201620

1621+
shmem_request_hook_type shmem_request_hook = NULL;
1622+
bool process_shmem_requests_in_progress = false;
1623+
16211624
/*
16221625
* load the shared libraries listed in 'libraries'
16231626
*
@@ -1701,6 +1704,18 @@ process_session_preload_libraries(void)
17011704
true);
17021705
}
17031706

1707+
/*
1708+
* process any shared memory requests from preloaded libraries
1709+
*/
1710+
void
1711+
process_shmem_requests(void)
1712+
{
1713+
process_shmem_requests_in_progress = true;
1714+
if (shmem_request_hook)
1715+
shmem_request_hook();
1716+
process_shmem_requests_in_progress = false;
1717+
}
1718+
17041719
void
17051720
pg_bindtextdomain(const char *domain)
17061721
{

‎src/include/miscadmin.h

Lines changed: 5 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -465,6 +465,7 @@ extern void BaseInit(void);
465465
extern PGDLLIMPORT bool IgnoreSystemIndexes;
466466
extern PGDLLIMPORT bool process_shared_preload_libraries_in_progress;
467467
extern PGDLLIMPORT bool process_shared_preload_libraries_done;
468+
extern PGDLLIMPORT bool process_shmem_requests_in_progress;
468469
extern PGDLLIMPORT char *session_preload_libraries_string;
469470
extern PGDLLIMPORT char *shared_preload_libraries_string;
470471
extern PGDLLIMPORT char *local_preload_libraries_string;
@@ -478,9 +479,13 @@ extern bool RecheckDataDirLockFile(void);
478479
extern void ValidatePgVersion(const char *path);
479480
extern void process_shared_preload_libraries(void);
480481
extern void process_session_preload_libraries(void);
482+
extern void process_shmem_requests(void);
481483
extern void pg_bindtextdomain(const char *domain);
482484
extern bool has_rolreplication(Oid roleid);
483485

486+
typedef void (*shmem_request_hook_type) (void);
487+
extern PGDLLIMPORT shmem_request_hook_type shmem_request_hook;
488+
484489
/* in executor/nodeHash.c */
485490
extern size_t get_hash_memory_limit(void);
486491

‎src/tools/pgindent/typedefs.list

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -3651,6 +3651,7 @@ shm_mq_result
36513651
shm_toc
36523652
shm_toc_entry
36533653
shm_toc_estimator
3654+
shmem_request_hook_type
36543655
shmem_startup_hook_type
36553656
sig_atomic_t
36563657
sigjmp_buf

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