pmchild.c

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/*-------------------------------------------------------------------------
 *
 * pmchild.c
 *    Functions for keeping track of postmaster child processes.
 *
 * Postmaster keeps track of all child processes so that when a process exits,
 * it knows what kind of a process it was and can clean up accordingly.  Every
 * child process is allocated a PMChild struct from a fixed pool of structs.
 * The size of the pool is determined by various settings that configure how
 * many worker processes and backend connections are allowed, i.e.
 * autovacuum_worker_slots, max_worker_processes, max_wal_senders, and
 * max_connections.
 *
 * Dead-end backends are handled slightly differently.  There is no limit
 * on the number of dead-end backends, and they do not need unique IDs, so
 * their PMChild structs are allocated dynamically, not from a pool.
 *
 * The structures and functions in this file are private to the postmaster
 * process.  But note that there is an array in shared memory, managed by
 * pmsignal.c, that mirrors this.
 *
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/postmaster/pmchild.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "miscadmin.h"
#include "postmaster/autovacuum.h"
#include "postmaster/postmaster.h"
#include "replication/walsender.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
 
/*
 * Freelists for different kinds of child processes.  We maintain separate
 * pools for each, so that for example launching a lot of regular backends
 * cannot prevent autovacuum or an aux process from launching.
 */
typedef struct PMChildPool
{
    int         size;           /* number of PMChild slots reserved for this
                                 * kind of processes */
    int         first_slotno;   /* first slot belonging to this pool */
    dlist_head  freelist;       /* currently unused PMChild entries */
} PMChildPool;
 
static PMChildPool pmchild_pools[BACKEND_NUM_TYPES];
NON_EXEC_STATIC int num_pmchild_slots = 0;
 
/*
 * List of active child processes.  This includes dead-end children.
 */
dlist_head  ActiveChildList;
 
/*
 * MaxLivePostmasterChildren
 *
 * This reports the number of postmaster child processes that can be active.
 * It includes all children except for dead-end children.  This allows the
 * array in shared memory (PMChildFlags) to have a fixed maximum size.
 */
int
MaxLivePostmasterChildren(void)
{
    if (num_pmchild_slots == 0)
        elog(ERROR, "PM child array not initialized yet");
    return num_pmchild_slots;
}
 
/*
 * Initialize at postmaster startup
 *
 * Note: This is not called on crash restart.  We rely on PMChild entries to
 * remain valid through the restart process.  This is important because the
 * syslogger survives through the crash restart process, so we must not
 * invalidate its PMChild slot.
 */
void
InitPostmasterChildSlots(void)
{
    int         slotno;
    PMChild    *slots;
 
    /*
     * We allow more connections here than we can have backends because some
     * might still be authenticating; they might fail auth, or some existing
     * backend might exit before the auth cycle is completed.  The exact
     * MaxConnections limit is enforced when a new backend tries to join the
     * PGPROC array.
     *
     * WAL senders start out as regular backends, so they share the same pool.
     */
    pmchild_pools[B_BACKEND].size = 2 * (MaxConnections + max_wal_senders);
 
    pmchild_pools[B_AUTOVAC_WORKER].size = autovacuum_worker_slots;
    pmchild_pools[B_BG_WORKER].size = max_worker_processes;
 
    /*
     * There can be only one of each of these running at a time.  They each
     * get their own pool of just one entry.
     */
    pmchild_pools[B_AUTOVAC_LAUNCHER].size = 1;
    pmchild_pools[B_SLOTSYNC_WORKER].size = 1;
    pmchild_pools[B_ARCHIVER].size = 1;
    pmchild_pools[B_BG_WRITER].size = 1;
    pmchild_pools[B_CHECKPOINTER].size = 1;
    pmchild_pools[B_STARTUP].size = 1;
    pmchild_pools[B_WAL_RECEIVER].size = 1;
    pmchild_pools[B_WAL_SUMMARIZER].size = 1;
    pmchild_pools[B_WAL_WRITER].size = 1;
    pmchild_pools[B_LOGGER].size = 1;
 
    /* The rest of the pmchild_pools are left at zero size */
 
    /* Count the total number of slots */
    num_pmchild_slots = 0;
    for (int i = 0; i < BACKEND_NUM_TYPES; i++)
        num_pmchild_slots += pmchild_pools[i].size;
 
    /* Initialize them */
    slots = palloc(num_pmchild_slots * sizeof(PMChild));
    slotno = 0;
    for (int btype = 0; btype < BACKEND_NUM_TYPES; btype++)
    {
        pmchild_pools[btype].first_slotno = slotno + 1;
        dlist_init(&pmchild_pools[btype].freelist);
 
        for (int j = 0; j < pmchild_pools[btype].size; j++)
        {
            slots[slotno].pid = 0;
            slots[slotno].child_slot = slotno + 1;
            slots[slotno].bkend_type = B_INVALID;
            slots[slotno].rw = NULL;
            slots[slotno].bgworker_notify = false;
            dlist_push_tail(&pmchild_pools[btype].freelist, &slots[slotno].elem);
            slotno++;
        }
    }
    Assert(slotno == num_pmchild_slots);
 
    /* Initialize other structures */
    dlist_init(&ActiveChildList);
}
 
/*
 * Allocate a PMChild entry for a postmaster child process of given type.
 *
 * The entry is taken from the right pool for the type.
 *
 * pmchild->child_slot in the returned struct is unique among all active child
 * processes.
 */
PMChild *
AssignPostmasterChildSlot(BackendType btype)
{
    dlist_head *freelist;
    PMChild    *pmchild;
 
    if (pmchild_pools[btype].size == 0)
        elog(ERROR, "cannot allocate a PMChild slot for backend type %d", btype);
 
    freelist = &pmchild_pools[btype].freelist;
    if (dlist_is_empty(freelist))
        return NULL;
 
    pmchild = dlist_container(PMChild, elem, dlist_pop_head_node(freelist));
    pmchild->pid = 0;
    pmchild->bkend_type = btype;
    pmchild->rw = NULL;
    pmchild->bgworker_notify = true;
 
    /*
     * pmchild->child_slot for each entry was initialized when the array of
     * slots was allocated.  Sanity check it.
     */
    if (!(pmchild->child_slot >= pmchild_pools[btype].first_slotno &&
          pmchild->child_slot < pmchild_pools[btype].first_slotno + pmchild_pools[btype].size))
    {
        elog(ERROR, "pmchild freelist for backend type %d is corrupt",
             pmchild->bkend_type);
    }
 
    dlist_push_head(&ActiveChildList, &pmchild->elem);
 
    /* Update the status in the shared memory array */
    MarkPostmasterChildSlotAssigned(pmchild->child_slot);
 
    elog(DEBUG2, "assigned pm child slot %d for %s",
         pmchild->child_slot, PostmasterChildName(btype));
 
    return pmchild;
}
 
/*
 * Allocate a PMChild struct for a dead-end backend.  Dead-end children are
 * not assigned a child_slot number.  The struct is palloc'd; returns NULL if
 * out of memory.
 */
PMChild *
AllocDeadEndChild(void)
{
    PMChild    *pmchild;
 
    elog(DEBUG2, "allocating dead-end child");
 
    pmchild = (PMChild *) palloc_extended(sizeof(PMChild), MCXT_ALLOC_NO_OOM);
    if (pmchild)
    {
        pmchild->pid = 0;
        pmchild->child_slot = 0;
        pmchild->bkend_type = B_DEAD_END_BACKEND;
        pmchild->rw = NULL;
        pmchild->bgworker_notify = false;
 
        dlist_push_head(&ActiveChildList, &pmchild->elem);
    }
 
    return pmchild;
}
 
/*
 * Release a PMChild slot, after the child process has exited.
 *
 * Returns true if the child detached cleanly from shared memory, false
 * otherwise (see MarkPostmasterChildSlotUnassigned).
 */
bool
ReleasePostmasterChildSlot(PMChild *pmchild)
{
    dlist_delete(&pmchild->elem);
    if (pmchild->bkend_type == B_DEAD_END_BACKEND)
    {
        elog(DEBUG2, "releasing dead-end backend");
        pfree(pmchild);
        return true;
    }
    else
    {
        PMChildPool *pool;
 
        elog(DEBUG2, "releasing pm child slot %d", pmchild->child_slot);
 
        /* WAL senders start out as regular backends, and share the pool */
        if (pmchild->bkend_type == B_WAL_SENDER)
            pool = &pmchild_pools[B_BACKEND];
        else
            pool = &pmchild_pools[pmchild->bkend_type];
 
        /* sanity check that we return the entry to the right pool */
        if (!(pmchild->child_slot >= pool->first_slotno &&
              pmchild->child_slot < pool->first_slotno + pool->size))
        {
            elog(ERROR, "pmchild freelist for backend type %d is corrupt",
                 pmchild->bkend_type);
        }
 
        dlist_push_head(&pool->freelist, &pmchild->elem);
        return MarkPostmasterChildSlotUnassigned(pmchild->child_slot);
    }
}
 
/*
 * Find the PMChild entry of a running child process by PID.
 */
PMChild *
FindPostmasterChildByPid(int pid)
{
    dlist_iter  iter;
 
    dlist_foreach(iter, &ActiveChildList)
    {
        PMChild    *bp = dlist_container(PMChild, elem, iter.cur);
 
        if (bp->pid == pid)
            return bp;
    }
    return NULL;
}