hash_page.c

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/*-
 * Copyright (c) 1990, 1993, 1994
 *  The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Margo Seltzer.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *  This product includes software developed by the University of
 *  California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */
 
#if defined(LIBC_SCCS) && !defined(lint)
static char sccsid[] = "@(#)hash_page.c 8.7 (Berkeley) 8/16/94";
#endif /* LIBC_SCCS and not lint */
 
/*
 * PACKAGE:  hashing
 *
 * DESCRIPTION:
 *  Page manipulation for hashing package.
 *
 * ROUTINES:
 *
 * External
 *  __get_page
 *  __add_ovflpage
 * Internal
 *  overflow_page
 *  open_temp
 */
 
#include <sys/types.h>
 
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#ifdef DEBUG
#include <assert.h>
#endif
 
#include "../include/db.h"
#include "hash.h"
#include "page.h"
#include "extern.h"
 
static u_int32_t    *fetch_bitmap __P((HTAB *, int));
static u_int32_t     first_free __P((u_int32_t));
static int   open_temp __P((HTAB *));
static u_int16_t     overflow_page __P((HTAB *));
static void  putpair __P((char *, const DBT *, const DBT *));
static void  squeeze_key __P((u_int16_t *, const DBT *, const DBT *));
static int   ugly_split
            __P((HTAB *, u_int32_t, BUFHEAD *, BUFHEAD *, int, int));
 
#define PAGE_INIT(P) { \
    ((u_int16_t *)(P))[0] = 0; \
    ((u_int16_t *)(P))[1] = hashp->BSIZE - 3 * sizeof(u_int16_t); \
    ((u_int16_t *)(P))[2] = hashp->BSIZE; \
}
 
/*
 * This is called AFTER we have verified that there is room on the page for
 * the pair (PAIRFITS has returned true) so we go right ahead and start moving
 * stuff on.
 */
static void
putpair(p, key, val)
    char *p;
    const DBT *key, *val;
{
    register u_int16_t *bp, n, off;
 
    bp = (u_int16_t *)p;
 
    /* Enter the key first. */
    n = bp[0];
 
    off = OFFSET(bp) - key->size;
    memmove(p + off, key->data, key->size);
    bp[++n] = off;
 
    /* Now the data. */
    off -= val->size;
    memmove(p + off, val->data, val->size);
    bp[++n] = off;
 
    /* Adjust page info. */
    bp[0] = n;
    bp[n + 1] = off - ((n + 3) * sizeof(u_int16_t));
    bp[n + 2] = off;
}
 
/*
 * Returns:
 *   0 OK
 *  -1 error
 */
extern int
__delpair(hashp, bufp, ndx)
    HTAB *hashp;
    BUFHEAD *bufp;
    register int ndx;
{
    register u_int16_t *bp, newoff;
    register int n;
    u_int16_t pairlen;
 
    bp = (u_int16_t *)bufp->page;
    n = bp[0];
 
    if (bp[ndx + 1] < REAL_KEY)
        return (__big_delete(hashp, bufp));
    if (ndx != 1)
        newoff = bp[ndx - 1];
    else
        newoff = hashp->BSIZE;
    pairlen = newoff - bp[ndx + 1];
 
    if (ndx != (n - 1)) {
        /* Hard Case -- need to shuffle keys */
        register int i;
        register char *src = bufp->page + (int)OFFSET(bp);
        register char *dst = src + (int)pairlen;
        memmove(dst, src, bp[ndx + 1] - OFFSET(bp));
 
        /* Now adjust the pointers */
        for (i = ndx + 2; i <= n; i += 2) {
            if (bp[i + 1] == OVFLPAGE) {
                bp[i - 2] = bp[i];
                bp[i - 1] = bp[i + 1];
            } else {
                bp[i - 2] = bp[i] + pairlen;
                bp[i - 1] = bp[i + 1] + pairlen;
            }
        }
    }
    /* Finally adjust the page data */
    bp[n] = OFFSET(bp) + pairlen;
    bp[n - 1] = bp[n + 1] + pairlen + 2 * sizeof(u_int16_t);
    bp[0] = n - 2;
    hashp->NKEYS--;
 
    bufp->flags |= BUF_MOD;
    return (0);
}
/*
 * Returns:
 *   0 ==> OK
 *  -1 ==> Error
 */
extern int
__split_page(hashp, obucket, nbucket)
    HTAB *hashp;
    u_int32_t obucket, nbucket;
{
    register BUFHEAD *new_bufp, *old_bufp;
    register u_int16_t *ino;
    register char *np;
    DBT key, val;
    int n, ndx, retval;
    u_int16_t copyto, diff, off, moved;
    char *op;
 
    copyto = (u_int16_t)hashp->BSIZE;
    off = (u_int16_t)hashp->BSIZE;
    old_bufp = __get_buf(hashp, obucket, NULL, 0);
    if (old_bufp == NULL)
        return (-1);
    new_bufp = __get_buf(hashp, nbucket, NULL, 0);
    if (new_bufp == NULL)
        return (-1);
 
    old_bufp->flags |= (BUF_MOD | BUF_PIN);
    new_bufp->flags |= (BUF_MOD | BUF_PIN);
 
    ino = (u_int16_t *)(op = old_bufp->page);
    np = new_bufp->page;
 
    moved = 0;
 
    for (n = 1, ndx = 1; n < ino[0]; n += 2) {
        if (ino[n + 1] < REAL_KEY) {
            retval = ugly_split(hashp, obucket, old_bufp, new_bufp,
                (int)copyto, (int)moved);
            old_bufp->flags &= ~BUF_PIN;
            new_bufp->flags &= ~BUF_PIN;
            return (retval);
 
        }
        key.data = (u_char *)op + ino[n];
        key.size = off - ino[n];
 
        if (__call_hash(hashp, key.data, key.size) == obucket) {
            /* Don't switch page */
            diff = copyto - off;
            if (diff) {
                copyto = ino[n + 1] + diff;
                memmove(op + copyto, op + ino[n + 1],
                    off - ino[n + 1]);
                ino[ndx] = copyto + ino[n] - ino[n + 1];
                ino[ndx + 1] = copyto;
            } else
                copyto = ino[n + 1];
            ndx += 2;
        } else {
            /* Switch page */
            val.data = (u_char *)op + ino[n + 1];
            val.size = ino[n] - ino[n + 1];
            putpair(np, &key, &val);
            moved += 2;
        }
 
        off = ino[n + 1];
    }
 
    /* Now clean up the page */
    ino[0] -= moved;
    FREESPACE(ino) = copyto - sizeof(u_int16_t) * (ino[0] + 3);
    OFFSET(ino) = copyto;
 
#ifdef DEBUG3
    (void)fprintf(stderr, "split %d/%d\n",
        ((u_int16_t *)np)[0] / 2,
        ((u_int16_t *)op)[0] / 2);
#endif
    /* unpin both pages */
    old_bufp->flags &= ~BUF_PIN;
    new_bufp->flags &= ~BUF_PIN;
    return (0);
}
 
/*
 * Called when we encounter an overflow or big key/data page during split
 * handling.  This is special cased since we have to begin checking whether
 * the key/data pairs fit on their respective pages and because we may need
 * overflow pages for both the old and new pages.
 *
 * The first page might be a page with regular key/data pairs in which case
 * we have a regular overflow condition and just need to go on to the next
 * page or it might be a big key/data pair in which case we need to fix the
 * big key/data pair.
 *
 * Returns:
 *   0 ==> success
 *  -1 ==> failure
 */
static int
ugly_split(hashp, obucket, old_bufp, new_bufp, copyto, moved)
    HTAB *hashp;
    u_int32_t obucket;  /* Same as __split_page. */
    BUFHEAD *old_bufp, *new_bufp;
    int copyto; /* First byte on page which contains key/data values. */
    int moved;  /* Number of pairs moved to new page. */
{
    register BUFHEAD *bufp; /* Buffer header for ino */
    register u_int16_t *ino;    /* Page keys come off of */
    register u_int16_t *np; /* New page */
    register u_int16_t *op; /* Page keys go on to if they aren't moving */
 
    BUFHEAD *last_bfp;  /* Last buf header OVFL needing to be freed */
    DBT key, val;
    SPLIT_RETURN ret;
    u_int16_t n, off, ov_addr, scopyto;
    char *cino;     /* Character value of ino */
 
    bufp = old_bufp;
    ino = (u_int16_t *)old_bufp->page;
    np = (u_int16_t *)new_bufp->page;
    op = (u_int16_t *)old_bufp->page;
    last_bfp = NULL;
    scopyto = (u_int16_t)copyto;    /* ANSI */
 
    n = ino[0] - 1;
    while (n < ino[0]) {
        if (ino[2] < REAL_KEY && ino[2] != OVFLPAGE) {
            if (__big_split(hashp, old_bufp,
                new_bufp, bufp, bufp->addr, obucket, &ret))
                return (-1);
            old_bufp = ret.oldp;
            if (!old_bufp)
                return (-1);
            op = (u_int16_t *)old_bufp->page;
            new_bufp = ret.newp;
            if (!new_bufp)
                return (-1);
            np = (u_int16_t *)new_bufp->page;
            bufp = ret.nextp;
            if (!bufp)
                return (0);
            cino = (char *)bufp->page;
            ino = (u_int16_t *)cino;
            last_bfp = ret.nextp;
        } else if (ino[n + 1] == OVFLPAGE) {
            ov_addr = ino[n];
            /*
             * Fix up the old page -- the extra 2 are the fields
             * which contained the overflow information.
             */
            ino[0] -= (moved + 2);
            FREESPACE(ino) =
                scopyto - sizeof(u_int16_t) * (ino[0] + 3);
            OFFSET(ino) = scopyto;
 
            bufp = __get_buf(hashp, ov_addr, bufp, 0);
            if (!bufp)
                return (-1);
 
            ino = (u_int16_t *)bufp->page;
            n = 1;
            scopyto = hashp->BSIZE;
            moved = 0;
 
            if (last_bfp)
                __free_ovflpage(hashp, last_bfp);
            last_bfp = bufp;
        }
        /* Move regular sized pairs of there are any */
        off = hashp->BSIZE;
        for (n = 1; (n < ino[0]) && (ino[n + 1] >= REAL_KEY); n += 2) {
            cino = (char *)ino;
            key.data = (u_char *)cino + ino[n];
            key.size = off - ino[n];
            val.data = (u_char *)cino + ino[n + 1];
            val.size = ino[n] - ino[n + 1];
            off = ino[n + 1];
 
            if (__call_hash(hashp, key.data, key.size) == obucket) {
                /* Keep on old page */
                if (PAIRFITS(op, (&key), (&val)))
                    putpair((char *)op, &key, &val);
                else {
                    old_bufp =
                        __add_ovflpage(hashp, old_bufp);
                    if (!old_bufp)
                        return (-1);
                    op = (u_int16_t *)old_bufp->page;
                    putpair((char *)op, &key, &val);
                }
                old_bufp->flags |= BUF_MOD;
            } else {
                /* Move to new page */
                if (PAIRFITS(np, (&key), (&val)))
                    putpair((char *)np, &key, &val);
                else {
                    new_bufp =
                        __add_ovflpage(hashp, new_bufp);
                    if (!new_bufp)
                        return (-1);
                    np = (u_int16_t *)new_bufp->page;
                    putpair((char *)np, &key, &val);
                }
                new_bufp->flags |= BUF_MOD;
            }
        }
    }
    if (last_bfp)
        __free_ovflpage(hashp, last_bfp);
    return (0);
}
 
/*
 * Add the given pair to the page
 *
 * Returns:
 *  0 ==> OK
 *  1 ==> failure
 */
extern int
__addel(hashp, bufp, key, val)
    HTAB *hashp;
    BUFHEAD *bufp;
    const DBT *key, *val;
{
    register u_int16_t *bp, *sop;
    int do_expand;
 
    bp = (u_int16_t *)bufp->page;
    do_expand = 0;
    while (bp[0] && (bp[2] < REAL_KEY || bp[bp[0]] < REAL_KEY))
        /* Exception case */
        if (bp[2] == FULL_KEY_DATA && bp[0] == 2)
            /* This is the last page of a big key/data pair
               and we need to add another page */
            break;
        else if (bp[2] < REAL_KEY && bp[bp[0]] != OVFLPAGE) {
            bufp = __get_buf(hashp, bp[bp[0] - 1], bufp, 0);
            if (!bufp)
                return (-1);
            bp = (u_int16_t *)bufp->page;
        } else
            /* Try to squeeze key on this page */
            if (FREESPACE(bp) > PAIRSIZE(key, val)) {
                squeeze_key(bp, key, val);
                return (0);
            } else {
                bufp = __get_buf(hashp, bp[bp[0] - 1], bufp, 0);
                if (!bufp)
                    return (-1);
                bp = (u_int16_t *)bufp->page;
            }
 
    if (PAIRFITS(bp, key, val))
        putpair(bufp->page, key, val);
    else {
        do_expand = 1;
        bufp = __add_ovflpage(hashp, bufp);
        if (!bufp)
            return (-1);
        sop = (u_int16_t *)bufp->page;
 
        if (PAIRFITS(sop, key, val))
            putpair((char *)sop, key, val);
        else
            if (__big_insert(hashp, bufp, key, val))
                return (-1);
    }
    bufp->flags |= BUF_MOD;
    /*
     * If the average number of keys per bucket exceeds the fill factor,
     * expand the table.
     */
    hashp->NKEYS++;
    if (do_expand ||
        (hashp->NKEYS / (hashp->MAX_BUCKET + 1) > hashp->FFACTOR))
        return (__expand_table(hashp));
    return (0);
}
 
/*
 *
 * Returns:
 *  pointer on success
 *  NULL on error
 */
extern BUFHEAD *
__add_ovflpage(hashp, bufp)
    HTAB *hashp;
    BUFHEAD *bufp;
{
    register u_int16_t *sp;
    u_int16_t ndx, ovfl_num;
#ifdef DEBUG1
    int tmp1, tmp2;
#endif
    sp = (u_int16_t *)bufp->page;
 
    /* Check if we are dynamically determining the fill factor */
    if (hashp->FFACTOR == DEF_FFACTOR) {
        hashp->FFACTOR = sp[0] >> 1;
        if (hashp->FFACTOR < MIN_FFACTOR)
            hashp->FFACTOR = MIN_FFACTOR;
    }
    bufp->flags |= BUF_MOD;
    ovfl_num = overflow_page(hashp);
#ifdef DEBUG1
    tmp1 = bufp->addr;
    tmp2 = bufp->ovfl ? bufp->ovfl->addr : 0;
#endif
    if (!ovfl_num || !(bufp->ovfl = __get_buf(hashp, ovfl_num, bufp, 1)))
        return (NULL);
    bufp->ovfl->flags |= BUF_MOD;
#ifdef DEBUG1
    (void)fprintf(stderr, "ADDOVFLPAGE: %d->ovfl was %d is now %d\n",
        tmp1, tmp2, bufp->ovfl->addr);
#endif
    ndx = sp[0];
    /*
     * Since a pair is allocated on a page only if there's room to add
     * an overflow page, we know that the OVFL information will fit on
     * the page.
     */
    sp[ndx + 4] = OFFSET(sp);
    sp[ndx + 3] = FREESPACE(sp) - OVFLSIZE;
    sp[ndx + 1] = ovfl_num;
    sp[ndx + 2] = OVFLPAGE;
    sp[0] = ndx + 2;
#ifdef HASH_STATISTICS
    hash_overflows++;
#endif
    return (bufp->ovfl);
}
 
/*
 * Returns:
 *   0 indicates SUCCESS
 *  -1 indicates FAILURE
 */
extern int
__get_page(hashp, p, bucket, is_bucket, is_disk, is_bitmap)
    HTAB *hashp;
    char *p;
    u_int32_t bucket;
    int is_bucket, is_disk, is_bitmap;
{
    register int fd, page, size;
    int rsize;
    u_int16_t *bp;
 
    fd = hashp->fp;
    size = hashp->BSIZE;
 
    if ((fd == -1) || !is_disk) {
        PAGE_INIT(p);
        return (0);
    }
    if (is_bucket)
        page = BUCKET_TO_PAGE(bucket);
    else
        page = OADDR_TO_PAGE(bucket);
    if ((lseek(fd, (off_t)page << hashp->BSHIFT, SEEK_SET) == -1) ||
        ((rsize = read(fd, p, size)) == -1))
        return (-1);
    bp = (u_int16_t *)p;
    if (!rsize)
        bp[0] = 0;  /* We hit the EOF, so initialize a new page */
    else
        if (rsize != size) {
            errno = EFTYPE;
            return (-1);
        }
    if (!is_bitmap && !bp[0]) {
        PAGE_INIT(p);
    } else
        if (hashp->LORDER != BYTE_ORDER) {
            register int i, max;
 
            if (is_bitmap) {
                max = hashp->BSIZE >> 2; /* divide by 4 */
                for (i = 0; i < max; i++)
                    M_32_SWAP(((int *)p)[i]);
            } else {
                M_16_SWAP(bp[0]);
                max = bp[0] + 2;
                for (i = 1; i <= max; i++)
                    M_16_SWAP(bp[i]);
            }
        }
    return (0);
}
 
/*
 * Write page p to disk
 *
 * Returns:
 *   0 ==> OK
 *  -1 ==>failure
 */
extern int
__put_page(hashp, p, bucket, is_bucket, is_bitmap)
    HTAB *hashp;
    char *p;
    u_int32_t bucket;
    int is_bucket, is_bitmap;
{
    register int fd, page, size;
    int wsize;
 
    size = hashp->BSIZE;
    if ((hashp->fp == -1) && open_temp(hashp))
        return (-1);
    fd = hashp->fp;
 
    if (hashp->LORDER != BYTE_ORDER) {
        register int i;
        register int max;
 
        if (is_bitmap) {
            max = hashp->BSIZE >> 2;    /* divide by 4 */
            for (i = 0; i < max; i++)
                M_32_SWAP(((int *)p)[i]);
        } else {
            max = ((u_int16_t *)p)[0] + 2;
            for (i = 0; i <= max; i++)
                M_16_SWAP(((u_int16_t *)p)[i]);
        }
    }
    if (is_bucket)
        page = BUCKET_TO_PAGE(bucket);
    else
        page = OADDR_TO_PAGE(bucket);
    if ((lseek(fd, (off_t)page << hashp->BSHIFT, SEEK_SET) == -1) ||
        ((wsize = write(fd, p, size)) == -1))
        /* Errno is set */
        return (-1);
    if (wsize != size) {
        errno = EFTYPE;
        return (-1);
    }
    return (0);
}
 
#define BYTE_MASK   ((1 << INT_BYTE_SHIFT) -1)
/*
 * Initialize a new bitmap page.  Bitmap pages are left in memory
 * once they are read in.
 */
extern int
__ibitmap(hashp, pnum, nbits, ndx)
    HTAB *hashp;
    int pnum, nbits, ndx;
{
    u_int32_t *ip;
    int clearbytes, clearints;
 
    if ((ip = (u_int32_t *)malloc(hashp->BSIZE)) == NULL)
        return (1);
    hashp->nmaps++;
    clearints = ((nbits - 1) >> INT_BYTE_SHIFT) + 1;
    clearbytes = clearints << INT_TO_BYTE;
    (void)memset((char *)ip, 0, clearbytes);
    (void)memset(((char *)ip) + clearbytes, 0xFF,
        hashp->BSIZE - clearbytes);
    ip[clearints - 1] = ALL_SET << (nbits & BYTE_MASK);
    SETBIT(ip, 0);
    hashp->BITMAPS[ndx] = (u_int16_t)pnum;
    hashp->mapp[ndx] = ip;
    return (0);
}
 
static u_int32_t
first_free(map)
    u_int32_t map;
{
    register u_int32_t i, mask;
 
    mask = 0x1;
    for (i = 0; i < BITS_PER_MAP; i++) {
        if (!(mask & map))
            return (i);
        mask = mask << 1;
    }
    return (i);
}
 
static u_int16_t
overflow_page(hashp)
    HTAB *hashp;
{
    register u_int32_t *freep = 0;
    register int max_free, offset, splitnum;
    u_int16_t addr;
    int bit, first_page, free_bit, free_page, i, in_use_bits, j;
#ifdef DEBUG2
    int tmp1, tmp2;
#endif
    splitnum = hashp->OVFL_POINT;
    max_free = hashp->SPARES[splitnum];
 
    free_page = (max_free - 1) >> (hashp->BSHIFT + BYTE_SHIFT);
    free_bit = (max_free - 1) & ((hashp->BSIZE << BYTE_SHIFT) - 1);
 
    /* Look through all the free maps to find the first free block */
    first_page = hashp->LAST_FREED >>(hashp->BSHIFT + BYTE_SHIFT);
    for ( i = first_page; i <= free_page; i++ ) {
        if (!(freep = (u_int32_t *)hashp->mapp[i]) &&
            !(freep = fetch_bitmap(hashp, i)))
            return (0);
        if (i == free_page)
            in_use_bits = free_bit;
        else
            in_use_bits = (hashp->BSIZE << BYTE_SHIFT) - 1;
 
        if (i == first_page) {
            bit = hashp->LAST_FREED &
                ((hashp->BSIZE << BYTE_SHIFT) - 1);
            j = bit / BITS_PER_MAP;
            bit = bit & ~(BITS_PER_MAP - 1);
        } else {
            bit = 0;
            j = 0;
        }
        for (; bit <= in_use_bits; j++, bit += BITS_PER_MAP)
            if (freep[j] != ALL_SET)
                goto found;
    }
 
    /* No Free Page Found */
    hashp->LAST_FREED = hashp->SPARES[splitnum];
    hashp->SPARES[splitnum]++;
    offset = hashp->SPARES[splitnum] -
        (splitnum ? hashp->SPARES[splitnum - 1] : 0);
 
#define OVMSG   "HASH: Out of overflow pages.  Increase page size\n"
    if (offset > SPLITMASK) {
        if (++splitnum >= NCACHED) {
            if (write(STDERR_FILENO, OVMSG, sizeof(OVMSG) - 1) < 0) {
            }
            return (0);
        }
        hashp->OVFL_POINT = splitnum;
        hashp->SPARES[splitnum] = hashp->SPARES[splitnum-1];
        hashp->SPARES[splitnum-1]--;
        offset = 1;
    }
 
    /* Check if we need to allocate a new bitmap page */
    if (free_bit == (hashp->BSIZE << BYTE_SHIFT) - 1) {
        free_page++;
        if (free_page >= NCACHED) {
            if (write(STDERR_FILENO, OVMSG, sizeof(OVMSG) - 1) < 0) {
            }
            return (0);
        }
        /*
         * This is tricky.  The 1 indicates that you want the new page
         * allocated with 1 clear bit.  Actually, you are going to
         * allocate 2 pages from this map.  The first is going to be
         * the map page, the second is the overflow page we were
         * looking for.  The init_bitmap routine automatically, sets
         * the first bit of itself to indicate that the bitmap itself
         * is in use.  We would explicitly set the second bit, but
         * don't have to if we tell init_bitmap not to leave it clear
         * in the first place.
         */
        if (__ibitmap(hashp,
            (int)OADDR_OF(splitnum, offset), 1, free_page))
            return (0);
        hashp->SPARES[splitnum]++;
#ifdef DEBUG2
        free_bit = 2;
#endif
        offset++;
        if (offset > SPLITMASK) {
            if (++splitnum >= NCACHED) {
                if (write(STDERR_FILENO, OVMSG, sizeof(OVMSG) - 1) < 0) {
                }
                return (0);
            }
            hashp->OVFL_POINT = splitnum;
            hashp->SPARES[splitnum] = hashp->SPARES[splitnum-1];
            hashp->SPARES[splitnum-1]--;
            offset = 0;
        }
    } else {
        /*
         * Free_bit addresses the last used bit.  Bump it to address
         * the first available bit.
         */
        free_bit++;
        SETBIT(freep, free_bit);
    }
 
    /* Calculate address of the new overflow page */
    addr = OADDR_OF(splitnum, offset);
#ifdef DEBUG2
    (void)fprintf(stderr, "OVERFLOW_PAGE: ADDR: %d BIT: %d PAGE %d\n",
        addr, free_bit, free_page);
#endif
    return (addr);
 
found:
    bit = bit + first_free(freep[j]);
    SETBIT(freep, bit);
#ifdef DEBUG2
    tmp1 = bit;
    tmp2 = i;
#endif
    /*
     * Bits are addressed starting with 0, but overflow pages are addressed
     * beginning at 1. Bit is a bit addressnumber, so we need to increment
     * it to convert it to a page number.
     */
    bit = 1 + bit + (i * (hashp->BSIZE << BYTE_SHIFT));
    if (bit >= hashp->LAST_FREED)
        hashp->LAST_FREED = bit - 1;
 
    /* Calculate the split number for this page */
    for (i = 0; (i < splitnum) && (bit > hashp->SPARES[i]); i++);
    offset = (i ? bit - hashp->SPARES[i - 1] : bit);
    if (offset >= SPLITMASK)
        return (0); /* Out of overflow pages */
    addr = OADDR_OF(i, offset);
#ifdef DEBUG2
    (void)fprintf(stderr, "OVERFLOW_PAGE: ADDR: %d BIT: %d PAGE %d\n",
        addr, tmp1, tmp2);
#endif
 
    /* Allocate and return the overflow page */
    return (addr);
}
 
/*
 * Mark this overflow page as free.
 */
extern void
__free_ovflpage(hashp, obufp)
    HTAB *hashp;
    BUFHEAD *obufp;
{
    register u_int16_t addr;
    u_int32_t *freep;
    int bit_address, free_page, free_bit;
    u_int16_t ndx;
 
    addr = obufp->addr;
#ifdef DEBUG1
    (void)fprintf(stderr, "Freeing %d\n", addr);
#endif
    ndx = (((u_int16_t)addr) >> SPLITSHIFT);
    bit_address =
        (ndx ? hashp->SPARES[ndx - 1] : 0) + (addr & SPLITMASK) - 1;
     if (bit_address < hashp->LAST_FREED)
        hashp->LAST_FREED = bit_address;
    free_page = (bit_address >> (hashp->BSHIFT + BYTE_SHIFT));
    free_bit = bit_address & ((hashp->BSIZE << BYTE_SHIFT) - 1);
 
    if (!(freep = hashp->mapp[free_page]))
        freep = fetch_bitmap(hashp, free_page);
#ifdef DEBUG
    /*
     * This had better never happen.  It means we tried to read a bitmap
     * that has already had overflow pages allocated off it, and we
     * failed to read it from the file.
     */
    if (!freep)
        assert(0);
#endif
    CLRBIT(freep, free_bit);
#ifdef DEBUG2
    (void)fprintf(stderr, "FREE_OVFLPAGE: ADDR: %d BIT: %d PAGE %d\n",
        obufp->addr, free_bit, free_page);
#endif
    __reclaim_buf(hashp, obufp);
}
 
/*
 * Returns:
 *   0 success
 *  -1 failure
 */
static int
open_temp(hashp)
    HTAB *hashp;
{
    sigset_t set, oset;
    static char namestr[] = "_hashXXXXXX";
 
    /* Block signals; make sure file goes away at process exit. */
    (void)sigfillset(&set);
    (void)sigprocmask(SIG_BLOCK, &set, &oset);
    if ((hashp->fp = mkstemp(namestr)) != -1) {
        (void)unlink(namestr);
        (void)fcntl(hashp->fp, F_SETFD, 1);
    }
    (void)sigprocmask(SIG_SETMASK, &oset, (sigset_t *)NULL);
    return (hashp->fp != -1 ? 0 : -1);
}
 
/*
 * We have to know that the key will fit, but the last entry on the page is
 * an overflow pair, so we need to shift things.
 */
static void
squeeze_key(sp, key, val)
    u_int16_t *sp;
    const DBT *key, *val;
{
    register char *p;
    u_int16_t free_space, n, off, pageno;
 
    p = (char *)sp;
    n = sp[0];
    free_space = FREESPACE(sp);
    off = OFFSET(sp);
 
    pageno = sp[n - 1];
    off -= key->size;
    sp[n - 1] = off;
    memmove(p + off, key->data, key->size);
    off -= val->size;
    sp[n] = off;
    memmove(p + off, val->data, val->size);
    sp[0] = n + 2;
    sp[n + 1] = pageno;
    sp[n + 2] = OVFLPAGE;
    FREESPACE(sp) = free_space - PAIRSIZE(key, val);
    OFFSET(sp) = off;
}
 
static u_int32_t *
fetch_bitmap(hashp, ndx)
    HTAB *hashp;
    int ndx;
{
    if (ndx >= hashp->nmaps)
        return (NULL);
    if ((hashp->mapp[ndx] = (u_int32_t *)malloc(hashp->BSIZE)) == NULL)
        return (NULL);
    if (__get_page(hashp,
        (char *)hashp->mapp[ndx], hashp->BITMAPS[ndx], 0, 1, 1)) {
        free(hashp->mapp[ndx]);
        return (NULL);
    }
    return (hashp->mapp[ndx]);
}
 
#ifdef DEBUG4
int
print_chain(addr)
    int addr;
{
    BUFHEAD *bufp;
    short *bp, oaddr;
 
    (void)fprintf(stderr, "%d ", addr);
    bufp = __get_buf(hashp, addr, NULL, 0);
    bp = (short *)bufp->page;
    while (bp[0] && ((bp[bp[0]] == OVFLPAGE) ||
        ((bp[0] > 2) && bp[2] < REAL_KEY))) {
        oaddr = bp[bp[0] - 1];
        (void)fprintf(stderr, "%d ", (int)oaddr);
        bufp = __get_buf(hashp, (int)oaddr, bufp, 0);
        bp = (short *)bufp->page;
    }
    (void)fprintf(stderr, "\n");
}
#endif