os_fid.c

/*-
 * See the file LICENSE for redistribution information.
 *
 * Copyright (c) 1996-2005
 *      Sleepycat Software.  All rights reserved.
 *
 * $Id: os_fid.c,v 12.4 2005/10/11 18:17:00 bostic Exp $
 */

#include "db_config.h"

#include "db_int.h"

#define SERIAL_INIT     0
static u_int32_t fid_serial = SERIAL_INIT;

/*
 * __os_fileid --
 *      Return a unique identifier for a file.
 */
int
__os_fileid(dbenv, fname, unique_okay, fidp)
        DB_ENV *dbenv;
        const char *fname;
        int unique_okay;
        u_int8_t *fidp;
{
        db_threadid_t tid;
        pid_t pid;
        size_t i;
        u_int32_t tmp;
        u_int8_t *p;
        int ret;

        /*
         * The documentation for GetFileInformationByHandle() states that the
         * inode-type numbers are not constant between processes.  Actually,
         * they are, they're the NTFS MFT indexes.  So, this works on NTFS,
         * but perhaps not on other platforms, and perhaps not over a network.
         * Can't think of a better solution right now.
         */
        DB_FH *fhp;
        BY_HANDLE_FILE_INFORMATION fi;
        BOOL retval = FALSE;

        DB_ASSERT(fname != NULL);

        /* Clear the buffer. */
        memset(fidp, 0, DB_FILE_ID_LEN);

        /*
         * Initialize/increment the serial number we use to help avoid
         * fileid collisions.  Note that we don't bother with locking;
         * it's unpleasant to do from down in here, and if we race on
         * this no real harm will be done, since the finished fileid
         * has so many other components.
         *
         * We use the bottom 32-bits of the process ID, hoping they
         * are more random than the top 32-bits (should we be on a
         * machine with 64-bit process IDs).
         *
         * We increment by 100000 on each call as a simple way of
         * randomizing;  simply incrementing seems potentially less useful
         * if pids are also simply incremented, since this is process-local
         * and we may be one of a set of processes starting up.  100000
         * pushes us out of pid space on most platforms, and has few
         * interesting properties in base 2.
         */
        if (fid_serial == SERIAL_INIT) {
                __os_id(dbenv, &pid, &tid);
                fid_serial = pid;
        } else
                fid_serial += 100000;

        /*
         * First we open the file, because we're not given a handle to it.
         * If we can't open it, we're in trouble.
         */
        if ((ret = __os_open(dbenv, fname, DB_OSO_RDONLY, _S_IREAD, &fhp)) != 0)
                return (ret);

        /* File open, get its info */
        if ((retval = GetFileInformationByHandle(fhp->handle, &fi)) == FALSE)
                ret = __os_get_errno();
        (void)__os_closehandle(dbenv, fhp);

        if (retval == FALSE)
                return (ret);

        /*
         * We want the three 32-bit words which tell us the volume ID and
         * the file ID.  We make a crude attempt to copy the bytes over to
         * the callers buffer.
         *
         * We don't worry about byte sexing or the actual variable sizes.
         *
         * When this routine is called from the DB access methods, it's only
         * called once -- whatever ID is generated when a database is created
         * is stored in the database file's metadata, and that is what is
         * saved in the mpool region's information to uniquely identify the
         * file.
         *
         * When called from the mpool layer this routine will be called each
         * time a new thread of control wants to share the file, which makes
         * things tougher.  As far as byte sexing goes, since the mpool region
         * lives on a single host, there's no issue of that -- the entire
         * region is byte sex dependent.  As far as variable sizes go, we make
         * the simplifying assumption that 32-bit and 64-bit processes will
         * get the same 32-bit values if we truncate any returned 64-bit value
         * to a 32-bit value.
         */
        tmp = (u_int32_t)fi.nFileIndexLow;
        for (p = (u_int8_t *)&tmp, i = sizeof(u_int32_t); i > 0; --i)
                *fidp++ = *p++;
        tmp = (u_int32_t)fi.nFileIndexHigh;
        for (p = (u_int8_t *)&tmp, i = sizeof(u_int32_t); i > 0; --i)
                *fidp++ = *p++;

        if (unique_okay) {
                /*
                 * Use the system time to try to get a unique value
                 * within this process.  A millisecond counter
                 * overflows 32 bits in about 49 days.  So we use 8
                 * bytes, and don't bother with the volume ID, which
                 * is not very useful for our purposes.
                 */
                SYSTEMTIME st;

                GetSystemTime(&st);
                tmp = (st.wYear - 1900) * 12 + (st.wMonth - 1);
                for (p = (u_int8_t *)&tmp, i = sizeof(u_int32_t); i > 0; --i)
                        *fidp++ = *p++;
                tmp = ((((st.wDay - 1) * 24 + st.wHour) * 60 +
                        st.wMinute) * 60 + st.wSecond) * 1000 +
                        st.wMilliseconds;
                for (p = (u_int8_t *)&tmp, i = sizeof(u_int32_t); i > 0; --i)
                        *fidp++ = *p++;
                for (p = (u_int8_t *)&fid_serial, i = sizeof(u_int32_t);
                    i > 0; --i)
                        *fidp++ = *p++;
        } else {
                tmp = (u_int32_t)fi.dwVolumeSerialNumber;
                for (p = (u_int8_t *)&tmp, i = sizeof(u_int32_t); i > 0; --i)
                        *fidp++ = *p++;
        }

        return (0);
}

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