dynahash.c

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/*-------------------------------------------------------------------------
 *
 * dynahash.c
 *    dynamic hash tables
 *
 * dynahash.c supports both local-to-a-backend hash tables and hash tables in
 * shared memory.  For shared hash tables, it is the caller's responsibility
 * to provide appropriate access interlocking.  The simplest convention is
 * that a single LWLock protects the whole hash table.  Searches (HASH_FIND or
 * hash_seq_search) need only shared lock, but any update requires exclusive
 * lock.  For heavily-used shared tables, the single-lock approach creates a
 * concurrency bottleneck, so we also support "partitioned" locking wherein
 * there are multiple LWLocks guarding distinct subsets of the table.  To use
 * a hash table in partitioned mode, the HASH_PARTITION flag must be given
 * to hash_create.  This prevents any attempt to split buckets on-the-fly.
 * Therefore, each hash bucket chain operates independently, and no fields
 * of the hash header change after init except nentries and freeList.
 * A partitioned table uses spinlocks to guard changes of those fields.
 * This lets any subset of the hash buckets be treated as a separately
 * lockable partition.  We expect callers to use the low-order bits of a
 * lookup key's hash value as a partition number --- this will work because
 * of the way calc_bucket() maps hash values to bucket numbers.
 *
 * For hash tables in shared memory, the memory allocator function should
 * match malloc's semantics of returning NULL on failure.  For hash tables
 * in local memory, we typically use palloc() which will throw error on
 * failure.  The code in this file has to cope with both cases.
 *
 * dynahash.c provides support for these types of lookup keys:
 *
 * 1. Null-terminated C strings (truncated if necessary to fit in keysize),
 * compared as though by strcmp().  This is the default behavior.
 *
 * 2. Arbitrary binary data of size keysize, compared as though by memcmp().
 * (Caller must ensure there are no undefined padding bits in the keys!)
 * This is selected by specifying HASH_BLOBS flag to hash_create.
 *
 * 3. More complex key behavior can be selected by specifying user-supplied
 * hashing, comparison, and/or key-copying functions.  At least a hashing
 * function must be supplied; comparison defaults to memcmp() and key copying
 * to memcpy() when a user-defined hashing function is selected.
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/utils/hash/dynahash.c
 *
 *-------------------------------------------------------------------------
 */

/*
 * Original comments:
 *
 * Dynamic hashing, after CACM April 1988 pp 446-457, by Per-Ake Larson.
 * Coded into C, with minor code improvements, and with hsearch(3) interface,
 * by ejp@ausmelb.oz, Jul 26, 1988: 13:16;
 * also, hcreate/hdestroy routines added to simulate hsearch(3).
 *
 * These routines simulate hsearch(3) and family, with the important
 * difference that the hash table is dynamic - can grow indefinitely
 * beyond its original size (as supplied to hcreate()).
 *
 * Performance appears to be comparable to that of hsearch(3).
 * The 'source-code' options referred to in hsearch(3)'s 'man' page
 * are not implemented; otherwise functionality is identical.
 *
 * Compilation controls:
 * HASH_DEBUG controls some informative traces, mainly for debugging.
 * HASH_STATISTICS causes HashAccesses and HashCollisions to be maintained;
 * when combined with HASH_DEBUG, these are displayed by hdestroy().
 *
 * Problems & fixes to ejp@ausmelb.oz. WARNING: relies on pre-processor
 * concatenation property, in probably unnecessary code 'optimization'.
 *
 * Modified margo@postgres.berkeley.edu February 1990
 *      added multiple table interface
 * Modified by sullivan@postgres.berkeley.edu April 1990
 *      changed ctl structure for shared memory
 */

#include "postgres.h"

#include <limits.h>

#include "access/xact.h"
#include "storage/shmem.h"
#include "storage/spin.h"
#include "utils/dynahash.h"
#include "utils/memutils.h"


/*
 * Constants
 *
 * A hash table has a top-level "directory", each of whose entries points
 * to a "segment" of ssize bucket headers.  The maximum number of hash
 * buckets is thus dsize * ssize (but dsize may be expansible).  Of course,
 * the number of records in the table can be larger, but we don't want a
 * whole lot of records per bucket or performance goes down.
 *
 * In a hash table allocated in shared memory, the directory cannot be
 * expanded because it must stay at a fixed address.  The directory size
 * should be selected using hash_select_dirsize (and you'd better have
 * a good idea of the maximum number of entries!).  For non-shared hash
 * tables, the initial directory size can be left at the default.
 */
#define DEF_SEGSIZE            256
#define DEF_SEGSIZE_SHIFT      8    /* must be log2(DEF_SEGSIZE) */
#define DEF_DIRSIZE            256
#define DEF_FFACTOR            1    /* default fill factor */

/* Number of freelists to be used for a partitioned hash table. */
#define NUM_FREELISTS           32

/* A hash bucket is a linked list of HASHELEMENTs */
typedef HASHELEMENT *HASHBUCKET;

/* A hash segment is an array of bucket headers */
typedef HASHBUCKET *HASHSEGMENT;

/*
 * Using array of FreeListData instead of separate arrays of mutexes, nentries
 * and freeLists prevents, at least partially, sharing one cache line between
 * different mutexes (see below).
 */
typedef struct
{
    slock_t     mutex;          /* spinlock */
    long        nentries;       /* number of entries */
    HASHELEMENT *freeList;      /* list of free elements */
} FreeListData;

/*
 * Header structure for a hash table --- contains all changeable info
 *
 * In a shared-memory hash table, the HASHHDR is in shared memory, while
 * each backend has a local HTAB struct.  For a non-shared table, there isn't
 * any functional difference between HASHHDR and HTAB, but we separate them
 * anyway to share code between shared and non-shared tables.
 */
struct HASHHDR
{
    /*
     * The freelist can become a point of contention on high-concurrency hash
     * tables, so we use an array of freelist, each with its own mutex and
     * nentries count, instead of just a single one.
     *
     * If hash table is not partitioned only freeList[0] is used and spinlocks
     * are not used at all.
     */
    FreeListData freeList[NUM_FREELISTS];

    /* These fields can change, but not in a partitioned table */
    /* Also, dsize can't change in a shared table, even if unpartitioned */
    long        dsize;          /* directory size */
    long        nsegs;          /* number of allocated segments (<= dsize) */
    uint32      max_bucket;     /* ID of maximum bucket in use */
    uint32      high_mask;      /* mask to modulo into entire table */
    uint32      low_mask;       /* mask to modulo into lower half of table */

    /* These fields are fixed at hashtable creation */
    Size        keysize;        /* hash key length in bytes */
    Size        entrysize;      /* total user element size in bytes */
    long        num_partitions; /* # partitions (must be power of 2), or 0 */
    long        ffactor;        /* target fill factor */
    long        max_dsize;      /* 'dsize' limit if directory is fixed size */
    long        ssize;          /* segment size --- must be power of 2 */
    int         sshift;         /* segment shift = log2(ssize) */
    int         nelem_alloc;    /* number of entries to allocate at once */

#ifdef HASH_STATISTICS

    /*
     * Count statistics here.  NB: stats code doesn't bother with mutex, so
     * counts could be corrupted a bit in a partitioned table.
     */
    long        accesses;
    long        collisions;
#endif
};

#define IS_PARTITIONED(hctl)  ((hctl)->num_partitions != 0)

#define FREELIST_IDX(hctl, hashcode) \
    (IS_PARTITIONED(hctl) ? hashcode % NUM_FREELISTS : 0)

/*
 * Top control structure for a hashtable --- in a shared table, each backend
 * has its own copy (OK since no fields change at runtime)
 */
struct HTAB
{
    HASHHDR    *hctl;           /* => shared control information */
    HASHSEGMENT *dir;           /* directory of segment starts */
    HashValueFunc hash;         /* hash function */
    HashCompareFunc match;      /* key comparison function */
    HashCopyFunc keycopy;       /* key copying function */
    HashAllocFunc alloc;        /* memory allocator */
    MemoryContext hcxt;         /* memory context if default allocator used */
    char       *tabname;        /* table name (for error messages) */
    bool        isshared;       /* true if table is in shared memory */
    bool        isfixed;        /* if true, don't enlarge */

    /* freezing a shared table isn't allowed, so we can keep state here */
    bool        frozen;         /* true = no more inserts allowed */

    /* We keep local copies of these fixed values to reduce contention */
    Size        keysize;        /* hash key length in bytes */
    long        ssize;          /* segment size --- must be power of 2 */
    int         sshift;         /* segment shift = log2(ssize) */
};

/*
 * Key (also entry) part of a HASHELEMENT
 */
#define ELEMENTKEY(helem)  (((char *)(helem)) + MAXALIGN(sizeof(HASHELEMENT)))

/*
 * Obtain element pointer given pointer to key
 */
#define ELEMENT_FROM_KEY(key)  \
    ((HASHELEMENT *) (((char *) (key)) - MAXALIGN(sizeof(HASHELEMENT))))

/*
 * Fast MOD arithmetic, assuming that y is a power of 2 !
 */
#define MOD(x,y)               ((x) & ((y)-1))

#if HASH_STATISTICS
static long hash_accesses,
            hash_collisions,
            hash_expansions;
#endif

/*
 * Private function prototypes
 */
static void *DynaHashAlloc(Size size);
static HASHSEGMENT seg_alloc(HTAB *hashp);
static bool element_alloc(HTAB *hashp, int nelem, int freelist_idx);
static bool dir_realloc(HTAB *hashp);
static bool expand_table(HTAB *hashp);
static HASHBUCKET get_hash_entry(HTAB *hashp, int freelist_idx);
static void hdefault(HTAB *hashp);
static int  choose_nelem_alloc(Size entrysize);
static bool init_htab(HTAB *hashp, long nelem);
static void hash_corrupted(HTAB *hashp);
static long next_pow2_long(long num);
static int  next_pow2_int(long num);
static void register_seq_scan(HTAB *hashp);
static void deregister_seq_scan(HTAB *hashp);
static bool has_seq_scans(HTAB *hashp);


/*
 * memory allocation support
 */
static MemoryContext CurrentDynaHashCxt = NULL;

static void *
DynaHashAlloc(Size size)
{
    Assert(MemoryContextIsValid(CurrentDynaHashCxt));
    return MemoryContextAlloc(CurrentDynaHashCxt, size);
}


/*
 * HashCompareFunc for string keys
 *
 * Because we copy keys with strlcpy(), they will be truncated at keysize-1
 * bytes, so we can only compare that many ... hence strncmp is almost but
 * not quite the right thing.
 */
static int
string_compare(const char *key1, const char *key2, Size keysize)
{
    return strncmp(key1, key2, keysize - 1);
}


/************************** CREATE ROUTINES **********************/

/*
 * hash_create -- create a new dynamic hash table
 *
 *  tabname: a name for the table (for debugging purposes)
 *  nelem: maximum number of elements expected
 *  *info: additional table parameters, as indicated by flags
 *  flags: bitmask indicating which parameters to take from *info
 *
 * Note: for a shared-memory hashtable, nelem needs to be a pretty good
 * estimate, since we can't expand the table on the fly.  But an unshared
 * hashtable can be expanded on-the-fly, so it's better for nelem to be
 * on the small side and let the table grow if it's exceeded.  An overly
 * large nelem will penalize hash_seq_search speed without buying much.
 */
HTAB *
hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
{
    HTAB       *hashp;
    HASHHDR    *hctl;

    /*
     * For shared hash tables, we have a local hash header (HTAB struct) that
     * we allocate in TopMemoryContext; all else is in shared memory.
     *
     * For non-shared hash tables, everything including the hash header is in
     * a memory context created specially for the hash table --- this makes
     * hash_destroy very simple.  The memory context is made a child of either
     * a context specified by the caller, or TopMemoryContext if nothing is
     * specified.
     */
    if (flags & HASH_SHARED_MEM)
    {
        /* Set up to allocate the hash header */
        CurrentDynaHashCxt = TopMemoryContext;
    }
    else
    {
        /* Create the hash table's private memory context */
        if (flags & HASH_CONTEXT)
            CurrentDynaHashCxt = info->hcxt;
        else
            CurrentDynaHashCxt = TopMemoryContext;
        CurrentDynaHashCxt = AllocSetContextCreate(CurrentDynaHashCxt,
                                                   tabname,
                                                   ALLOCSET_DEFAULT_SIZES);
    }

    /* Initialize the hash header, plus a copy of the table name */
    hashp = (HTAB *) DynaHashAlloc(sizeof(HTAB) + strlen(tabname) + 1);
    MemSet(hashp, 0, sizeof(HTAB));

    hashp->tabname = (char *) (hashp + 1);
    strcpy(hashp->tabname, tabname);

    /*
     * Select the appropriate hash function (see comments at head of file).
     */
    if (flags & HASH_FUNCTION)
        hashp->hash = info->hash;
    else if (flags & HASH_BLOBS)
    {
        /* We can optimize hashing for common key sizes */
        Assert(flags & HASH_ELEM);
        if (info->keysize == sizeof(uint32))
            hashp->hash = uint32_hash;
        else
            hashp->hash = tag_hash;
    }
    else
        hashp->hash = string_hash;  /* default hash function */

    /*
     * If you don't specify a match function, it defaults to string_compare if
     * you used string_hash (either explicitly or by default) and to memcmp
     * otherwise.
     *
     * Note: explicitly specifying string_hash is deprecated, because this
     * might not work for callers in loadable modules on some platforms due to
     * referencing a trampoline instead of the string_hash function proper.
     * Just let it default, eh?
     */
    if (flags & HASH_COMPARE)
        hashp->match = info->match;
    else if (hashp->hash == string_hash)
        hashp->match = (HashCompareFunc) string_compare;
    else
        hashp->match = memcmp;

    /*
     * Similarly, the key-copying function defaults to strlcpy or memcpy.
     */
    if (flags & HASH_KEYCOPY)
        hashp->keycopy = info->keycopy;
    else if (hashp->hash == string_hash)
        hashp->keycopy = (HashCopyFunc) strlcpy;
    else
        hashp->keycopy = memcpy;

    /* And select the entry allocation function, too. */
    if (flags & HASH_ALLOC)
        hashp->alloc = info->alloc;
    else
        hashp->alloc = DynaHashAlloc;

    if (flags & HASH_SHARED_MEM)
    {
        /*
         * ctl structure and directory are preallocated for shared memory
         * tables.  Note that HASH_DIRSIZE and HASH_ALLOC had better be set as
         * well.
         */
        hashp->hctl = info->hctl;
        hashp->dir = (HASHSEGMENT *) (((char *) info->hctl) + sizeof(HASHHDR));
        hashp->hcxt = NULL;
        hashp->isshared = true;

        /* hash table already exists, we're just attaching to it */
        if (flags & HASH_ATTACH)
        {
            /* make local copies of some heavily-used values */
            hctl = hashp->hctl;
            hashp->keysize = hctl->keysize;
            hashp->ssize = hctl->ssize;
            hashp->sshift = hctl->sshift;

            return hashp;
        }
    }
    else
    {
        /* setup hash table defaults */
        hashp->hctl = NULL;
        hashp->dir = NULL;
        hashp->hcxt = CurrentDynaHashCxt;
        hashp->isshared = false;
    }

    if (!hashp->hctl)
    {
        hashp->hctl = (HASHHDR *) hashp->alloc(sizeof(HASHHDR));
        if (!hashp->hctl)
            ereport(ERROR,
                    (errcode(ERRCODE_OUT_OF_MEMORY),
                     errmsg("out of memory")));
    }

    hashp->frozen = false;

    hdefault(hashp);

    hctl = hashp->hctl;

    if (flags & HASH_PARTITION)
    {
        /* Doesn't make sense to partition a local hash table */
        Assert(flags & HASH_SHARED_MEM);

        /*
         * The number of partitions had better be a power of 2. Also, it must
         * be less than INT_MAX (see init_htab()), so call the int version of
         * next_pow2.
         */
        Assert(info->num_partitions == next_pow2_int(info->num_partitions));

        hctl->num_partitions = info->num_partitions;
    }

    if (flags & HASH_SEGMENT)
    {
        hctl->ssize = info->ssize;
        hctl->sshift = my_log2(info->ssize);
        /* ssize had better be a power of 2 */
        Assert(hctl->ssize == (1L << hctl->sshift));
    }
    if (flags & HASH_FFACTOR)
        hctl->ffactor = info->ffactor;

    /*
     * SHM hash tables have fixed directory size passed by the caller.
     */
    if (flags & HASH_DIRSIZE)
    {
        hctl->max_dsize = info->max_dsize;
        hctl->dsize = info->dsize;
    }

    /*
     * hash table now allocates space for key and data but you have to say how
     * much space to allocate
     */
    if (flags & HASH_ELEM)
    {
        Assert(info->entrysize >= info->keysize);
        hctl->keysize = info->keysize;
        hctl->entrysize = info->entrysize;
    }

    /* make local copies of heavily-used constant fields */
    hashp->keysize = hctl->keysize;
    hashp->ssize = hctl->ssize;
    hashp->sshift = hctl->sshift;

    /* Build the hash directory structure */
    if (!init_htab(hashp, nelem))
        elog(ERROR, "failed to initialize hash table \"%s\"", hashp->tabname);

    /*
     * For a shared hash table, preallocate the requested number of elements.
     * This reduces problems with run-time out-of-shared-memory conditions.
     *
     * For a non-shared hash table, preallocate the requested number of
     * elements if it's less than our chosen nelem_alloc.  This avoids wasting
     * space if the caller correctly estimates a small table size.
     */
    if ((flags & HASH_SHARED_MEM) ||
        nelem < hctl->nelem_alloc)
    {
        int         i,
                    freelist_partitions,
                    nelem_alloc,
                    nelem_alloc_first;

        /*
         * If hash table is partitioned all freeLists have equal number of
         * elements. Otherwise only freeList[0] is used.
         */
        if (IS_PARTITIONED(hashp->hctl))
            freelist_partitions = NUM_FREELISTS;
        else
            freelist_partitions = 1;

        nelem_alloc = nelem / freelist_partitions;
        if (nelem_alloc == 0)
            nelem_alloc = 1;

        /* Make sure all memory will be used */
        if (nelem_alloc * freelist_partitions < nelem)
            nelem_alloc_first =
                nelem - nelem_alloc * (freelist_partitions - 1);
        else
            nelem_alloc_first = nelem_alloc;

        for (i = 0; i < freelist_partitions; i++)
        {
            int         temp = (i == 0) ? nelem_alloc_first : nelem_alloc;

            if (!element_alloc(hashp, temp, i))
                ereport(ERROR,
                        (errcode(ERRCODE_OUT_OF_MEMORY),
                         errmsg("out of memory")));
        }
    }

    if (flags & HASH_FIXED_SIZE)
        hashp->isfixed = true;
    return hashp;
}

/*
 * Set default HASHHDR parameters.
 */
static void
hdefault(HTAB *hashp)
{
    HASHHDR    *hctl = hashp->hctl;

    MemSet(hctl, 0, sizeof(HASHHDR));

    hctl->dsize = DEF_DIRSIZE;
    hctl->nsegs = 0;

    /* rather pointless defaults for key & entry size */
    hctl->keysize = sizeof(char *);
    hctl->entrysize = 2 * sizeof(char *);

    hctl->num_partitions = 0;   /* not partitioned */

    hctl->ffactor = DEF_FFACTOR;

    /* table has no fixed maximum size */
    hctl->max_dsize = NO_MAX_DSIZE;

    hctl->ssize = DEF_SEGSIZE;
    hctl->sshift = DEF_SEGSIZE_SHIFT;

#ifdef HASH_STATISTICS
    hctl->accesses = hctl->collisions = 0;
#endif
}

/*
 * Given the user-specified entry size, choose nelem_alloc, ie, how many
 * elements to add to the hash table when we need more.
 */
static int
choose_nelem_alloc(Size entrysize)
{
    int         nelem_alloc;
    Size        elementSize;
    Size        allocSize;

    /* Each element has a HASHELEMENT header plus user data. */
    /* NB: this had better match element_alloc() */
    elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(entrysize);

    /*
     * The idea here is to choose nelem_alloc at least 32, but round up so
     * that the allocation request will be a power of 2 or just less. This
     * makes little difference for hash tables in shared memory, but for hash
     * tables managed by palloc, the allocation request will be rounded up to
     * a power of 2 anyway.  If we fail to take this into account, we'll waste
     * as much as half the allocated space.
     */
    allocSize = 32 * 4;         /* assume elementSize at least 8 */
    do
    {
        allocSize <<= 1;
        nelem_alloc = allocSize / elementSize;
    } while (nelem_alloc < 32);

    return nelem_alloc;
}

/*
 * Compute derived fields of hctl and build the initial directory/segment
 * arrays
 */
static bool
init_htab(HTAB *hashp, long nelem)
{
    HASHHDR    *hctl = hashp->hctl;
    HASHSEGMENT *segp;
    int         nbuckets;
    int         nsegs;
    int         i;

    /*
     * initialize mutex if it's a partitioned table
     */
    if (IS_PARTITIONED(hctl))
        for (i = 0; i < NUM_FREELISTS; i++)
            SpinLockInit(&(hctl->freeList[i].mutex));

    /*
     * Divide number of elements by the fill factor to determine a desired
     * number of buckets.  Allocate space for the next greater power of two
     * number of buckets
     */
    nbuckets = next_pow2_int((nelem - 1) / hctl->ffactor + 1);

    /*
     * In a partitioned table, nbuckets must be at least equal to
     * num_partitions; were it less, keys with apparently different partition
     * numbers would map to the same bucket, breaking partition independence.
     * (Normally nbuckets will be much bigger; this is just a safety check.)
     */
    while (nbuckets < hctl->num_partitions)
        nbuckets <<= 1;

    hctl->max_bucket = hctl->low_mask = nbuckets - 1;
    hctl->high_mask = (nbuckets << 1) - 1;

    /*
     * Figure number of directory segments needed, round up to a power of 2
     */
    nsegs = (nbuckets - 1) / hctl->ssize + 1;
    nsegs = next_pow2_int(nsegs);

    /*
     * Make sure directory is big enough. If pre-allocated directory is too
     * small, choke (caller screwed up).
     */
    if (nsegs > hctl->dsize)
    {
        if (!(hashp->dir))
            hctl->dsize = nsegs;
        else
            return false;
    }

    /* Allocate a directory */
    if (!(hashp->dir))
    {
        CurrentDynaHashCxt = hashp->hcxt;
        hashp->dir = (HASHSEGMENT *)
            hashp->alloc(hctl->dsize * sizeof(HASHSEGMENT));
        if (!hashp->dir)
            return false;
    }

    /* Allocate initial segments */
    for (segp = hashp->dir; hctl->nsegs < nsegs; hctl->nsegs++, segp++)
    {
        *segp = seg_alloc(hashp);
        if (*segp == NULL)
            return false;
    }

    /* Choose number of entries to allocate at a time */
    hctl->nelem_alloc = choose_nelem_alloc(hctl->entrysize);

#if HASH_DEBUG
    fprintf(stderr, "init_htab:\n%s%p\n%s%ld\n%s%ld\n%s%d\n%s%ld\n%s%u\n%s%x\n%s%x\n%s%ld\n%s%ld\n",
            "TABLE POINTER   ", hashp,
            "DIRECTORY SIZE  ", hctl->dsize,
            "SEGMENT SIZE    ", hctl->ssize,
            "SEGMENT SHIFT   ", hctl->sshift,
            "FILL FACTOR     ", hctl->ffactor,
            "MAX BUCKET      ", hctl->max_bucket,
            "HIGH MASK       ", hctl->high_mask,
            "LOW  MASK       ", hctl->low_mask,
            "NSEGS           ", hctl->nsegs,
            "NENTRIES        ", hash_get_num_entries(hctl));
#endif
    return true;
}

/*
 * Estimate the space needed for a hashtable containing the given number
 * of entries of given size.
 * NOTE: this is used to estimate the footprint of hashtables in shared
 * memory; therefore it does not count HTAB which is in local memory.
 * NB: assumes that all hash structure parameters have default values!
 */
Size
hash_estimate_size(long num_entries, Size entrysize)
{
    Size        size;
    long        nBuckets,
                nSegments,
                nDirEntries,
                nElementAllocs,
                elementSize,
                elementAllocCnt;

    /* estimate number of buckets wanted */
    nBuckets = next_pow2_long((num_entries - 1) / DEF_FFACTOR + 1);
    /* # of segments needed for nBuckets */
    nSegments = next_pow2_long((nBuckets - 1) / DEF_SEGSIZE + 1);
    /* directory entries */
    nDirEntries = DEF_DIRSIZE;
    while (nDirEntries < nSegments)
        nDirEntries <<= 1;      /* dir_alloc doubles dsize at each call */

    /* fixed control info */
    size = MAXALIGN(sizeof(HASHHDR));   /* but not HTAB, per above */
    /* directory */
    size = add_size(size, mul_size(nDirEntries, sizeof(HASHSEGMENT)));
    /* segments */
    size = add_size(size, mul_size(nSegments,
                                   MAXALIGN(DEF_SEGSIZE * sizeof(HASHBUCKET))));
    /* elements --- allocated in groups of choose_nelem_alloc() entries */
    elementAllocCnt = choose_nelem_alloc(entrysize);
    nElementAllocs = (num_entries - 1) / elementAllocCnt + 1;
    elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(entrysize);
    size = add_size(size,
                    mul_size(nElementAllocs,
                             mul_size(elementAllocCnt, elementSize)));

    return size;
}

/*
 * Select an appropriate directory size for a hashtable with the given
 * maximum number of entries.
 * This is only needed for hashtables in shared memory, whose directories
 * cannot be expanded dynamically.
 * NB: assumes that all hash structure parameters have default values!
 *
 * XXX this had better agree with the behavior of init_htab()...
 */
long
hash_select_dirsize(long num_entries)
{
    long        nBuckets,
                nSegments,
                nDirEntries;

    /* estimate number of buckets wanted */
    nBuckets = next_pow2_long((num_entries - 1) / DEF_FFACTOR + 1);
    /* # of segments needed for nBuckets */
    nSegments = next_pow2_long((nBuckets - 1) / DEF_SEGSIZE + 1);
    /* directory entries */
    nDirEntries = DEF_DIRSIZE;
    while (nDirEntries < nSegments)
        nDirEntries <<= 1;      /* dir_alloc doubles dsize at each call */

    return nDirEntries;
}

/*
 * Compute the required initial memory allocation for a shared-memory
 * hashtable with the given parameters.  We need space for the HASHHDR
 * and for the (non expansible) directory.
 */
Size
hash_get_shared_size(HASHCTL *info, int flags)
{
    Assert(flags & HASH_DIRSIZE);
    Assert(info->dsize == info->max_dsize);
    return sizeof(HASHHDR) + info->dsize * sizeof(HASHSEGMENT);
}


/********************** DESTROY ROUTINES ************************/

void
hash_destroy(HTAB *hashp)
{
    if (hashp != NULL)
    {
        /* allocation method must be one we know how to free, too */
        Assert(hashp->alloc == DynaHashAlloc);
        /* so this hashtable must have it's own context */
        Assert(hashp->hcxt != NULL);

        hash_stats("destroy", hashp);

        /*
         * Free everything by destroying the hash table's memory context.
         */
        MemoryContextDelete(hashp->hcxt);
    }
}

void
hash_stats(const char *where, HTAB *hashp)
{
#if HASH_STATISTICS
    fprintf(stderr, "%s: this HTAB -- accesses %ld collisions %ld\n",
            where, hashp->hctl->accesses, hashp->hctl->collisions);

    fprintf(stderr, "hash_stats: entries %ld keysize %ld maxp %u segmentcount %ld\n",
            hash_get_num_entries(hashp), (long) hashp->hctl->keysize,
            hashp->hctl->max_bucket, hashp->hctl->nsegs);
    fprintf(stderr, "%s: total accesses %ld total collisions %ld\n",
            where, hash_accesses, hash_collisions);
    fprintf(stderr, "hash_stats: total expansions %ld\n",
            hash_expansions);
#endif
}

/*******************************SEARCH ROUTINES *****************************/


/*
 * get_hash_value -- exported routine to calculate a key's hash value
 *
 * We export this because for partitioned tables, callers need to compute
 * the partition number (from the low-order bits of the hash value) before
 * searching.
 */
uint32
get_hash_value(HTAB *hashp, const void *keyPtr)
{
    return hashp->hash(keyPtr, hashp->keysize);
}

/* Convert a hash value to a bucket number */
static inline uint32
calc_bucket(HASHHDR *hctl, uint32 hash_val)
{
    uint32      bucket;

    bucket = hash_val & hctl->high_mask;
    if (bucket > hctl->max_bucket)
        bucket = bucket & hctl->low_mask;

    return bucket;
}

/*
 * hash_search -- look up key in table and perform action
 * hash_search_with_hash_value -- same, with key's hash value already computed
 *
 * action is one of:
 *      HASH_FIND: look up key in table
 *      HASH_ENTER: look up key in table, creating entry if not present
 *      HASH_ENTER_NULL: same, but return NULL if out of memory
 *      HASH_REMOVE: look up key in table, remove entry if present
 *
 * Return value is a pointer to the element found/entered/removed if any,
 * or NULL if no match was found.  (NB: in the case of the REMOVE action,
 * the result is a dangling pointer that shouldn't be dereferenced!)
 *
 * HASH_ENTER will normally ereport a generic "out of memory" error if
 * it is unable to create a new entry.  The HASH_ENTER_NULL operation is
 * the same except it will return NULL if out of memory.  Note that
 * HASH_ENTER_NULL cannot be used with the default palloc-based allocator,
 * since palloc internally ereports on out-of-memory.
 *
 * If foundPtr isn't NULL, then *foundPtr is set TRUE if we found an
 * existing entry in the table, FALSE otherwise.  This is needed in the
 * HASH_ENTER case, but is redundant with the return value otherwise.
 *
 * For hash_search_with_hash_value, the hashvalue parameter must have been
 * calculated with get_hash_value().
 */
void *
hash_search(HTAB *hashp,
            const void *keyPtr,
            HASHACTION action,
            bool *foundPtr)
{
    return hash_search_with_hash_value(hashp,
                                       keyPtr,
                                       hashp->hash(keyPtr, hashp->keysize),
                                       action,
                                       foundPtr);
}

void *
hash_search_with_hash_value(HTAB *hashp,
                            const void *keyPtr,
                            uint32 hashvalue,
                            HASHACTION action,
                            bool *foundPtr)
{
    HASHHDR    *hctl = hashp->hctl;
    Size        keysize;
    uint32      bucket;
    long        segment_num;
    long        segment_ndx;
    HASHSEGMENT segp;
    HASHBUCKET  currBucket;
    HASHBUCKET *prevBucketPtr;
    HashCompareFunc match;
    int         freelist_idx = FREELIST_IDX(hctl, hashvalue);

#if HASH_STATISTICS
    hash_accesses++;
    hctl->accesses++;
#endif

    /*
     * If inserting, check if it is time to split a bucket.
     *
     * NOTE: failure to expand table is not a fatal error, it just means we
     * have to run at higher fill factor than we wanted.  However, if we're
     * using the palloc allocator then it will throw error anyway on
     * out-of-memory, so we must do this before modifying the table.
     */
    if (action == HASH_ENTER || action == HASH_ENTER_NULL)
    {
        /*
         * Can't split if running in partitioned mode, nor if frozen, nor if
         * table is the subject of any active hash_seq_search scans.  Strange
         * order of these tests is to try to check cheaper conditions first.
         */
        if (!IS_PARTITIONED(hctl) && !hashp->frozen &&
            hctl->freeList[0].nentries / (long) (hctl->max_bucket + 1) >= hctl->ffactor &&
            !has_seq_scans(hashp))
            (void) expand_table(hashp);
    }

    /*
     * Do the initial lookup
     */
    bucket = calc_bucket(hctl, hashvalue);

    segment_num = bucket >> hashp->sshift;
    segment_ndx = MOD(bucket, hashp->ssize);

    segp = hashp->dir[segment_num];

    if (segp == NULL)
        hash_corrupted(hashp);

    prevBucketPtr = &segp[segment_ndx];
    currBucket = *prevBucketPtr;

    /*
     * Follow collision chain looking for matching key
     */
    match = hashp->match;       /* save one fetch in inner loop */
    keysize = hashp->keysize;   /* ditto */

    while (currBucket != NULL)
    {
        if (currBucket->hashvalue == hashvalue &&
            match(ELEMENTKEY(currBucket), keyPtr, keysize) == 0)
            break;
        prevBucketPtr = &(currBucket->link);
        currBucket = *prevBucketPtr;
#if HASH_STATISTICS
        hash_collisions++;
        hctl->collisions++;
#endif
    }

    if (foundPtr)
        *foundPtr = (bool) (currBucket != NULL);

    /*
     * OK, now what?
     */
    switch (action)
    {
        case HASH_FIND:
            if (currBucket != NULL)
                return (void *) ELEMENTKEY(currBucket);
            return NULL;

        case HASH_REMOVE:
            if (currBucket != NULL)
            {
                /* if partitioned, must lock to touch nentries and freeList */
                if (IS_PARTITIONED(hctl))
                    SpinLockAcquire(&(hctl->freeList[freelist_idx].mutex));

                Assert(hctl->freeList[freelist_idx].nentries > 0);
                hctl->freeList[freelist_idx].nentries--;

                /* remove record from hash bucket's chain. */
                *prevBucketPtr = currBucket->link;

                /* add the record to the freelist for this table.  */
                currBucket->link = hctl->freeList[freelist_idx].freeList;
                hctl->freeList[freelist_idx].freeList = currBucket;

                if (IS_PARTITIONED(hctl))
                    SpinLockRelease(&hctl->freeList[freelist_idx].mutex);

                /*
                 * better hope the caller is synchronizing access to this
                 * element, because someone else is going to reuse it the next
                 * time something is added to the table
                 */
                return (void *) ELEMENTKEY(currBucket);
            }
            return NULL;

        case HASH_ENTER_NULL:
            /* ENTER_NULL does not work with palloc-based allocator */
            Assert(hashp->alloc != DynaHashAlloc);
            /* FALL THRU */

        case HASH_ENTER:
            /* Return existing element if found, else create one */
            if (currBucket != NULL)
                return (void *) ELEMENTKEY(currBucket);

            /* disallow inserts if frozen */
            if (hashp->frozen)
                elog(ERROR, "cannot insert into frozen hashtable \"%s\"",
                     hashp->tabname);

            currBucket = get_hash_entry(hashp, freelist_idx);
            if (currBucket == NULL)
            {
                /* out of memory */
                if (action == HASH_ENTER_NULL)
                    return NULL;
                /* report a generic message */
                if (hashp->isshared)
                    ereport(ERROR,
                            (errcode(ERRCODE_OUT_OF_MEMORY),
                             errmsg("out of shared memory")));
                else
                    ereport(ERROR,
                            (errcode(ERRCODE_OUT_OF_MEMORY),
                             errmsg("out of memory")));
            }

            /* link into hashbucket chain */
            *prevBucketPtr = currBucket;
            currBucket->link = NULL;

            /* copy key into record */
            currBucket->hashvalue = hashvalue;
            hashp->keycopy(ELEMENTKEY(currBucket), keyPtr, keysize);

            /*
             * Caller is expected to fill the data field on return.  DO NOT
             * insert any code that could possibly throw error here, as doing
             * so would leave the table entry incomplete and hence corrupt the
             * caller's data structure.
             */

            return (void *) ELEMENTKEY(currBucket);
    }

    elog(ERROR, "unrecognized hash action code: %d", (int) action);

    return NULL;                /* keep compiler quiet */
}

/*
 * hash_update_hash_key -- change the hash key of an existing table entry
 *
 * This is equivalent to removing the entry, making a new entry, and copying
 * over its data, except that the entry never goes to the table's freelist.
 * Therefore this cannot suffer an out-of-memory failure, even if there are
 * other processes operating in other partitions of the hashtable.
 *
 * Returns TRUE if successful, FALSE if the requested new hash key is already
 * present.  Throws error if the specified entry pointer isn't actually a
 * table member.
 *
 * NB: currently, there is no special case for old and new hash keys being
 * identical, which means we'll report FALSE for that situation.  This is
 * preferable for existing uses.
 *
 * NB: for a partitioned hashtable, caller must hold lock on both relevant
 * partitions, if the new hash key would belong to a different partition.
 */
bool
hash_update_hash_key(HTAB *hashp,
                     void *existingEntry,
                     const void *newKeyPtr)
{
    HASHELEMENT *existingElement = ELEMENT_FROM_KEY(existingEntry);
    HASHHDR    *hctl = hashp->hctl;
    uint32      newhashvalue;
    Size        keysize;
    uint32      bucket;
    uint32      newbucket;
    long        segment_num;
    long        segment_ndx;
    HASHSEGMENT segp;
    HASHBUCKET  currBucket;
    HASHBUCKET *prevBucketPtr;
    HASHBUCKET *oldPrevPtr;
    HashCompareFunc match;

#if HASH_STATISTICS
    hash_accesses++;
    hctl->accesses++;
#endif

    /* disallow updates if frozen */
    if (hashp->frozen)
        elog(ERROR, "cannot update in frozen hashtable \"%s\"",
             hashp->tabname);

    /*
     * Lookup the existing element using its saved hash value.  We need to do
     * this to be able to unlink it from its hash chain, but as a side benefit
     * we can verify the validity of the passed existingEntry pointer.
     */
    bucket = calc_bucket(hctl, existingElement->hashvalue);

    segment_num = bucket >> hashp->sshift;
    segment_ndx = MOD(bucket, hashp->ssize);

    segp = hashp->dir[segment_num];

    if (segp == NULL)
        hash_corrupted(hashp);

    prevBucketPtr = &segp[segment_ndx];
    currBucket = *prevBucketPtr;

    while (currBucket != NULL)
    {
        if (currBucket == existingElement)
            break;
        prevBucketPtr = &(currBucket->link);
        currBucket = *prevBucketPtr;
    }

    if (currBucket == NULL)
        elog(ERROR, "hash_update_hash_key argument is not in hashtable \"%s\"",
             hashp->tabname);

    oldPrevPtr = prevBucketPtr;

    /*
     * Now perform the equivalent of a HASH_ENTER operation to locate the hash
     * chain we want to put the entry into.
     */
    newhashvalue = hashp->hash(newKeyPtr, hashp->keysize);

    newbucket = calc_bucket(hctl, newhashvalue);

    segment_num = newbucket >> hashp->sshift;
    segment_ndx = MOD(newbucket, hashp->ssize);

    segp = hashp->dir[segment_num];

    if (segp == NULL)
        hash_corrupted(hashp);

    prevBucketPtr = &segp[segment_ndx];
    currBucket = *prevBucketPtr;

    /*
     * Follow collision chain looking for matching key
     */
    match = hashp->match;       /* save one fetch in inner loop */
    keysize = hashp->keysize;   /* ditto */

    while (currBucket != NULL)
    {
        if (currBucket->hashvalue == newhashvalue &&
            match(ELEMENTKEY(currBucket), newKeyPtr, keysize) == 0)
            break;
        prevBucketPtr = &(currBucket->link);
        currBucket = *prevBucketPtr;
#if HASH_STATISTICS
        hash_collisions++;
        hctl->collisions++;
#endif
    }

    if (currBucket != NULL)
        return false;           /* collision with an existing entry */

    currBucket = existingElement;

    /*
     * If old and new hash values belong to the same bucket, we need not
     * change any chain links, and indeed should not since this simplistic
     * update will corrupt the list if currBucket is the last element.  (We
     * cannot fall out earlier, however, since we need to scan the bucket to
     * check for duplicate keys.)
     */
    if (bucket != newbucket)
    {
        /* OK to remove record from old hash bucket's chain. */
        *oldPrevPtr = currBucket->link;

        /* link into new hashbucket chain */
        *prevBucketPtr = currBucket;
        currBucket->link = NULL;
    }

    /* copy new key into record */
    currBucket->hashvalue = newhashvalue;
    hashp->keycopy(ELEMENTKEY(currBucket), newKeyPtr, keysize);

    /* rest of record is untouched */

    return true;
}

/*
 * create a new entry if possible
 */
static HASHBUCKET
get_hash_entry(HTAB *hashp, int freelist_idx)
{
    HASHHDR    *hctl = hashp->hctl;
    HASHBUCKET  newElement;
    int         borrow_from_idx;

    for (;;)
    {
        /* if partitioned, must lock to touch nentries and freeList */
        if (IS_PARTITIONED(hctl))
            SpinLockAcquire(&hctl->freeList[freelist_idx].mutex);

        /* try to get an entry from the freelist */
        newElement = hctl->freeList[freelist_idx].freeList;

        if (newElement != NULL)
            break;

        if (IS_PARTITIONED(hctl))
            SpinLockRelease(&hctl->freeList[freelist_idx].mutex);

        /* no free elements.  allocate another chunk of buckets */
        if (!element_alloc(hashp, hctl->nelem_alloc, freelist_idx))
        {
            if (!IS_PARTITIONED(hctl))
                return NULL;    /* out of memory */

            /* try to borrow element from another partition */
            borrow_from_idx = freelist_idx;
            for (;;)
            {
                borrow_from_idx = (borrow_from_idx + 1) % NUM_FREELISTS;
                if (borrow_from_idx == freelist_idx)
                    break;

                SpinLockAcquire(&(hctl->freeList[borrow_from_idx].mutex));
                newElement = hctl->freeList[borrow_from_idx].freeList;

                if (newElement != NULL)
                {
                    hctl->freeList[borrow_from_idx].freeList = newElement->link;
                    SpinLockRelease(&(hctl->freeList[borrow_from_idx].mutex));

                    SpinLockAcquire(&hctl->freeList[freelist_idx].mutex);
                    hctl->freeList[freelist_idx].nentries++;
                    SpinLockRelease(&hctl->freeList[freelist_idx].mutex);

                    break;
                }

                SpinLockRelease(&(hctl->freeList[borrow_from_idx].mutex));
            }

            return newElement;
        }
    }

    /* remove entry from freelist, bump nentries */
    hctl->freeList[freelist_idx].freeList = newElement->link;
    hctl->freeList[freelist_idx].nentries++;

    if (IS_PARTITIONED(hctl))
        SpinLockRelease(&hctl->freeList[freelist_idx].mutex);

    return newElement;
}

/*
 * hash_get_num_entries -- get the number of entries in a hashtable
 */
long
hash_get_num_entries(HTAB *hashp)
{
    int         i;
    long        sum = hashp->hctl->freeList[0].nentries;

    /*
     * We currently don't bother with the mutex; it's only sensible to call
     * this function if you've got lock on all partitions of the table.
     */

    if (!IS_PARTITIONED(hashp->hctl))
        return sum;

    for (i = 1; i < NUM_FREELISTS; i++)
        sum += hashp->hctl->freeList[i].nentries;

    return sum;
}

/*
 * hash_seq_init/_search/_term
 *          Sequentially search through hash table and return
 *          all the elements one by one, return NULL when no more.
 *
 * hash_seq_term should be called if and only if the scan is abandoned before
 * completion; if hash_seq_search returns NULL then it has already done the
 * end-of-scan cleanup.
 *
 * NOTE: caller may delete the returned element before continuing the scan.
 * However, deleting any other element while the scan is in progress is
 * UNDEFINED (it might be the one that curIndex is pointing at!).  Also,
 * if elements are added to the table while the scan is in progress, it is
 * unspecified whether they will be visited by the scan or not.
 *
 * NOTE: it is possible to use hash_seq_init/hash_seq_search without any
 * worry about hash_seq_term cleanup, if the hashtable is first locked against
 * further insertions by calling hash_freeze.
 *
 * NOTE: to use this with a partitioned hashtable, caller had better hold
 * at least shared lock on all partitions of the table throughout the scan!
 * We can cope with insertions or deletions by our own backend, but *not*
 * with concurrent insertions or deletions by another.
 */
void
hash_seq_init(HASH_SEQ_STATUS *status, HTAB *hashp)
{
    status->hashp = hashp;
    status->curBucket = 0;
    status->curEntry = NULL;
    if (!hashp->frozen)
        register_seq_scan(hashp);
}

void *
hash_seq_search(HASH_SEQ_STATUS *status)
{
    HTAB       *hashp;
    HASHHDR    *hctl;
    uint32      max_bucket;
    long        ssize;
    long        segment_num;
    long        segment_ndx;
    HASHSEGMENT segp;
    uint32      curBucket;
    HASHELEMENT *curElem;

    if ((curElem = status->curEntry) != NULL)
    {
        /* Continuing scan of curBucket... */
        status->curEntry = curElem->link;
        if (status->curEntry == NULL)   /* end of this bucket */
            ++status->curBucket;
        return (void *) ELEMENTKEY(curElem);
    }

    /*
     * Search for next nonempty bucket starting at curBucket.
     */
    curBucket = status->curBucket;
    hashp = status->hashp;
    hctl = hashp->hctl;
    ssize = hashp->ssize;
    max_bucket = hctl->max_bucket;

    if (curBucket > max_bucket)
    {
        hash_seq_term(status);
        return NULL;            /* search is done */
    }

    /*
     * first find the right segment in the table directory.
     */
    segment_num = curBucket >> hashp->sshift;
    segment_ndx = MOD(curBucket, ssize);

    segp = hashp->dir[segment_num];

    /*
     * Pick up the first item in this bucket's chain.  If chain is not empty
     * we can begin searching it.  Otherwise we have to advance to find the
     * next nonempty bucket.  We try to optimize that case since searching a
     * near-empty hashtable has to iterate this loop a lot.
     */
    while ((curElem = segp[segment_ndx]) == NULL)
    {
        /* empty bucket, advance to next */
        if (++curBucket > max_bucket)
        {
            status->curBucket = curBucket;
            hash_seq_term(status);
            return NULL;        /* search is done */
        }
        if (++segment_ndx >= ssize)
        {
            segment_num++;
            segment_ndx = 0;
            segp = hashp->dir[segment_num];
        }
    }

    /* Begin scan of curBucket... */
    status->curEntry = curElem->link;
    if (status->curEntry == NULL)   /* end of this bucket */
        ++curBucket;
    status->curBucket = curBucket;
    return (void *) ELEMENTKEY(curElem);
}

void
hash_seq_term(HASH_SEQ_STATUS *status)
{
    if (!status->hashp->frozen)
        deregister_seq_scan(status->hashp);
}

/*
 * hash_freeze
 *          Freeze a hashtable against future insertions (deletions are
 *          still allowed)
 *
 * The reason for doing this is that by preventing any more bucket splits,
 * we no longer need to worry about registering hash_seq_search scans,
 * and thus caller need not be careful about ensuring hash_seq_term gets
 * called at the right times.
 *
 * Multiple calls to hash_freeze() are allowed, but you can't freeze a table
 * with active scans (since hash_seq_term would then do the wrong thing).
 */
void
hash_freeze(HTAB *hashp)
{
    if (hashp->isshared)
        elog(ERROR, "cannot freeze shared hashtable \"%s\"", hashp->tabname);
    if (!hashp->frozen && has_seq_scans(hashp))
        elog(ERROR, "cannot freeze hashtable \"%s\" because it has active scans",
             hashp->tabname);
    hashp->frozen = true;
}


/********************************* UTILITIES ************************/

/*
 * Expand the table by adding one more hash bucket.
 */
static bool
expand_table(HTAB *hashp)
{
    HASHHDR    *hctl = hashp->hctl;
    HASHSEGMENT old_seg,
                new_seg;
    long        old_bucket,
                new_bucket;
    long        new_segnum,
                new_segndx;
    long        old_segnum,
                old_segndx;
    HASHBUCKET *oldlink,
               *newlink;
    HASHBUCKET  currElement,
                nextElement;

    Assert(!IS_PARTITIONED(hctl));

#ifdef HASH_STATISTICS
    hash_expansions++;
#endif

    new_bucket = hctl->max_bucket + 1;
    new_segnum = new_bucket >> hashp->sshift;
    new_segndx = MOD(new_bucket, hashp->ssize);

    if (new_segnum >= hctl->nsegs)
    {
        /* Allocate new segment if necessary -- could fail if dir full */
        if (new_segnum >= hctl->dsize)
            if (!dir_realloc(hashp))
                return false;
        if (!(hashp->dir[new_segnum] = seg_alloc(hashp)))
            return false;
        hctl->nsegs++;
    }

    /* OK, we created a new bucket */
    hctl->max_bucket++;

    /*
     * *Before* changing masks, find old bucket corresponding to same hash
     * values; values in that bucket may need to be relocated to new bucket.
     * Note that new_bucket is certainly larger than low_mask at this point,
     * so we can skip the first step of the regular hash mask calc.
     */
    old_bucket = (new_bucket & hctl->low_mask);

    /*
     * If we crossed a power of 2, readjust masks.
     */
    if ((uint32) new_bucket > hctl->high_mask)
    {
        hctl->low_mask = hctl->high_mask;
        hctl->high_mask = (uint32) new_bucket | hctl->low_mask;
    }

    /*
     * Relocate records to the new bucket.  NOTE: because of the way the hash
     * masking is done in calc_bucket, only one old bucket can need to be
     * split at this point.  With a different way of reducing the hash value,
     * that might not be true!
     */
    old_segnum = old_bucket >> hashp->sshift;
    old_segndx = MOD(old_bucket, hashp->ssize);

    old_seg = hashp->dir[old_segnum];
    new_seg = hashp->dir[new_segnum];

    oldlink = &old_seg[old_segndx];
    newlink = &new_seg[new_segndx];

    for (currElement = *oldlink;
         currElement != NULL;
         currElement = nextElement)
    {
        nextElement = currElement->link;
        if ((long) calc_bucket(hctl, currElement->hashvalue) == old_bucket)
        {
            *oldlink = currElement;
            oldlink = &currElement->link;
        }
        else
        {
            *newlink = currElement;
            newlink = &currElement->link;
        }
    }
    /* don't forget to terminate the rebuilt hash chains... */
    *oldlink = NULL;
    *newlink = NULL;

    return true;
}


static bool
dir_realloc(HTAB *hashp)
{
    HASHSEGMENT *p;
    HASHSEGMENT *old_p;
    long        new_dsize;
    long        old_dirsize;
    long        new_dirsize;

    if (hashp->hctl->max_dsize != NO_MAX_DSIZE)
        return false;

    /* Reallocate directory */
    new_dsize = hashp->hctl->dsize << 1;
    old_dirsize = hashp->hctl->dsize * sizeof(HASHSEGMENT);
    new_dirsize = new_dsize * sizeof(HASHSEGMENT);

    old_p = hashp->dir;
    CurrentDynaHashCxt = hashp->hcxt;
    p = (HASHSEGMENT *) hashp->alloc((Size) new_dirsize);

    if (p != NULL)
    {
        memcpy(p, old_p, old_dirsize);
        MemSet(((char *) p) + old_dirsize, 0, new_dirsize - old_dirsize);
        hashp->dir = p;
        hashp->hctl->dsize = new_dsize;

        /* XXX assume the allocator is palloc, so we know how to free */
        Assert(hashp->alloc == DynaHashAlloc);
        pfree(old_p);

        return true;
    }

    return false;
}


static HASHSEGMENT
seg_alloc(HTAB *hashp)
{
    HASHSEGMENT segp;

    CurrentDynaHashCxt = hashp->hcxt;
    segp = (HASHSEGMENT) hashp->alloc(sizeof(HASHBUCKET) * hashp->ssize);

    if (!segp)
        return NULL;

    MemSet(segp, 0, sizeof(HASHBUCKET) * hashp->ssize);

    return segp;
}

/*
 * allocate some new elements and link them into the indicated free list
 */
static bool
element_alloc(HTAB *hashp, int nelem, int freelist_idx)
{
    HASHHDR    *hctl = hashp->hctl;
    Size        elementSize;
    HASHELEMENT *firstElement;
    HASHELEMENT *tmpElement;
    HASHELEMENT *prevElement;
    int         i;

    if (hashp->isfixed)
        return false;

    /* Each element has a HASHELEMENT header plus user data. */
    elementSize = MAXALIGN(sizeof(HASHELEMENT)) + MAXALIGN(hctl->entrysize);

    CurrentDynaHashCxt = hashp->hcxt;
    firstElement = (HASHELEMENT *) hashp->alloc(nelem * elementSize);

    if (!firstElement)
        return false;

    /* prepare to link all the new entries into the freelist */
    prevElement = NULL;
    tmpElement = firstElement;
    for (i = 0; i < nelem; i++)
    {
        tmpElement->link = prevElement;
        prevElement = tmpElement;
        tmpElement = (HASHELEMENT *) (((char *) tmpElement) + elementSize);
    }

    /* if partitioned, must lock to touch freeList */
    if (IS_PARTITIONED(hctl))
        SpinLockAcquire(&hctl->freeList[freelist_idx].mutex);

    /* freelist could be nonempty if two backends did this concurrently */
    firstElement->link = hctl->freeList[freelist_idx].freeList;
    hctl->freeList[freelist_idx].freeList = prevElement;

    if (IS_PARTITIONED(hctl))
        SpinLockRelease(&hctl->freeList[freelist_idx].mutex);

    return true;
}

/* complain when we have detected a corrupted hashtable */
static void
hash_corrupted(HTAB *hashp)
{
    /*
     * If the corruption is in a shared hashtable, we'd better force a
     * systemwide restart.  Otherwise, just shut down this one backend.
     */
    if (hashp->isshared)
        elog(PANIC, "hash table \"%s\" corrupted", hashp->tabname);
    else
        elog(FATAL, "hash table \"%s\" corrupted", hashp->tabname);
}

/* calculate ceil(log base 2) of num */
int
my_log2(long num)
{
    int         i;
    long        limit;

    /* guard against too-large input, which would put us into infinite loop */
    if (num > LONG_MAX / 2)
        num = LONG_MAX / 2;

    for (i = 0, limit = 1; limit < num; i++, limit <<= 1)
        ;
    return i;
}

/* calculate first power of 2 >= num, bounded to what will fit in a long */
static long
next_pow2_long(long num)
{
    /* my_log2's internal range check is sufficient */
    return 1L << my_log2(num);
}

/* calculate first power of 2 >= num, bounded to what will fit in an int */
static int
next_pow2_int(long num)
{
    if (num > INT_MAX / 2)
        num = INT_MAX / 2;
    return 1 << my_log2(num);
}


/************************* SEQ SCAN TRACKING ************************/

/*
 * We track active hash_seq_search scans here.  The need for this mechanism
 * comes from the fact that a scan will get confused if a bucket split occurs
 * while it's in progress: it might visit entries twice, or even miss some
 * entirely (if it's partway through the same bucket that splits).  Hence
 * we want to inhibit bucket splits if there are any active scans on the
 * table being inserted into.  This is a fairly rare case in current usage,
 * so just postponing the split until the next insertion seems sufficient.
 *
 * Given present usages of the function, only a few scans are likely to be
 * open concurrently; so a finite-size stack of open scans seems sufficient,
 * and we don't worry that linear search is too slow.  Note that we do
 * allow multiple scans of the same hashtable to be open concurrently.
 *
 * This mechanism can support concurrent scan and insertion in a shared
 * hashtable if it's the same backend doing both.  It would fail otherwise,
 * but locking reasons seem to preclude any such scenario anyway, so we don't
 * worry.
 *
 * This arrangement is reasonably robust if a transient hashtable is deleted
 * without notifying us.  The absolute worst case is we might inhibit splits
 * in another table created later at exactly the same address.  We will give
 * a warning at transaction end for reference leaks, so any bugs leading to
 * lack of notification should be easy to catch.
 */

#define MAX_SEQ_SCANS 100

static HTAB *seq_scan_tables[MAX_SEQ_SCANS];    /* tables being scanned */
static int  seq_scan_level[MAX_SEQ_SCANS];  /* subtransaction nest level */
static int  num_seq_scans = 0;


/* Register a table as having an active hash_seq_search scan */
static void
register_seq_scan(HTAB *hashp)
{
    if (num_seq_scans >= MAX_SEQ_SCANS)
        elog(ERROR, "too many active hash_seq_search scans, cannot start one on \"%s\"",
             hashp->tabname);
    seq_scan_tables[num_seq_scans] = hashp;
    seq_scan_level[num_seq_scans] = GetCurrentTransactionNestLevel();
    num_seq_scans++;
}

/* Deregister an active scan */
static void
deregister_seq_scan(HTAB *hashp)
{
    int         i;

    /* Search backward since it's most likely at the stack top */
    for (i = num_seq_scans - 1; i >= 0; i--)
    {
        if (seq_scan_tables[i] == hashp)
        {
            seq_scan_tables[i] = seq_scan_tables[num_seq_scans - 1];
            seq_scan_level[i] = seq_scan_level[num_seq_scans - 1];
            num_seq_scans--;
            return;
        }
    }
    elog(ERROR, "no hash_seq_search scan for hash table \"%s\"",
         hashp->tabname);
}

/* Check if a table has any active scan */
static bool
has_seq_scans(HTAB *hashp)
{
    int         i;

    for (i = 0; i < num_seq_scans; i++)
    {
        if (seq_scan_tables[i] == hashp)
            return true;
    }
    return false;
}

/* Clean up any open scans at end of transaction */
void
AtEOXact_HashTables(bool isCommit)
{
    /*
     * During abort cleanup, open scans are expected; just silently clean 'em
     * out.  An open scan at commit means someone forgot a hash_seq_term()
     * call, so complain.
     *
     * Note: it's tempting to try to print the tabname here, but refrain for
     * fear of touching deallocated memory.  This isn't a user-facing message
     * anyway, so it needn't be pretty.
     */
    if (isCommit)
    {
        int         i;

        for (i = 0; i < num_seq_scans; i++)
        {
            elog(WARNING, "leaked hash_seq_search scan for hash table %p",
                 seq_scan_tables[i]);
        }
    }
    num_seq_scans = 0;
}

/* Clean up any open scans at end of subtransaction */
void
AtEOSubXact_HashTables(bool isCommit, int nestDepth)
{
    int         i;

    /*
     * Search backward to make cleanup easy.  Note we must check all entries,
     * not only those at the end of the array, because deletion technique
     * doesn't keep them in order.
     */
    for (i = num_seq_scans - 1; i >= 0; i--)
    {
        if (seq_scan_level[i] >= nestDepth)
        {
            if (isCommit)
                elog(WARNING, "leaked hash_seq_search scan for hash table %p",
                     seq_scan_tables[i]);
            seq_scan_tables[i] = seq_scan_tables[num_seq_scans - 1];
            seq_scan_level[i] = seq_scan_level[num_seq_scans - 1];
            num_seq_scans--;
        }
    }
}