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Add a new shmem_request_hook hook.

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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
This commit is contained in:
Robert Haas
2022-05-13 09:31:06 -04:00
parent 8c8d307f82
commit 4f2400cb3f
9 changed files with 81 additions and 38 deletions
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18
src/backend/storage/lmgr/lwlock.c
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@@ -243,8 +243,6 @@ int NamedLWLockTrancheRequests = 0;
/* points to data in shared memory: */
NamedLWLockTranche *NamedLWLockTrancheArray = NULL;
static bool lock_named_request_allowed = true;
static void InitializeLWLocks(void);
static inline void LWLockReportWaitStart(LWLock *lock);
static inline void LWLockReportWaitEnd(void);
@@ -458,9 +456,6 @@ LWLockShmemSize(void)
for (i = 0; i < NamedLWLockTrancheRequests; i++)
size = add_size(size, strlen(NamedLWLockTrancheRequestArray[i].tranche_name) + 1);
/* Disallow adding any more named tranches. */
lock_named_request_allowed = false;
return size;
}
@@ -691,12 +686,9 @@ LWLockRegisterTranche(int tranche_id, const char *tranche_name)
* Request that extra LWLocks be allocated during postmaster
* startup.
*
* This is only useful for extensions if called from the _PG_init hook
* of a library that is loaded into the postmaster via
* shared_preload_libraries. Once shared memory has been allocated, calls
* will be ignored. (We could raise an error, but it seems better to make
* it a no-op, so that libraries containing such calls can be reloaded if
* needed.)
* This may only be called via the shmem_request_hook of a library that is
* loaded into the postmaster via shared_preload_libraries. Calls from
* elsewhere will fail.
*
* The tranche name will be user-visible as a wait event name, so try to
* use a name that fits the style for those.
@@ -706,8 +698,8 @@ RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
{
NamedLWLockTrancheRequest *request;
if (IsUnderPostmaster || !lock_named_request_allowed)
return; /* too late */
if (!process_shmem_requests_in_progress)
elog(FATAL, "cannot request additional LWLocks outside shmem_request_hook");
if (NamedLWLockTrancheRequestArray == NULL)
{