dsm_impl.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * dsm_impl.c
 *    manage dynamic shared memory segments
 *
 * This file provides low-level APIs for creating and destroying shared
 * memory segments using several different possible techniques.  We refer
 * to these segments as dynamic because they can be created, altered, and
 * destroyed at any point during the server life cycle.  This is unlike
 * the main shared memory segment, of which there is always exactly one
 * and which is always mapped at a fixed address in every PostgreSQL
 * background process.
 *
 * Because not all systems provide the same primitives in this area, nor
 * do all primitives behave the same way on all systems, we provide
 * several implementations of this facility.  Many systems implement
 * POSIX shared memory (shm_open etc.), which is well-suited to our needs
 * in this area, with the exception that shared memory identifiers live
 * in a flat system-wide namespace, raising the uncomfortable prospect of
 * name collisions with other processes (including other copies of
 * PostgreSQL) running on the same system.  Some systems only support
 * the older System V shared memory interface (shmget etc.) which is
 * also usable; however, the default allocation limits are often quite
 * small, and the namespace is even more restricted.
 *
 * We also provide an mmap-based shared memory implementation.  This may
 * be useful on systems that provide shared memory via a special-purpose
 * filesystem; by opting for this implementation, the user can even
 * control precisely where their shared memory segments are placed.  It
 * can also be used as a fallback for systems where shm_open and shmget
 * are not available or can't be used for some reason.  Of course,
 * mapping a file residing on an actual spinning disk is a fairly poor
 * approximation for shared memory because writeback may hurt performance
 * substantially, but there should be few systems where we must make do
 * with such poor tools.
 *
 * As ever, Windows requires its own implementation.
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/ipc/dsm_impl.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <fcntl.h>
#include <unistd.h>
#ifndef WIN32
#include <sys/mman.h>
#endif
#include <sys/stat.h>
#ifdef HAVE_SYS_IPC_H
#include <sys/ipc.h>
#endif
#ifdef HAVE_SYS_SHM_H
#include <sys/shm.h>
#endif

#include "common/file_perm.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "portability/mem.h"
#include "postmaster/postmaster.h"
#include "storage/dsm_impl.h"
#include "storage/fd.h"
#include "utils/guc.h"
#include "utils/memutils.h"

#ifdef USE_DSM_POSIX
static bool dsm_impl_posix(dsm_op op, dsm_handle handle, Size request_size,
                           void **impl_private, void **mapped_address,
                           Size *mapped_size, int elevel);
static int  dsm_impl_posix_resize(int fd, off_t size);
#endif
#ifdef USE_DSM_SYSV
static bool dsm_impl_sysv(dsm_op op, dsm_handle handle, Size request_size,
                          void **impl_private, void **mapped_address,
                          Size *mapped_size, int elevel);
#endif
#ifdef USE_DSM_WINDOWS
static bool dsm_impl_windows(dsm_op op, dsm_handle handle, Size request_size,
                             void **impl_private, void **mapped_address,
                             Size *mapped_size, int elevel);
#endif
#ifdef USE_DSM_MMAP
static bool dsm_impl_mmap(dsm_op op, dsm_handle handle, Size request_size,
                          void **impl_private, void **mapped_address,
                          Size *mapped_size, int elevel);
#endif
static int  errcode_for_dynamic_shared_memory(void);

const struct config_enum_entry dynamic_shared_memory_options[] = {
#ifdef USE_DSM_POSIX
    {"posix", DSM_IMPL_POSIX, false},
#endif
#ifdef USE_DSM_SYSV
    {"sysv", DSM_IMPL_SYSV, false},
#endif
#ifdef USE_DSM_WINDOWS
    {"windows", DSM_IMPL_WINDOWS, false},
#endif
#ifdef USE_DSM_MMAP
    {"mmap", DSM_IMPL_MMAP, false},
#endif
    {NULL, 0, false}
};

/* Implementation selector. */
int         dynamic_shared_memory_type;

/* Amount of space reserved for DSM segments in the main area. */
int         min_dynamic_shared_memory;

/* Size of buffer to be used for zero-filling. */
#define ZBUFFER_SIZE                8192

#define SEGMENT_NAME_PREFIX         "Global/PostgreSQL"

/*------
 * Perform a low-level shared memory operation in a platform-specific way,
 * as dictated by the selected implementation.  Each implementation is
 * required to implement the following primitives.
 *
 * DSM_OP_CREATE.  Create a segment whose size is the request_size and
 * map it.
 *
 * DSM_OP_ATTACH.  Map the segment, whose size must be the request_size.
 *
 * DSM_OP_DETACH.  Unmap the segment.
 *
 * DSM_OP_DESTROY.  Unmap the segment, if it is mapped.  Destroy the
 * segment.
 *
 * Arguments:
 *   op: The operation to be performed.
 *   handle: The handle of an existing object, or for DSM_OP_CREATE, the
 *     a new handle the caller wants created.
 *   request_size: For DSM_OP_CREATE, the requested size.  Otherwise, 0.
 *   impl_private: Private, implementation-specific data.  Will be a pointer
 *     to NULL for the first operation on a shared memory segment within this
 *     backend; thereafter, it will point to the value to which it was set
 *     on the previous call.
 *   mapped_address: Pointer to start of current mapping; pointer to NULL
 *     if none.  Updated with new mapping address.
 *   mapped_size: Pointer to size of current mapping; pointer to 0 if none.
 *     Updated with new mapped size.
 *   elevel: Level at which to log errors.
 *
 * Return value: true on success, false on failure.  When false is returned,
 * a message should first be logged at the specified elevel, except in the
 * case where DSM_OP_CREATE experiences a name collision, which should
 * silently return false.
 *-----
 */
bool
dsm_impl_op(dsm_op op, dsm_handle handle, Size request_size,
            void **impl_private, void **mapped_address, Size *mapped_size,
            int elevel)
{
    Assert(op == DSM_OP_CREATE || request_size == 0);
    Assert((op != DSM_OP_CREATE && op != DSM_OP_ATTACH) ||
           (*mapped_address == NULL && *mapped_size == 0));

    switch (dynamic_shared_memory_type)
    {
#ifdef USE_DSM_POSIX
        case DSM_IMPL_POSIX:
            return dsm_impl_posix(op, handle, request_size, impl_private,
                                  mapped_address, mapped_size, elevel);
#endif
#ifdef USE_DSM_SYSV
        case DSM_IMPL_SYSV:
            return dsm_impl_sysv(op, handle, request_size, impl_private,
                                 mapped_address, mapped_size, elevel);
#endif
#ifdef USE_DSM_WINDOWS
        case DSM_IMPL_WINDOWS:
            return dsm_impl_windows(op, handle, request_size, impl_private,
                                    mapped_address, mapped_size, elevel);
#endif
#ifdef USE_DSM_MMAP
        case DSM_IMPL_MMAP:
            return dsm_impl_mmap(op, handle, request_size, impl_private,
                                 mapped_address, mapped_size, elevel);
#endif
        default:
            elog(ERROR, "unexpected dynamic shared memory type: %d",
                 dynamic_shared_memory_type);
            return false;
    }
}

#ifdef USE_DSM_POSIX
/*
 * Operating system primitives to support POSIX shared memory.
 *
 * POSIX shared memory segments are created and attached using shm_open()
 * and shm_unlink(); other operations, such as sizing or mapping the
 * segment, are performed as if the shared memory segments were files.
 *
 * Indeed, on some platforms, they may be implemented that way.  While
 * POSIX shared memory segments seem intended to exist in a flat namespace,
 * some operating systems may implement them as files, even going so far
 * to treat a request for /xyz as a request to create a file by that name
 * in the root directory.  Users of such broken platforms should select
 * a different shared memory implementation.
 */
static bool
dsm_impl_posix(dsm_op op, dsm_handle handle, Size request_size,
               void **impl_private, void **mapped_address, Size *mapped_size,
               int elevel)
{
    char        name[64];
    int         flags;
    int         fd;
    char       *address;

    snprintf(name, 64, "/PostgreSQL.%u", handle);

    /* Handle teardown cases. */
    if (op == DSM_OP_DETACH || op == DSM_OP_DESTROY)
    {
        if (*mapped_address != NULL
            && munmap(*mapped_address, *mapped_size) != 0)
        {
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not unmap shared memory segment \"%s\": %m",
                            name)));
            return false;
        }
        *mapped_address = NULL;
        *mapped_size = 0;
        if (op == DSM_OP_DESTROY && shm_unlink(name) != 0)
        {
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not remove shared memory segment \"%s\": %m",
                            name)));
            return false;
        }
        return true;
    }

    /*
     * Create new segment or open an existing one for attach.
     *
     * Even though we will close the FD before returning, it seems desirable
     * to use Reserve/ReleaseExternalFD, to reduce the probability of EMFILE
     * failure.  The fact that we won't hold the FD open long justifies using
     * ReserveExternalFD rather than AcquireExternalFD, though.
     */
    ReserveExternalFD();

    flags = O_RDWR | (op == DSM_OP_CREATE ? O_CREAT | O_EXCL : 0);
    if ((fd = shm_open(name, flags, PG_FILE_MODE_OWNER)) == -1)
    {
        ReleaseExternalFD();
        if (errno != EEXIST)
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not open shared memory segment \"%s\": %m",
                            name)));
        return false;
    }

    /*
     * If we're attaching the segment, determine the current size; if we are
     * creating the segment, set the size to the requested value.
     */
    if (op == DSM_OP_ATTACH)
    {
        struct stat st;

        if (fstat(fd, &st) != 0)
        {
            int         save_errno;

            /* Back out what's already been done. */
            save_errno = errno;
            close(fd);
            ReleaseExternalFD();
            errno = save_errno;

            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not stat shared memory segment \"%s\": %m",
                            name)));
            return false;
        }
        request_size = st.st_size;
    }
    else if (dsm_impl_posix_resize(fd, request_size) != 0)
    {
        int         save_errno;

        /* Back out what's already been done. */
        save_errno = errno;
        close(fd);
        ReleaseExternalFD();
        shm_unlink(name);
        errno = save_errno;

        /*
         * If we received a query cancel or termination signal, we will have
         * EINTR set here.  If the caller said that errors are OK here, check
         * for interrupts immediately.
         */
        if (errno == EINTR && elevel >= ERROR)
            CHECK_FOR_INTERRUPTS();

        ereport(elevel,
                (errcode_for_dynamic_shared_memory(),
                 errmsg("could not resize shared memory segment \"%s\" to %zu bytes: %m",
                        name, request_size)));
        return false;
    }

    /* Map it. */
    address = mmap(NULL, request_size, PROT_READ | PROT_WRITE,
                   MAP_SHARED | MAP_HASSEMAPHORE | MAP_NOSYNC, fd, 0);
    if (address == MAP_FAILED)
    {
        int         save_errno;

        /* Back out what's already been done. */
        save_errno = errno;
        close(fd);
        ReleaseExternalFD();
        if (op == DSM_OP_CREATE)
            shm_unlink(name);
        errno = save_errno;

        ereport(elevel,
                (errcode_for_dynamic_shared_memory(),
                 errmsg("could not map shared memory segment \"%s\": %m",
                        name)));
        return false;
    }
    *mapped_address = address;
    *mapped_size = request_size;
    close(fd);
    ReleaseExternalFD();

    return true;
}

/*
 * Set the size of a virtual memory region associated with a file descriptor.
 * If necessary, also ensure that virtual memory is actually allocated by the
 * operating system, to avoid nasty surprises later.
 *
 * Returns non-zero if either truncation or allocation fails, and sets errno.
 */
static int
dsm_impl_posix_resize(int fd, off_t size)
{
    int         rc;

    /* Truncate (or extend) the file to the requested size. */
    rc = ftruncate(fd, size);

    /*
     * On Linux, a shm_open fd is backed by a tmpfs file.  After resizing with
     * ftruncate, the file may contain a hole.  Accessing memory backed by a
     * hole causes tmpfs to allocate pages, which fails with SIGBUS if there
     * is no more tmpfs space available.  So we ask tmpfs to allocate pages
     * here, so we can fail gracefully with ENOSPC now rather than risking
     * SIGBUS later.
     */
#if defined(HAVE_POSIX_FALLOCATE) && defined(__linux__)
    if (rc == 0)
    {
        /*
         * We may get interrupted.  If so, just retry unless there is an
         * interrupt pending.  This avoids the possibility of looping forever
         * if another backend is repeatedly trying to interrupt us.
         */
        pgstat_report_wait_start(WAIT_EVENT_DSM_FILL_ZERO_WRITE);
        do
        {
            rc = posix_fallocate(fd, 0, size);
        } while (rc == EINTR && !(ProcDiePending || QueryCancelPending));
        pgstat_report_wait_end();

        /*
         * The caller expects errno to be set, but posix_fallocate() doesn't
         * set it.  Instead it returns error numbers directly.  So set errno,
         * even though we'll also return rc to indicate success or failure.
         */
        errno = rc;
    }
#endif                          /* HAVE_POSIX_FALLOCATE && __linux__ */

    return rc;
}

#endif                          /* USE_DSM_POSIX */

#ifdef USE_DSM_SYSV
/*
 * Operating system primitives to support System V shared memory.
 *
 * System V shared memory segments are manipulated using shmget(), shmat(),
 * shmdt(), and shmctl().  As the default allocation limits for System V
 * shared memory are usually quite low, the POSIX facilities may be
 * preferable; but those are not supported everywhere.
 */
static bool
dsm_impl_sysv(dsm_op op, dsm_handle handle, Size request_size,
              void **impl_private, void **mapped_address, Size *mapped_size,
              int elevel)
{
    key_t       key;
    int         ident;
    char       *address;
    char        name[64];
    int        *ident_cache;

    /*
     * POSIX shared memory and mmap-based shared memory identify segments with
     * names.  To avoid needless error message variation, we use the handle as
     * the name.
     */
    snprintf(name, 64, "%u", handle);

    /*
     * The System V shared memory namespace is very restricted; names are of
     * type key_t, which is expected to be some sort of integer data type, but
     * not necessarily the same one as dsm_handle.  Since we use dsm_handle to
     * identify shared memory segments across processes, this might seem like
     * a problem, but it's really not.  If dsm_handle is bigger than key_t,
     * the cast below might truncate away some bits from the handle the
     * user-provided, but it'll truncate exactly the same bits away in exactly
     * the same fashion every time we use that handle, which is all that
     * really matters.  Conversely, if dsm_handle is smaller than key_t, we
     * won't use the full range of available key space, but that's no big deal
     * either.
     *
     * We do make sure that the key isn't negative, because that might not be
     * portable.
     */
    key = (key_t) handle;
    if (key < 1)                /* avoid compiler warning if type is unsigned */
        key = -key;

    /*
     * There's one special key, IPC_PRIVATE, which can't be used.  If we end
     * up with that value by chance during a create operation, just pretend it
     * already exists, so that caller will retry.  If we run into it anywhere
     * else, the caller has passed a handle that doesn't correspond to
     * anything we ever created, which should not happen.
     */
    if (key == IPC_PRIVATE)
    {
        if (op != DSM_OP_CREATE)
            elog(DEBUG4, "System V shared memory key may not be IPC_PRIVATE");
        errno = EEXIST;
        return false;
    }

    /*
     * Before we can do anything with a shared memory segment, we have to map
     * the shared memory key to a shared memory identifier using shmget(). To
     * avoid repeated lookups, we store the key using impl_private.
     */
    if (*impl_private != NULL)
    {
        ident_cache = *impl_private;
        ident = *ident_cache;
    }
    else
    {
        int         flags = IPCProtection;
        size_t      segsize;

        /*
         * Allocate the memory BEFORE acquiring the resource, so that we don't
         * leak the resource if memory allocation fails.
         */
        ident_cache = MemoryContextAlloc(TopMemoryContext, sizeof(int));

        /*
         * When using shmget to find an existing segment, we must pass the
         * size as 0.  Passing a non-zero size which is greater than the
         * actual size will result in EINVAL.
         */
        segsize = 0;

        if (op == DSM_OP_CREATE)
        {
            flags |= IPC_CREAT | IPC_EXCL;
            segsize = request_size;
        }

        if ((ident = shmget(key, segsize, flags)) == -1)
        {
            if (errno != EEXIST)
            {
                int         save_errno = errno;

                pfree(ident_cache);
                errno = save_errno;
                ereport(elevel,
                        (errcode_for_dynamic_shared_memory(),
                         errmsg("could not get shared memory segment: %m")));
            }
            return false;
        }

        *ident_cache = ident;
        *impl_private = ident_cache;
    }

    /* Handle teardown cases. */
    if (op == DSM_OP_DETACH || op == DSM_OP_DESTROY)
    {
        pfree(ident_cache);
        *impl_private = NULL;
        if (*mapped_address != NULL && shmdt(*mapped_address) != 0)
        {
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not unmap shared memory segment \"%s\": %m",
                            name)));
            return false;
        }
        *mapped_address = NULL;
        *mapped_size = 0;
        if (op == DSM_OP_DESTROY && shmctl(ident, IPC_RMID, NULL) < 0)
        {
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not remove shared memory segment \"%s\": %m",
                            name)));
            return false;
        }
        return true;
    }

    /* If we're attaching it, we must use IPC_STAT to determine the size. */
    if (op == DSM_OP_ATTACH)
    {
        struct shmid_ds shm;

        if (shmctl(ident, IPC_STAT, &shm) != 0)
        {
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not stat shared memory segment \"%s\": %m",
                            name)));
            return false;
        }
        request_size = shm.shm_segsz;
    }

    /* Map it. */
    address = shmat(ident, NULL, PG_SHMAT_FLAGS);
    if (address == (void *) -1)
    {
        int         save_errno;

        /* Back out what's already been done. */
        save_errno = errno;
        if (op == DSM_OP_CREATE)
            shmctl(ident, IPC_RMID, NULL);
        errno = save_errno;

        ereport(elevel,
                (errcode_for_dynamic_shared_memory(),
                 errmsg("could not map shared memory segment \"%s\": %m",
                        name)));
        return false;
    }
    *mapped_address = address;
    *mapped_size = request_size;

    return true;
}
#endif

#ifdef USE_DSM_WINDOWS
/*
 * Operating system primitives to support Windows shared memory.
 *
 * Windows shared memory implementation is done using file mapping
 * which can be backed by either physical file or system paging file.
 * Current implementation uses system paging file as other effects
 * like performance are not clear for physical file and it is used in similar
 * way for main shared memory in windows.
 *
 * A memory mapping object is a kernel object - they always get deleted when
 * the last reference to them goes away, either explicitly via a CloseHandle or
 * when the process containing the reference exits.
 */
static bool
dsm_impl_windows(dsm_op op, dsm_handle handle, Size request_size,
                 void **impl_private, void **mapped_address,
                 Size *mapped_size, int elevel)
{
    char       *address;
    HANDLE      hmap;
    char        name[64];
    MEMORY_BASIC_INFORMATION info;

    /*
     * Storing the shared memory segment in the Global\ namespace, can allow
     * any process running in any session to access that file mapping object
     * provided that the caller has the required access rights. But to avoid
     * issues faced in main shared memory, we are using the naming convention
     * similar to main shared memory. We can change here once issue mentioned
     * in GetSharedMemName is resolved.
     */
    snprintf(name, 64, "%s.%u", SEGMENT_NAME_PREFIX, handle);

    /*
     * Handle teardown cases.  Since Windows automatically destroys the object
     * when no references remain, we can treat it the same as detach.
     */
    if (op == DSM_OP_DETACH || op == DSM_OP_DESTROY)
    {
        if (*mapped_address != NULL
            && UnmapViewOfFile(*mapped_address) == 0)
        {
            _dosmaperr(GetLastError());
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not unmap shared memory segment \"%s\": %m",
                            name)));
            return false;
        }
        if (*impl_private != NULL
            && CloseHandle(*impl_private) == 0)
        {
            _dosmaperr(GetLastError());
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not remove shared memory segment \"%s\": %m",
                            name)));
            return false;
        }

        *impl_private = NULL;
        *mapped_address = NULL;
        *mapped_size = 0;
        return true;
    }

    /* Create new segment or open an existing one for attach. */
    if (op == DSM_OP_CREATE)
    {
        DWORD       size_high;
        DWORD       size_low;
        DWORD       errcode;

        /* Shifts >= the width of the type are undefined. */
#ifdef _WIN64
        size_high = request_size >> 32;
#else
        size_high = 0;
#endif
        size_low = (DWORD) request_size;

        /* CreateFileMapping might not clear the error code on success */
        SetLastError(0);

        hmap = CreateFileMapping(INVALID_HANDLE_VALUE,  /* Use the pagefile */
                                 NULL,  /* Default security attrs */
                                 PAGE_READWRITE,    /* Memory is read/write */
                                 size_high, /* Upper 32 bits of size */
                                 size_low,  /* Lower 32 bits of size */
                                 name);

        errcode = GetLastError();
        if (errcode == ERROR_ALREADY_EXISTS || errcode == ERROR_ACCESS_DENIED)
        {
            /*
             * On Windows, when the segment already exists, a handle for the
             * existing segment is returned.  We must close it before
             * returning.  However, if the existing segment is created by a
             * service, then it returns ERROR_ACCESS_DENIED. We don't do
             * _dosmaperr here, so errno won't be modified.
             */
            if (hmap)
                CloseHandle(hmap);
            return false;
        }

        if (!hmap)
        {
            _dosmaperr(errcode);
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not create shared memory segment \"%s\": %m",
                            name)));
            return false;
        }
    }
    else
    {
        hmap = OpenFileMapping(FILE_MAP_WRITE | FILE_MAP_READ,
                               FALSE,   /* do not inherit the name */
                               name);   /* name of mapping object */
        if (!hmap)
        {
            _dosmaperr(GetLastError());
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not open shared memory segment \"%s\": %m",
                            name)));
            return false;
        }
    }

    /* Map it. */
    address = MapViewOfFile(hmap, FILE_MAP_WRITE | FILE_MAP_READ,
                            0, 0, 0);
    if (!address)
    {
        int         save_errno;

        _dosmaperr(GetLastError());
        /* Back out what's already been done. */
        save_errno = errno;
        CloseHandle(hmap);
        errno = save_errno;

        ereport(elevel,
                (errcode_for_dynamic_shared_memory(),
                 errmsg("could not map shared memory segment \"%s\": %m",
                        name)));
        return false;
    }

    /*
     * VirtualQuery gives size in page_size units, which is 4K for Windows. We
     * need size only when we are attaching, but it's better to get the size
     * when creating new segment to keep size consistent both for
     * DSM_OP_CREATE and DSM_OP_ATTACH.
     */
    if (VirtualQuery(address, &info, sizeof(info)) == 0)
    {
        int         save_errno;

        _dosmaperr(GetLastError());
        /* Back out what's already been done. */
        save_errno = errno;
        UnmapViewOfFile(address);
        CloseHandle(hmap);
        errno = save_errno;

        ereport(elevel,
                (errcode_for_dynamic_shared_memory(),
                 errmsg("could not stat shared memory segment \"%s\": %m",
                        name)));
        return false;
    }

    *mapped_address = address;
    *mapped_size = info.RegionSize;
    *impl_private = hmap;

    return true;
}
#endif

#ifdef USE_DSM_MMAP
/*
 * Operating system primitives to support mmap-based shared memory.
 *
 * Calling this "shared memory" is somewhat of a misnomer, because what
 * we're really doing is creating a bunch of files and mapping them into
 * our address space.  The operating system may feel obliged to
 * synchronize the contents to disk even if nothing is being paged out,
 * which will not serve us well.  The user can relocate the pg_dynshmem
 * directory to a ramdisk to avoid this problem, if available.
 */
static bool
dsm_impl_mmap(dsm_op op, dsm_handle handle, Size request_size,
              void **impl_private, void **mapped_address, Size *mapped_size,
              int elevel)
{
    char        name[64];
    int         flags;
    int         fd;
    char       *address;

    snprintf(name, 64, PG_DYNSHMEM_DIR "/" PG_DYNSHMEM_MMAP_FILE_PREFIX "%u",
             handle);

    /* Handle teardown cases. */
    if (op == DSM_OP_DETACH || op == DSM_OP_DESTROY)
    {
        if (*mapped_address != NULL
            && munmap(*mapped_address, *mapped_size) != 0)
        {
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not unmap shared memory segment \"%s\": %m",
                            name)));
            return false;
        }
        *mapped_address = NULL;
        *mapped_size = 0;
        if (op == DSM_OP_DESTROY && unlink(name) != 0)
        {
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not remove shared memory segment \"%s\": %m",
                            name)));
            return false;
        }
        return true;
    }

    /* Create new segment or open an existing one for attach. */
    flags = O_RDWR | (op == DSM_OP_CREATE ? O_CREAT | O_EXCL : 0);
    if ((fd = OpenTransientFile(name, flags)) == -1)
    {
        if (errno != EEXIST)
            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not open shared memory segment \"%s\": %m",
                            name)));
        return false;
    }

    /*
     * If we're attaching the segment, determine the current size; if we are
     * creating the segment, set the size to the requested value.
     */
    if (op == DSM_OP_ATTACH)
    {
        struct stat st;

        if (fstat(fd, &st) != 0)
        {
            int         save_errno;

            /* Back out what's already been done. */
            save_errno = errno;
            CloseTransientFile(fd);
            errno = save_errno;

            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not stat shared memory segment \"%s\": %m",
                            name)));
            return false;
        }
        request_size = st.st_size;
    }
    else
    {
        /*
         * Allocate a buffer full of zeros.
         *
         * Note: palloc zbuffer, instead of just using a local char array, to
         * ensure it is reasonably well-aligned; this may save a few cycles
         * transferring data to the kernel.
         */
        char       *zbuffer = (char *) palloc0(ZBUFFER_SIZE);
        uint32      remaining = request_size;
        bool        success = true;

        /*
         * Zero-fill the file. We have to do this the hard way to ensure that
         * all the file space has really been allocated, so that we don't
         * later seg fault when accessing the memory mapping.  This is pretty
         * pessimal.
         */
        while (success && remaining > 0)
        {
            Size        goal = remaining;

            if (goal > ZBUFFER_SIZE)
                goal = ZBUFFER_SIZE;
            pgstat_report_wait_start(WAIT_EVENT_DSM_FILL_ZERO_WRITE);
            if (write(fd, zbuffer, goal) == goal)
                remaining -= goal;
            else
                success = false;
            pgstat_report_wait_end();
        }

        if (!success)
        {
            int         save_errno;

            /* Back out what's already been done. */
            save_errno = errno;
            CloseTransientFile(fd);
            unlink(name);
            errno = save_errno ? save_errno : ENOSPC;

            ereport(elevel,
                    (errcode_for_dynamic_shared_memory(),
                     errmsg("could not resize shared memory segment \"%s\" to %zu bytes: %m",
                            name, request_size)));
            return false;
        }
    }

    /* Map it. */
    address = mmap(NULL, request_size, PROT_READ | PROT_WRITE,
                   MAP_SHARED | MAP_HASSEMAPHORE | MAP_NOSYNC, fd, 0);
    if (address == MAP_FAILED)
    {
        int         save_errno;

        /* Back out what's already been done. */
        save_errno = errno;
        CloseTransientFile(fd);
        if (op == DSM_OP_CREATE)
            unlink(name);
        errno = save_errno;

        ereport(elevel,
                (errcode_for_dynamic_shared_memory(),
                 errmsg("could not map shared memory segment \"%s\": %m",
                        name)));
        return false;
    }
    *mapped_address = address;
    *mapped_size = request_size;

    if (CloseTransientFile(fd) != 0)
    {
        ereport(elevel,
                (errcode_for_file_access(),
                 errmsg("could not close shared memory segment \"%s\": %m",
                        name)));
        return false;
    }

    return true;
}
#endif

/*
 * Implementation-specific actions that must be performed when a segment is to
 * be preserved even when no backend has it attached.
 *
 * Except on Windows, we don't need to do anything at all.  But since Windows
 * cleans up segments automatically when no references remain, we duplicate
 * the segment handle into the postmaster process.  The postmaster needn't
 * do anything to receive the handle; Windows transfers it automatically.
 */
void
dsm_impl_pin_segment(dsm_handle handle, void *impl_private,
                     void **impl_private_pm_handle)
{
    switch (dynamic_shared_memory_type)
    {
#ifdef USE_DSM_WINDOWS
        case DSM_IMPL_WINDOWS:
            {
                HANDLE      hmap;

                if (!DuplicateHandle(GetCurrentProcess(), impl_private,
                                     PostmasterHandle, &hmap, 0, FALSE,
                                     DUPLICATE_SAME_ACCESS))
                {
                    char        name[64];

                    snprintf(name, 64, "%s.%u", SEGMENT_NAME_PREFIX, handle);
                    _dosmaperr(GetLastError());
                    ereport(ERROR,
                            (errcode_for_dynamic_shared_memory(),
                             errmsg("could not duplicate handle for \"%s\": %m",
                                    name)));
                }

                /*
                 * Here, we remember the handle that we created in the
                 * postmaster process.  This handle isn't actually usable in
                 * any process other than the postmaster, but that doesn't
                 * matter.  We're just holding onto it so that, if the segment
                 * is unpinned, dsm_impl_unpin_segment can close it.
                 */
                *impl_private_pm_handle = hmap;
                break;
            }
#endif
        default:
            break;
    }
}

/*
 * Implementation-specific actions that must be performed when a segment is no
 * longer to be preserved, so that it will be cleaned up when all backends
 * have detached from it.
 *
 * Except on Windows, we don't need to do anything at all.  For Windows, we
 * close the extra handle that dsm_impl_pin_segment created in the
 * postmaster's process space.
 */
void
dsm_impl_unpin_segment(dsm_handle handle, void **impl_private)
{
    switch (dynamic_shared_memory_type)
    {
#ifdef USE_DSM_WINDOWS
        case DSM_IMPL_WINDOWS:
            {
                if (*impl_private &&
                    !DuplicateHandle(PostmasterHandle, *impl_private,
                                     NULL, NULL, 0, FALSE,
                                     DUPLICATE_CLOSE_SOURCE))
                {
                    char        name[64];

                    snprintf(name, 64, "%s.%u", SEGMENT_NAME_PREFIX, handle);
                    _dosmaperr(GetLastError());
                    ereport(ERROR,
                            (errcode_for_dynamic_shared_memory(),
                             errmsg("could not duplicate handle for \"%s\": %m",
                                    name)));
                }

                *impl_private = NULL;
                break;
            }
#endif
        default:
            break;
    }
}

static int
errcode_for_dynamic_shared_memory(void)
{
    if (errno == EFBIG || errno == ENOMEM)
        return errcode(ERRCODE_OUT_OF_MEMORY);
    else
        return errcode_for_file_access();
}