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postgresql/src/backend/utils/sort/logtape.c

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/*-------------------------------------------------------------------------
*
* logtape.c
* Management of "logical tapes" within temporary files.
*
* This module exists to support sorting via multiple merge passes (see
* tuplesort.c). Merging is an ideal algorithm for tape devices, but if
* we implement it on disk by creating a separate file for each "tape",
* there is an annoying problem: the peak space usage is at least twice
* the volume of actual data to be sorted. (This must be so because each
* datum will appear in both the input and output tapes of the final
* merge pass. For seven-tape polyphase merge, which is otherwise a
* pretty good algorithm, peak usage is more like 4x actual data volume.)
*
* We can work around this problem by recognizing that any one tape
* dataset (with the possible exception of the final output) is written
* and read exactly once in a perfectly sequential manner. Therefore,
* a datum once read will not be required again, and we can recycle its
* space for use by the new tape dataset(s) being generated. In this way,
* the total space usage is essentially just the actual data volume, plus
* insignificant bookkeeping and start/stop overhead.
*
* Few OSes allow arbitrary parts of a file to be released back to the OS,
* so we have to implement this space-recycling ourselves within a single
* logical file. logtape.c exists to perform this bookkeeping and provide
* the illusion of N independent tape devices to tuplesort.c. Note that
* logtape.c itself depends on buffile.c to provide a "logical file" of
* larger size than the underlying OS may support.
*
* For simplicity, we allocate and release space in the underlying file
* in BLCKSZ-size blocks. Space allocation boils down to keeping track
* of which blocks in the underlying file belong to which logical tape,
* plus any blocks that are free (recycled and not yet reused). Normally
* there are not very many free blocks, so we just keep those in a list.
* The blocks in each logical tape are remembered using a method borrowed
* from the Unix HFS filesystem: we store data block numbers in an
* "indirect block". If an indirect block fills up, we write it out to
* the underlying file and remember its location in a second-level indirect
* block. In the same way second-level blocks are remembered in third-
* level blocks, and so on if necessary (of course we're talking huge
* amounts of data here). The topmost indirect block of a given logical
* tape is never actually written out to the physical file, but all lower-
* level indirect blocks will be.
*
* The initial write pass is guaranteed to fill the underlying file
* perfectly sequentially, no matter how data is divided into logical tapes.
* Once we begin merge passes, the access pattern becomes considerably
* less predictable --- but the seeking involved should be comparable to
* what would happen if we kept each logical tape in a separate file,
* so there's no serious performance penalty paid to obtain the space
* savings of recycling. We try to localize the write accesses by always
* writing to the lowest-numbered free block when we have a choice; it's
* not clear this helps much, but it can't hurt. (XXX perhaps a LIFO
* policy for free blocks would be better?)
*
* Since all the bookkeeping and buffer memory is allocated with palloc(),
* and the underlying file(s) are made with OpenTemporaryFile, all resources
* for a logical tape set are certain to be cleaned up even if processing
* is aborted by elog(ERROR). To avoid confusion, the caller should take
* care that all calls for a single LogicalTapeSet are made in the same
* palloc context.
*
* Portions Copyright (c) 1996-2000, PostgreSQL, Inc
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/utils/sort/logtape.c,v 1.5 2000/04/12 17:16:11 momjian Exp $
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "storage/buffile.h"
#include "utils/logtape.h"
/*
* Block indexes are "long"s, so we can fit this many per indirect block.
* NB: we assume this is an exact fit!
*/
#define BLOCKS_PER_INDIR_BLOCK ((int) (BLCKSZ / sizeof(long)))
/*
* We use a struct like this for each active indirection level of each
* logical tape. If the indirect block is not the highest level of its
* tape, the "nextup" link points to the next higher level. Only the
* "ptrs" array is written out if we have to dump the indirect block to
* disk. If "ptrs" is not completely full, we store -1L in the first
* unused slot at completion of the write phase for the logical tape.
*/
typedef struct IndirectBlock
{
int nextSlot; /* next pointer slot to write or read */
struct IndirectBlock *nextup; /* parent indirect level, or NULL
* if top */
long ptrs[BLOCKS_PER_INDIR_BLOCK]; /* indexes of contained
* blocks */
} IndirectBlock;
/*
* This data structure represents a single "logical tape" within the set
* of logical tapes stored in the same file. We must keep track of the
* current partially-read-or-written data block as well as the active
* indirect block level(s).
*/
typedef struct LogicalTape
{
IndirectBlock *indirect; /* bottom of my indirect-block hierarchy */
bool writing; /* T while in write phase */
bool frozen; /* T if blocks should not be freed when
* read */
bool dirty; /* does buffer need to be written? */
/*
* The total data volume in the logical tape is numFullBlocks * BLCKSZ
* + lastBlockBytes. BUT: we do not update lastBlockBytes during
* writing, only at completion of a write phase.
*/
long numFullBlocks; /* number of complete blocks in log tape */
int lastBlockBytes; /* valid bytes in last (incomplete) block */
/*
* Buffer for current data block. Note we don't bother to store the
* actual file block number of the data block (during the write phase
* it hasn't been assigned yet, and during read we don't care
* anymore). But we do need the relative block number so we can detect
* end-of-tape while reading.
*/
long curBlockNumber; /* this block's logical blk# within tape */
int pos; /* next read/write position in buffer */
int nbytes; /* total # of valid bytes in buffer */
char buffer[BLCKSZ];
} LogicalTape;
/*
* This data structure represents a set of related "logical tapes" sharing
* space in a single underlying file. (But that "file" may be multiple files
* if needed to escape OS limits on file size; buffile.c handles that for us.)
* The number of tapes is fixed at creation.
*/
struct LogicalTapeSet
{
BufFile *pfile; /* underlying file for whole tape set */
long nFileBlocks; /* # of blocks used in underlying file */
/*
* We store the numbers of recycled-and-available blocks in
* freeBlocks[]. When there are no such blocks, we extend the
* underlying file. Note that the block numbers in freeBlocks are
* always in *decreasing* order, so that removing the last entry gives
* us the lowest free block.
*/
long *freeBlocks; /* resizable array */
int nFreeBlocks; /* # of currently free blocks */
int freeBlocksLen; /* current allocated length of
* freeBlocks[] */
/*
* tapes[] is declared size 1 since C wants a fixed size, but actually
* it is of length nTapes.
*/
int nTapes; /* # of logical tapes in set */
LogicalTape *tapes[1]; /* must be last in struct! */
};
static void ltsWriteBlock(LogicalTapeSet *lts, long blocknum, void *buffer);
static void ltsReadBlock(LogicalTapeSet *lts, long blocknum, void *buffer);
static long ltsGetFreeBlock(LogicalTapeSet *lts);
static void ltsReleaseBlock(LogicalTapeSet *lts, long blocknum);
static void ltsRecordBlockNum(LogicalTapeSet *lts, IndirectBlock *indirect,
long blocknum);
static long ltsRewindIndirectBlock(LogicalTapeSet *lts,
IndirectBlock *indirect,
bool freezing);
static long ltsRewindFrozenIndirectBlock(LogicalTapeSet *lts,
IndirectBlock *indirect);
static long ltsRecallNextBlockNum(LogicalTapeSet *lts,
IndirectBlock *indirect,
bool frozen);
static long ltsRecallPrevBlockNum(LogicalTapeSet *lts,
IndirectBlock *indirect);
static void ltsDumpBuffer(LogicalTapeSet *lts, LogicalTape *lt);
/*
* Write a block-sized buffer to the specified block of the underlying file.
*
* NB: should not attempt to write beyond current end of file (ie, create
* "holes" in file), since BufFile doesn't allow that. The first write pass
* must write blocks sequentially.
*
* No need for an error return convention; we elog() on any error.
*/
static void
ltsWriteBlock(LogicalTapeSet *lts, long blocknum, void *buffer)
{
if (BufFileSeekBlock(lts->pfile, blocknum) != 0 ||
BufFileWrite(lts->pfile, buffer, BLCKSZ) != BLCKSZ)
elog(ERROR, "ltsWriteBlock: failed to write block %ld of temporary file\n\t\tPerhaps out of disk space?",
blocknum);
}
/*
* Read a block-sized buffer from the specified block of the underlying file.
*
* No need for an error return convention; we elog() on any error. This
* module should never attempt to read a block it doesn't know is there.
*/
static void
ltsReadBlock(LogicalTapeSet *lts, long blocknum, void *buffer)
{
if (BufFileSeekBlock(lts->pfile, blocknum) != 0 ||
BufFileRead(lts->pfile, buffer, BLCKSZ) != BLCKSZ)
elog(ERROR, "ltsReadBlock: failed to read block %ld of temporary file",
blocknum);
}
/*
* Select a currently unused block for writing to.
*
* NB: should only be called when writer is ready to write immediately,
* to ensure that first write pass is sequential.
*/
static long
ltsGetFreeBlock(LogicalTapeSet *lts)
{
/*
* If there are multiple free blocks, we select the one appearing last
* in freeBlocks[]. If there are none, assign the next block at the
* end of the file.
*/
if (lts->nFreeBlocks > 0)
return lts->freeBlocks[--lts->nFreeBlocks];
else
return lts->nFileBlocks++;
}
/*
* Return a block# to the freelist.
*/
static void
ltsReleaseBlock(LogicalTapeSet *lts, long blocknum)
{
int ndx;
long *ptr;
/*
* Enlarge freeBlocks array if full.
*/
if (lts->nFreeBlocks >= lts->freeBlocksLen)
{
lts->freeBlocksLen *= 2;
lts->freeBlocks = (long *) repalloc(lts->freeBlocks,
lts->freeBlocksLen * sizeof(long));
}
/*
* Insert blocknum into array, preserving decreasing order (so that
* ltsGetFreeBlock returns the lowest available block number). This
* could get fairly slow if there were many free blocks, but we don't
* expect there to be very many at one time.
*/
ndx = lts->nFreeBlocks++;
ptr = lts->freeBlocks + ndx;
while (ndx > 0 && ptr[-1] < blocknum)
{
ptr[0] = ptr[-1];
ndx--, ptr--;
}
ptr[0] = blocknum;
}
/*
* These routines manipulate indirect-block hierarchies. All are recursive
* so that they don't have any specific limit on the depth of hierarchy.
*/
/*
* Record a data block number in a logical tape's lowest indirect block,
* or record an indirect block's number in the next higher indirect level.
*/
static void
ltsRecordBlockNum(LogicalTapeSet *lts, IndirectBlock *indirect,
long blocknum)
{
if (indirect->nextSlot >= BLOCKS_PER_INDIR_BLOCK)
{
/*
* This indirect block is full, so dump it out and recursively
* save its address in the next indirection level. Create a new
* indirection level if there wasn't one before.
*/
long indirblock = ltsGetFreeBlock(lts);
ltsWriteBlock(lts, indirblock, (void *) indirect->ptrs);
if (indirect->nextup == NULL)
{
indirect->nextup = (IndirectBlock *) palloc(sizeof(IndirectBlock));
indirect->nextup->nextSlot = 0;
indirect->nextup->nextup = NULL;
}
ltsRecordBlockNum(lts, indirect->nextup, indirblock);
/*
* Reset to fill another indirect block at this level.
*/
indirect->nextSlot = 0;
}
indirect->ptrs[indirect->nextSlot++] = blocknum;
}
/*
* Reset a logical tape's indirect-block hierarchy after a write pass
* to prepare for reading. We dump out partly-filled blocks except
* at the top of the hierarchy, and we rewind each level to the start.
* This call returns the first data block number, or -1L if the tape
* is empty.
*
* Unless 'freezing' is true, release indirect blocks to the free pool after
* reading them.
*/
static long
ltsRewindIndirectBlock(LogicalTapeSet *lts,
IndirectBlock *indirect,
bool freezing)
{
/* Insert sentinel if block is not full */
if (indirect->nextSlot < BLOCKS_PER_INDIR_BLOCK)
indirect->ptrs[indirect->nextSlot] = -1L;
/*
* If block is not topmost, write it out, and recurse to obtain
* address of first block in this hierarchy level. Read that one in.
*/
if (indirect->nextup != NULL)
{
long indirblock = ltsGetFreeBlock(lts);
ltsWriteBlock(lts, indirblock, (void *) indirect->ptrs);
ltsRecordBlockNum(lts, indirect->nextup, indirblock);
indirblock = ltsRewindIndirectBlock(lts, indirect->nextup, freezing);
Assert(indirblock != -1L);
ltsReadBlock(lts, indirblock, (void *) indirect->ptrs);
if (!freezing)
ltsReleaseBlock(lts, indirblock);
}
/*
* Reset my next-block pointer, and then fetch a block number if any.
*/
indirect->nextSlot = 0;
if (indirect->ptrs[0] == -1L)
return -1L;
return indirect->ptrs[indirect->nextSlot++];
}
/*
* Rewind a previously-frozen indirect-block hierarchy for another read pass.
* This call returns the first data block number, or -1L if the tape
* is empty.
*/
static long
ltsRewindFrozenIndirectBlock(LogicalTapeSet *lts,
IndirectBlock *indirect)
{
/*
* If block is not topmost, recurse to obtain address of first block
* in this hierarchy level. Read that one in.
*/
if (indirect->nextup != NULL)
{
long indirblock;
indirblock = ltsRewindFrozenIndirectBlock(lts, indirect->nextup);
Assert(indirblock != -1L);
ltsReadBlock(lts, indirblock, (void *) indirect->ptrs);
}
/*
* Reset my next-block pointer, and then fetch a block number if any.
*/
indirect->nextSlot = 0;
if (indirect->ptrs[0] == -1L)
return -1L;
return indirect->ptrs[indirect->nextSlot++];
}
/*
* Obtain next data block number in the forward direction, or -1L if no more.
*
* Unless 'frozen' is true, release indirect blocks to the free pool after
* reading them.
*/
static long
ltsRecallNextBlockNum(LogicalTapeSet *lts,
IndirectBlock *indirect,
bool frozen)
{
if (indirect->nextSlot >= BLOCKS_PER_INDIR_BLOCK ||
indirect->ptrs[indirect->nextSlot] == -1L)
{
long indirblock;
if (indirect->nextup == NULL)
return -1L; /* nothing left at this level */
indirblock = ltsRecallNextBlockNum(lts, indirect->nextup, frozen);
if (indirblock == -1L)
return -1L; /* nothing left at this level */
ltsReadBlock(lts, indirblock, (void *) indirect->ptrs);
if (!frozen)
ltsReleaseBlock(lts, indirblock);
indirect->nextSlot = 0;
}
if (indirect->ptrs[indirect->nextSlot] == -1L)
return -1L;
return indirect->ptrs[indirect->nextSlot++];
}
/*
* Obtain next data block number in the reverse direction, or -1L if no more.
*
* Note this fetches the block# before the one last returned, no matter which
* direction of call returned that one. If we fail, no change in state.
*
* This routine can only be used in 'frozen' state, so there's no need to
* pass a parameter telling whether to release blocks ... we never do.
*/
static long
ltsRecallPrevBlockNum(LogicalTapeSet *lts,
IndirectBlock *indirect)
{
if (indirect->nextSlot <= 1)
{
long indirblock;
if (indirect->nextup == NULL)
return -1L; /* nothing left at this level */
indirblock = ltsRecallPrevBlockNum(lts, indirect->nextup);
if (indirblock == -1L)
return -1L; /* nothing left at this level */
ltsReadBlock(lts, indirblock, (void *) indirect->ptrs);
/*
* The previous block would only have been written out if full, so
* we need not search it for a -1 sentinel.
*/
indirect->nextSlot = BLOCKS_PER_INDIR_BLOCK + 1;
}
indirect->nextSlot--;
return indirect->ptrs[indirect->nextSlot - 1];
}
/*
* Create a set of logical tapes in a temporary underlying file.
*
* Each tape is initialized in write state.
*/
LogicalTapeSet *
LogicalTapeSetCreate(int ntapes)
{
LogicalTapeSet *lts;
LogicalTape *lt;
int i;
/*
* Create top-level struct. First LogicalTape pointer is already
* counted in sizeof(LogicalTapeSet).
*/
Assert(ntapes > 0);
lts = (LogicalTapeSet *) palloc(sizeof(LogicalTapeSet) +
(ntapes - 1) *sizeof(LogicalTape *));
lts->pfile = BufFileCreateTemp();
lts->nFileBlocks = 0L;
lts->freeBlocksLen = 32; /* reasonable initial guess */
lts->freeBlocks = (long *) palloc(lts->freeBlocksLen * sizeof(long));
lts->nFreeBlocks = 0;
lts->nTapes = ntapes;
/*
* Create per-tape structs, including first-level indirect blocks.
*/
for (i = 0; i < ntapes; i++)
{
lt = (LogicalTape *) palloc(sizeof(LogicalTape));
lts->tapes[i] = lt;
lt->indirect = (IndirectBlock *) palloc(sizeof(IndirectBlock));
lt->indirect->nextSlot = 0;
lt->indirect->nextup = NULL;
lt->writing = true;
lt->frozen = false;
lt->dirty = false;
lt->numFullBlocks = 0L;
lt->lastBlockBytes = 0;
lt->curBlockNumber = 0L;
lt->pos = 0;
lt->nbytes = 0;
}
return lts;
}
/*
* Close a logical tape set and release all resources.
*/
void
LogicalTapeSetClose(LogicalTapeSet *lts)
{
LogicalTape *lt;
IndirectBlock *ib,
*nextib;
int i;
BufFileClose(lts->pfile);
for (i = 0; i < lts->nTapes; i++)
{
lt = lts->tapes[i];
for (ib = lt->indirect; ib != NULL; ib = nextib)
{
nextib = ib->nextup;
pfree(ib);
}
pfree(lt);
}
pfree(lts->freeBlocks);
pfree(lts);
}
/*
* Dump the dirty buffer of a logical tape.
*/
static void
ltsDumpBuffer(LogicalTapeSet *lts, LogicalTape *lt)
{
long datablock = ltsGetFreeBlock(lts);
Assert(lt->dirty);
ltsWriteBlock(lts, datablock, (void *) lt->buffer);
ltsRecordBlockNum(lts, lt->indirect, datablock);
lt->dirty = false;
/* Caller must do other state update as needed */
}
/*
* Write to a logical tape.
*
* There are no error returns; we elog() on failure.
*/
void
LogicalTapeWrite(LogicalTapeSet *lts, int tapenum,
void *ptr, size_t size)
{
LogicalTape *lt;
size_t nthistime;
Assert(tapenum >= 0 && tapenum < lts->nTapes);
lt = lts->tapes[tapenum];
Assert(lt->writing);
while (size > 0)
{
if (lt->pos >= BLCKSZ)
{
/* Buffer full, dump it out */
if (lt->dirty)
ltsDumpBuffer(lts, lt);
else
{
/* Hmm, went directly from reading to writing? */
elog(ERROR, "LogicalTapeWrite: impossible state");
}
lt->numFullBlocks++;
lt->curBlockNumber++;
lt->pos = 0;
lt->nbytes = 0;
}
nthistime = BLCKSZ - lt->pos;
if (nthistime > size)
nthistime = size;
Assert(nthistime > 0);
memcpy(lt->buffer + lt->pos, ptr, nthistime);
lt->dirty = true;
lt->pos += nthistime;
if (lt->nbytes < lt->pos)
lt->nbytes = lt->pos;
ptr = (void *) ((char *) ptr + nthistime);
size -= nthistime;
}
}
/*
* Rewind logical tape and switch from writing to reading or vice versa.
*
* Unless the tape has been "frozen" in read state, forWrite must be the
* opposite of the previous tape state.
*/
void
LogicalTapeRewind(LogicalTapeSet *lts, int tapenum, bool forWrite)
{
LogicalTape *lt;
long datablocknum;
Assert(tapenum >= 0 && tapenum < lts->nTapes);
lt = lts->tapes[tapenum];
if (!forWrite)
{
if (lt->writing)
{
/*
* Completion of a write phase. Flush last partial data
* block, flush any partial indirect blocks, rewind for normal
* (destructive) read.
*/
if (lt->dirty)
ltsDumpBuffer(lts, lt);
lt->lastBlockBytes = lt->nbytes;
lt->writing = false;
datablocknum = ltsRewindIndirectBlock(lts, lt->indirect, false);
}
else
{
/*
* This is only OK if tape is frozen; we rewind for (another)
* read pass.
*/
Assert(lt->frozen);
datablocknum = ltsRewindFrozenIndirectBlock(lts, lt->indirect);
}
/* Read the first block, or reset if tape is empty */
lt->curBlockNumber = 0L;
lt->pos = 0;
lt->nbytes = 0;
if (datablocknum != -1L)
{
ltsReadBlock(lts, datablocknum, (void *) lt->buffer);
if (!lt->frozen)
ltsReleaseBlock(lts, datablocknum);
lt->nbytes = (lt->curBlockNumber < lt->numFullBlocks) ?
BLCKSZ : lt->lastBlockBytes;
}
}
else
{
/*
* Completion of a read phase. Rewind and prepare for write.
*
* NOTE: we assume the caller has read the tape to the end; otherwise
* untouched data and indirect blocks will not have been freed. We
* could add more code to free any unread blocks, but in current
* usage of this module it'd be useless code.
*/
IndirectBlock *ib,
*nextib;
Assert(!lt->writing && !lt->frozen);
/* Must truncate the indirect-block hierarchy down to one level. */
for (ib = lt->indirect->nextup; ib != NULL; ib = nextib)
{
nextib = ib->nextup;
pfree(ib);
}
lt->indirect->nextSlot = 0;
lt->indirect->nextup = NULL;
lt->writing = true;
lt->dirty = false;
lt->numFullBlocks = 0L;
lt->lastBlockBytes = 0;
lt->curBlockNumber = 0L;
lt->pos = 0;
lt->nbytes = 0;
}
}
/*
* Read from a logical tape.
*
* Early EOF is indicated by return value less than #bytes requested.
*/
size_t
LogicalTapeRead(LogicalTapeSet *lts, int tapenum,
void *ptr, size_t size)
{
LogicalTape *lt;
size_t nread = 0;
size_t nthistime;
Assert(tapenum >= 0 && tapenum < lts->nTapes);
lt = lts->tapes[tapenum];
Assert(!lt->writing);
while (size > 0)
{
if (lt->pos >= lt->nbytes)
{
/* Try to load more data into buffer. */
long datablocknum = ltsRecallNextBlockNum(lts, lt->indirect,
lt->frozen);
if (datablocknum == -1L)
break; /* EOF */
lt->curBlockNumber++;
lt->pos = 0;
ltsReadBlock(lts, datablocknum, (void *) lt->buffer);
if (!lt->frozen)
ltsReleaseBlock(lts, datablocknum);
lt->nbytes = (lt->curBlockNumber < lt->numFullBlocks) ?
BLCKSZ : lt->lastBlockBytes;
if (lt->nbytes <= 0)
break; /* EOF (possible here?) */
}
nthistime = lt->nbytes - lt->pos;
if (nthistime > size)
nthistime = size;
Assert(nthistime > 0);
memcpy(ptr, lt->buffer + lt->pos, nthistime);
lt->pos += nthistime;
ptr = (void *) ((char *) ptr + nthistime);
size -= nthistime;
nread += nthistime;
}
return nread;
}
/*
* "Freeze" the contents of a tape so that it can be read multiple times
* and/or read backwards. Once a tape is frozen, its contents will not
* be released until the LogicalTapeSet is destroyed. This is expected
* to be used only for the final output pass of a merge.
*
* This *must* be called just at the end of a write pass, before the
* tape is rewound (after rewind is too late!). It performs a rewind
* and switch to read mode "for free". An immediately following rewind-
* for-read call is OK but not necessary.
*/
void
LogicalTapeFreeze(LogicalTapeSet *lts, int tapenum)
{
LogicalTape *lt;
long datablocknum;
Assert(tapenum >= 0 && tapenum < lts->nTapes);
lt = lts->tapes[tapenum];
Assert(lt->writing);
/*
* Completion of a write phase. Flush last partial data block, flush
* any partial indirect blocks, rewind for nondestructive read.
*/
if (lt->dirty)
ltsDumpBuffer(lts, lt);
lt->lastBlockBytes = lt->nbytes;
lt->writing = false;
lt->frozen = true;
datablocknum = ltsRewindIndirectBlock(lts, lt->indirect, true);
/* Read the first block, or reset if tape is empty */
lt->curBlockNumber = 0L;
lt->pos = 0;
lt->nbytes = 0;
if (datablocknum != -1L)
{
ltsReadBlock(lts, datablocknum, (void *) lt->buffer);
lt->nbytes = (lt->curBlockNumber < lt->numFullBlocks) ?
BLCKSZ : lt->lastBlockBytes;
}
}
/*
* Backspace the tape a given number of bytes. (We also support a more
* general seek interface, see below.)
*
* *Only* a frozen-for-read tape can be backed up; we don't support
* random access during write, and an unfrozen read tape may have
* already discarded the desired data!
*
* Return value is TRUE if seek successful, FALSE if there isn't that much
* data before the current point (in which case there's no state change).
*/
bool
LogicalTapeBackspace(LogicalTapeSet *lts, int tapenum, size_t size)
{
LogicalTape *lt;
long nblocks;
int newpos;
Assert(tapenum >= 0 && tapenum < lts->nTapes);
lt = lts->tapes[tapenum];
Assert(lt->frozen);
/*
* Easy case for seek within current block.
*/
if (size <= (size_t) lt->pos)
{
lt->pos -= (int) size;
return true;
}
/*
* Not-so-easy case. Figure out whether it's possible at all.
*/
size -= (size_t) lt->pos; /* part within this block */
nblocks = size / BLCKSZ;
size = size % BLCKSZ;
if (size)
{
nblocks++;
newpos = (int) (BLCKSZ - size);
}
else
newpos = 0;
if (nblocks > lt->curBlockNumber)
return false; /* a seek too far... */
/*
* OK, we need to back up nblocks blocks. This implementation would
* be pretty inefficient for long seeks, but we really aren't
* expecting that (a seek over one tuple is typical).
*/
while (nblocks-- > 0)
{
long datablocknum = ltsRecallPrevBlockNum(lts, lt->indirect);
if (datablocknum == -1L)
elog(ERROR, "LogicalTapeBackspace: unexpected end of tape");
lt->curBlockNumber--;
if (nblocks == 0)
{
ltsReadBlock(lts, datablocknum, (void *) lt->buffer);
lt->nbytes = BLCKSZ;
}
}
lt->pos = newpos;
return true;
}
/*
* Seek to an arbitrary position in a logical tape.
*
* *Only* a frozen-for-read tape can be seeked.
*
* Return value is TRUE if seek successful, FALSE if there isn't that much
* data in the tape (in which case there's no state change).
*/
bool
LogicalTapeSeek(LogicalTapeSet *lts, int tapenum,
long blocknum, int offset)
{
LogicalTape *lt;
Assert(tapenum >= 0 && tapenum < lts->nTapes);
lt = lts->tapes[tapenum];
Assert(lt->frozen);
Assert(offset >= 0 && offset <= BLCKSZ);
/*
* Easy case for seek within current block.
*/
if (blocknum == lt->curBlockNumber && offset <= lt->nbytes)
{
lt->pos = offset;
return true;
}
/*
* Not-so-easy case. Figure out whether it's possible at all.
*/
if (blocknum < 0 || blocknum > lt->numFullBlocks ||
(blocknum == lt->numFullBlocks && offset > lt->lastBlockBytes))
return false;
/*
* OK, advance or back up to the target block. This implementation
* would be pretty inefficient for long seeks, but we really aren't
* expecting that (a seek over one tuple is typical).
*/
while (lt->curBlockNumber > blocknum)
{
long datablocknum = ltsRecallPrevBlockNum(lts, lt->indirect);
if (datablocknum == -1L)
elog(ERROR, "LogicalTapeSeek: unexpected end of tape");
if (--lt->curBlockNumber == blocknum)
ltsReadBlock(lts, datablocknum, (void *) lt->buffer);
}
while (lt->curBlockNumber < blocknum)
{
long datablocknum = ltsRecallNextBlockNum(lts, lt->indirect,
lt->frozen);
if (datablocknum == -1L)
elog(ERROR, "LogicalTapeSeek: unexpected end of tape");
if (++lt->curBlockNumber == blocknum)
ltsReadBlock(lts, datablocknum, (void *) lt->buffer);
}
lt->nbytes = (lt->curBlockNumber < lt->numFullBlocks) ?
BLCKSZ : lt->lastBlockBytes;
lt->pos = offset;
return true;
}
/*
* Obtain current position in a form suitable for a later LogicalTapeSeek.
*
* NOTE: it'd be OK to do this during write phase with intention of using
* the position for a seek after freezing. Not clear if anyone needs that.
*/
void
LogicalTapeTell(LogicalTapeSet *lts, int tapenum,
long *blocknum, int *offset)
{
LogicalTape *lt;
Assert(tapenum >= 0 && tapenum < lts->nTapes);
lt = lts->tapes[tapenum];
*blocknum = lt->curBlockNumber;
*offset = lt->pos;
}
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