Don't create "holes" in BufFiles, in the new logtape code.

The "Simplify tape block format" commit ignored the rule that blocks
returned by ltsGetFreeBlock() must be written out in the same order, at
least in the first write pass. To fix, relax that requirement, by making
ltsWriteBlock() to detect if it's about to create a "hole" in the
underlying BufFile, and fill it with zeros instead.

Reported, analysed, and reviewed by Peter Geoghegan.

Discussion: https://www.postgresql.org/message-id/CAM3SWZRWdNtkhiG0GyiX_1mUAypiK3dV6-6542pYe2iEL-foTA@mail.gmail.com

src/backend/utils/sort/logtape.c

@@ -152,7 +152,17 @@ typedef struct LogicalTape
 struct LogicalTapeSet
 {
 	BufFile    *pfile;			/* underlying file for whole tape set */
-	long		nFileBlocks;	/* # of blocks used in underlying file */
+
+	/*
+	 * File size tracking.  nBlocksWritten is the size of the underlying file,
+	 * in BLCKSZ blocks.  nBlocksAllocated is the number of blocks allocated
+	 * by ltsGetFreeBlock(), and it is always greater than or equal to
+	 * nBlocksWritten.  Blocks between nBlocksAllocated and nBlocksWritten are
+	 * blocks that have been allocated for a tape, but have not been written
+	 * to the underlying file yet.
+	 */
+	long		nBlocksAllocated;		/* # of blocks allocated */
+	long		nBlocksWritten; /* # of blocks used in underlying file */
 
 	/*
 	 * We store the numbers of recycled-and-available blocks in freeBlocks[].
@@ -187,21 +197,43 @@ static void ltsReleaseBlock(LogicalTapeSet *lts, long blocknum);
 /*
  * 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 ereport() on any error.
  */
 static void
 ltsWriteBlock(LogicalTapeSet *lts, long blocknum, void *buffer)
 {
+	/*
+	 * BufFile does not support "holes", so if we're about to write a block
+	 * that's past the current end of file, fill the space between the current
+	 * end of file and the target block with zeros.
+	 *
+	 * This should happen rarely, otherwise you are not writing very
+	 * sequentially.  In current use, this only happens when the sort ends
+	 * writing a run, and switches to another tape.  The last block of the
+	 * previous tape isn't flushed to disk until the end of the sort, so you
+	 * get one-block hole, where the last block of the previous tape will
+	 * later go.
+	 */
+	while (blocknum > lts->nBlocksWritten)
+	{
+		char		zerobuf[BLCKSZ];
+
+		MemSet(zerobuf, 0, sizeof(zerobuf));
+
+		ltsWriteBlock(lts, lts->nBlocksWritten, zerobuf);
+	}
+
+	/* Write the requested block */
 	if (BufFileSeekBlock(lts->pfile, blocknum) != 0 ||
 		BufFileWrite(lts->pfile, buffer, BLCKSZ) != BLCKSZ)
 		ereport(ERROR,
 				(errcode_for_file_access(),
 				 errmsg("could not write block %ld of temporary file: %m",
 						blocknum)));
+
+	/* Update nBlocksWritten, if we extended the file */
+	if (blocknum == lts->nBlocksWritten)
+		lts->nBlocksWritten++;
 }
 
 /*
@@ -281,9 +313,6 @@ freeBlocks_cmp(const void *a, const void *b)
 
 /*
  * 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)
@@ -304,7 +333,7 @@ ltsGetFreeBlock(LogicalTapeSet *lts)
 		return lts->freeBlocks[--lts->nFreeBlocks];
 	}
 	else
-		return lts->nFileBlocks++;
+		return lts->nBlocksAllocated++;
 }
 
 /*
@@ -360,7 +389,8 @@ LogicalTapeSetCreate(int ntapes)
 	lts = (LogicalTapeSet *) palloc(offsetof(LogicalTapeSet, tapes) +
 									ntapes * sizeof(LogicalTape));
 	lts->pfile = BufFileCreateTemp(false);
-	lts->nFileBlocks = 0L;
+	lts->nBlocksAllocated = 0L;
+	lts->nBlocksWritten = 0L;
 	lts->forgetFreeSpace = false;
 	lts->blocksSorted = true;	/* a zero-length array is sorted ... */
 	lts->freeBlocksLen = 32;	/* reasonable initial guess */
@@ -858,5 +888,5 @@ LogicalTapeTell(LogicalTapeSet *lts, int tapenum,
 long
 LogicalTapeSetBlocks(LogicalTapeSet *lts)
 {
-	return lts->nFileBlocks;
+	return lts->nBlocksAllocated;
 }