sysv_shmem.c

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/*-------------------------------------------------------------------------
 *
 * sysv_shmem.c
 *    Implement shared memory using SysV facilities
 *
 * These routines used to be a fairly thin layer on top of SysV shared
 * memory functionality.  With the addition of anonymous-shmem logic,
 * they're a bit fatter now.  We still require a SysV shmem block to
 * exist, though, because mmap'd shmem provides no way to find out how
 * many processes are attached, which we need for interlocking purposes.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/port/sysv_shmem.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include <signal.h>
#include <unistd.h>
#include <sys/file.h>
#include <sys/ipc.h>
#include <sys/mman.h>
#include <sys/shm.h>
#include <sys/stat.h>
 
#include "miscadmin.h"
#include "port/pg_bitutils.h"
#include "portability/mem.h"
#include "storage/dsm.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/pg_shmem.h"
#include "utils/guc.h"
#include "utils/guc_hooks.h"
#include "utils/pidfile.h"
 
 
/*
 * As of PostgreSQL 9.3, we normally allocate only a very small amount of
 * System V shared memory, and only for the purposes of providing an
 * interlock to protect the data directory.  The real shared memory block
 * is allocated using mmap().  This works around the problem that many
 * systems have very low limits on the amount of System V shared memory
 * that can be allocated.  Even a limit of a few megabytes will be enough
 * to run many copies of PostgreSQL without needing to adjust system settings.
 *
 * We assume that no one will attempt to run PostgreSQL 9.3 or later on
 * systems that are ancient enough that anonymous shared memory is not
 * supported, such as pre-2.4 versions of Linux.  If that turns out to be
 * false, we might need to add compile and/or run-time tests here and do this
 * only if the running kernel supports it.
 *
 * However, we must always disable this logic in the EXEC_BACKEND case, and
 * fall back to the old method of allocating the entire segment using System V
 * shared memory, because there's no way to attach an anonymous mmap'd segment
 * to a process after exec().  Since EXEC_BACKEND is intended only for
 * developer use, this shouldn't be a big problem.  Because of this, we do
 * not worry about supporting anonymous shmem in the EXEC_BACKEND cases below.
 *
 * As of PostgreSQL 12, we regained the ability to use a large System V shared
 * memory region even in non-EXEC_BACKEND builds, if shared_memory_type is set
 * to sysv (though this is not the default).
 */
 
 
typedef key_t IpcMemoryKey;     /* shared memory key passed to shmget(2) */
typedef int IpcMemoryId;        /* shared memory ID returned by shmget(2) */
 
/*
 * How does a given IpcMemoryId relate to this PostgreSQL process?
 *
 * One could recycle unattached segments of different data directories if we
 * distinguished that case from other SHMSTATE_FOREIGN cases.  Doing so would
 * cause us to visit less of the key space, making us less likely to detect a
 * SHMSTATE_ATTACHED key.  It would also complicate the concurrency analysis,
 * in that postmasters of different data directories could simultaneously
 * attempt to recycle a given key.  We'll waste keys longer in some cases, but
 * avoiding the problems of the alternative justifies that loss.
 */
typedef enum
{
    SHMSTATE_ANALYSIS_FAILURE,  /* unexpected failure to analyze the ID */
    SHMSTATE_ATTACHED,          /* pertinent to DataDir, has attached PIDs */
    SHMSTATE_ENOENT,            /* no segment of that ID */
    SHMSTATE_FOREIGN,           /* exists, but not pertinent to DataDir */
    SHMSTATE_UNATTACHED,        /* pertinent to DataDir, no attached PIDs */
} IpcMemoryState;
 
 
unsigned long UsedShmemSegID = 0;
void       *UsedShmemSegAddr = NULL;
 
static Size AnonymousShmemSize;
static void *AnonymousShmem = NULL;
 
static void *InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size);
static void IpcMemoryDetach(int status, Datum shmaddr);
static void IpcMemoryDelete(int status, Datum shmId);
static IpcMemoryState PGSharedMemoryAttach(IpcMemoryId shmId,
                                           void *attachAt,
                                           PGShmemHeader **addr);
 
 
/*
 *  InternalIpcMemoryCreate(memKey, size)
 *
 * Attempt to create a new shared memory segment with the specified key.
 * Will fail (return NULL) if such a segment already exists.  If successful,
 * attach the segment to the current process and return its attached address.
 * On success, callbacks are registered with on_shmem_exit to detach and
 * delete the segment when on_shmem_exit is called.
 *
 * If we fail with a failure code other than collision-with-existing-segment,
 * print out an error and abort.  Other types of errors are not recoverable.
 */
static void *
InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size)
{
    IpcMemoryId shmid;
    void       *requestedAddress = NULL;
    void       *memAddress;
 
    /*
     * Normally we just pass requestedAddress = NULL to shmat(), allowing the
     * system to choose where the segment gets mapped.  But in an EXEC_BACKEND
     * build, it's possible for whatever is chosen in the postmaster to not
     * work for backends, due to variations in address space layout.  As a
     * rather klugy workaround, allow the user to specify the address to use
     * via setting the environment variable PG_SHMEM_ADDR.  (If this were of
     * interest for anything except debugging, we'd probably create a cleaner
     * and better-documented way to set it, such as a GUC.)
     */
#ifdef EXEC_BACKEND
    {
        char       *pg_shmem_addr = getenv("PG_SHMEM_ADDR");
 
        if (pg_shmem_addr)
            requestedAddress = (void *) strtoul(pg_shmem_addr, NULL, 0);
        else
        {
#if defined(__darwin__) && SIZEOF_VOID_P == 8
            /*
             * Provide a default value that is believed to avoid problems with
             * ASLR on the current macOS release.
             */
            requestedAddress = (void *) 0x80000000000;
#endif
        }
    }
#endif
 
    shmid = shmget(memKey, size, IPC_CREAT | IPC_EXCL | IPCProtection);
 
    if (shmid < 0)
    {
        int         shmget_errno = errno;
 
        /*
         * Fail quietly if error indicates a collision with existing segment.
         * One would expect EEXIST, given that we said IPC_EXCL, but perhaps
         * we could get a permission violation instead?  Also, EIDRM might
         * occur if an old seg is slated for destruction but not gone yet.
         */
        if (shmget_errno == EEXIST || shmget_errno == EACCES
#ifdef EIDRM
            || shmget_errno == EIDRM
#endif
            )
            return NULL;
 
        /*
         * Some BSD-derived kernels are known to return EINVAL, not EEXIST, if
         * there is an existing segment but it's smaller than "size" (this is
         * a result of poorly-thought-out ordering of error tests). To
         * distinguish between collision and invalid size in such cases, we
         * make a second try with size = 0.  These kernels do not test size
         * against SHMMIN in the preexisting-segment case, so we will not get
         * EINVAL a second time if there is such a segment.
         */
        if (shmget_errno == EINVAL)
        {
            shmid = shmget(memKey, 0, IPC_CREAT | IPC_EXCL | IPCProtection);
 
            if (shmid < 0)
            {
                /* As above, fail quietly if we verify a collision */
                if (errno == EEXIST || errno == EACCES
#ifdef EIDRM
                    || errno == EIDRM
#endif
                    )
                    return NULL;
                /* Otherwise, fall through to report the original error */
            }
            else
            {
                /*
                 * On most platforms we cannot get here because SHMMIN is
                 * greater than zero.  However, if we do succeed in creating a
                 * zero-size segment, free it and then fall through to report
                 * the original error.
                 */
                if (shmctl(shmid, IPC_RMID, NULL) < 0)
                    elog(LOG, "shmctl(%d, %d, 0) failed: %m",
                         (int) shmid, IPC_RMID);
            }
        }
 
        /*
         * Else complain and abort.
         *
         * Note: at this point EINVAL should mean that either SHMMIN or SHMMAX
         * is violated.  SHMALL violation might be reported as either ENOMEM
         * (BSDen) or ENOSPC (Linux); the Single Unix Spec fails to say which
         * it should be.  SHMMNI violation is ENOSPC, per spec.  Just plain
         * not-enough-RAM is ENOMEM.
         */
        errno = shmget_errno;
        ereport(FATAL,
                (errmsg("could not create shared memory segment: %m"),
                 errdetail("Failed system call was shmget(key=%lu, size=%zu, 0%o).",
                           (unsigned long) memKey, size,
                           IPC_CREAT | IPC_EXCL | IPCProtection),
                 (shmget_errno == EINVAL) ?
                 errhint("This error usually means that PostgreSQL's request for a shared memory "
                         "segment exceeded your kernel's SHMMAX parameter, or possibly that "
                         "it is less than "
                         "your kernel's SHMMIN parameter.\n"
                         "The PostgreSQL documentation contains more information about shared "
                         "memory configuration.") : 0,
                 (shmget_errno == ENOMEM) ?
                 errhint("This error usually means that PostgreSQL's request for a shared "
                         "memory segment exceeded your kernel's SHMALL parameter.  You might need "
                         "to reconfigure the kernel with larger SHMALL.\n"
                         "The PostgreSQL documentation contains more information about shared "
                         "memory configuration.") : 0,
                 (shmget_errno == ENOSPC) ?
                 errhint("This error does *not* mean that you have run out of disk space.  "
                         "It occurs either if all available shared memory IDs have been taken, "
                         "in which case you need to raise the SHMMNI parameter in your kernel, "
                         "or because the system's overall limit for shared memory has been "
                         "reached.\n"
                         "The PostgreSQL documentation contains more information about shared "
                         "memory configuration.") : 0));
    }
 
    /* Register on-exit routine to delete the new segment */
    on_shmem_exit(IpcMemoryDelete, Int32GetDatum(shmid));
 
    /* OK, should be able to attach to the segment */
    memAddress = shmat(shmid, requestedAddress, PG_SHMAT_FLAGS);
 
    if (memAddress == (void *) -1)
        elog(FATAL, "shmat(id=%d, addr=%p, flags=0x%x) failed: %m",
             shmid, requestedAddress, PG_SHMAT_FLAGS);
 
    /* Register on-exit routine to detach new segment before deleting */
    on_shmem_exit(IpcMemoryDetach, PointerGetDatum(memAddress));
 
    /*
     * Store shmem key and ID in data directory lockfile.  Format to try to
     * keep it the same length always (trailing junk in the lockfile won't
     * hurt, but might confuse humans).
     */
    {
        char        line[64];
 
        sprintf(line, "%9lu %9lu",
                (unsigned long) memKey, (unsigned long) shmid);
        AddToDataDirLockFile(LOCK_FILE_LINE_SHMEM_KEY, line);
    }
 
    return memAddress;
}
 
/****************************************************************************/
/*  IpcMemoryDetach(status, shmaddr)    removes a shared memory segment     */
/*                                      from process' address space         */
/*  (called as an on_shmem_exit callback, hence funny argument list)        */
/****************************************************************************/
static void
IpcMemoryDetach(int status, Datum shmaddr)
{
    /* Detach System V shared memory block. */
    if (shmdt(DatumGetPointer(shmaddr)) < 0)
        elog(LOG, "shmdt(%p) failed: %m", DatumGetPointer(shmaddr));
}
 
/****************************************************************************/
/*  IpcMemoryDelete(status, shmId)      deletes a shared memory segment     */
/*  (called as an on_shmem_exit callback, hence funny argument list)        */
/****************************************************************************/
static void
IpcMemoryDelete(int status, Datum shmId)
{
    if (shmctl(DatumGetInt32(shmId), IPC_RMID, NULL) < 0)
        elog(LOG, "shmctl(%d, %d, 0) failed: %m",
             DatumGetInt32(shmId), IPC_RMID);
}
 
/*
 * PGSharedMemoryIsInUse
 *
 * Is a previously-existing shmem segment still existing and in use?
 *
 * The point of this exercise is to detect the case where a prior postmaster
 * crashed, but it left child backends that are still running.  Therefore
 * we only care about shmem segments that are associated with the intended
 * DataDir.  This is an important consideration since accidental matches of
 * shmem segment IDs are reasonably common.
 */
bool
PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2)
{
    PGShmemHeader *memAddress;
    IpcMemoryState state;
 
    state = PGSharedMemoryAttach((IpcMemoryId) id2, NULL, &memAddress);
    if (memAddress && shmdt(memAddress) < 0)
        elog(LOG, "shmdt(%p) failed: %m", memAddress);
    switch (state)
    {
        case SHMSTATE_ENOENT:
        case SHMSTATE_FOREIGN:
        case SHMSTATE_UNATTACHED:
            return false;
        case SHMSTATE_ANALYSIS_FAILURE:
        case SHMSTATE_ATTACHED:
            return true;
    }
    return true;
}
 
/*
 * Test for a segment with id shmId; see comment at IpcMemoryState.
 *
 * If the segment exists, we'll attempt to attach to it, using attachAt
 * if that's not NULL (but it's best to pass NULL if possible).
 *
 * *addr is set to the segment memory address if we attached to it, else NULL.
 */
static IpcMemoryState
PGSharedMemoryAttach(IpcMemoryId shmId,
                     void *attachAt,
                     PGShmemHeader **addr)
{
    struct shmid_ds shmStat;
    struct stat statbuf;
    PGShmemHeader *hdr;
 
    *addr = NULL;
 
    /*
     * First, try to stat the shm segment ID, to see if it exists at all.
     */
    if (shmctl(shmId, IPC_STAT, &shmStat) < 0)
    {
        /*
         * EINVAL actually has multiple possible causes documented in the
         * shmctl man page, but we assume it must mean the segment no longer
         * exists.
         */
        if (errno == EINVAL)
            return SHMSTATE_ENOENT;
 
        /*
         * EACCES implies we have no read permission, which means it is not a
         * Postgres shmem segment (or at least, not one that is relevant to
         * our data directory).
         */
        if (errno == EACCES)
            return SHMSTATE_FOREIGN;
 
        /*
         * Some Linux kernel versions (in fact, all of them as of July 2007)
         * sometimes return EIDRM when EINVAL is correct.  The Linux kernel
         * actually does not have any internal state that would justify
         * returning EIDRM, so we can get away with assuming that EIDRM is
         * equivalent to EINVAL on that platform.
         */
#ifdef HAVE_LINUX_EIDRM_BUG
        if (errno == EIDRM)
            return SHMSTATE_ENOENT;
#endif
 
        /*
         * Otherwise, we had better assume that the segment is in use.  The
         * only likely case is (non-Linux, assumed spec-compliant) EIDRM,
         * which implies that the segment has been IPC_RMID'd but there are
         * still processes attached to it.
         */
        return SHMSTATE_ANALYSIS_FAILURE;
    }
 
    /*
     * Try to attach to the segment and see if it matches our data directory.
     * This avoids any risk of duplicate-shmem-key conflicts on machines that
     * are running several postmasters under the same userid.
     *
     * (When we're called from PGSharedMemoryCreate, this stat call is
     * duplicative; but since this isn't a high-traffic case it's not worth
     * trying to optimize.)
     */
    if (stat(DataDir, &statbuf) < 0)
        return SHMSTATE_ANALYSIS_FAILURE;   /* can't stat; be conservative */
 
    hdr = (PGShmemHeader *) shmat(shmId, attachAt, PG_SHMAT_FLAGS);
    if (hdr == (PGShmemHeader *) -1)
    {
        /*
         * Attachment failed.  The cases we're interested in are the same as
         * for the shmctl() call above.  In particular, note that the owning
         * postmaster could have terminated and removed the segment between
         * shmctl() and shmat().
         *
         * If attachAt isn't NULL, it's possible that EINVAL reflects a
         * problem with that address not a vanished segment, so it's best to
         * pass NULL when probing for conflicting segments.
         */
        if (errno == EINVAL)
            return SHMSTATE_ENOENT; /* segment disappeared */
        if (errno == EACCES)
            return SHMSTATE_FOREIGN;    /* must be non-Postgres */
#ifdef HAVE_LINUX_EIDRM_BUG
        if (errno == EIDRM)
            return SHMSTATE_ENOENT; /* segment disappeared */
#endif
        /* Otherwise, be conservative. */
        return SHMSTATE_ANALYSIS_FAILURE;
    }
    *addr = hdr;
 
    if (hdr->magic != PGShmemMagic ||
        hdr->device != statbuf.st_dev ||
        hdr->inode != statbuf.st_ino)
    {
        /*
         * It's either not a Postgres segment, or not one for my data
         * directory.
         */
        return SHMSTATE_FOREIGN;
    }
 
    /*
     * It does match our data directory, so now test whether any processes are
     * still attached to it.  (We are, now, but the shm_nattch result is from
     * before we attached to it.)
     */
    return shmStat.shm_nattch == 0 ? SHMSTATE_UNATTACHED : SHMSTATE_ATTACHED;
}
 
/*
 * Identify the huge page size to use, and compute the related mmap flags.
 *
 * Some Linux kernel versions have a bug causing mmap() to fail on requests
 * that are not a multiple of the hugepage size.  Versions without that bug
 * instead silently round the request up to the next hugepage multiple ---
 * and then munmap() fails when we give it a size different from that.
 * So we have to round our request up to a multiple of the actual hugepage
 * size to avoid trouble.
 *
 * Doing the round-up ourselves also lets us make use of the extra memory,
 * rather than just wasting it.  Currently, we just increase the available
 * space recorded in the shmem header, which will make the extra usable for
 * purposes such as additional locktable entries.  Someday, for very large
 * hugepage sizes, we might want to think about more invasive strategies,
 * such as increasing shared_buffers to absorb the extra space.
 *
 * Returns the (real, assumed or config provided) page size into
 * *hugepagesize, and the hugepage-related mmap flags to use into
 * *mmap_flags if requested by the caller.  If huge pages are not supported,
 * *hugepagesize and *mmap_flags are set to 0.
 */
void
GetHugePageSize(Size *hugepagesize, int *mmap_flags)
{
#ifdef MAP_HUGETLB
 
    Size        default_hugepagesize = 0;
    Size        hugepagesize_local = 0;
    int         mmap_flags_local = 0;
 
    /*
     * System-dependent code to find out the default huge page size.
     *
     * On Linux, read /proc/meminfo looking for a line like "Hugepagesize:
     * nnnn kB".  Ignore any failures, falling back to the preset default.
     */
#ifdef __linux__
 
    {
        FILE       *fp = AllocateFile("/proc/meminfo", "r");
        char        buf[128];
        unsigned int sz;
        char        ch;
 
        if (fp)
        {
            while (fgets(buf, sizeof(buf), fp))
            {
                if (sscanf(buf, "Hugepagesize: %u %c", &sz, &ch) == 2)
                {
                    if (ch == 'k')
                    {
                        default_hugepagesize = sz * (Size) 1024;
                        break;
                    }
                    /* We could accept other units besides kB, if needed */
                }
            }
            FreeFile(fp);
        }
    }
#endif                          /* __linux__ */
 
    if (huge_page_size != 0)
    {
        /* If huge page size is requested explicitly, use that. */
        hugepagesize_local = (Size) huge_page_size * 1024;
    }
    else if (default_hugepagesize != 0)
    {
        /* Otherwise use the system default, if we have it. */
        hugepagesize_local = default_hugepagesize;
    }
    else
    {
        /*
         * If we fail to find out the system's default huge page size, or no
         * huge page size is requested explicitly, assume it is 2MB. This will
         * work fine when the actual size is less.  If it's more, we might get
         * mmap() or munmap() failures due to unaligned requests; but at this
         * writing, there are no reports of any non-Linux systems being picky
         * about that.
         */
        hugepagesize_local = 2 * 1024 * 1024;
    }
 
    mmap_flags_local = MAP_HUGETLB;
 
    /*
     * On recent enough Linux, also include the explicit page size, if
     * necessary.
     */
#if defined(MAP_HUGE_MASK) && defined(MAP_HUGE_SHIFT)
    if (hugepagesize_local != default_hugepagesize)
    {
        int         shift = pg_ceil_log2_64(hugepagesize_local);
 
        mmap_flags_local |= (shift & MAP_HUGE_MASK) << MAP_HUGE_SHIFT;
    }
#endif
 
    /* assign the results found */
    if (mmap_flags)
        *mmap_flags = mmap_flags_local;
    if (hugepagesize)
        *hugepagesize = hugepagesize_local;
 
#else
 
    if (hugepagesize)
        *hugepagesize = 0;
    if (mmap_flags)
        *mmap_flags = 0;
 
#endif                          /* MAP_HUGETLB */
}
 
/*
 * GUC check_hook for huge_page_size
 */
bool
check_huge_page_size(int *newval, void **extra, GucSource source)
{
#if !(defined(MAP_HUGE_MASK) && defined(MAP_HUGE_SHIFT))
    /* Recent enough Linux only, for now.  See GetHugePageSize(). */
    if (*newval != 0)
    {
        GUC_check_errdetail("\"huge_page_size\" must be 0 on this platform.");
        return false;
    }
#endif
    return true;
}
 
/*
 * Creates an anonymous mmap()ed shared memory segment.
 *
 * Pass the requested size in *size.  This function will modify *size to the
 * actual size of the allocation, if it ends up allocating a segment that is
 * larger than requested.
 */
static void *
CreateAnonymousSegment(Size *size)
{
    Size        allocsize = *size;
    void       *ptr = MAP_FAILED;
    int         mmap_errno = 0;
 
#ifndef MAP_HUGETLB
    /* PGSharedMemoryCreate should have dealt with this case */
    Assert(huge_pages != HUGE_PAGES_ON);
#else
    if (huge_pages == HUGE_PAGES_ON || huge_pages == HUGE_PAGES_TRY)
    {
        /*
         * Round up the request size to a suitable large value.
         */
        Size        hugepagesize;
        int         mmap_flags;
 
        GetHugePageSize(&hugepagesize, &mmap_flags);
 
        if (allocsize % hugepagesize != 0)
            allocsize += hugepagesize - (allocsize % hugepagesize);
 
        ptr = mmap(NULL, allocsize, PROT_READ | PROT_WRITE,
                   PG_MMAP_FLAGS | mmap_flags, -1, 0);
        mmap_errno = errno;
        if (huge_pages == HUGE_PAGES_TRY && ptr == MAP_FAILED)
            elog(DEBUG1, "mmap(%zu) with MAP_HUGETLB failed, huge pages disabled: %m",
                 allocsize);
    }
#endif
 
    /*
     * Report whether huge pages are in use.  This needs to be tracked before
     * the second mmap() call if attempting to use huge pages failed
     * previously.
     */
    SetConfigOption("huge_pages_status", (ptr == MAP_FAILED) ? "off" : "on",
                    PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
 
    if (ptr == MAP_FAILED && huge_pages != HUGE_PAGES_ON)
    {
        /*
         * Use the original size, not the rounded-up value, when falling back
         * to non-huge pages.
         */
        allocsize = *size;
        ptr = mmap(NULL, allocsize, PROT_READ | PROT_WRITE,
                   PG_MMAP_FLAGS, -1, 0);
        mmap_errno = errno;
    }
 
    if (ptr == MAP_FAILED)
    {
        errno = mmap_errno;
        ereport(FATAL,
                (errmsg("could not map anonymous shared memory: %m"),
                 (mmap_errno == ENOMEM) ?
                 errhint("This error usually means that PostgreSQL's request "
                         "for a shared memory segment exceeded available memory, "
                         "swap space, or huge pages. To reduce the request size "
                         "(currently %zu bytes), reduce PostgreSQL's shared "
                         "memory usage, perhaps by reducing \"shared_buffers\" or "
                         "\"max_connections\".",
                         allocsize) : 0));
    }
 
    *size = allocsize;
    return ptr;
}
 
/*
 * AnonymousShmemDetach --- detach from an anonymous mmap'd block
 * (called as an on_shmem_exit callback, hence funny argument list)
 */
static void
AnonymousShmemDetach(int status, Datum arg)
{
    /* Release anonymous shared memory block, if any. */
    if (AnonymousShmem != NULL)
    {
        if (munmap(AnonymousShmem, AnonymousShmemSize) < 0)
            elog(LOG, "munmap(%p, %zu) failed: %m",
                 AnonymousShmem, AnonymousShmemSize);
        AnonymousShmem = NULL;
    }
}
 
/*
 * PGSharedMemoryCreate
 *
 * Create a shared memory segment of the given size and initialize its
 * standard header.  Also, register an on_shmem_exit callback to release
 * the storage.
 *
 * Dead Postgres segments pertinent to this DataDir are recycled if found, but
 * we do not fail upon collision with foreign shmem segments.  The idea here
 * is to detect and re-use keys that may have been assigned by a crashed
 * postmaster or backend.
 */
PGShmemHeader *
PGSharedMemoryCreate(Size size,
                     PGShmemHeader **shim)
{
    IpcMemoryKey NextShmemSegID;
    void       *memAddress;
    PGShmemHeader *hdr;
    struct stat statbuf;
    Size        sysvsize;
 
    /*
     * We use the data directory's ID info (inode and device numbers) to
     * positively identify shmem segments associated with this data dir, and
     * also as seeds for searching for a free shmem key.
     */
    if (stat(DataDir, &statbuf) < 0)
        ereport(FATAL,
                (errcode_for_file_access(),
                 errmsg("could not stat data directory \"%s\": %m",
                        DataDir)));
 
    /* Complain if hugepages demanded but we can't possibly support them */
#if !defined(MAP_HUGETLB)
    if (huge_pages == HUGE_PAGES_ON)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("huge pages not supported on this platform")));
#endif
 
    /* For now, we don't support huge pages in SysV memory */
    if (huge_pages == HUGE_PAGES_ON && shared_memory_type != SHMEM_TYPE_MMAP)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("huge pages not supported with the current \"shared_memory_type\" setting")));
 
    /* Room for a header? */
    Assert(size > MAXALIGN(sizeof(PGShmemHeader)));
 
    if (shared_memory_type == SHMEM_TYPE_MMAP)
    {
        AnonymousShmem = CreateAnonymousSegment(&size);
        AnonymousShmemSize = size;
 
        /* Register on-exit routine to unmap the anonymous segment */
        on_shmem_exit(AnonymousShmemDetach, (Datum) 0);
 
        /* Now we need only allocate a minimal-sized SysV shmem block. */
        sysvsize = sizeof(PGShmemHeader);
    }
    else
    {
        sysvsize = size;
 
        /* huge pages are only available with mmap */
        SetConfigOption("huge_pages_status", "off",
                        PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
    }
 
    /*
     * Loop till we find a free IPC key.  Trust CreateDataDirLockFile() to
     * ensure no more than one postmaster per data directory can enter this
     * loop simultaneously.  (CreateDataDirLockFile() does not entirely ensure
     * that, but prefer fixing it over coping here.)
     */
    NextShmemSegID = statbuf.st_ino;
 
    for (;;)
    {
        IpcMemoryId shmid;
        PGShmemHeader *oldhdr;
        IpcMemoryState state;
 
        /* Try to create new segment */
        memAddress = InternalIpcMemoryCreate(NextShmemSegID, sysvsize);
        if (memAddress)
            break;              /* successful create and attach */
 
        /* Check shared memory and possibly remove and recreate */
 
        /*
         * shmget() failure is typically EACCES, hence SHMSTATE_FOREIGN.
         * ENOENT, a narrow possibility, implies SHMSTATE_ENOENT, but one can
         * safely treat SHMSTATE_ENOENT like SHMSTATE_FOREIGN.
         */
        shmid = shmget(NextShmemSegID, sizeof(PGShmemHeader), 0);
        if (shmid < 0)
        {
            oldhdr = NULL;
            state = SHMSTATE_FOREIGN;
        }
        else
            state = PGSharedMemoryAttach(shmid, NULL, &oldhdr);
 
        switch (state)
        {
            case SHMSTATE_ANALYSIS_FAILURE:
            case SHMSTATE_ATTACHED:
                ereport(FATAL,
                        (errcode(ERRCODE_LOCK_FILE_EXISTS),
                         errmsg("pre-existing shared memory block (key %lu, ID %lu) is still in use",
                                (unsigned long) NextShmemSegID,
                                (unsigned long) shmid),
                         errhint("Terminate any old server processes associated with data directory \"%s\".",
                                 DataDir)));
                break;
            case SHMSTATE_ENOENT:
 
                /*
                 * To our surprise, some other process deleted since our last
                 * InternalIpcMemoryCreate().  Moments earlier, we would have
                 * seen SHMSTATE_FOREIGN.  Try that same ID again.
                 */
                elog(LOG,
                     "shared memory block (key %lu, ID %lu) deleted during startup",
                     (unsigned long) NextShmemSegID,
                     (unsigned long) shmid);
                break;
            case SHMSTATE_FOREIGN:
                NextShmemSegID++;
                break;
            case SHMSTATE_UNATTACHED:
 
                /*
                 * The segment pertains to DataDir, and every process that had
                 * used it has died or detached.  Zap it, if possible, and any
                 * associated dynamic shared memory segments, as well.  This
                 * shouldn't fail, but if it does, assume the segment belongs
                 * to someone else after all, and try the next candidate.
                 * Otherwise, try again to create the segment.  That may fail
                 * if some other process creates the same shmem key before we
                 * do, in which case we'll try the next key.
                 */
                if (oldhdr->dsm_control != 0)
                    dsm_cleanup_using_control_segment(oldhdr->dsm_control);
                if (shmctl(shmid, IPC_RMID, NULL) < 0)
                    NextShmemSegID++;
                break;
        }
 
        if (oldhdr && shmdt(oldhdr) < 0)
            elog(LOG, "shmdt(%p) failed: %m", oldhdr);
    }
 
    /* Initialize new segment. */
    hdr = (PGShmemHeader *) memAddress;
    hdr->creatorPID = getpid();
    hdr->magic = PGShmemMagic;
    hdr->dsm_control = 0;
 
    /* Fill in the data directory ID info, too */
    hdr->device = statbuf.st_dev;
    hdr->inode = statbuf.st_ino;
 
    /*
     * Initialize space allocation status for segment.
     */
    hdr->totalsize = size;
    hdr->freeoffset = MAXALIGN(sizeof(PGShmemHeader));
    *shim = hdr;
 
    /* Save info for possible future use */
    UsedShmemSegAddr = memAddress;
    UsedShmemSegID = (unsigned long) NextShmemSegID;
 
    /*
     * If AnonymousShmem is NULL here, then we're not using anonymous shared
     * memory, and should return a pointer to the System V shared memory
     * block. Otherwise, the System V shared memory block is only a shim, and
     * we must return a pointer to the real block.
     */
    if (AnonymousShmem == NULL)
        return hdr;
    memcpy(AnonymousShmem, hdr, sizeof(PGShmemHeader));
    return (PGShmemHeader *) AnonymousShmem;
}
 
#ifdef EXEC_BACKEND
 
/*
 * PGSharedMemoryReAttach
 *
 * This is called during startup of a postmaster child process to re-attach to
 * an already existing shared memory segment.  This is needed only in the
 * EXEC_BACKEND case; otherwise postmaster children inherit the shared memory
 * segment attachment via fork().
 *
 * UsedShmemSegID and UsedShmemSegAddr are implicit parameters to this
 * routine.  The caller must have already restored them to the postmaster's
 * values.
 */
void
PGSharedMemoryReAttach(void)
{
    IpcMemoryId shmid;
    PGShmemHeader *hdr;
    IpcMemoryState state;
    void       *origUsedShmemSegAddr = UsedShmemSegAddr;
 
    Assert(UsedShmemSegAddr != NULL);
    Assert(IsUnderPostmaster);
 
#ifdef __CYGWIN__
    /* cygipc (currently) appears to not detach on exec. */
    PGSharedMemoryDetach();
    UsedShmemSegAddr = origUsedShmemSegAddr;
#endif
 
    elog(DEBUG3, "attaching to %p", UsedShmemSegAddr);
    shmid = shmget(UsedShmemSegID, sizeof(PGShmemHeader), 0);
    if (shmid < 0)
        state = SHMSTATE_FOREIGN;
    else
        state = PGSharedMemoryAttach(shmid, UsedShmemSegAddr, &hdr);
    if (state != SHMSTATE_ATTACHED)
        elog(FATAL, "could not reattach to shared memory (key=%d, addr=%p): %m",
             (int) UsedShmemSegID, UsedShmemSegAddr);
    if (hdr != origUsedShmemSegAddr)
        elog(FATAL, "reattaching to shared memory returned unexpected address (got %p, expected %p)",
             hdr, origUsedShmemSegAddr);
    dsm_set_control_handle(hdr->dsm_control);
 
    UsedShmemSegAddr = hdr;     /* probably redundant */
}
 
/*
 * PGSharedMemoryNoReAttach
 *
 * This is called during startup of a postmaster child process when we choose
 * *not* to re-attach to the existing shared memory segment.  We must clean up
 * to leave things in the appropriate state.  This is not used in the non
 * EXEC_BACKEND case, either.
 *
 * The child process startup logic might or might not call PGSharedMemoryDetach
 * after this; make sure that it will be a no-op if called.
 *
 * UsedShmemSegID and UsedShmemSegAddr are implicit parameters to this
 * routine.  The caller must have already restored them to the postmaster's
 * values.
 */
void
PGSharedMemoryNoReAttach(void)
{
    Assert(UsedShmemSegAddr != NULL);
    Assert(IsUnderPostmaster);
 
#ifdef __CYGWIN__
    /* cygipc (currently) appears to not detach on exec. */
    PGSharedMemoryDetach();
#endif
 
    /* For cleanliness, reset UsedShmemSegAddr to show we're not attached. */
    UsedShmemSegAddr = NULL;
    /* And the same for UsedShmemSegID. */
    UsedShmemSegID = 0;
}
 
#endif                          /* EXEC_BACKEND */
 
/*
 * PGSharedMemoryDetach
 *
 * Detach from the shared memory segment, if still attached.  This is not
 * intended to be called explicitly by the process that originally created the
 * segment (it will have on_shmem_exit callback(s) registered to do that).
 * Rather, this is for subprocesses that have inherited an attachment and want
 * to get rid of it.
 *
 * UsedShmemSegID and UsedShmemSegAddr are implicit parameters to this
 * routine, also AnonymousShmem and AnonymousShmemSize.
 */
void
PGSharedMemoryDetach(void)
{
    if (UsedShmemSegAddr != NULL)
    {
        if ((shmdt(UsedShmemSegAddr) < 0)
#if defined(EXEC_BACKEND) && defined(__CYGWIN__)
        /* Work-around for cygipc exec bug */
            && shmdt(NULL) < 0
#endif
            )
            elog(LOG, "shmdt(%p) failed: %m", UsedShmemSegAddr);
        UsedShmemSegAddr = NULL;
    }
 
    if (AnonymousShmem != NULL)
    {
        if (munmap(AnonymousShmem, AnonymousShmemSize) < 0)
            elog(LOG, "munmap(%p, %zu) failed: %m",
                 AnonymousShmem, AnonymousShmemSize);
        AnonymousShmem = NULL;
    }
}