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db_include/utils/logtape.h

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+/*-------------------------------------------------------------------------
+ *
+ * logtape.h
+ *	  Management of "logical tapes" within temporary files.
+ *
+ * See logtape.c for explanations.
+ *
+ * Portions Copyright (c) 1996-2021, 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, bool preallocate,
+											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 void LogicalTapeSetExtend(LogicalTapeSet *lts, int nAdditional);
+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 */