On Thu, Apr 14, 2022 at 12:39:46PM -0400, Tom Lane wrote: > Robert Haas <robertmh...@gmail.com> writes: >> On Thu, Apr 14, 2022 at 12:22 PM Nathan Bossart >> <nathandboss...@gmail.com> wrote: >>>> I'd be in favor of a hard break. > >>> Yeah, this is a good point. If we're okay with breaking existing >>> extensions like this, I will work on a patch. > >> I tend to think it's a good idea. > > I've come around to that view as well.
Here is a new patch set that introduces the aforementioned "hard break." I noticed that requests for more LWLocks follow a similar pattern as regular shared memory requests, and I figured that we would want to do something similar for those, but I wasn't sure exactly how to proceed. I saw two options: 1) use shmem_request_hook for both regular requests and LWLock requests or 2) introduce an lwlock_request_hook. My instinct was that option 1 was preferable, but AFAICT this requires introducing a new external variable for inspecting whether the request is made at a valid time. This would be similar to process_shared_preload_libraries_in_progress, which I believe means a determined extension author could easily hack around the request restrictions. I thought option 2 added too much machinery to work around this problem. For now, I haven't made any changes for LWLock requests. What are your thoughts? -- Nathan Bossart Amazon Web Services: https://aws.amazon.com
>From 55af4473761a8bcfe8c2ff940a5df4625a40653a Mon Sep 17 00:00:00 2001 From: Nathan Bossart <nathandboss...@gmail.com> Date: Tue, 12 Apr 2022 14:57:00 -0700 Subject: [PATCH v4 1/2] Fix comments about bgworker registration before MaxBackends initialization Since 6bc8ef0b, InitializeMaxBackends() has used max_worker_processes instead of adapting MaxBackends to the number of background workers registered by modules loaded in shared_preload_libraries (at this time, bgworkers were only static, but gained dynamic capabilities as a matter of supporting parallel queries meaning that a control cap was necessary). Some comments referred to the past registration logic, making them confusing and incorrect, so fix these. Some of the out-of-core modules that could be loaded in this path sometimes like to manipulate dynamically some of the resource-related GUCs for their own needs, this commit adds a note about that. Author: Nathan Bossart Discussion: https://postgr.es/m/20220127181815.GA551692@nathanxps13 --- src/backend/postmaster/postmaster.c | 10 ++++------ src/backend/utils/init/postinit.c | 5 ++--- 2 files changed, 6 insertions(+), 9 deletions(-) diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c index 964a56dec4..ce4007bb2c 100644 --- a/src/backend/postmaster/postmaster.c +++ b/src/backend/postmaster/postmaster.c @@ -1005,10 +1005,8 @@ PostmasterMain(int argc, char *argv[]) LocalProcessControlFile(false); /* - * Register the apply launcher. Since it registers a background worker, - * it needs to be called before InitializeMaxBackends(), and it's probably - * a good idea to call it before any modules had chance to take the - * background worker slots. + * Register the apply launcher. It's probably a good idea to call this + * before any modules had a chance to take the background worker slots. */ ApplyLauncherRegister(); @@ -1029,8 +1027,8 @@ PostmasterMain(int argc, char *argv[]) #endif /* - * Now that loadable modules have had their chance to register background - * workers, calculate MaxBackends. + * Now that loadable modules have had their chance to alter any GUCs, + * calculate MaxBackends. */ InitializeMaxBackends(); diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c index 9139fe895c..a28612b375 100644 --- a/src/backend/utils/init/postinit.c +++ b/src/backend/utils/init/postinit.c @@ -538,9 +538,8 @@ pg_split_opts(char **argv, int *argcp, const char *optstr) /* * Initialize MaxBackends value from config options. * - * This must be called after modules have had the chance to register background - * workers in shared_preload_libraries, and before shared memory size is - * determined. + * This must be called after modules have had the chance to alter GUCs in + * shared_preload_libraries and before shared memory size is determined. * * Note that in EXEC_BACKEND environment, the value is passed down from * postmaster to subprocesses via BackendParameters in SubPostmasterMain; only -- 2.25.1
>From 135c1363d0ff2aaa1886d4ed59016c3b610c375a Mon Sep 17 00:00:00 2001 From: Nathan Bossart <nathandboss...@gmail.com> Date: Mon, 18 Apr 2022 15:25:37 -0700 Subject: [PATCH v4 2/2] Add a new shmem_request_hook hook. Currently, preloaded libraries are expected to request additional shared memory 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. Furthermore, this change restricts shared memory requests by preloaded libraries to this hook. Previously, libraries could request additional shared memory until the size of the main shared memory segment was calculated. Besides decoupling a common library task from _PG_init(), this ensures that shared memory requests are only allowed when MaxBackends is initialized and GUCs should not be changed. Unlike before, we no longer silently ignore requests received at invalid times. Instead, we ERROR if someone tries to request additional shared memory outside of the hook. Authors: Julien Rouhaud, Nathan Bossart Discussion: https://postgr.es/m/20220412210112.GA2065815%40nathanxps13 --- contrib/pg_prewarm/autoprewarm.c | 27 ++++++++++++- .../pg_stat_statements/pg_stat_statements.c | 27 +++++++++---- src/backend/postmaster/postmaster.c | 5 +++ src/backend/storage/ipc/ipci.c | 39 +++++++++++++------ src/include/storage/ipc.h | 2 + src/include/storage/shmem.h | 1 + src/tools/pgindent/typedefs.list | 1 + 7 files changed, 81 insertions(+), 21 deletions(-) diff --git a/contrib/pg_prewarm/autoprewarm.c b/contrib/pg_prewarm/autoprewarm.c index 45e012a63a..14345d060a 100644 --- a/contrib/pg_prewarm/autoprewarm.c +++ b/contrib/pg_prewarm/autoprewarm.c @@ -83,6 +83,7 @@ typedef struct AutoPrewarmSharedState } AutoPrewarmSharedState; void _PG_init(void); +void _PG_fini(void); void autoprewarm_main(Datum main_arg); void autoprewarm_database_main(Datum main_arg); @@ -96,6 +97,8 @@ static void apw_start_database_worker(void); static bool apw_init_shmem(void); static void apw_detach_shmem(int code, Datum arg); static int apw_compare_blockinfo(const void *p, const void *q); +static void autoprewarm_shmem_request(void); +static shmem_request_hook_type prev_shmem_request_hook = NULL; /* Pointer to shared-memory state. */ static AutoPrewarmSharedState *apw_state = NULL; @@ -139,13 +142,35 @@ _PG_init(void) MarkGUCPrefixReserved("pg_prewarm"); - RequestAddinShmemSpace(MAXALIGN(sizeof(AutoPrewarmSharedState))); + prev_shmem_request_hook = shmem_request_hook; + shmem_request_hook = autoprewarm_shmem_request; /* Register autoprewarm worker, if enabled. */ if (autoprewarm) apw_start_leader_worker(); } +/* + * Module unload callback. + */ +void +_PG_fini(void) +{ + shmem_request_hook = prev_shmem_request_hook; +} + +/* + * Requests any additional shared memory required for autoprewarm. + */ +static void +autoprewarm_shmem_request(void) +{ + if (prev_shmem_request_hook) + prev_shmem_request_hook(); + + RequestAddinShmemSpace(MAXALIGN(sizeof(AutoPrewarmSharedState))); +} + /* * Main entry point for the leader autoprewarm process. Per-database workers * have a separate entry point. diff --git a/contrib/pg_stat_statements/pg_stat_statements.c b/contrib/pg_stat_statements/pg_stat_statements.c index df2ce63790..87b75d779e 100644 --- a/contrib/pg_stat_statements/pg_stat_statements.c +++ b/contrib/pg_stat_statements/pg_stat_statements.c @@ -252,6 +252,7 @@ static int exec_nested_level = 0; static int plan_nested_level = 0; /* Saved hook values in case of unload */ +static shmem_request_hook_type prev_shmem_request_hook = NULL; static shmem_startup_hook_type prev_shmem_startup_hook = NULL; static post_parse_analyze_hook_type prev_post_parse_analyze_hook = NULL; static planner_hook_type prev_planner_hook = NULL; @@ -317,6 +318,7 @@ PG_FUNCTION_INFO_V1(pg_stat_statements_1_10); PG_FUNCTION_INFO_V1(pg_stat_statements); PG_FUNCTION_INFO_V1(pg_stat_statements_info); +static void pgss_shmem_request(void); static void pgss_shmem_startup(void); static void pgss_shmem_shutdown(int code, Datum arg); static void pgss_post_parse_analyze(ParseState *pstate, Query *query, @@ -452,17 +454,11 @@ _PG_init(void) MarkGUCPrefixReserved("pg_stat_statements"); - /* - * Request additional shared resources. (These are no-ops if we're not in - * the postmaster process.) We'll allocate or attach to the shared - * resources in pgss_shmem_startup(). - */ - RequestAddinShmemSpace(pgss_memsize()); - RequestNamedLWLockTranche("pg_stat_statements", 1); - /* * Install hooks. */ + prev_shmem_request_hook = shmem_request_hook; + shmem_request_hook = pgss_shmem_request; prev_shmem_startup_hook = shmem_startup_hook; shmem_startup_hook = pgss_shmem_startup; prev_post_parse_analyze_hook = post_parse_analyze_hook; @@ -488,6 +484,7 @@ void _PG_fini(void) { /* Uninstall hooks. */ + shmem_request_hook = prev_shmem_request_hook; shmem_startup_hook = prev_shmem_startup_hook; post_parse_analyze_hook = prev_post_parse_analyze_hook; planner_hook = prev_planner_hook; @@ -498,6 +495,20 @@ _PG_fini(void) ProcessUtility_hook = prev_ProcessUtility; } +/* + * shmem_request hook: request additional shared resources. We'll allocate or + * attach to the shared resources in pgss_shmem_startup(). + */ +static void +pgss_shmem_request(void) +{ + if (prev_shmem_request_hook) + prev_shmem_request_hook(); + + RequestAddinShmemSpace(pgss_memsize()); + RequestNamedLWLockTranche("pg_stat_statements", 1); +} + /* * shmem_startup hook: allocate or attach to shared memory, * then load any pre-existing statistics from file. diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c index ce4007bb2c..57663ddc6a 100644 --- a/src/backend/postmaster/postmaster.c +++ b/src/backend/postmaster/postmaster.c @@ -1032,6 +1032,11 @@ PostmasterMain(int argc, char *argv[]) */ InitializeMaxBackends(); + /* + * Give preloaded libraries a chance to request additional shared memory. + */ + ProcessShmemRequests(); + /* * Now that loadable modules have had their chance to request additional * shared memory, determine the value of any runtime-computed GUCs that diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c index 75e456360b..117aacba7d 100644 --- a/src/backend/storage/ipc/ipci.c +++ b/src/backend/storage/ipc/ipci.c @@ -52,28 +52,45 @@ /* GUCs */ int shared_memory_type = DEFAULT_SHARED_MEMORY_TYPE; +shmem_request_hook_type shmem_request_hook = NULL; shmem_startup_hook_type shmem_startup_hook = NULL; static Size total_addin_request = 0; -static bool addin_request_allowed = true; +static bool addin_request_allowed = false; +/* + * ProcessShmemRequests + * + * Calls to RequestAddinShmemSpace() by preloaded libraries are only allowed in + * the shmem_request_hook. + */ +void +ProcessShmemRequests(void) +{ + Assert(MaxBackends > 0); + + addin_request_allowed = true; + + if (shmem_request_hook) + shmem_request_hook(); + + addin_request_allowed = false; +} /* * RequestAddinShmemSpace * Request that extra shmem space be allocated for use by * a loadable module. * - * This is only useful 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 ERROR. */ void RequestAddinShmemSpace(Size size) { if (IsUnderPostmaster || !addin_request_allowed) - return; /* too late */ + elog(ERROR, "cannot request additional shared memory outside shmem_request_hook"); total_addin_request = add_size(total_addin_request, size); } @@ -83,9 +100,6 @@ RequestAddinShmemSpace(Size size) * * If num_semaphores is not NULL, it will be set to the number of semaphores * required. - * - * Note that this function freezes the additional shared memory request size - * from loadable modules. */ Size CalculateShmemSize(int *num_semaphores) @@ -93,6 +107,8 @@ CalculateShmemSize(int *num_semaphores) Size size; int numSemas; + Assert(!addin_request_allowed); + /* Compute number of semaphores we'll need */ numSemas = ProcGlobalSemas(); numSemas += SpinlockSemas(); @@ -152,8 +168,7 @@ CalculateShmemSize(int *num_semaphores) size = add_size(size, ShmemBackendArraySize()); #endif - /* freeze the addin request size and include it */ - addin_request_allowed = false; + /* include additional requested shmem from preload libraries */ size = add_size(size, total_addin_request); /* might as well round it off to a multiple of a typical page size */ diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h index fade4dbe63..5f2c6683db 100644 --- a/src/include/storage/ipc.h +++ b/src/include/storage/ipc.h @@ -19,6 +19,7 @@ #define IPC_H typedef void (*pg_on_exit_callback) (int code, Datum arg); +typedef void (*shmem_request_hook_type) (void); typedef void (*shmem_startup_hook_type) (void); /*---------- @@ -75,6 +76,7 @@ extern void on_exit_reset(void); extern void check_on_shmem_exit_lists_are_empty(void); /* ipci.c */ +extern PGDLLIMPORT shmem_request_hook_type shmem_request_hook; extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook; extern Size CalculateShmemSize(int *num_semaphores); diff --git a/src/include/storage/shmem.h b/src/include/storage/shmem.h index de9e7c6e73..bdb0acfedf 100644 --- a/src/include/storage/shmem.h +++ b/src/include/storage/shmem.h @@ -46,6 +46,7 @@ extern Size add_size(Size s1, Size s2); extern Size mul_size(Size s1, Size s2); /* ipci.c */ +extern void ProcessShmemRequests(void); extern void RequestAddinShmemSpace(Size size); /* size constants for the shmem index table */ diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list index 87ee7bf866..71a97654e0 100644 --- a/src/tools/pgindent/typedefs.list +++ b/src/tools/pgindent/typedefs.list @@ -3549,6 +3549,7 @@ shm_mq_result shm_toc shm_toc_entry shm_toc_estimator +shmem_request_hook_type shmem_startup_hook_type sig_atomic_t sigjmp_buf -- 2.25.1
