Adding upstream version 14.5.upstream/14.5upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 949 insertions, 0 deletions

diff --git a/src/backend/storage/file/buffile.c b/src/backend/storage/file/buffile.c
new file mode 100644
index 0000000..a4be5fe
--- /dev/null
+++ b/src/backend/storage/file/buffile.c
@@ -0,0 +1,949 @@
+/*-------------------------------------------------------------------------
+ *
+ * buffile.c
+ *	  Management of large buffered temporary files.
+ *
+ * Portions Copyright (c) 1996-2021, 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 query/transaction end.  Since the underlying
+ * virtual Files are made with OpenTemporaryFile, all resources for
+ * the file are certain to be cleaned up even if processing is aborted
+ * by ereport(ERROR).  The data structures required are made in the
+ * palloc context that was current when the BufFile was created, and
+ * any external resources such as temp files are owned by the ResourceOwner
+ * that was current at that time.
+ *
+ * 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.
+ *
+ * BufFile supports temporary files that can be shared with other backends, as
+ * infrastructure for parallel execution.  Such files need to be created as a
+ * member of a SharedFileSet that all participants are attached to.
+ *
+ * BufFile also supports temporary files that can be used by the single backend
+ * when the corresponding files need to be survived across the transaction and
+ * need to be opened and closed multiple times.  Such files need to be created
+ * as a member of a SharedFileSet.
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "commands/tablespace.h"
+#include "executor/instrument.h"
+#include "miscadmin.h"
+#include "pgstat.h"
+#include "storage/buf_internals.h"
+#include "storage/buffile.h"
+#include "storage/fd.h"
+#include "utils/resowner.h"
+
+/*
+ * We break BufFiles into gigabyte-sized segments, regardless of RELSEG_SIZE.
+ * The reason is that we'd like large 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 */
+
+	bool		isInterXact;	/* keep open over transactions? */
+	bool		dirty;			/* does buffer need to be written? */
+	bool		readOnly;		/* has the file been set to read only? */
+
+	SharedFileSet *fileset;		/* space for segment files if shared */
+	const char *name;			/* name of this BufFile if shared */
+
+	/*
+	 * resowner is the ResourceOwner to use for underlying temp files.  (We
+	 * don't need to remember the memory context we're using explicitly,
+	 * because after creation we only repalloc our arrays larger.)
+	 */
+	ResourceOwner resowner;
+
+	/*
+	 * "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 */
+	PGAlignedBlock buffer;
+};
+
+static BufFile *makeBufFileCommon(int nfiles);
+static BufFile *makeBufFile(File firstfile);
+static void extendBufFile(BufFile *file);
+static void BufFileLoadBuffer(BufFile *file);
+static void BufFileDumpBuffer(BufFile *file);
+static void BufFileFlush(BufFile *file);
+static File MakeNewSharedSegment(BufFile *file, int segment);
+
+/*
+ * Create BufFile and perform the common initialization.
+ */
+static BufFile *
+makeBufFileCommon(int nfiles)
+{
+	BufFile    *file = (BufFile *) palloc(sizeof(BufFile));
+
+	file->numFiles = nfiles;
+	file->isInterXact = false;
+	file->dirty = false;
+	file->resowner = CurrentResourceOwner;
+	file->curFile = 0;
+	file->curOffset = 0L;
+	file->pos = 0;
+	file->nbytes = 0;
+
+	return file;
+}
+
+/*
+ * Create a BufFile given the first underlying physical file.
+ * NOTE: caller must set isInterXact if appropriate.
+ */
+static BufFile *
+makeBufFile(File firstfile)
+{
+	BufFile    *file = makeBufFileCommon(1);
+
+	file->files = (File *) palloc(sizeof(File));
+	file->files[0] = firstfile;
+	file->readOnly = false;
+	file->fileset = NULL;
+	file->name = NULL;
+
+	return file;
+}
+
+/*
+ * Add another component temp file.
+ */
+static void
+extendBufFile(BufFile *file)
+{
+	File		pfile;
+	ResourceOwner oldowner;
+
+	/* Be sure to associate the file with the BufFile's resource owner */
+	oldowner = CurrentResourceOwner;
+	CurrentResourceOwner = file->resowner;
+
+	if (file->fileset == NULL)
+		pfile = OpenTemporaryFile(file->isInterXact);
+	else
+		pfile = MakeNewSharedSegment(file, file->numFiles);
+
+	Assert(pfile >= 0);
+
+	CurrentResourceOwner = oldowner;
+
+	file->files = (File *) repalloc(file->files,
+									(file->numFiles + 1) * sizeof(File));
+	file->files[file->numFiles] = pfile;
+	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, and with a resource owner, that will survive across
+ * transaction boundaries.
+ */
+BufFile *
+BufFileCreateTemp(bool interXact)
+{
+	BufFile    *file;
+	File		pfile;
+
+	/*
+	 * Ensure that temp tablespaces are set up for OpenTemporaryFile to use.
+	 * Possibly the caller will have done this already, but it seems useful to
+	 * double-check here.  Failure to do this at all would result in the temp
+	 * files always getting placed in the default tablespace, which is a
+	 * pretty hard-to-detect bug.  Callers may prefer to do it earlier if they
+	 * want to be sure that any required catalog access is done in some other
+	 * resource context.
+	 */
+	PrepareTempTablespaces();
+
+	pfile = OpenTemporaryFile(interXact);
+	Assert(pfile >= 0);
+
+	file = makeBufFile(pfile);
+	file->isInterXact = interXact;
+
+	return file;
+}
+
+/*
+ * Build the name for a given segment of a given BufFile.
+ */
+static void
+SharedSegmentName(char *name, const char *buffile_name, int segment)
+{
+	snprintf(name, MAXPGPATH, "%s.%d", buffile_name, segment);
+}
+
+/*
+ * Create a new segment file backing a shared BufFile.
+ */
+static File
+MakeNewSharedSegment(BufFile *buffile, int segment)
+{
+	char		name[MAXPGPATH];
+	File		file;
+
+	/*
+	 * It is possible that there are files left over from before a crash
+	 * restart with the same name.  In order for BufFileOpenShared() not to
+	 * get confused about how many segments there are, we'll unlink the next
+	 * segment number if it already exists.
+	 */
+	SharedSegmentName(name, buffile->name, segment + 1);
+	SharedFileSetDelete(buffile->fileset, name, true);
+
+	/* Create the new segment. */
+	SharedSegmentName(name, buffile->name, segment);
+	file = SharedFileSetCreate(buffile->fileset, name);
+
+	/* SharedFileSetCreate would've errored out */
+	Assert(file > 0);
+
+	return file;
+}
+
+/*
+ * Create a BufFile that can be discovered and opened read-only by other
+ * backends that are attached to the same SharedFileSet using the same name.
+ *
+ * The naming scheme for shared BufFiles is left up to the calling code.  The
+ * name will appear as part of one or more filenames on disk, and might
+ * provide clues to administrators about which subsystem is generating
+ * temporary file data.  Since each SharedFileSet object is backed by one or
+ * more uniquely named temporary directory, names don't conflict with
+ * unrelated SharedFileSet objects.
+ */
+BufFile *
+BufFileCreateShared(SharedFileSet *fileset, const char *name)
+{
+	BufFile    *file;
+
+	file = makeBufFileCommon(1);
+	file->fileset = fileset;
+	file->name = pstrdup(name);
+	file->files = (File *) palloc(sizeof(File));
+	file->files[0] = MakeNewSharedSegment(file, 0);
+	file->readOnly = false;
+
+	return file;
+}
+
+/*
+ * Open a file that was previously created in another backend (or this one)
+ * with BufFileCreateShared in the same SharedFileSet using the same name.
+ * The backend that created the file must have called BufFileClose() or
+ * BufFileExportShared() to make sure that it is ready to be opened by other
+ * backends and render it read-only.
+ */
+BufFile *
+BufFileOpenShared(SharedFileSet *fileset, const char *name, int mode)
+{
+	BufFile    *file;
+	char		segment_name[MAXPGPATH];
+	Size		capacity = 16;
+	File	   *files;
+	int			nfiles = 0;
+
+	files = palloc(sizeof(File) * capacity);
+
+	/*
+	 * We don't know how many segments there are, so we'll probe the
+	 * filesystem to find out.
+	 */
+	for (;;)
+	{
+		/* See if we need to expand our file segment array. */
+		if (nfiles + 1 > capacity)
+		{
+			capacity *= 2;
+			files = repalloc(files, sizeof(File) * capacity);
+		}
+		/* Try to load a segment. */
+		SharedSegmentName(segment_name, name, nfiles);
+		files[nfiles] = SharedFileSetOpen(fileset, segment_name, mode);
+		if (files[nfiles] <= 0)
+			break;
+		++nfiles;
+
+		CHECK_FOR_INTERRUPTS();
+	}
+
+	/*
+	 * If we didn't find any files at all, then no BufFile exists with this
+	 * name.
+	 */
+	if (nfiles == 0)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not open temporary file \"%s\" from BufFile \"%s\": %m",
+						segment_name, name)));
+
+	file = makeBufFileCommon(nfiles);
+	file->files = files;
+	file->readOnly = (mode == O_RDONLY) ? true : false;
+	file->fileset = fileset;
+	file->name = pstrdup(name);
+
+	return file;
+}
+
+/*
+ * Delete a BufFile that was created by BufFileCreateShared in the given
+ * SharedFileSet using the given name.
+ *
+ * It is not necessary to delete files explicitly with this function.  It is
+ * provided only as a way to delete files proactively, rather than waiting for
+ * the SharedFileSet to be cleaned up.
+ *
+ * Only one backend should attempt to delete a given name, and should know
+ * that it exists and has been exported or closed.
+ */
+void
+BufFileDeleteShared(SharedFileSet *fileset, const char *name)
+{
+	char		segment_name[MAXPGPATH];
+	int			segment = 0;
+	bool		found = false;
+
+	/*
+	 * We don't know how many segments the file has.  We'll keep deleting
+	 * until we run out.  If we don't manage to find even an initial segment,
+	 * raise an error.
+	 */
+	for (;;)
+	{
+		SharedSegmentName(segment_name, name, segment);
+		if (!SharedFileSetDelete(fileset, segment_name, true))
+			break;
+		found = true;
+		++segment;
+
+		CHECK_FOR_INTERRUPTS();
+	}
+
+	if (!found)
+		elog(ERROR, "could not delete unknown shared BufFile \"%s\"", name);
+}
+
+/*
+ * BufFileExportShared --- flush and make read-only, in preparation for sharing.
+ */
+void
+BufFileExportShared(BufFile *file)
+{
+	/* Must be a file belonging to a SharedFileSet. */
+	Assert(file->fileset != NULL);
+
+	/* It's probably a bug if someone calls this twice. */
+	Assert(!file->readOnly);
+
+	BufFileFlush(file);
+	file->readOnly = true;
+}
+
+/*
+ * 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 and delete the underlying file(s) */
+	for (i = 0; i < file->numFiles; i++)
+		FileClose(file->files[i]);
+	/* release the buffer space */
+	pfree(file->files);
+	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.
+	 */
+	if (file->curOffset >= MAX_PHYSICAL_FILESIZE &&
+		file->curFile + 1 < file->numFiles)
+	{
+		file->curFile++;
+		file->curOffset = 0L;
+	}
+
+	/*
+	 * Read whatever we can get, up to a full bufferload.
+	 */
+	thisfile = file->files[file->curFile];
+	file->nbytes = FileRead(thisfile,
+							file->buffer.data,
+							sizeof(file->buffer),
+							file->curOffset,
+							WAIT_EVENT_BUFFILE_READ);
+	if (file->nbytes < 0)
+	{
+		file->nbytes = 0;
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not read file \"%s\": %m",
+						FilePathName(thisfile))));
+	}
+
+	/* we choose not to advance curOffset here */
+
+	if (file->nbytes > 0)
+		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)
+	{
+		off_t		availbytes;
+
+		/*
+		 * Advance to next component file if necessary and possible.
+		 */
+		if (file->curOffset >= MAX_PHYSICAL_FILESIZE)
+		{
+			while (file->curFile + 1 >= file->numFiles)
+				extendBufFile(file);
+			file->curFile++;
+			file->curOffset = 0L;
+		}
+
+		/*
+		 * Determine how much we need to write into this file.
+		 */
+		bytestowrite = file->nbytes - wpos;
+		availbytes = MAX_PHYSICAL_FILESIZE - file->curOffset;
+
+		if ((off_t) bytestowrite > availbytes)
+			bytestowrite = (int) availbytes;
+
+		thisfile = file->files[file->curFile];
+		bytestowrite = FileWrite(thisfile,
+								 file->buffer.data + wpos,
+								 bytestowrite,
+								 file->curOffset,
+								 WAIT_EVENT_BUFFILE_WRITE);
+		if (bytestowrite <= 0)
+			ereport(ERROR,
+					(errcode_for_file_access(),
+					 errmsg("could not write to file \"%s\": %m",
+							FilePathName(thisfile))));
+		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 and report I/O errors via
+ * ereport().
+ */
+size_t
+BufFileRead(BufFile *file, void *ptr, size_t size)
+{
+	size_t		nread = 0;
+	size_t		nthistime;
+
+	BufFileFlush(file);
+
+	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.data + 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 and report errors via
+ * ereport().
+ */
+void
+BufFileWrite(BufFile *file, void *ptr, size_t size)
+{
+	size_t		nthistime;
+
+	Assert(!file->readOnly);
+
+	while (size > 0)
+	{
+		if (file->pos >= BLCKSZ)
+		{
+			/* Buffer full, dump it out */
+			if (file->dirty)
+				BufFileDumpBuffer(file);
+			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.data + 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;
+	}
+}
+
+/*
+ * BufFileFlush
+ *
+ * Like fflush(), except that I/O errors are reported with ereport().
+ */
+static void
+BufFileFlush(BufFile *file)
+{
+	if (file->dirty)
+		BufFileDumpBuffer(file);
+
+	Assert(!file->dirty);
+}
+
+/*
+ * BufFileSeek
+ *
+ * Like fseek(), except that target position needs two values in order to
+ * work when logical filesize exceeds maximum value representable by off_t.
+ * We do not support relative seeks across more than that, however.
+ * I/O errors are reported by ereport().
+ *
+ * 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 GB) risk overflow in this
+			 * add, unless we have 64-bit off_t.
+			 */
+			newFile = file->curFile;
+			newOffset = (file->curOffset + file->pos) + offset;
+			break;
+		case SEEK_END:
+
+			/*
+			 * The file size of the last file gives us the end offset of that
+			 * file.
+			 */
+			newFile = file->numFiles - 1;
+			newOffset = FileSize(file->files[file->numFiles - 1]);
+			if (newOffset < 0)
+				ereport(ERROR,
+						(errcode_for_file_access(),
+						 errmsg("could not determine size of temporary file \"%s\" from BufFile \"%s\": %m",
+								FilePathName(file->files[file->numFiles - 1]),
+								file->name)));
+			break;
+		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 */
+	BufFileFlush(file);
+
+	/*
+	 * 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).
+	 */
+
+	/* 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
+
+/*
+ * Return the current shared BufFile size.
+ *
+ * Counts any holes left behind by BufFileAppend as part of the size.
+ * ereport()s on failure.
+ */
+int64
+BufFileSize(BufFile *file)
+{
+	int64		lastFileSize;
+
+	Assert(file->fileset != NULL);
+
+	/* Get the size of the last physical file. */
+	lastFileSize = FileSize(file->files[file->numFiles - 1]);
+	if (lastFileSize < 0)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not determine size of temporary file \"%s\" from BufFile \"%s\": %m",
+						FilePathName(file->files[file->numFiles - 1]),
+						file->name)));
+
+	return ((file->numFiles - 1) * (int64) MAX_PHYSICAL_FILESIZE) +
+		lastFileSize;
+}
+
+/*
+ * Append the contents of source file (managed within shared fileset) to
+ * end of target file (managed within same shared fileset).
+ *
+ * Note that operation subsumes ownership of underlying resources from
+ * "source".  Caller should never call BufFileClose against source having
+ * called here first.  Resource owners for source and target must match,
+ * too.
+ *
+ * This operation works by manipulating lists of segment files, so the
+ * file content is always appended at a MAX_PHYSICAL_FILESIZE-aligned
+ * boundary, typically creating empty holes before the boundary.  These
+ * areas do not contain any interesting data, and cannot be read from by
+ * caller.
+ *
+ * Returns the block number within target where the contents of source
+ * begins.  Caller should apply this as an offset when working off block
+ * positions that are in terms of the original BufFile space.
+ */
+long
+BufFileAppend(BufFile *target, BufFile *source)
+{
+	long		startBlock = target->numFiles * BUFFILE_SEG_SIZE;
+	int			newNumFiles = target->numFiles + source->numFiles;
+	int			i;
+
+	Assert(target->fileset != NULL);
+	Assert(source->readOnly);
+	Assert(!source->dirty);
+	Assert(source->fileset != NULL);
+
+	if (target->resowner != source->resowner)
+		elog(ERROR, "could not append BufFile with non-matching resource owner");
+
+	target->files = (File *)
+		repalloc(target->files, sizeof(File) * newNumFiles);
+	for (i = target->numFiles; i < newNumFiles; i++)
+		target->files[i] = source->files[i - target->numFiles];
+	target->numFiles = newNumFiles;
+
+	return startBlock;
+}
+
+/*
+ * Truncate a BufFile created by BufFileCreateShared up to the given fileno and
+ * the offset.
+ */
+void
+BufFileTruncateShared(BufFile *file, int fileno, off_t offset)
+{
+	int			numFiles = file->numFiles;
+	int			newFile = fileno;
+	off_t		newOffset = file->curOffset;
+	char		segment_name[MAXPGPATH];
+	int			i;
+
+	/*
+	 * Loop over all the files up to the given fileno and remove the files
+	 * that are greater than the fileno and truncate the given file up to the
+	 * offset. Note that we also remove the given fileno if the offset is 0
+	 * provided it is not the first file in which we truncate it.
+	 */
+	for (i = file->numFiles - 1; i >= fileno; i--)
+	{
+		if ((i != fileno || offset == 0) && i != 0)
+		{
+			SharedSegmentName(segment_name, file->name, i);
+			FileClose(file->files[i]);
+			if (!SharedFileSetDelete(file->fileset, segment_name, true))
+				ereport(ERROR,
+						(errcode_for_file_access(),
+						 errmsg("could not delete shared fileset \"%s\": %m",
+								segment_name)));
+			numFiles--;
+			newOffset = MAX_PHYSICAL_FILESIZE;
+
+			/*
+			 * This is required to indicate that we have deleted the given
+			 * fileno.
+			 */
+			if (i == fileno)
+				newFile--;
+		}
+		else
+		{
+			if (FileTruncate(file->files[i], offset,
+							 WAIT_EVENT_BUFFILE_TRUNCATE) < 0)
+				ereport(ERROR,
+						(errcode_for_file_access(),
+						 errmsg("could not truncate file \"%s\": %m",
+								FilePathName(file->files[i]))));
+			newOffset = offset;
+		}
+	}
+
+	file->numFiles = numFiles;
+
+	/*
+	 * If the truncate point is within existing buffer then we can just adjust
+	 * pos within buffer.
+	 */
+	if (newFile == file->curFile &&
+		newOffset >= file->curOffset &&
+		newOffset <= file->curOffset + file->nbytes)
+	{
+		/* No need to reset the current pos if the new pos is greater. */
+		if (newOffset <= file->curOffset + file->pos)
+			file->pos = (int) (newOffset - file->curOffset);
+
+		/* Adjust the nbytes for the current buffer. */
+		file->nbytes = (int) (newOffset - file->curOffset);
+	}
+	else if (newFile == file->curFile &&
+			 newOffset < file->curOffset)
+	{
+		/*
+		 * The truncate point is within the existing file but prior to the
+		 * current position, so we can forget the current buffer and reset the
+		 * current position.
+		 */
+		file->curOffset = newOffset;
+		file->pos = 0;
+		file->nbytes = 0;
+	}
+	else if (newFile < file->curFile)
+	{
+		/*
+		 * The truncate point is prior to the current file, so need to reset
+		 * the current position accordingly.
+		 */
+		file->curFile = newFile;
+		file->curOffset = newOffset;
+		file->pos = 0;
+		file->nbytes = 0;
+	}
+	/* Nothing to do, if the truncate point is beyond current file. */
+}