dsm.c

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/*-------------------------------------------------------------------------
 *
 * dsm.c
 *    manage dynamic shared memory segments
 *
 * This file provides a set of services to make programming with dynamic
 * shared memory segments more convenient.  Unlike the low-level
 * facilities provided by dsm_impl.h and dsm_impl.c, mappings and segments
 * created using this module will be cleaned up automatically.  Mappings
 * will be removed when the resource owner under which they were created
 * is cleaned up, unless dsm_pin_mapping() is used, in which case they
 * have session lifespan.  Segments will be removed when there are no
 * remaining mappings, or at postmaster shutdown in any case.  After a
 * hard postmaster crash, remaining segments will be removed, if they
 * still exist, at the next postmaster startup.
 *
 * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/ipc/dsm.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <fcntl.h>
#include <unistd.h>
#ifndef WIN32
#include <sys/mman.h>
#endif
#include <sys/stat.h>

#include "lib/ilist.h"
#include "miscadmin.h"
#include "storage/dsm.h"
#include "storage/ipc.h"
#include "storage/lwlock.h"
#include "storage/pg_shmem.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/resowner_private.h"

#define PG_DYNSHMEM_CONTROL_MAGIC       0x9a503d32

/*
 * There's no point in getting too cheap here, because the minimum allocation
 * is one OS page, which is probably at least 4KB and could easily be as high
 * as 64KB.  Each currently sizeof(dsm_control_item), currently 8 bytes.
 */
#define PG_DYNSHMEM_FIXED_SLOTS         64
#define PG_DYNSHMEM_SLOTS_PER_BACKEND   2

#define INVALID_CONTROL_SLOT        ((uint32) -1)

/* Backend-local tracking for on-detach callbacks. */
typedef struct dsm_segment_detach_callback
{
    on_dsm_detach_callback function;
    Datum       arg;
    slist_node  node;
} dsm_segment_detach_callback;

/* Backend-local state for a dynamic shared memory segment. */
struct dsm_segment
{
    dlist_node  node;           /* List link in dsm_segment_list. */
    ResourceOwner resowner;     /* Resource owner. */
    dsm_handle  handle;         /* Segment name. */
    uint32      control_slot;   /* Slot in control segment. */
    void       *impl_private;   /* Implementation-specific private data. */
    void       *mapped_address; /* Mapping address, or NULL if unmapped. */
    Size        mapped_size;    /* Size of our mapping. */
    slist_head  on_detach;      /* On-detach callbacks. */
};

/* Shared-memory state for a dynamic shared memory segment. */
typedef struct dsm_control_item
{
    dsm_handle  handle;
    uint32      refcnt;         /* 2+ = active, 1 = moribund, 0 = gone */
    void       *impl_private_pm_handle; /* only needed on Windows */
    bool        pinned;
} dsm_control_item;

/* Layout of the dynamic shared memory control segment. */
typedef struct dsm_control_header
{
    uint32      magic;
    uint32      nitems;
    uint32      maxitems;
    dsm_control_item item[FLEXIBLE_ARRAY_MEMBER];
} dsm_control_header;

static void dsm_cleanup_for_mmap(void);
static void dsm_postmaster_shutdown(int code, Datum arg);
static dsm_segment *dsm_create_descriptor(void);
static bool dsm_control_segment_sane(dsm_control_header *control,
                         Size mapped_size);
static uint64 dsm_control_bytes_needed(uint32 nitems);

/* Has this backend initialized the dynamic shared memory system yet? */
static bool dsm_init_done = false;

/*
 * List of dynamic shared memory segments used by this backend.
 *
 * At process exit time, we must decrement the reference count of each
 * segment we have attached; this list makes it possible to find all such
 * segments.
 *
 * This list should always be empty in the postmaster.  We could probably
 * allow the postmaster to map dynamic shared memory segments before it
 * begins to start child processes, provided that each process adjusted
 * the reference counts for those segments in the control segment at
 * startup time, but there's no obvious need for such a facility, which
 * would also be complex to handle in the EXEC_BACKEND case.  Once the
 * postmaster has begun spawning children, there's an additional problem:
 * each new mapping would require an update to the control segment,
 * which requires locking, in which the postmaster must not be involved.
 */
static dlist_head dsm_segment_list = DLIST_STATIC_INIT(dsm_segment_list);

/*
 * Control segment information.
 *
 * Unlike ordinary shared memory segments, the control segment is not
 * reference counted; instead, it lasts for the postmaster's entire
 * life cycle.  For simplicity, it doesn't have a dsm_segment object either.
 */
static dsm_handle dsm_control_handle;
static dsm_control_header *dsm_control;
static Size dsm_control_mapped_size = 0;
static void *dsm_control_impl_private = NULL;

/*
 * Start up the dynamic shared memory system.
 *
 * This is called just once during each cluster lifetime, at postmaster
 * startup time.
 */
void
dsm_postmaster_startup(PGShmemHeader *shim)
{
    void       *dsm_control_address = NULL;
    uint32      maxitems;
    Size        segsize;

    Assert(!IsUnderPostmaster);

    /* If dynamic shared memory is disabled, there's nothing to do. */
    if (dynamic_shared_memory_type == DSM_IMPL_NONE)
        return;

    /*
     * If we're using the mmap implementations, clean up any leftovers.
     * Cleanup isn't needed on Windows, and happens earlier in startup for
     * POSIX and System V shared memory, via a direct call to
     * dsm_cleanup_using_control_segment.
     */
    if (dynamic_shared_memory_type == DSM_IMPL_MMAP)
        dsm_cleanup_for_mmap();

    /* Determine size for new control segment. */
    maxitems = PG_DYNSHMEM_FIXED_SLOTS
        + PG_DYNSHMEM_SLOTS_PER_BACKEND * MaxBackends;
    elog(DEBUG2, "dynamic shared memory system will support %u segments",
         maxitems);
    segsize = dsm_control_bytes_needed(maxitems);

    /*
     * Loop until we find an unused identifier for the new control segment. We
     * sometimes use 0 as a sentinel value indicating that no control segment
     * is known to exist, so avoid using that value for a real control
     * segment.
     */
    for (;;)
    {
        Assert(dsm_control_address == NULL);
        Assert(dsm_control_mapped_size == 0);
        dsm_control_handle = random();
        if (dsm_control_handle == DSM_HANDLE_INVALID)
            continue;
        if (dsm_impl_op(DSM_OP_CREATE, dsm_control_handle, segsize,
                        &dsm_control_impl_private, &dsm_control_address,
                        &dsm_control_mapped_size, ERROR))
            break;
    }
    dsm_control = dsm_control_address;
    on_shmem_exit(dsm_postmaster_shutdown, PointerGetDatum(shim));
    elog(DEBUG2,
         "created dynamic shared memory control segment %u (%zu bytes)",
         dsm_control_handle, segsize);
    shim->dsm_control = dsm_control_handle;

    /* Initialize control segment. */
    dsm_control->magic = PG_DYNSHMEM_CONTROL_MAGIC;
    dsm_control->nitems = 0;
    dsm_control->maxitems = maxitems;
}

/*
 * Determine whether the control segment from the previous postmaster
 * invocation still exists.  If so, remove the dynamic shared memory
 * segments to which it refers, and then the control segment itself.
 */
void
dsm_cleanup_using_control_segment(dsm_handle old_control_handle)
{
    void       *mapped_address = NULL;
    void       *junk_mapped_address = NULL;
    void       *impl_private = NULL;
    void       *junk_impl_private = NULL;
    Size        mapped_size = 0;
    Size        junk_mapped_size = 0;
    uint32      nitems;
    uint32      i;
    dsm_control_header *old_control;

    /* If dynamic shared memory is disabled, there's nothing to do. */
    if (dynamic_shared_memory_type == DSM_IMPL_NONE)
        return;

    /*
     * Try to attach the segment.  If this fails, it probably just means that
     * the operating system has been rebooted and the segment no longer
     * exists, or an unrelated process has used the same shm ID.  So just fall
     * out quietly.
     */
    if (!dsm_impl_op(DSM_OP_ATTACH, old_control_handle, 0, &impl_private,
                     &mapped_address, &mapped_size, DEBUG1))
        return;

    /*
     * We've managed to reattach it, but the contents might not be sane. If
     * they aren't, we disregard the segment after all.
     */
    old_control = (dsm_control_header *) mapped_address;
    if (!dsm_control_segment_sane(old_control, mapped_size))
    {
        dsm_impl_op(DSM_OP_DETACH, old_control_handle, 0, &impl_private,
                    &mapped_address, &mapped_size, LOG);
        return;
    }

    /*
     * OK, the control segment looks basically valid, so we can use it to get
     * a list of segments that need to be removed.
     */
    nitems = old_control->nitems;
    for (i = 0; i < nitems; ++i)
    {
        dsm_handle  handle;
        uint32      refcnt;

        /* If the reference count is 0, the slot is actually unused. */
        refcnt = old_control->item[i].refcnt;
        if (refcnt == 0)
            continue;

        /* Log debugging information. */
        handle = old_control->item[i].handle;
        elog(DEBUG2, "cleaning up orphaned dynamic shared memory with ID %u (reference count %u)",
             handle, refcnt);

        /* Destroy the referenced segment. */
        dsm_impl_op(DSM_OP_DESTROY, handle, 0, &junk_impl_private,
                    &junk_mapped_address, &junk_mapped_size, LOG);
    }

    /* Destroy the old control segment, too. */
    elog(DEBUG2,
         "cleaning up dynamic shared memory control segment with ID %u",
         old_control_handle);
    dsm_impl_op(DSM_OP_DESTROY, old_control_handle, 0, &impl_private,
                &mapped_address, &mapped_size, LOG);
}

/*
 * When we're using the mmap shared memory implementation, "shared memory"
 * segments might even manage to survive an operating system reboot.
 * But there's no guarantee as to exactly what will survive: some segments
 * may survive, and others may not, and the contents of some may be out
 * of date.  In particular, the control segment may be out of date, so we
 * can't rely on it to figure out what to remove.  However, since we know
 * what directory contains the files we used as shared memory, we can simply
 * scan the directory and blow everything away that shouldn't be there.
 */
static void
dsm_cleanup_for_mmap(void)
{
    DIR        *dir;
    struct dirent *dent;

    /* Scan the directory for something with a name of the correct format. */
    dir = AllocateDir(PG_DYNSHMEM_DIR);

    while ((dent = ReadDir(dir, PG_DYNSHMEM_DIR)) != NULL)
    {
        if (strncmp(dent->d_name, PG_DYNSHMEM_MMAP_FILE_PREFIX,
                    strlen(PG_DYNSHMEM_MMAP_FILE_PREFIX)) == 0)
        {
            char        buf[MAXPGPATH + sizeof(PG_DYNSHMEM_DIR)];

            snprintf(buf, sizeof(buf), PG_DYNSHMEM_DIR "/%s", dent->d_name);

            elog(DEBUG2, "removing file \"%s\"", buf);

            /* We found a matching file; so remove it. */
            if (unlink(buf) != 0)
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not remove file \"%s\": %m", buf)));
        }
    }

    /* Cleanup complete. */
    FreeDir(dir);
}

/*
 * At shutdown time, we iterate over the control segment and remove all
 * remaining dynamic shared memory segments.  We avoid throwing errors here;
 * the postmaster is shutting down either way, and this is just non-critical
 * resource cleanup.
 */
static void
dsm_postmaster_shutdown(int code, Datum arg)
{
    uint32      nitems;
    uint32      i;
    void       *dsm_control_address;
    void       *junk_mapped_address = NULL;
    void       *junk_impl_private = NULL;
    Size        junk_mapped_size = 0;
    PGShmemHeader *shim = (PGShmemHeader *) DatumGetPointer(arg);

    /*
     * If some other backend exited uncleanly, it might have corrupted the
     * control segment while it was dying.  In that case, we warn and ignore
     * the contents of the control segment.  This may end up leaving behind
     * stray shared memory segments, but there's not much we can do about that
     * if the metadata is gone.
     */
    nitems = dsm_control->nitems;
    if (!dsm_control_segment_sane(dsm_control, dsm_control_mapped_size))
    {
        ereport(LOG,
                (errmsg("dynamic shared memory control segment is corrupt")));
        return;
    }

    /* Remove any remaining segments. */
    for (i = 0; i < nitems; ++i)
    {
        dsm_handle  handle;

        /* If the reference count is 0, the slot is actually unused. */
        if (dsm_control->item[i].refcnt == 0)
            continue;

        /* Log debugging information. */
        handle = dsm_control->item[i].handle;
        elog(DEBUG2, "cleaning up orphaned dynamic shared memory with ID %u",
             handle);

        /* Destroy the segment. */
        dsm_impl_op(DSM_OP_DESTROY, handle, 0, &junk_impl_private,
                    &junk_mapped_address, &junk_mapped_size, LOG);
    }

    /* Remove the control segment itself. */
    elog(DEBUG2,
         "cleaning up dynamic shared memory control segment with ID %u",
         dsm_control_handle);
    dsm_control_address = dsm_control;
    dsm_impl_op(DSM_OP_DESTROY, dsm_control_handle, 0,
                &dsm_control_impl_private, &dsm_control_address,
                &dsm_control_mapped_size, LOG);
    dsm_control = dsm_control_address;
    shim->dsm_control = 0;
}

/*
 * Prepare this backend for dynamic shared memory usage.  Under EXEC_BACKEND,
 * we must reread the state file and map the control segment; in other cases,
 * we'll have inherited the postmaster's mapping and global variables.
 */
static void
dsm_backend_startup(void)
{
    /* If dynamic shared memory is disabled, reject this. */
    if (dynamic_shared_memory_type == DSM_IMPL_NONE)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("dynamic shared memory is disabled"),
                 errhint("Set dynamic_shared_memory_type to a value other than \"none\".")));

#ifdef EXEC_BACKEND
    {
        void       *control_address = NULL;

        /* Attach control segment. */
        Assert(dsm_control_handle != 0);
        dsm_impl_op(DSM_OP_ATTACH, dsm_control_handle, 0,
                    &dsm_control_impl_private, &control_address,
                    &dsm_control_mapped_size, ERROR);
        dsm_control = control_address;
        /* If control segment doesn't look sane, something is badly wrong. */
        if (!dsm_control_segment_sane(dsm_control, dsm_control_mapped_size))
        {
            dsm_impl_op(DSM_OP_DETACH, dsm_control_handle, 0,
                        &dsm_control_impl_private, &control_address,
                        &dsm_control_mapped_size, WARNING);
            ereport(FATAL,
                    (errcode(ERRCODE_INTERNAL_ERROR),
                     errmsg("dynamic shared memory control segment is not valid")));
        }
    }
#endif

    dsm_init_done = true;
}

#ifdef EXEC_BACKEND
/*
 * When running under EXEC_BACKEND, we get a callback here when the main
 * shared memory segment is re-attached, so that we can record the control
 * handle retrieved from it.
 */
void
dsm_set_control_handle(dsm_handle h)
{
    Assert(dsm_control_handle == 0 && h != 0);
    dsm_control_handle = h;
}
#endif

/*
 * Create a new dynamic shared memory segment.
 *
 * If there is a non-NULL CurrentResourceOwner, the new segment is associated
 * with it and must be detached before the resource owner releases, or a
 * warning will be logged.  If CurrentResourceOwner is NULL, the segment
 * remains attached until explicitely detached or the session ends.
 * Creating with a NULL CurrentResourceOwner is equivalent to creating
 * with a non-NULL CurrentResourceOwner and then calling dsm_pin_mapping.
 */
dsm_segment *
dsm_create(Size size, int flags)
{
    dsm_segment *seg;
    uint32      i;
    uint32      nitems;

    /* Unsafe in postmaster (and pointless in a stand-alone backend). */
    Assert(IsUnderPostmaster);

    if (!dsm_init_done)
        dsm_backend_startup();

    /* Create a new segment descriptor. */
    seg = dsm_create_descriptor();

    /* Loop until we find an unused segment identifier. */
    for (;;)
    {
        Assert(seg->mapped_address == NULL && seg->mapped_size == 0);
        seg->handle = random();
        if (seg->handle == DSM_HANDLE_INVALID)  /* Reserve sentinel */
            continue;
        if (dsm_impl_op(DSM_OP_CREATE, seg->handle, size, &seg->impl_private,
                        &seg->mapped_address, &seg->mapped_size, ERROR))
            break;
    }

    /* Lock the control segment so we can register the new segment. */
    LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);

    /* Search the control segment for an unused slot. */
    nitems = dsm_control->nitems;
    for (i = 0; i < nitems; ++i)
    {
        if (dsm_control->item[i].refcnt == 0)
        {
            dsm_control->item[i].handle = seg->handle;
            /* refcnt of 1 triggers destruction, so start at 2 */
            dsm_control->item[i].refcnt = 2;
            dsm_control->item[i].impl_private_pm_handle = NULL;
            dsm_control->item[i].pinned = false;
            seg->control_slot = i;
            LWLockRelease(DynamicSharedMemoryControlLock);
            return seg;
        }
    }

    /* Verify that we can support an additional mapping. */
    if (nitems >= dsm_control->maxitems)
    {
        if ((flags & DSM_CREATE_NULL_IF_MAXSEGMENTS) != 0)
        {
            LWLockRelease(DynamicSharedMemoryControlLock);
            dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private,
                        &seg->mapped_address, &seg->mapped_size, WARNING);
            if (seg->resowner != NULL)
                ResourceOwnerForgetDSM(seg->resowner, seg);
            dlist_delete(&seg->node);
            pfree(seg);
            return NULL;
        }
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_RESOURCES),
                 errmsg("too many dynamic shared memory segments")));
    }

    /* Enter the handle into a new array slot. */
    dsm_control->item[nitems].handle = seg->handle;
    /* refcnt of 1 triggers destruction, so start at 2 */
    dsm_control->item[nitems].refcnt = 2;
    dsm_control->item[nitems].impl_private_pm_handle = NULL;
    dsm_control->item[nitems].pinned = false;
    seg->control_slot = nitems;
    dsm_control->nitems++;
    LWLockRelease(DynamicSharedMemoryControlLock);

    return seg;
}

/*
 * Attach a dynamic shared memory segment.
 *
 * See comments for dsm_segment_handle() for an explanation of how this
 * is intended to be used.
 *
 * This function will return NULL if the segment isn't known to the system.
 * This can happen if we're asked to attach the segment, but then everyone
 * else detaches it (causing it to be destroyed) before we get around to
 * attaching it.
 *
 * If there is a non-NULL CurrentResourceOwner, the attached segment is
 * associated with it and must be detached before the resource owner releases,
 * or a warning will be logged.  Otherwise the segment remains attached until
 * explicitely detached or the session ends.  See the note atop dsm_create().
 */
dsm_segment *
dsm_attach(dsm_handle h)
{
    dsm_segment *seg;
    dlist_iter  iter;
    uint32      i;
    uint32      nitems;

    /* Unsafe in postmaster (and pointless in a stand-alone backend). */
    Assert(IsUnderPostmaster);

    if (!dsm_init_done)
        dsm_backend_startup();

    /*
     * Since this is just a debugging cross-check, we could leave it out
     * altogether, or include it only in assert-enabled builds.  But since the
     * list of attached segments should normally be very short, let's include
     * it always for right now.
     *
     * If you're hitting this error, you probably want to attempt to find an
     * existing mapping via dsm_find_mapping() before calling dsm_attach() to
     * create a new one.
     */
    dlist_foreach(iter, &dsm_segment_list)
    {
        seg = dlist_container(dsm_segment, node, iter.cur);
        if (seg->handle == h)
            elog(ERROR, "can't attach the same segment more than once");
    }

    /* Create a new segment descriptor. */
    seg = dsm_create_descriptor();
    seg->handle = h;

    /* Bump reference count for this segment in shared memory. */
    LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
    nitems = dsm_control->nitems;
    for (i = 0; i < nitems; ++i)
    {
        /* If the reference count is 0, the slot is actually unused. */
        if (dsm_control->item[i].refcnt == 0)
            continue;

        /* If the handle doesn't match, it's not the slot we want. */
        if (dsm_control->item[i].handle != seg->handle)
            continue;

        /*
         * If the reference count is 1, the slot is still in use, but the
         * segment is in the process of going away.  Treat that as if we
         * didn't find a match.
         */
        if (dsm_control->item[i].refcnt == 1)
            break;

        /* Otherwise we've found a match. */
        dsm_control->item[i].refcnt++;
        seg->control_slot = i;
        break;
    }
    LWLockRelease(DynamicSharedMemoryControlLock);

    /*
     * If we didn't find the handle we're looking for in the control segment,
     * it probably means that everyone else who had it mapped, including the
     * original creator, died before we got to this point. It's up to the
     * caller to decide what to do about that.
     */
    if (seg->control_slot == INVALID_CONTROL_SLOT)
    {
        dsm_detach(seg);
        return NULL;
    }

    /* Here's where we actually try to map the segment. */
    dsm_impl_op(DSM_OP_ATTACH, seg->handle, 0, &seg->impl_private,
                &seg->mapped_address, &seg->mapped_size, ERROR);

    return seg;
}

/*
 * At backend shutdown time, detach any segments that are still attached.
 * (This is similar to dsm_detach_all, except that there's no reason to
 * unmap the control segment before exiting, so we don't bother.)
 */
void
dsm_backend_shutdown(void)
{
    while (!dlist_is_empty(&dsm_segment_list))
    {
        dsm_segment *seg;

        seg = dlist_head_element(dsm_segment, node, &dsm_segment_list);
        dsm_detach(seg);
    }
}

/*
 * Detach all shared memory segments, including the control segments.  This
 * should be called, along with PGSharedMemoryDetach, in processes that
 * might inherit mappings but are not intended to be connected to dynamic
 * shared memory.
 */
void
dsm_detach_all(void)
{
    void       *control_address = dsm_control;

    while (!dlist_is_empty(&dsm_segment_list))
    {
        dsm_segment *seg;

        seg = dlist_head_element(dsm_segment, node, &dsm_segment_list);
        dsm_detach(seg);
    }

    if (control_address != NULL)
        dsm_impl_op(DSM_OP_DETACH, dsm_control_handle, 0,
                    &dsm_control_impl_private, &control_address,
                    &dsm_control_mapped_size, ERROR);
}

/*
 * Resize an existing shared memory segment.
 *
 * This may cause the shared memory segment to be remapped at a different
 * address.  For the caller's convenience, we return the mapped address.
 */
void *
dsm_resize(dsm_segment *seg, Size size)
{
    Assert(seg->control_slot != INVALID_CONTROL_SLOT);
    dsm_impl_op(DSM_OP_RESIZE, seg->handle, size, &seg->impl_private,
                &seg->mapped_address, &seg->mapped_size, ERROR);
    return seg->mapped_address;
}

/*
 * Remap an existing shared memory segment.
 *
 * This is intended to be used when some other process has extended the
 * mapping using dsm_resize(), but we've still only got the initial
 * portion mapped.  Since this might change the address at which the
 * segment is mapped, we return the new mapped address.
 */
void *
dsm_remap(dsm_segment *seg)
{
    dsm_impl_op(DSM_OP_ATTACH, seg->handle, 0, &seg->impl_private,
                &seg->mapped_address, &seg->mapped_size, ERROR);

    return seg->mapped_address;
}

/*
 * Detach from a shared memory segment, destroying the segment if we
 * remove the last reference.
 *
 * This function should never fail.  It will often be invoked when aborting
 * a transaction, and a further error won't serve any purpose.  It's not a
 * complete disaster if we fail to unmap or destroy the segment; it means a
 * resource leak, but that doesn't necessarily preclude further operations.
 */
void
dsm_detach(dsm_segment *seg)
{
    /*
     * Invoke registered callbacks.  Just in case one of those callbacks
     * throws a further error that brings us back here, pop the callback
     * before invoking it, to avoid infinite error recursion.
     */
    while (!slist_is_empty(&seg->on_detach))
    {
        slist_node *node;
        dsm_segment_detach_callback *cb;
        on_dsm_detach_callback function;
        Datum       arg;

        node = slist_pop_head_node(&seg->on_detach);
        cb = slist_container(dsm_segment_detach_callback, node, node);
        function = cb->function;
        arg = cb->arg;
        pfree(cb);

        function(seg, arg);
    }

    /*
     * Try to remove the mapping, if one exists.  Normally, there will be, but
     * maybe not, if we failed partway through a create or attach operation.
     * We remove the mapping before decrementing the reference count so that
     * the process that sees a zero reference count can be certain that no
     * remaining mappings exist.  Even if this fails, we pretend that it
     * works, because retrying is likely to fail in the same way.
     */
    if (seg->mapped_address != NULL)
    {
        dsm_impl_op(DSM_OP_DETACH, seg->handle, 0, &seg->impl_private,
                    &seg->mapped_address, &seg->mapped_size, WARNING);
        seg->impl_private = NULL;
        seg->mapped_address = NULL;
        seg->mapped_size = 0;
    }

    /* Reduce reference count, if we previously increased it. */
    if (seg->control_slot != INVALID_CONTROL_SLOT)
    {
        uint32      refcnt;
        uint32      control_slot = seg->control_slot;

        LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
        Assert(dsm_control->item[control_slot].handle == seg->handle);
        Assert(dsm_control->item[control_slot].refcnt > 1);
        refcnt = --dsm_control->item[control_slot].refcnt;
        seg->control_slot = INVALID_CONTROL_SLOT;
        LWLockRelease(DynamicSharedMemoryControlLock);

        /* If new reference count is 1, try to destroy the segment. */
        if (refcnt == 1)
        {
            /* A pinned segment should never reach 1. */
            Assert(!dsm_control->item[control_slot].pinned);

            /*
             * If we fail to destroy the segment here, or are killed before we
             * finish doing so, the reference count will remain at 1, which
             * will mean that nobody else can attach to the segment.  At
             * postmaster shutdown time, or when a new postmaster is started
             * after a hard kill, another attempt will be made to remove the
             * segment.
             *
             * The main case we're worried about here is being killed by a
             * signal before we can finish removing the segment.  In that
             * case, it's important to be sure that the segment still gets
             * removed. If we actually fail to remove the segment for some
             * other reason, the postmaster may not have any better luck than
             * we did.  There's not much we can do about that, though.
             */
            if (dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private,
                            &seg->mapped_address, &seg->mapped_size, WARNING))
            {
                LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
                Assert(dsm_control->item[control_slot].handle == seg->handle);
                Assert(dsm_control->item[control_slot].refcnt == 1);
                dsm_control->item[control_slot].refcnt = 0;
                LWLockRelease(DynamicSharedMemoryControlLock);
            }
        }
    }

    /* Clean up our remaining backend-private data structures. */
    if (seg->resowner != NULL)
        ResourceOwnerForgetDSM(seg->resowner, seg);
    dlist_delete(&seg->node);
    pfree(seg);
}

/*
 * Keep a dynamic shared memory mapping until end of session.
 *
 * By default, mappings are owned by the current resource owner, which
 * typically means they stick around for the duration of the current query
 * only.
 */
void
dsm_pin_mapping(dsm_segment *seg)
{
    if (seg->resowner != NULL)
    {
        ResourceOwnerForgetDSM(seg->resowner, seg);
        seg->resowner = NULL;
    }
}

/*
 * Arrange to remove a dynamic shared memory mapping at cleanup time.
 *
 * dsm_pin_mapping() can be used to preserve a mapping for the entire
 * lifetime of a process; this function reverses that decision, making
 * the segment owned by the current resource owner.  This may be useful
 * just before performing some operation that will invalidate the segment
 * for future use by this backend.
 */
void
dsm_unpin_mapping(dsm_segment *seg)
{
    Assert(seg->resowner == NULL);
    ResourceOwnerEnlargeDSMs(CurrentResourceOwner);
    seg->resowner = CurrentResourceOwner;
    ResourceOwnerRememberDSM(seg->resowner, seg);
}

/*
 * Keep a dynamic shared memory segment until postmaster shutdown, or until
 * dsm_unpin_segment is called.
 *
 * This function should not be called more than once per segment, unless the
 * segment is explicitly unpinned with dsm_unpin_segment in between calls.
 *
 * Note that this function does not arrange for the current process to
 * keep the segment mapped indefinitely; if that behavior is desired,
 * dsm_pin_mapping() should be used from each process that needs to
 * retain the mapping.
 */
void
dsm_pin_segment(dsm_segment *seg)
{
    void       *handle;

    /*
     * Bump reference count for this segment in shared memory. This will
     * ensure that even if there is no session which is attached to this
     * segment, it will remain until postmaster shutdown or an explicit call
     * to unpin.
     */
    LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
    if (dsm_control->item[seg->control_slot].pinned)
        elog(ERROR, "cannot pin a segment that is already pinned");
    dsm_impl_pin_segment(seg->handle, seg->impl_private, &handle);
    dsm_control->item[seg->control_slot].pinned = true;
    dsm_control->item[seg->control_slot].refcnt++;
    dsm_control->item[seg->control_slot].impl_private_pm_handle = handle;
    LWLockRelease(DynamicSharedMemoryControlLock);
}

/*
 * Unpin a dynamic shared memory segment that was previously pinned with
 * dsm_pin_segment.  This function should not be called unless dsm_pin_segment
 * was previously called for this segment.
 *
 * The argument is a dsm_handle rather than a dsm_segment in case you want
 * to unpin a segment to which you haven't attached.  This turns out to be
 * useful if, for example, a reference to one shared memory segment is stored
 * within another shared memory segment.  You might want to unpin the
 * referenced segment before destroying the referencing segment.
 */
void
dsm_unpin_segment(dsm_handle handle)
{
    uint32      control_slot = INVALID_CONTROL_SLOT;
    bool        destroy = false;
    uint32      i;

    /* Find the control slot for the given handle. */
    LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
    for (i = 0; i < dsm_control->nitems; ++i)
    {
        /* Skip unused slots. */
        if (dsm_control->item[i].refcnt == 0)
            continue;

        /* If we've found our handle, we can stop searching. */
        if (dsm_control->item[i].handle == handle)
        {
            control_slot = i;
            break;
        }
    }

    /*
     * We should definitely have found the slot, and it should not already be
     * in the process of going away, because this function should only be
     * called on a segment which is pinned.
     */
    if (control_slot == INVALID_CONTROL_SLOT)
        elog(ERROR, "cannot unpin unknown segment handle");
    if (!dsm_control->item[control_slot].pinned)
        elog(ERROR, "cannot unpin a segment that is not pinned");
    Assert(dsm_control->item[control_slot].refcnt > 1);

    /*
     * Allow implementation-specific code to run.  We have to do this before
     * releasing the lock, because impl_private_pm_handle may get modified by
     * dsm_impl_unpin_segment.
     */
    dsm_impl_unpin_segment(handle,
                           &dsm_control->item[control_slot].impl_private_pm_handle);

    /* Note that 1 means no references (0 means unused slot). */
    if (--dsm_control->item[control_slot].refcnt == 1)
        destroy = true;
    dsm_control->item[control_slot].pinned = false;

    /* Now we can release the lock. */
    LWLockRelease(DynamicSharedMemoryControlLock);

    /* Clean up resources if that was the last reference. */
    if (destroy)
    {
        void       *junk_impl_private = NULL;
        void       *junk_mapped_address = NULL;
        Size        junk_mapped_size = 0;

        /*
         * For an explanation of how error handling works in this case, see
         * comments in dsm_detach.  Note that if we reach this point, the
         * current process certainly does not have the segment mapped, because
         * if it did, the reference count would have still been greater than 1
         * even after releasing the reference count held by the pin.  The fact
         * that there can't be a dsm_segment for this handle makes it OK to
         * pass the mapped size, mapped address, and private data as NULL
         * here.
         */
        if (dsm_impl_op(DSM_OP_DESTROY, handle, 0, &junk_impl_private,
                        &junk_mapped_address, &junk_mapped_size, WARNING))
        {
            LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
            Assert(dsm_control->item[control_slot].handle == handle);
            Assert(dsm_control->item[control_slot].refcnt == 1);
            dsm_control->item[control_slot].refcnt = 0;
            LWLockRelease(DynamicSharedMemoryControlLock);
        }
    }
}

/*
 * Find an existing mapping for a shared memory segment, if there is one.
 */
dsm_segment *
dsm_find_mapping(dsm_handle h)
{
    dlist_iter  iter;
    dsm_segment *seg;

    dlist_foreach(iter, &dsm_segment_list)
    {
        seg = dlist_container(dsm_segment, node, iter.cur);
        if (seg->handle == h)
            return seg;
    }

    return NULL;
}

/*
 * Get the address at which a dynamic shared memory segment is mapped.
 */
void *
dsm_segment_address(dsm_segment *seg)
{
    Assert(seg->mapped_address != NULL);
    return seg->mapped_address;
}

/*
 * Get the size of a mapping.
 */
Size
dsm_segment_map_length(dsm_segment *seg)
{
    Assert(seg->mapped_address != NULL);
    return seg->mapped_size;
}

/*
 * Get a handle for a mapping.
 *
 * To establish communication via dynamic shared memory between two backends,
 * one of them should first call dsm_create() to establish a new shared
 * memory mapping.  That process should then call dsm_segment_handle() to
 * obtain a handle for the mapping, and pass that handle to the
 * coordinating backend via some means (e.g. bgw_main_arg, or via the
 * main shared memory segment).  The recipient, once in possession of the
 * handle, should call dsm_attach().
 */
dsm_handle
dsm_segment_handle(dsm_segment *seg)
{
    return seg->handle;
}

/*
 * Register an on-detach callback for a dynamic shared memory segment.
 */
void
on_dsm_detach(dsm_segment *seg, on_dsm_detach_callback function, Datum arg)
{
    dsm_segment_detach_callback *cb;

    cb = MemoryContextAlloc(TopMemoryContext,
                            sizeof(dsm_segment_detach_callback));
    cb->function = function;
    cb->arg = arg;
    slist_push_head(&seg->on_detach, &cb->node);
}

/*
 * Unregister an on-detach callback for a dynamic shared memory segment.
 */
void
cancel_on_dsm_detach(dsm_segment *seg, on_dsm_detach_callback function,
                     Datum arg)
{
    slist_mutable_iter iter;

    slist_foreach_modify(iter, &seg->on_detach)
    {
        dsm_segment_detach_callback *cb;

        cb = slist_container(dsm_segment_detach_callback, node, iter.cur);
        if (cb->function == function && cb->arg == arg)
        {
            slist_delete_current(&iter);
            pfree(cb);
            break;
        }
    }
}

/*
 * Discard all registered on-detach callbacks without executing them.
 */
void
reset_on_dsm_detach(void)
{
    dlist_iter  iter;

    dlist_foreach(iter, &dsm_segment_list)
    {
        dsm_segment *seg = dlist_container(dsm_segment, node, iter.cur);

        /* Throw away explicit on-detach actions one by one. */
        while (!slist_is_empty(&seg->on_detach))
        {
            slist_node *node;
            dsm_segment_detach_callback *cb;

            node = slist_pop_head_node(&seg->on_detach);
            cb = slist_container(dsm_segment_detach_callback, node, node);
            pfree(cb);
        }

        /*
         * Decrementing the reference count is a sort of implicit on-detach
         * action; make sure we don't do that, either.
         */
        seg->control_slot = INVALID_CONTROL_SLOT;
    }
}

/*
 * Create a segment descriptor.
 */
static dsm_segment *
dsm_create_descriptor(void)
{
    dsm_segment *seg;

    if (CurrentResourceOwner)
        ResourceOwnerEnlargeDSMs(CurrentResourceOwner);

    seg = MemoryContextAlloc(TopMemoryContext, sizeof(dsm_segment));
    dlist_push_head(&dsm_segment_list, &seg->node);

    /* seg->handle must be initialized by the caller */
    seg->control_slot = INVALID_CONTROL_SLOT;
    seg->impl_private = NULL;
    seg->mapped_address = NULL;
    seg->mapped_size = 0;

    seg->resowner = CurrentResourceOwner;
    if (CurrentResourceOwner)
        ResourceOwnerRememberDSM(CurrentResourceOwner, seg);

    slist_init(&seg->on_detach);

    return seg;
}

/*
 * Sanity check a control segment.
 *
 * The goal here isn't to detect everything that could possibly be wrong with
 * the control segment; there's not enough information for that.  Rather, the
 * goal is to make sure that someone can iterate over the items in the segment
 * without overrunning the end of the mapping and crashing.  We also check
 * the magic number since, if that's messed up, this may not even be one of
 * our segments at all.
 */
static bool
dsm_control_segment_sane(dsm_control_header *control, Size mapped_size)
{
    if (mapped_size < offsetof(dsm_control_header, item))
        return false;           /* Mapped size too short to read header. */
    if (control->magic != PG_DYNSHMEM_CONTROL_MAGIC)
        return false;           /* Magic number doesn't match. */
    if (dsm_control_bytes_needed(control->maxitems) > mapped_size)
        return false;           /* Max item count won't fit in map. */
    if (control->nitems > control->maxitems)
        return false;           /* Overfull. */
    return true;
}

/*
 * Compute the number of control-segment bytes needed to store a given
 * number of items.
 */
static uint64
dsm_control_bytes_needed(uint32 nitems)
{
    return offsetof(dsm_control_header, item)
        + sizeof(dsm_control_item) * (uint64) nitems;
}