postgresql/src/include/utils/logtape.h

/*-------------------------------------------------------------------------
 *
 * logtape.h
 *	  Management of "logical tapes" within temporary files.
 *
 * See logtape.c for explanations.
 *
 * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/utils/logtape.h
 *
 *-------------------------------------------------------------------------
 */

#ifndef LOGTAPE_H
#define LOGTAPE_H

#include "storage/sharedfileset.h"

/* LogicalTapeSet is an opaque type whose details are not known outside logtape.c. */

typedef struct LogicalTapeSet LogicalTapeSet;

/*
 * The approach tuplesort.c takes to parallel external sorts is that workers,
 * whose state is almost the same as independent serial sorts, are made to
 * produce a final materialized tape of sorted output in all cases.  This is
 * frozen, just like any case requiring a final materialized tape.  However,
 * there is one difference, which is that freezing will also export an
 * underlying shared fileset BufFile for sharing.  Freezing produces TapeShare
 * metadata for the worker when this happens, which is passed along through
 * shared memory to leader.
 *
 * The leader process can then pass an array of TapeShare metadata (one per
 * worker participant) to LogicalTapeSetCreate(), alongside a handle to a
 * shared fileset, which is sufficient to construct a new logical tapeset that
 * consists of each of the tapes materialized by workers.
 *
 * Note that while logtape.c does create an empty leader tape at the end of the
 * tapeset in the leader case, it can never be written to due to a restriction
 * in the shared buffile infrastructure.
 */
typedef struct TapeShare
{
	/*
	 * Currently, all the leader process needs is the location of the
	 * materialized tape's first block.
	 */
	long		firstblocknumber;
} TapeShare;

/*
 * prototypes for functions in logtape.c
 */

extern LogicalTapeSet *LogicalTapeSetCreate(int ntapes, TapeShare *shared,
					 SharedFileSet *fileset, int worker);
extern void LogicalTapeSetClose(LogicalTapeSet *lts);
extern void LogicalTapeSetForgetFreeSpace(LogicalTapeSet *lts);
extern size_t LogicalTapeRead(LogicalTapeSet *lts, int tapenum,
				void *ptr, size_t size);
extern void LogicalTapeWrite(LogicalTapeSet *lts, int tapenum,
				 void *ptr, size_t size);
extern void LogicalTapeRewindForRead(LogicalTapeSet *lts, int tapenum,
						 size_t buffer_size);
extern void LogicalTapeRewindForWrite(LogicalTapeSet *lts, int tapenum);
extern void LogicalTapeFreeze(LogicalTapeSet *lts, int tapenum,
				  TapeShare *share);
extern size_t LogicalTapeBackspace(LogicalTapeSet *lts, int tapenum,
					 size_t size);
extern void LogicalTapeSeek(LogicalTapeSet *lts, int tapenum,
				long blocknum, int offset);
extern void LogicalTapeTell(LogicalTapeSet *lts, int tapenum,
				long *blocknum, int *offset);
extern long LogicalTapeSetBlocks(LogicalTapeSet *lts);

#endif							/* LOGTAPE_H */