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Allow background workers to be started dynamically.
There is a new API, RegisterDynamicBackgroundWorker, which allows an ordinary user backend to register a new background writer during normal running. This means that it's no longer necessary for all background workers to be registered during processing of shared_preload_libraries, although the option of registering workers at that time remains available. When a background worker exits and will not be restarted, the slot previously used by that background worker is automatically released and becomes available for reuse. Slots used by background workers that are configured for automatic restart can't (yet) be released without shutting down the system. This commit adds a new source file, bgworker.c, and moves some of the existing control logic for background workers there. Previously, there was little enough logic that it made sense to keep everything in postmaster.c, but not any more. This commit also makes the worker_spi contrib module into an extension and adds a new function, worker_spi_launch, which can be used to demonstrate the new facility.
This commit is contained in:
Robert Haas
483
src/backend/postmaster/bgworker.c
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483
src/backend/postmaster/bgworker.c
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/*--------------------------------------------------------------------
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* bgworker.c
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* POSTGRES pluggable background workers implementation
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*
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* Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
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*
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* IDENTIFICATION
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* src/backend/postmaster/bgworker.c
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*
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*-------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include "miscadmin.h"
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#include "postmaster/bgworker_internals.h"
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#include "storage/barrier.h"
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#include "storage/lwlock.h"
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#include "storage/pmsignal.h"
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#include "storage/shmem.h"
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#include "utils/ascii.h"
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/*
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* The postmaster's list of registered background workers, in private memory.
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*/
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slist_head BackgroundWorkerList = SLIST_STATIC_INIT(BackgroundWorkerList);
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/*
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* BackgroundWorkerSlots exist in shared memory and can be accessed (via
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* the BackgroundWorkerArray) by both the postmaster and by regular backends.
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* However, the postmaster cannot take locks, even spinlocks, because this
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* might allow it to crash or become wedged if shared memory gets corrupted.
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* Such an outcome is intolerable. Therefore, we need a lockless protocol
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* for coordinating access to this data.
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*
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* The 'in_use' flag is used to hand off responsibility for the slot between
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* the postmaster and the rest of the system. When 'in_use' is false,
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* the postmaster will ignore the slot entirely, except for the 'in_use' flag
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* itself, which it may read. In this state, regular backends may modify the
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* slot. Once a backend sets 'in_use' to true, the slot becomes the
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* responsibility of the postmaster. Regular backends may no longer modify it,
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* but the postmaster may examine it. Thus, a backend initializing a slot
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* must fully initialize the slot - and insert a write memory barrier - before
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* marking it as in use.
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*
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* In addition to coordinating with the postmaster, backends modifying this
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* data structure must coordinate with each other. Since they can take locks,
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* this is straightforward: any backend wishing to manipulate a slot must
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* take BackgroundWorkerLock in exclusive mode. Backends wishing to read
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* data that might get concurrently modified by other backends should take
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* this lock in shared mode. No matter what, backends reading this data
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* structure must be able to tolerate concurrent modifications by the
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* postmaster.
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*/
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typedef struct BackgroundWorkerSlot
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{
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bool in_use;
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BackgroundWorker worker;
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} BackgroundWorkerSlot;
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typedef struct BackgroundWorkerArray
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{
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int total_slots;
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BackgroundWorkerSlot slot[FLEXIBLE_ARRAY_MEMBER];
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} BackgroundWorkerArray;
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BackgroundWorkerArray *BackgroundWorkerData;
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/*
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* Calculate shared memory needed.
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*/
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Size
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BackgroundWorkerShmemSize(void)
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{
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Size size;
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/* Array of workers is variably sized. */
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size = offsetof(BackgroundWorkerArray, slot);
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size = add_size(size, mul_size(max_worker_processes,
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sizeof(BackgroundWorkerSlot)));
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return size;
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}
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/*
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* Initialize shared memory.
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*/
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void
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BackgroundWorkerShmemInit(void)
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{
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bool found;
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BackgroundWorkerData = ShmemInitStruct("Background Worker Data",
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BackgroundWorkerShmemSize(),
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&found);
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if (!IsUnderPostmaster)
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{
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slist_iter siter;
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int slotno = 0;
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BackgroundWorkerData->total_slots = max_worker_processes;
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/*
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* Copy contents of worker list into shared memory. Record the
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* shared memory slot assigned to each worker. This ensures
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* a 1-to-1 correspondence betwen the postmaster's private list and
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* the array in shared memory.
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*/
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slist_foreach(siter, &BackgroundWorkerList)
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{
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BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
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RegisteredBgWorker *rw;
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rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
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Assert(slotno < max_worker_processes);
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slot->in_use = true;
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rw->rw_shmem_slot = slotno;
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memcpy(&slot->worker, &rw->rw_worker, sizeof(BackgroundWorker));
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++slotno;
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}
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/*
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* Mark any remaining slots as not in use.
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*/
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while (slotno < max_worker_processes)
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{
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BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
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slot->in_use = false;
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++slotno;
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}
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}
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else
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Assert(found);
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}
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static RegisteredBgWorker *
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FindRegisteredWorkerBySlotNumber(int slotno)
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{
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slist_iter siter;
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/*
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* Copy contents of worker list into shared memory. Record the
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* shared memory slot assigned to each worker. This ensures
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* a 1-to-1 correspondence betwen the postmaster's private list and
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* the array in shared memory.
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*/
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slist_foreach(siter, &BackgroundWorkerList)
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{
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RegisteredBgWorker *rw;
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rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
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if (rw->rw_shmem_slot == slotno)
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return rw;
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}
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return NULL;
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}
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/*
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* Notice changes to shared_memory made by other backends. This code
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* runs in the postmaster, so we must be very careful not to assume that
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* shared memory contents are sane. Otherwise, a rogue backend could take
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* out the postmaster.
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*/
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void
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BackgroundWorkerStateChange(void)
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{
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int slotno;
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/*
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* The total number of slots stored in shared memory should match our
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* notion of max_worker_processes. If it does not, something is very
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* wrong. Further down, we always refer to this value as
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* max_worker_processes, in case shared memory gets corrupted while
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* we're looping.
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*/
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if (max_worker_processes != BackgroundWorkerData->total_slots)
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{
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elog(LOG,
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"inconsistent background worker state (max_worker_processes=%d, total_slots=%d",
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max_worker_processes,
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BackgroundWorkerData->total_slots);
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return;
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}
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/*
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* Iterate through slots, looking for newly-registered workers or
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* workers who must die.
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*/
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for (slotno = 0; slotno < max_worker_processes; ++slotno)
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{
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BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
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RegisteredBgWorker *rw;
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if (!slot->in_use)
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continue;
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/*
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* Make sure we don't see the in_use flag before the updated slot
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* contents.
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*/
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pg_read_barrier();
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/*
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* See whether we already know about this worker. If not, we need
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* to update our backend-private BackgroundWorkerList to match shared
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* memory.
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*/
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rw = FindRegisteredWorkerBySlotNumber(slotno);
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if (rw != NULL)
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continue;
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/*
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* Copy the registration data into the registered workers list.
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*/
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rw = malloc(sizeof(RegisteredBgWorker));
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if (rw == NULL)
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{
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ereport(LOG,
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(errcode(ERRCODE_OUT_OF_MEMORY),
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errmsg("out of memory")));
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return;
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}
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/*
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* Copy strings in a paranoid way. If shared memory is corrupted,
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* the source data might not even be NUL-terminated.
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*/
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ascii_safe_strlcpy(rw->rw_worker.bgw_name,
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slot->worker.bgw_name, BGW_MAXLEN);
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ascii_safe_strlcpy(rw->rw_worker.bgw_library_name,
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slot->worker.bgw_library_name, BGW_MAXLEN);
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ascii_safe_strlcpy(rw->rw_worker.bgw_function_name,
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slot->worker.bgw_function_name, BGW_MAXLEN);
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/*
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* Copy remaining fields.
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*
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* flags, start_time, and restart_time are examined by the
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* postmaster, but nothing too bad will happen if they are
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* corrupted. The remaining fields will only be examined by the
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* child process. It might crash, but we won't.
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*/
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rw->rw_worker.bgw_flags = slot->worker.bgw_flags;
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rw->rw_worker.bgw_start_time = slot->worker.bgw_start_time;
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rw->rw_worker.bgw_restart_time = slot->worker.bgw_restart_time;
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rw->rw_worker.bgw_main = slot->worker.bgw_main;
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rw->rw_worker.bgw_main_arg = slot->worker.bgw_main_arg;
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rw->rw_worker.bgw_sighup = slot->worker.bgw_sighup;
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rw->rw_worker.bgw_sigterm = slot->worker.bgw_sigterm;
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/* Initialize postmaster bookkeeping. */
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rw->rw_backend = NULL;
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rw->rw_pid = 0;
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rw->rw_child_slot = 0;
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rw->rw_crashed_at = 0;
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rw->rw_shmem_slot = slotno;
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/* Log it! */
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ereport(LOG,
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(errmsg("registering background worker: %s",
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rw->rw_worker.bgw_name)));
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slist_push_head(&BackgroundWorkerList, &rw->rw_lnode);
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}
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}
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/*
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* Forget about a background worker that's no longer needed.
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*
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* At present, this only happens when a background worker marked
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* BGW_NEVER_RESTART exits. This function should only be invoked in
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* the postmaster.
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*/
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void
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ForgetBackgroundWorker(RegisteredBgWorker *rw)
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{
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BackgroundWorkerSlot *slot;
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Assert(rw->rw_shmem_slot < max_worker_processes);
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slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
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slot->in_use = false;
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ereport(LOG,
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(errmsg("unregistering background worker: %s",
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rw->rw_worker.bgw_name)));
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slist_delete(&BackgroundWorkerList, &rw->rw_lnode);
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free(rw);
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}
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#ifdef EXEC_BACKEND
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/*
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* In EXEC_BACKEND mode, workers use this to retrieve their details from
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* shared memory.
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*/
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BackgroundWorker *
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BackgroundWorkerEntry(int slotno)
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{
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BackgroundWorkerSlot *slot;
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Assert(slotno < BackgroundWorkerData->total_slots);
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slot = &BackgroundWorkerData->slot[slotno];
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Assert(slot->in_use);
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return &slot->worker; /* can't become free while we're still here */
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}
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#endif
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/*
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* Complain about the BackgroundWorker definition using error level elevel.
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* Return true if it looks ok, false if not (unless elevel >= ERROR, in
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* which case we won't return at all in the not-OK case).
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*/
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static bool
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SanityCheckBackgroundWorker(BackgroundWorker *worker, int elevel)
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{
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/* sanity check for flags */
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if (worker->bgw_flags & BGWORKER_BACKEND_DATABASE_CONNECTION)
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{
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if (!(worker->bgw_flags & BGWORKER_SHMEM_ACCESS))
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{
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ereport(elevel,
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(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
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errmsg("background worker \"%s\": must attach to shared memory in order to request a database connection",
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worker->bgw_name)));
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return false;
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}
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if (worker->bgw_start_time == BgWorkerStart_PostmasterStart)
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{
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ereport(elevel,
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(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
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errmsg("background worker \"%s\": cannot request database access if starting at postmaster start",
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worker->bgw_name)));
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return false;
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}
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/* XXX other checks? */
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}
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if ((worker->bgw_restart_time < 0 &&
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worker->bgw_restart_time != BGW_NEVER_RESTART) ||
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(worker->bgw_restart_time > USECS_PER_DAY / 1000))
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{
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ereport(elevel,
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(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
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errmsg("background worker \"%s\": invalid restart interval",
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worker->bgw_name)));
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return false;
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}
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return true;
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}
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/*
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* Register a new background worker while processing shared_preload_libraries.
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*
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* This can only be called in the _PG_init function of a module library
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* that's loaded by shared_preload_libraries; otherwise it has no effect.
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*/
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void
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RegisterBackgroundWorker(BackgroundWorker *worker)
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{
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RegisteredBgWorker *rw;
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static int numworkers = 0;
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if (!IsUnderPostmaster)
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ereport(LOG,
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(errmsg("registering background worker: %s", worker->bgw_name)));
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if (!process_shared_preload_libraries_in_progress)
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{
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if (!IsUnderPostmaster)
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ereport(LOG,
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(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
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errmsg("background worker \"%s\": must be registered in shared_preload_libraries",
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worker->bgw_name)));
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return;
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}
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if (!SanityCheckBackgroundWorker(worker, LOG))
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return;
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/*
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* Enforce maximum number of workers. Note this is overly restrictive: we
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* could allow more non-shmem-connected workers, because these don't count
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* towards the MAX_BACKENDS limit elsewhere. For now, it doesn't seem
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* important to relax this restriction.
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*/
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if (++numworkers > max_worker_processes)
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{
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ereport(LOG,
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(errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
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errmsg("too many background workers"),
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errdetail_plural("Up to %d background worker can be registered with the current settings.",
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"Up to %d background workers can be registered with the current settings.",
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max_worker_processes,
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max_worker_processes),
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errhint("Consider increasing the configuration parameter \"max_worker_processes\".")));
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return;
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}
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/*
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* Copy the registration data into the registered workers list.
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*/
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rw = malloc(sizeof(RegisteredBgWorker));
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if (rw == NULL)
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{
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ereport(LOG,
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(errcode(ERRCODE_OUT_OF_MEMORY),
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errmsg("out of memory")));
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return;
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}
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rw->rw_worker = *worker;
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rw->rw_backend = NULL;
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rw->rw_pid = 0;
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rw->rw_child_slot = 0;
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rw->rw_crashed_at = 0;
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slist_push_head(&BackgroundWorkerList, &rw->rw_lnode);
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}
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/*
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* Register a new background worker from a regular backend.
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*
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* Returns true on success and false on failure. Failure typically indicates
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* that no background worker slots are currently available.
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*/
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bool
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RegisterDynamicBackgroundWorker(BackgroundWorker *worker)
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{
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int slotno;
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bool success = false;
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/*
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* We can't register dynamic background workers from the postmaster.
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* If this is a standalone backend, we're the only process and can't
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* start any more. In a multi-process environement, it might be
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* theoretically possible, but we don't currently support it due to
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* locking considerations; see comments on the BackgroundWorkerSlot
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* data structure.
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*/
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if (!IsUnderPostmaster)
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return false;
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if (!SanityCheckBackgroundWorker(worker, ERROR))
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return false;
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LWLockAcquire(BackgroundWorkerLock, LW_EXCLUSIVE);
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/*
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* Look for an unused slot. If we find one, grab it.
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*/
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for (slotno = 0; slotno < BackgroundWorkerData->total_slots; ++slotno)
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{
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BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
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if (!slot->in_use)
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{
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memcpy(&slot->worker, worker, sizeof(BackgroundWorker));
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/*
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* Make sure postmaster doesn't see the slot as in use before
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* it sees the new contents.
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*/
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pg_write_barrier();
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slot->in_use = true;
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success = true;
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break;
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}
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}
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LWLockRelease(BackgroundWorkerLock);
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/* If we found a slot, tell the postmaster to notice the change. */
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if (success)
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SendPostmasterSignal(PMSIGNAL_BACKGROUND_WORKER_CHANGE);
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return success;
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}
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