src/backend/access/transam/generic_xlog.c

/*-------------------------------------------------------------------------
 *
 * generic_xlog.c
 *	 Implementation of generic xlog records.
 *
 *
 * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/generic_xlog.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/bufmask.h"
#include "access/generic_xlog.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "utils/memutils.h"

/*-------------------------------------------------------------------------
 * Internally, a delta between pages consists of a set of fragments.  Each
 * fragment represents changes made in a given region of a page.  A fragment
 * is made up as follows:
 *
 * - offset of page region (OffsetNumber)
 * - length of page region (OffsetNumber)
 * - data - the data to place into the region ('length' number of bytes)
 *
 * Unchanged regions of a page are not represented in its delta.  As a result,
 * a delta can be more compact than the full page image.  But having an
 * unchanged region between two fragments that is smaller than the fragment
 * header (offset+length) does not pay off in terms of the overall size of
 * the delta.  For this reason, we merge adjacent fragments if the unchanged
 * region between them is <= MATCH_THRESHOLD bytes.
 *
 * We do not bother to merge fragments across the "lower" and "upper" parts
 * of a page; it's very seldom the case that pd_lower and pd_upper are within
 * MATCH_THRESHOLD bytes of each other, and handling that infrequent case
 * would complicate and slow down the delta-computation code unduly.
 * Therefore, the worst-case delta size includes two fragment headers plus
 * a full page's worth of data.
 *-------------------------------------------------------------------------
 */
#define FRAGMENT_HEADER_SIZE	(2 * sizeof(OffsetNumber))
#define MATCH_THRESHOLD			FRAGMENT_HEADER_SIZE
#define MAX_DELTA_SIZE			(BLCKSZ + 2 * FRAGMENT_HEADER_SIZE)

/* Struct of generic xlog data for single page */
typedef struct
{
	Buffer		buffer;			/* registered buffer */
	int			flags;			/* flags for this buffer */
	int			deltaLen;		/* space consumed in delta field */
	char	   *image;			/* copy of page image for modification, do not
								 * do it in-place to have aligned memory chunk */
	char		delta[MAX_DELTA_SIZE];	/* delta between page images */
} PageData;

/*
 * State of generic xlog record construction.  Must be allocated at an I/O
 * aligned address.
 */
struct GenericXLogState
{
	/* Page images (properly aligned, must be first) */
	PGIOAlignedBlock images[MAX_GENERIC_XLOG_PAGES];
	/* Info about each page, see above */
	PageData	pages[MAX_GENERIC_XLOG_PAGES];
	bool		isLogged;
};

static void writeFragment(PageData *pageData, OffsetNumber offset,
						  OffsetNumber length, const char *data);
static void computeRegionDelta(PageData *pageData,
							   const char *curpage, const char *targetpage,
							   int targetStart, int targetEnd,
							   int validStart, int validEnd);
static void computeDelta(PageData *pageData, Page curpage, Page targetpage);
static void applyPageRedo(Page page, const char *delta, Size deltaSize);


/*
 * Write next fragment into pageData's delta.
 *
 * The fragment has the given offset and length, and data points to the
 * actual data (of length length).
 */
static void
writeFragment(PageData *pageData, OffsetNumber offset, OffsetNumber length,
			  const char *data)
{
	char	   *ptr = pageData->delta + pageData->deltaLen;

	/* Verify we have enough space */
	Assert(pageData->deltaLen + sizeof(offset) +
		   sizeof(length) + length <= sizeof(pageData->delta));

	/* Write fragment data */
	memcpy(ptr, &offset, sizeof(offset));
	ptr += sizeof(offset);
	memcpy(ptr, &length, sizeof(length));
	ptr += sizeof(length);
	memcpy(ptr, data, length);
	ptr += length;

	pageData->deltaLen = ptr - pageData->delta;
}

/*
 * Compute the XLOG fragments needed to transform a region of curpage into the
 * corresponding region of targetpage, and append them to pageData's delta
 * field.  The region to transform runs from targetStart to targetEnd-1.
 * Bytes in curpage outside the range validStart to validEnd-1 should be
 * considered invalid, and always overwritten with target data.
 *
 * This function is a hot spot, so it's worth being as tense as possible
 * about the data-matching loops.
 */
static void
computeRegionDelta(PageData *pageData,
				   const char *curpage, const char *targetpage,
				   int targetStart, int targetEnd,
				   int validStart, int validEnd)
{
	int			i,
				loopEnd,
				fragmentBegin = -1,
				fragmentEnd = -1;

	/* Deal with any invalid start region by including it in first fragment */
	if (validStart > targetStart)
	{
		fragmentBegin = targetStart;
		targetStart = validStart;
	}

	/* We'll deal with any invalid end region after the main loop */
	loopEnd = Min(targetEnd, validEnd);

	/* Examine all the potentially matchable bytes */
	i = targetStart;
	while (i < loopEnd)
	{
		if (curpage[i] != targetpage[i])
		{
			/* On unmatched byte, start new fragment if not already in one */
			if (fragmentBegin < 0)
				fragmentBegin = i;
			/* Mark unmatched-data endpoint as uncertain */
			fragmentEnd = -1;
			/* Extend the fragment as far as possible in a tight loop */
			i++;
			while (i < loopEnd && curpage[i] != targetpage[i])
				i++;
			if (i >= loopEnd)
				break;
		}

		/* Found a matched byte, so remember end of unmatched fragment */
		fragmentEnd = i;

		/*
		 * Extend the match as far as possible in a tight loop.  (On typical
		 * workloads, this inner loop is the bulk of this function's runtime.)
		 */
		i++;
		while (i < loopEnd && curpage[i] == targetpage[i])
			i++;

		/*
		 * There are several possible cases at this point:
		 *
		 * 1. We have no unwritten fragment (fragmentBegin < 0).  There's
		 * nothing to write; and it doesn't matter what fragmentEnd is.
		 *
		 * 2. We found more than MATCH_THRESHOLD consecutive matching bytes.
		 * Dump out the unwritten fragment, stopping at fragmentEnd.
		 *
		 * 3. The match extends to loopEnd.  We'll do nothing here, exit the
		 * loop, and then dump the unwritten fragment, after merging it with
		 * the invalid end region if any.  If we don't so merge, fragmentEnd
		 * establishes how much the final writeFragment call needs to write.
		 *
		 * 4. We found an unmatched byte before loopEnd.  The loop will repeat
		 * and will enter the unmatched-byte stanza above.  So in this case
		 * also, it doesn't matter what fragmentEnd is.  The matched bytes
		 * will get merged into the continuing unmatched fragment.
		 *
		 * Only in case 3 do we reach the bottom of the loop with a meaningful
		 * fragmentEnd value, which is why it's OK that we unconditionally
		 * assign "fragmentEnd = i" above.
		 */
		if (fragmentBegin >= 0 && i - fragmentEnd > MATCH_THRESHOLD)
		{
			writeFragment(pageData, fragmentBegin,
						  fragmentEnd - fragmentBegin,
						  targetpage + fragmentBegin);
			fragmentBegin = -1;
			fragmentEnd = -1;	/* not really necessary */
		}
	}

	/* Deal with any invalid end region by including it in final fragment */
	if (loopEnd < targetEnd)
	{
		if (fragmentBegin < 0)
			fragmentBegin = loopEnd;
		fragmentEnd = targetEnd;
	}

	/* Write final fragment if any */
	if (fragmentBegin >= 0)
	{
		if (fragmentEnd < 0)
			fragmentEnd = targetEnd;
		writeFragment(pageData, fragmentBegin,
					  fragmentEnd - fragmentBegin,
					  targetpage + fragmentBegin);
	}
}

/*
 * Compute the XLOG delta record needed to transform curpage into targetpage,
 * and store it in pageData's delta field.
 */
static void
computeDelta(PageData *pageData, Page curpage, Page targetpage)
{
	int			targetLower = ((PageHeader) targetpage)->pd_lower,
				targetUpper = ((PageHeader) targetpage)->pd_upper,
				curLower = ((PageHeader) curpage)->pd_lower,
				curUpper = ((PageHeader) curpage)->pd_upper;

	pageData->deltaLen = 0;

	/* Compute delta records for lower part of page ... */
	computeRegionDelta(pageData, curpage, targetpage,
					   0, targetLower,
					   0, curLower);
	/* ... and for upper part, ignoring what's between */
	computeRegionDelta(pageData, curpage, targetpage,
					   targetUpper, BLCKSZ,
					   curUpper, BLCKSZ);

	/*
	 * If xlog debug is enabled, then check produced delta.  Result of delta
	 * application to curpage should be equivalent to targetpage.
	 */
#ifdef WAL_DEBUG
	if (XLOG_DEBUG)
	{
		PGAlignedBlock tmp;

		memcpy(tmp.data, curpage, BLCKSZ);
		applyPageRedo(tmp.data, pageData->delta, pageData->deltaLen);
		if (memcmp(tmp.data, targetpage, targetLower) != 0 ||
			memcmp(tmp.data + targetUpper, targetpage + targetUpper,
				   BLCKSZ - targetUpper) != 0)
			elog(ERROR, "result of generic xlog apply does not match");
	}
#endif
}

/*
 * Start new generic xlog record for modifications to specified relation.
 */
GenericXLogState *
GenericXLogStart(Relation relation)
{
	GenericXLogState *state;
	int			i;

	state = (GenericXLogState *) palloc_aligned(sizeof(GenericXLogState),
												PG_IO_ALIGN_SIZE,
												0);
	state->isLogged = RelationNeedsWAL(relation);

	for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
	{
		state->pages[i].image = state->images[i].data;
		state->pages[i].buffer = InvalidBuffer;
	}

	return state;
}

/*
 * Register new buffer for generic xlog record.
 *
 * Returns pointer to the page's image in the GenericXLogState, which
 * is what the caller should modify.
 *
 * If the buffer is already registered, just return its existing entry.
 * (It's not very clear what to do with the flags in such a case, but
 * for now we stay with the original flags.)
 */
Page
GenericXLogRegisterBuffer(GenericXLogState *state, Buffer buffer, int flags)
{
	int			block_id;

	/* Search array for existing entry or first unused slot */
	for (block_id = 0; block_id < MAX_GENERIC_XLOG_PAGES; block_id++)
	{
		PageData   *page = &state->pages[block_id];

		if (BufferIsInvalid(page->buffer))
		{
			/* Empty slot, so use it (there cannot be a match later) */
			page->buffer = buffer;
			page->flags = flags;
			memcpy(page->image, BufferGetPage(buffer), BLCKSZ);
			return (Page) page->image;
		}
		else if (page->buffer == buffer)
		{
			/*
			 * Buffer is already registered.  Just return the image, which is
			 * already prepared.
			 */
			return (Page) page->image;
		}
	}

	elog(ERROR, "maximum number %d of generic xlog buffers is exceeded",
		 MAX_GENERIC_XLOG_PAGES);
	/* keep compiler quiet */
	return NULL;
}

/*
 * Apply changes represented by GenericXLogState to the actual buffers,
 * and emit a generic xlog record.
 */
XLogRecPtr
GenericXLogFinish(GenericXLogState *state)
{
	XLogRecPtr	lsn;
	int			i;

	if (state->isLogged)
	{
		/* Logged relation: make xlog record in critical section. */
		XLogBeginInsert();

		START_CRIT_SECTION();

		/*
		 * Compute deltas if necessary, write changes to buffers, mark
		 * buffers dirty, and register changes.
		 */
		for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
		{
			PageData   *pageData = &state->pages[i];
			Page		page;
			PageHeader	pageHeader;

			if (BufferIsInvalid(pageData->buffer))
				continue;

			page = BufferGetPage(pageData->buffer);
			pageHeader = (PageHeader) pageData->image;

			/*
			 * Compute delta while we still have both the unmodified page and
			 * the new image. Not needed if we are logging the full image.
			 */
			if (!(pageData->flags & GENERIC_XLOG_FULL_IMAGE))
				computeDelta(pageData, page, (Page) pageData->image);

			/*
			 * Apply the image, being careful to zero the "hole" between
			 * pd_lower and pd_upper in order to avoid divergence between
			 * actual page state and what replay would produce.
			 */
			memcpy(page, pageData->image, pageHeader->pd_lower);
			memset(page + pageHeader->pd_lower, 0,
				   pageHeader->pd_upper - pageHeader->pd_lower);
			memcpy(page + pageHeader->pd_upper,
				   pageData->image + pageHeader->pd_upper,
				   BLCKSZ - pageHeader->pd_upper);

			MarkBufferDirty(pageData->buffer);

			if (pageData->flags & GENERIC_XLOG_FULL_IMAGE)
			{
				XLogRegisterBuffer(i, pageData->buffer,
								   REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
			}
			else
			{
				XLogRegisterBuffer(i, pageData->buffer, REGBUF_STANDARD);
				XLogRegisterBufData(i, pageData->delta, pageData->deltaLen);
			}
		}

		/* Insert xlog record */
		lsn = XLogInsert(RM_GENERIC_ID, 0);

		/* Set LSN */
		for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
		{
			PageData   *pageData = &state->pages[i];

			if (BufferIsInvalid(pageData->buffer))
				continue;
			PageSetLSN(BufferGetPage(pageData->buffer), lsn);
		}
		END_CRIT_SECTION();
	}
	else
	{
		/* Unlogged relation: skip xlog-related stuff */
		START_CRIT_SECTION();
		for (i = 0; i < MAX_GENERIC_XLOG_PAGES; i++)
		{
			PageData   *pageData = &state->pages[i];

			if (BufferIsInvalid(pageData->buffer))
				continue;
			memcpy(BufferGetPage(pageData->buffer),
				   pageData->image,
				   BLCKSZ);
			/* We don't worry about zeroing the "hole" in this case */
			MarkBufferDirty(pageData->buffer);
		}
		END_CRIT_SECTION();
		/* We don't have a LSN to return, in this case */
		lsn = InvalidXLogRecPtr;
	}

	pfree(state);

	return lsn;
}

/*
 * Abort generic xlog record construction.  No changes are applied to buffers.
 *
 * Note: caller is responsible for releasing locks/pins on buffers, if needed.
 */
void
GenericXLogAbort(GenericXLogState *state)
{
	pfree(state);
}

/*
 * Apply delta to given page image.
 */
static void
applyPageRedo(Page page, const char *delta, Size deltaSize)
{
	const char *ptr = delta;
	const char *end = delta + deltaSize;

	while (ptr < end)
	{
		OffsetNumber offset,
					length;

		memcpy(&offset, ptr, sizeof(offset));
		ptr += sizeof(offset);
		memcpy(&length, ptr, sizeof(length));
		ptr += sizeof(length);

		memcpy(page + offset, ptr, length);

		ptr += length;
	}
}

/*
 * Redo function for generic xlog record.
 */
void
generic_redo(XLogReaderState *record)
{
	XLogRecPtr	lsn = record->EndRecPtr;
	Buffer		buffers[MAX_GENERIC_XLOG_PAGES];
	uint8		block_id;

	/* Protect limited size of buffers[] array */
	Assert(XLogRecMaxBlockId(record) < MAX_GENERIC_XLOG_PAGES);

	/* Iterate over blocks */
	for (block_id = 0; block_id <= XLogRecMaxBlockId(record); block_id++)
	{
		XLogRedoAction action;

		if (!XLogRecHasBlockRef(record, block_id))
		{
			buffers[block_id] = InvalidBuffer;
			continue;
		}

		action = XLogReadBufferForRedo(record, block_id, &buffers[block_id]);

		/* Apply redo to given block if needed */
		if (action == BLK_NEEDS_REDO)
		{
			Page		page;
			PageHeader	pageHeader;
			char	   *blockDelta;
			Size		blockDeltaSize;

			page = BufferGetPage(buffers[block_id]);
			blockDelta = XLogRecGetBlockData(record, block_id, &blockDeltaSize);
			applyPageRedo(page, blockDelta, blockDeltaSize);

			/*
			 * Since the delta contains no information about what's in the
			 * "hole" between pd_lower and pd_upper, set that to zero to
			 * ensure we produce the same page state that application of the
			 * logged action by GenericXLogFinish did.
			 */
			pageHeader = (PageHeader) page;
			memset(page + pageHeader->pd_lower, 0,
				   pageHeader->pd_upper - pageHeader->pd_lower);

			PageSetLSN(page, lsn);
			MarkBufferDirty(buffers[block_id]);
		}
	}

	/* Changes are done: unlock and release all buffers */
	for (block_id = 0; block_id <= XLogRecMaxBlockId(record); block_id++)
	{
		if (BufferIsValid(buffers[block_id]))
			UnlockReleaseBuffer(buffers[block_id]);
	}
}

/*
 * Mask a generic page before performing consistency checks on it.
 */
void
generic_mask(char *page, BlockNumber blkno)
{
	mask_page_lsn_and_checksum(page);

	mask_unused_space(page);
}