buffile.c

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/*-------------------------------------------------------------------------
 *
 * buffile.c
 *    Management of large buffered files, primarily temporary files.
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/storage/file/buffile.c
 *
 * NOTES:
 *
 * BufFiles provide a very incomplete emulation of stdio atop virtual Files
 * (as managed by fd.c).  Currently, we only support the buffered-I/O
 * aspect of stdio: a read or write of the low-level File occurs only
 * when the buffer is filled or emptied.  This is an even bigger win
 * for virtual Files than for ordinary kernel files, since reducing the
 * frequency with which a virtual File is touched reduces "thrashing"
 * of opening/closing file descriptors.
 *
 * Note that BufFile structs are allocated with palloc(), and therefore
 * will go away automatically at transaction end.  If the underlying
 * virtual File is made with OpenTemporaryFile, then all resources for
 * the file are certain to be cleaned up even if processing is aborted
 * by ereport(ERROR).   To avoid confusion, the caller should take care that
 * all calls for a single BufFile are made in the same palloc context.
 *
 * BufFile also supports temporary files that exceed the OS file size limit
 * (by opening multiple fd.c temporary files).  This is an essential feature
 * for sorts and hashjoins on large amounts of data.
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "executor/instrument.h"
#include "storage/fd.h"
#include "storage/buffile.h"
#include "storage/buf_internals.h"

/*
 * We break BufFiles into gigabyte-sized segments, regardless of RELSEG_SIZE.
 * The reason is that we'd like large temporary BufFiles to be spread across
 * multiple tablespaces when available.
 */
#define MAX_PHYSICAL_FILESIZE   0x40000000
#define BUFFILE_SEG_SIZE        (MAX_PHYSICAL_FILESIZE / BLCKSZ)

/*
 * This data structure represents a buffered file that consists of one or
 * more physical files (each accessed through a virtual file descriptor
 * managed by fd.c).
 */
struct BufFile
{
    int         numFiles;       /* number of physical files in set */
    /* all files except the last have length exactly MAX_PHYSICAL_FILESIZE */
    File       *files;          /* palloc'd array with numFiles entries */
    off_t      *offsets;        /* palloc'd array with numFiles entries */

    /*
     * offsets[i] is the current seek position of files[i].  We use this to
     * avoid making redundant FileSeek calls.
     */

    bool        isTemp;         /* can only add files if this is TRUE */
    bool        isInterXact;    /* keep open over transactions? */
    bool        dirty;          /* does buffer need to be written? */

    /*
     * "current pos" is position of start of buffer within the logical file.
     * Position as seen by user of BufFile is (curFile, curOffset + pos).
     */
    int         curFile;        /* file index (0..n) part of current pos */
    off_t       curOffset;      /* offset part of current pos */
    int         pos;            /* next read/write position in buffer */
    int         nbytes;         /* total # of valid bytes in buffer */
    char        buffer[BLCKSZ];
};

static BufFile *makeBufFile(File firstfile);
static void extendBufFile(BufFile *file);
static void BufFileLoadBuffer(BufFile *file);
static void BufFileDumpBuffer(BufFile *file);
static int  BufFileFlush(BufFile *file);


/*
 * Create a BufFile given the first underlying physical file.
 * NOTE: caller must set isTemp and isInterXact if appropriate.
 */
static BufFile *
makeBufFile(File firstfile)
{
    BufFile    *file = (BufFile *) palloc(sizeof(BufFile));

    file->numFiles = 1;
    file->files = (File *) palloc(sizeof(File));
    file->files[0] = firstfile;
    file->offsets = (off_t *) palloc(sizeof(off_t));
    file->offsets[0] = 0L;
    file->isTemp = false;
    file->isInterXact = false;
    file->dirty = false;
    file->curFile = 0;
    file->curOffset = 0L;
    file->pos = 0;
    file->nbytes = 0;

    return file;
}

/*
 * Add another component temp file.
 */
static void
extendBufFile(BufFile *file)
{
    File        pfile;

    Assert(file->isTemp);
    pfile = OpenTemporaryFile(file->isInterXact);
    Assert(pfile >= 0);

    file->files = (File *) repalloc(file->files,
                                    (file->numFiles + 1) * sizeof(File));
    file->offsets = (off_t *) repalloc(file->offsets,
                                       (file->numFiles + 1) * sizeof(off_t));
    file->files[file->numFiles] = pfile;
    file->offsets[file->numFiles] = 0L;
    file->numFiles++;
}

/*
 * Create a BufFile for a new temporary file (which will expand to become
 * multiple temporary files if more than MAX_PHYSICAL_FILESIZE bytes are
 * written to it).
 *
 * If interXact is true, the temp file will not be automatically deleted
 * at end of transaction.
 *
 * Note: if interXact is true, the caller had better be calling us in a
 * memory context that will survive across transaction boundaries.
 */
BufFile *
BufFileCreateTemp(bool interXact)
{
    BufFile    *file;
    File        pfile;

    pfile = OpenTemporaryFile(interXact);
    Assert(pfile >= 0);

    file = makeBufFile(pfile);
    file->isTemp = true;
    file->isInterXact = interXact;

    return file;
}

#ifdef NOT_USED
/*
 * Create a BufFile and attach it to an already-opened virtual File.
 *
 * This is comparable to fdopen() in stdio.  This is the only way at present
 * to attach a BufFile to a non-temporary file.  Note that BufFiles created
 * in this way CANNOT be expanded into multiple files.
 */
BufFile *
BufFileCreate(File file)
{
    return makeBufFile(file);
}
#endif

/*
 * Close a BufFile
 *
 * Like fclose(), this also implicitly FileCloses the underlying File.
 */
void
BufFileClose(BufFile *file)
{
    int         i;

    /* flush any unwritten data */
    BufFileFlush(file);
    /* close the underlying file(s) (with delete if it's a temp file) */
    for (i = 0; i < file->numFiles; i++)
        FileClose(file->files[i]);
    /* release the buffer space */
    pfree(file->files);
    pfree(file->offsets);
    pfree(file);
}

/*
 * BufFileLoadBuffer
 *
 * Load some data into buffer, if possible, starting from curOffset.
 * At call, must have dirty = false, pos and nbytes = 0.
 * On exit, nbytes is number of bytes loaded.
 */
static void
BufFileLoadBuffer(BufFile *file)
{
    File        thisfile;

    /*
     * Advance to next component file if necessary and possible.
     *
     * This path can only be taken if there is more than one component, so it
     * won't interfere with reading a non-temp file that is over
     * MAX_PHYSICAL_FILESIZE.
     */
    if (file->curOffset >= MAX_PHYSICAL_FILESIZE &&
        file->curFile + 1 < file->numFiles)
    {
        file->curFile++;
        file->curOffset = 0L;
    }

    /*
     * May need to reposition physical file.
     */
    thisfile = file->files[file->curFile];
    if (file->curOffset != file->offsets[file->curFile])
    {
        if (FileSeek(thisfile, file->curOffset, SEEK_SET) != file->curOffset)
            return;             /* seek failed, read nothing */
        file->offsets[file->curFile] = file->curOffset;
    }

    /*
     * Read whatever we can get, up to a full bufferload.
     */
    file->nbytes = FileRead(thisfile, file->buffer, sizeof(file->buffer));
    if (file->nbytes < 0)
        file->nbytes = 0;
    file->offsets[file->curFile] += file->nbytes;
    /* we choose not to advance curOffset here */

    pgBufferUsage.temp_blks_read++;
}

/*
 * BufFileDumpBuffer
 *
 * Dump buffer contents starting at curOffset.
 * At call, should have dirty = true, nbytes > 0.
 * On exit, dirty is cleared if successful write, and curOffset is advanced.
 */
static void
BufFileDumpBuffer(BufFile *file)
{
    int         wpos = 0;
    int         bytestowrite;
    File        thisfile;

    /*
     * Unlike BufFileLoadBuffer, we must dump the whole buffer even if it
     * crosses a component-file boundary; so we need a loop.
     */
    while (wpos < file->nbytes)
    {
        /*
         * Advance to next component file if necessary and possible.
         */
        if (file->curOffset >= MAX_PHYSICAL_FILESIZE && file->isTemp)
        {
            while (file->curFile + 1 >= file->numFiles)
                extendBufFile(file);
            file->curFile++;
            file->curOffset = 0L;
        }

        /*
         * Enforce per-file size limit only for temp files, else just try to
         * write as much as asked...
         */
        bytestowrite = file->nbytes - wpos;
        if (file->isTemp)
        {
            off_t       availbytes = MAX_PHYSICAL_FILESIZE - file->curOffset;

            if ((off_t) bytestowrite > availbytes)
                bytestowrite = (int) availbytes;
        }

        /*
         * May need to reposition physical file.
         */
        thisfile = file->files[file->curFile];
        if (file->curOffset != file->offsets[file->curFile])
        {
            if (FileSeek(thisfile, file->curOffset, SEEK_SET) != file->curOffset)
                return;         /* seek failed, give up */
            file->offsets[file->curFile] = file->curOffset;
        }
        bytestowrite = FileWrite(thisfile, file->buffer + wpos, bytestowrite);
        if (bytestowrite <= 0)
            return;             /* failed to write */
        file->offsets[file->curFile] += bytestowrite;
        file->curOffset += bytestowrite;
        wpos += bytestowrite;

        pgBufferUsage.temp_blks_written++;
    }
    file->dirty = false;

    /*
     * At this point, curOffset has been advanced to the end of the buffer,
     * ie, its original value + nbytes.  We need to make it point to the
     * logical file position, ie, original value + pos, in case that is less
     * (as could happen due to a small backwards seek in a dirty buffer!)
     */
    file->curOffset -= (file->nbytes - file->pos);
    if (file->curOffset < 0)    /* handle possible segment crossing */
    {
        file->curFile--;
        Assert(file->curFile >= 0);
        file->curOffset += MAX_PHYSICAL_FILESIZE;
    }

    /*
     * Now we can set the buffer empty without changing the logical position
     */
    file->pos = 0;
    file->nbytes = 0;
}

/*
 * BufFileRead
 *
 * Like fread() except we assume 1-byte element size.
 */
size_t
BufFileRead(BufFile *file, void *ptr, size_t size)
{
    size_t      nread = 0;
    size_t      nthistime;

    if (file->dirty)
    {
        if (BufFileFlush(file) != 0)
            return 0;           /* could not flush... */
        Assert(!file->dirty);
    }

    while (size > 0)
    {
        if (file->pos >= file->nbytes)
        {
            /* Try to load more data into buffer. */
            file->curOffset += file->pos;
            file->pos = 0;
            file->nbytes = 0;
            BufFileLoadBuffer(file);
            if (file->nbytes <= 0)
                break;          /* no more data available */
        }

        nthistime = file->nbytes - file->pos;
        if (nthistime > size)
            nthistime = size;
        Assert(nthistime > 0);

        memcpy(ptr, file->buffer + file->pos, nthistime);

        file->pos += nthistime;
        ptr = (void *) ((char *) ptr + nthistime);
        size -= nthistime;
        nread += nthistime;
    }

    return nread;
}

/*
 * BufFileWrite
 *
 * Like fwrite() except we assume 1-byte element size.
 */
size_t
BufFileWrite(BufFile *file, void *ptr, size_t size)
{
    size_t      nwritten = 0;
    size_t      nthistime;

    while (size > 0)
    {
        if (file->pos >= BLCKSZ)
        {
            /* Buffer full, dump it out */
            if (file->dirty)
            {
                BufFileDumpBuffer(file);
                if (file->dirty)
                    break;      /* I/O error */
            }
            else
            {
                /* Hmm, went directly from reading to writing? */
                file->curOffset += file->pos;
                file->pos = 0;
                file->nbytes = 0;
            }
        }

        nthistime = BLCKSZ - file->pos;
        if (nthistime > size)
            nthistime = size;
        Assert(nthistime > 0);

        memcpy(file->buffer + file->pos, ptr, nthistime);

        file->dirty = true;
        file->pos += nthistime;
        if (file->nbytes < file->pos)
            file->nbytes = file->pos;
        ptr = (void *) ((char *) ptr + nthistime);
        size -= nthistime;
        nwritten += nthistime;
    }

    return nwritten;
}

/*
 * BufFileFlush
 *
 * Like fflush()
 */
static int
BufFileFlush(BufFile *file)
{
    if (file->dirty)
    {
        BufFileDumpBuffer(file);
        if (file->dirty)
            return EOF;
    }

    return 0;
}

/*
 * BufFileSeek
 *
 * Like fseek(), except that target position needs two values in order to
 * work when logical filesize exceeds maximum value representable by long.
 * We do not support relative seeks across more than LONG_MAX, however.
 *
 * Result is 0 if OK, EOF if not.  Logical position is not moved if an
 * impossible seek is attempted.
 */
int
BufFileSeek(BufFile *file, int fileno, off_t offset, int whence)
{
    int         newFile;
    off_t       newOffset;

    switch (whence)
    {
        case SEEK_SET:
            if (fileno < 0)
                return EOF;
            newFile = fileno;
            newOffset = offset;
            break;
        case SEEK_CUR:

            /*
             * Relative seek considers only the signed offset, ignoring
             * fileno. Note that large offsets (> 1 gig) risk overflow in this
             * add, unless we have 64-bit off_t.
             */
            newFile = file->curFile;
            newOffset = (file->curOffset + file->pos) + offset;
            break;
#ifdef NOT_USED
        case SEEK_END:
            /* could be implemented, not needed currently */
            break;
#endif
        default:
            elog(ERROR, "invalid whence: %d", whence);
            return EOF;
    }
    while (newOffset < 0)
    {
        if (--newFile < 0)
            return EOF;
        newOffset += MAX_PHYSICAL_FILESIZE;
    }
    if (newFile == file->curFile &&
        newOffset >= file->curOffset &&
        newOffset <= file->curOffset + file->nbytes)
    {
        /*
         * Seek is to a point within existing buffer; we can just adjust
         * pos-within-buffer, without flushing buffer.  Note this is OK
         * whether reading or writing, but buffer remains dirty if we were
         * writing.
         */
        file->pos = (int) (newOffset - file->curOffset);
        return 0;
    }
    /* Otherwise, must reposition buffer, so flush any dirty data */
    if (BufFileFlush(file) != 0)
        return EOF;

    /*
     * At this point and no sooner, check for seek past last segment. The
     * above flush could have created a new segment, so checking sooner would
     * not work (at least not with this code).
     */
    if (file->isTemp)
    {
        /* convert seek to "start of next seg" to "end of last seg" */
        if (newFile == file->numFiles && newOffset == 0)
        {
            newFile--;
            newOffset = MAX_PHYSICAL_FILESIZE;
        }
        while (newOffset > MAX_PHYSICAL_FILESIZE)
        {
            if (++newFile >= file->numFiles)
                return EOF;
            newOffset -= MAX_PHYSICAL_FILESIZE;
        }
    }
    if (newFile >= file->numFiles)
        return EOF;
    /* Seek is OK! */
    file->curFile = newFile;
    file->curOffset = newOffset;
    file->pos = 0;
    file->nbytes = 0;
    return 0;
}

void
BufFileTell(BufFile *file, int *fileno, off_t *offset)
{
    *fileno = file->curFile;
    *offset = file->curOffset + file->pos;
}

/*
 * BufFileSeekBlock --- block-oriented seek
 *
 * Performs absolute seek to the start of the n'th BLCKSZ-sized block of
 * the file.  Note that users of this interface will fail if their files
 * exceed BLCKSZ * LONG_MAX bytes, but that is quite a lot; we don't work
 * with tables bigger than that, either...
 *
 * Result is 0 if OK, EOF if not.  Logical position is not moved if an
 * impossible seek is attempted.
 */
int
BufFileSeekBlock(BufFile *file, long blknum)
{
    return BufFileSeek(file,
                       (int) (blknum / BUFFILE_SEG_SIZE),
                       (off_t) (blknum % BUFFILE_SEG_SIZE) * BLCKSZ,
                       SEEK_SET);
}

#ifdef NOT_USED
/*
 * BufFileTellBlock --- block-oriented tell
 *
 * Any fractional part of a block in the current seek position is ignored.
 */
long
BufFileTellBlock(BufFile *file)
{
    long        blknum;

    blknum = (file->curOffset + file->pos) / BLCKSZ;
    blknum += file->curFile * BUFFILE_SEG_SIZE;
    return blknum;
}

#endif