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[pgsql.git] / src / backend / utils / hash / dynahash.c

/*-------------------------------------------------------------------------
 *
 * dynahash.c
 *        dynamic chained 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 multiple copies of those fields, guarded by
 * spinlocks, for additional concurrency.)
 * 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 selected by specifying the
 * HASH_STRINGS flag to hash_create.
 *
 * 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 the 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.
 *
 * Compared to simplehash, dynahash has the following benefits:
 *
 * - It supports partitioning, which is useful for shared memory access using
 *   locks.
 * - Shared memory hashes are allocated in a fixed size area at startup and
 *   are discoverable by name from other processes.
 * - Because entries don't need to be moved in the case of hash conflicts,
 *   dynahash has better performance for large entries.
 * - Guarantees stable pointers to entries.
 *
 * Portions Copyright (c) 1996-2022, 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 "common/hashfn.h"
#include "port/pg_bitutils.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

/* 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;

/*
 * Per-freelist data.
 *
 * In a partitioned hash table, each freelist is associated with a specific
 * set of hashcodes, as determined by the FREELIST_IDX() macro below.
 * nentries tracks the number of live hashtable entries having those hashcodes
 * (NOT the number of entries in the freelist, as you might expect).
 *
 * The coverage of a freelist might be more or less than one partition, so it
 * needs its own lock rather than relying on caller locking.  Relying on that
 * wouldn't work even if the coverage was the same, because of the occasional
 * need to "borrow" entries from another freelist; see get_hash_entry().
 *
 * Using an array of FreeListData instead of separate arrays of mutexes,
 * nentries and freeLists helps to reduce sharing of cache lines between
 * different mutexes.
 */
typedef struct
{
        slock_t         mutex;                  /* spinlock for this freelist */
        long            nentries;               /* number of entries in associated buckets */
        HASHELEMENT *freeList;          /* chain 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 in high-concurrency hash
         * tables, so we use an array of freelists, each with its own mutex and
         * nentries count, instead of just a single one.  Although the freelists
         * normally operate independently, we will scavenge entries from freelists
         * other than a hashcode's default freelist when necessary.
         *
         * If the hash table is not partitioned, only freeList[0] is used and its
         * spinlock is not used at all; callers' locking is assumed sufficient.
         */
        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            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))

#ifdef 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
 *
 * The flags value *must* include HASH_ELEM.  (Formerly, this was nominally
 * optional, but the default keysize and entrysize values were useless.)
 * The flags value must also include exactly one of HASH_STRINGS, HASH_BLOBS,
 * or HASH_FUNCTION, to define the key hashing semantics (C strings,
 * binary blobs, or custom, respectively).  Callers specifying a custom
 * hash function will likely also want to use HASH_COMPARE, and perhaps
 * also HASH_KEYCOPY, to control key comparison and copying.
 * Another often-used flag is HASH_CONTEXT, to allocate the hash table
 * under info->hcxt rather than under TopMemoryContext; the default
 * behavior is only suitable for session-lifespan hash tables.
 * Other flags bits are special-purpose and seldom used, except for those
 * associated with shared-memory hash tables, for which see ShmemInitHash().
 *
 * Fields in *info are read only when the associated flags bit is set.
 * It is not necessary to initialize other fields of *info.
 * Neither tabname nor *info need persist after the hash_create() call.
 *
 * Note: It is deprecated for callers of hash_create() to explicitly specify
 * string_hash, tag_hash, uint32_hash, or oid_hash.  Just set HASH_STRINGS or
 * HASH_BLOBS.  Use HASH_FUNCTION only when you want something other than
 * one of these.
 *
 * 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, const HASHCTL *info, int flags)
{
        HTAB       *hashp;
        HASHHDR    *hctl;

        /*
         * Hash tables now allocate space for key and data, but you have to say
         * how much space to allocate.
         */
        Assert(flags & HASH_ELEM);
        Assert(info->keysize > 0);
        Assert(info->entrysize >= info->keysize);

        /*
         * 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,
                                                                                                   "dynahash",
                                                                                                   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);

        /* If we have a private context, label it with hashtable's name */
        if (!(flags & HASH_SHARED_MEM))
                MemoryContextSetIdentifier(CurrentDynaHashCxt, hashp->tabname);

        /*
         * Select the appropriate hash function (see comments at head of file).
         */
        if (flags & HASH_FUNCTION)
        {
                Assert(!(flags & (HASH_BLOBS | HASH_STRINGS)));
                hashp->hash = info->hash;
        }
        else if (flags & HASH_BLOBS)
        {
                Assert(!(flags & HASH_STRINGS));
                /* We can optimize hashing for common key sizes */
                if (info->keysize == sizeof(uint32))
                        hashp->hash = uint32_hash;
                else
                        hashp->hash = tag_hash;
        }
        else
        {
                /*
                 * string_hash used to be considered the default hash method, and in a
                 * non-assert build it effectively still is.  But we now consider it
                 * an assertion error to not say HASH_STRINGS explicitly.  To help
                 * catch mistaken usage of HASH_STRINGS, we also insist on a
                 * reasonably long string length: if the keysize is only 4 or 8 bytes,
                 * it's almost certainly an integer or pointer not a string.
                 */
                Assert(flags & HASH_STRINGS);
                Assert(info->keysize > 8);

                hashp->hash = string_hash;
        }

        /*
         * If you don't specify a match function, it defaults to string_compare if
         * you used string_hash, 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.
         * Specify HASH_STRINGS instead.
         */
        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)
        {
                /*
                 * The signature of keycopy is meant for memcpy(), which returns
                 * void*, but strlcpy() returns size_t.  Since we never use the return
                 * value of keycopy, and size_t is pretty much always the same size as
                 * void *, this should be safe.  The extra cast in the middle is to
                 * avoid warnings from -Wcast-function-type.
                 */
                hashp->keycopy = (HashCopyFunc) (pg_funcptr_t) 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));
        }

        /*
         * 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;
        }

        /* remember the entry sizes, too */
        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, give each freelist an equal share of
                 * the initial allocation.  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 we'll allocate all the requested elements; freeList[0]
                 * gets the excess if the request isn't divisible by NUM_FREELISTS.
                 */
                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;

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

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

        /*
         * Allocate space for the next greater power of two number of buckets,
         * assuming a desired maximum load factor of 1.
         */
        nbuckets = next_pow2_int(nelem);

        /*
         * 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);

#ifdef 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",
                        "TABLE POINTER   ", hashp,
                        "DIRECTORY SIZE  ", hctl->dsize,
                        "SEGMENT SIZE    ", hctl->ssize,
                        "SEGMENT SHIFT   ", hctl->sshift,
                        "MAX BUCKET      ", hctl->max_bucket,
                        "HIGH MASK       ", hctl->high_mask,
                        "LOW  MASK       ", hctl->low_mask,
                        "NSEGS           ", hctl->nsegs);
#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);
        /* # 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);
        /* # 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 its 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)
{
#ifdef 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;
        int                     freelist_idx = FREELIST_IDX(hctl, hashvalue);
        Size            keysize;
        uint32          bucket;
        long            segment_num;
        long            segment_ndx;
        HASHSEGMENT segp;
        HASHBUCKET      currBucket;
        HASHBUCKET *prevBucketPtr;
        HashCompareFunc match;

#ifdef 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.
                 */
                if (hctl->freeList[0].nentries > (long) hctl->max_bucket &&
                        !IS_PARTITIONED(hctl) && !hashp->frozen &&
                        !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;
#ifdef 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));

                                /* delete the record from the appropriate nentries counter. */
                                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 appropriate freelist. */
                                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;

#ifdef 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;
#ifdef 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;
}

/*
 * Allocate a new hashtable entry if possible; return NULL if out of memory.
 * (Or, if the underlying space allocator throws error for out-of-memory,
 * we won't return at all.)
 */
static HASHBUCKET
get_hash_entry(HTAB *hashp, int freelist_idx)
{
        HASHHDR    *hctl = hashp->hctl;
        HASHBUCKET      newElement;

        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 in this freelist.  In a partitioned table, there
                 * might be entries in other freelists, but to reduce contention we
                 * prefer to first try to get another chunk of buckets from the main
                 * shmem allocator.  If that fails, though, we *MUST* root through all
                 * the other freelists before giving up.  There are multiple callers
                 * that assume that they can allocate every element in the initially
                 * requested table size, or that deleting an element guarantees they
                 * can insert a new element, even if shared memory is entirely full.
                 * Failing because the needed element is in a different freelist is
                 * not acceptable.
                 */
                if (!element_alloc(hashp, hctl->nelem_alloc, freelist_idx))
                {
                        int                     borrow_from_idx;

                        if (!IS_PARTITIONED(hctl))
                                return NULL;    /* out of memory */

                        /* try to borrow element from another freelist */
                        borrow_from_idx = freelist_idx;
                        for (;;)
                        {
                                borrow_from_idx = (borrow_from_idx + 1) % NUM_FREELISTS;
                                if (borrow_from_idx == freelist_idx)
                                        break;          /* examined all freelists, fail */

                                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));

                                        /* careful: count the new element in its proper freelist */
                                        SpinLockAcquire(&hctl->freeList[freelist_idx].mutex);
                                        hctl->freeList[freelist_idx].nentries++;
                                        SpinLockRelease(&hctl->freeList[freelist_idx].mutex);

                                        return newElement;
                                }

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

                        /* no elements available to borrow either, so out of memory */
                        return NULL;
                }
        }

        /* 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 acquiring the mutexes; it's only
         * sensible to call this function if you've got lock on all partitions of
         * the table.
         */
        if (IS_PARTITIONED(hashp->hctl))
        {
                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)
{
        /*
         * guard against too-large input, which would be invalid for
         * pg_ceil_log2_*()
         */
        if (num > LONG_MAX / 2)
                num = LONG_MAX / 2;

#if SIZEOF_LONG < 8
        return pg_ceil_log2_32(num);
#else
        return pg_ceil_log2_64(num);
#endif
}

/* 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--;
                }
        }
}