xloginsert.c

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/*-------------------------------------------------------------------------
 *
 * xloginsert.c
 *      Functions for constructing WAL records
 *
 * Constructing a WAL record begins with a call to XLogBeginInsert,
 * followed by a number of XLogRegister* calls. The registered data is
 * collected in private working memory, and finally assembled into a chain
 * of XLogRecData structs by a call to XLogRecordAssemble(). See
 * access/transam/README for details.
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/xloginsert.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#ifdef USE_LZ4
#include <lz4.h>
#endif
 
#ifdef USE_ZSTD
#include <zstd.h>
#endif
 
#include "access/xact.h"
#include "access/xlog.h"
#include "access/xlog_internal.h"
#include "access/xloginsert.h"
#include "catalog/pg_control.h"
#include "common/pg_lzcompress.h"
#include "executor/instrument.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "replication/origin.h"
#include "storage/bufmgr.h"
#include "storage/proc.h"
#include "utils/memutils.h"
#include "utils/pgstat_internal.h"
#include "utils/rel.h"
 
/*
 * Guess the maximum buffer size required to store a compressed version of
 * backup block image.
 */
#ifdef USE_LZ4
#define LZ4_MAX_BLCKSZ      LZ4_COMPRESSBOUND(BLCKSZ)
#else
#define LZ4_MAX_BLCKSZ      0
#endif
 
#ifdef USE_ZSTD
#define ZSTD_MAX_BLCKSZ     ZSTD_COMPRESSBOUND(BLCKSZ)
#else
#define ZSTD_MAX_BLCKSZ     0
#endif
 
#define PGLZ_MAX_BLCKSZ     PGLZ_MAX_OUTPUT(BLCKSZ)
 
/* Buffer size required to store a compressed version of backup block image */
#define COMPRESS_BUFSIZE    Max(Max(PGLZ_MAX_BLCKSZ, LZ4_MAX_BLCKSZ), ZSTD_MAX_BLCKSZ)
 
/*
 * For each block reference registered with XLogRegisterBuffer, we fill in
 * a registered_buffer struct.
 */
typedef struct
{
    bool        in_use;         /* is this slot in use? */
    uint8       flags;          /* REGBUF_* flags */
    RelFileLocator rlocator;    /* identifies the relation and block */
    ForkNumber  forkno;
    BlockNumber block;
    const PageData *page;       /* page content */
    uint32      rdata_len;      /* total length of data in rdata chain */
    XLogRecData *rdata_head;    /* head of the chain of data registered with
                                 * this block */
    XLogRecData *rdata_tail;    /* last entry in the chain, or &rdata_head if
                                 * empty */
 
    XLogRecData bkp_rdatas[2];  /* temporary rdatas used to hold references to
                                 * backup block data in XLogRecordAssemble() */
 
    /* buffer to store a compressed version of backup block image */
    char        compressed_page[COMPRESS_BUFSIZE];
} registered_buffer;
 
static registered_buffer *registered_buffers;
static int  max_registered_buffers; /* allocated size */
static int  max_registered_block_id = 0;    /* highest block_id + 1 currently
                                             * registered */
 
/*
 * A chain of XLogRecDatas to hold the "main data" of a WAL record, registered
 * with XLogRegisterData(...).
 */
static XLogRecData *mainrdata_head;
static XLogRecData *mainrdata_last = (XLogRecData *) &mainrdata_head;
static uint64 mainrdata_len;    /* total # of bytes in chain */
 
/* flags for the in-progress insertion */
static uint8 curinsert_flags = 0;
 
/*
 * These are used to hold the record header while constructing a record.
 * 'hdr_scratch' is not a plain variable, but is palloc'd at initialization,
 * because we want it to be MAXALIGNed and padding bytes zeroed.
 *
 * For simplicity, it's allocated large enough to hold the headers for any
 * WAL record.
 */
static XLogRecData hdr_rdt;
static char *hdr_scratch = NULL;
 
#define SizeOfXlogOrigin    (sizeof(ReplOriginId) + sizeof(char))
#define SizeOfXLogTransactionId (sizeof(TransactionId) + sizeof(char))
 
#define HEADER_SCRATCH_SIZE \
    (SizeOfXLogRecord + \
     MaxSizeOfXLogRecordBlockHeader * (XLR_MAX_BLOCK_ID + 1) + \
     SizeOfXLogRecordDataHeaderLong + SizeOfXlogOrigin + \
     SizeOfXLogTransactionId)
 
/*
 * An array of XLogRecData structs, to hold registered data.
 */
static XLogRecData *rdatas;
static int  num_rdatas;         /* entries currently used */
static int  max_rdatas;         /* allocated size */
 
static bool begininsert_called = false;
 
/* Memory context to hold the registered buffer and data references. */
static MemoryContext xloginsert_cxt;
 
static XLogRecData *XLogRecordAssemble(RmgrId rmid, uint8 info,
                                       XLogRecPtr RedoRecPtr, bool doPageWrites,
                                       XLogRecPtr *fpw_lsn, int *num_fpi,
                                       uint64 *fpi_bytes,
                                       bool *topxid_included);
static bool XLogCompressBackupBlock(const PageData *page, uint16 hole_offset,
                                    uint16 hole_length, void *dest, uint16 *dlen);
 
/*
 * Begin constructing a WAL record. This must be called before the
 * XLogRegister* functions and XLogInsert().
 */
void
XLogBeginInsert(void)
{
    Assert(max_registered_block_id == 0);
    Assert(mainrdata_last == (XLogRecData *) &mainrdata_head);
    Assert(mainrdata_len == 0);
 
    /* cross-check on whether we should be here or not */
    if (!XLogInsertAllowed())
        elog(ERROR, "cannot make new WAL entries during recovery");
 
    if (begininsert_called)
        elog(ERROR, "XLogBeginInsert was already called");
 
    begininsert_called = true;
}
 
/*
 * Ensure that there are enough buffer and data slots in the working area,
 * for subsequent XLogRegisterBuffer, XLogRegisterData and XLogRegisterBufData
 * calls.
 *
 * There is always space for a small number of buffers and data chunks, enough
 * for most record types. This function is for the exceptional cases that need
 * more.
 */
void
XLogEnsureRecordSpace(int max_block_id, int ndatas)
{
    int         nbuffers;
 
    /*
     * This must be called before entering a critical section, because
     * allocating memory inside a critical section can fail. repalloc() will
     * check the same, but better to check it here too so that we fail
     * consistently even if the arrays happen to be large enough already.
     */
    Assert(CritSectionCount == 0);
 
    /* the minimum values can't be decreased */
    if (max_block_id < XLR_NORMAL_MAX_BLOCK_ID)
        max_block_id = XLR_NORMAL_MAX_BLOCK_ID;
    if (ndatas < XLR_NORMAL_RDATAS)
        ndatas = XLR_NORMAL_RDATAS;
 
    if (max_block_id > XLR_MAX_BLOCK_ID)
        elog(ERROR, "maximum number of WAL record block references exceeded");
    nbuffers = max_block_id + 1;
 
    if (nbuffers > max_registered_buffers)
    {
        registered_buffers = (registered_buffer *)
            repalloc(registered_buffers, sizeof(registered_buffer) * nbuffers);
 
        /*
         * At least the padding bytes in the structs must be zeroed, because
         * they are included in WAL data, but initialize it all for tidiness.
         */
        MemSet(&registered_buffers[max_registered_buffers], 0,
               (nbuffers - max_registered_buffers) * sizeof(registered_buffer));
        max_registered_buffers = nbuffers;
    }
 
    if (ndatas > max_rdatas)
    {
        rdatas = (XLogRecData *) repalloc(rdatas, sizeof(XLogRecData) * ndatas);
        max_rdatas = ndatas;
    }
}
 
/*
 * Reset WAL record construction buffers.
 */
void
XLogResetInsertion(void)
{
    int         i;
 
    for (i = 0; i < max_registered_block_id; i++)
        registered_buffers[i].in_use = false;
 
    num_rdatas = 0;
    max_registered_block_id = 0;
    mainrdata_len = 0;
    mainrdata_last = (XLogRecData *) &mainrdata_head;
    curinsert_flags = 0;
    begininsert_called = false;
}
 
/*
 * Register a reference to a buffer with the WAL record being constructed.
 * This must be called for every page that the WAL-logged operation modifies.
 */
void
XLogRegisterBuffer(uint8 block_id, Buffer buffer, uint8 flags)
{
    registered_buffer *regbuf;
 
    /* NO_IMAGE doesn't make sense with FORCE_IMAGE */
    Assert(!((flags & REGBUF_FORCE_IMAGE) && (flags & (REGBUF_NO_IMAGE))));
    Assert(begininsert_called);
 
    /*
     * Ordinarily, the buffer should be exclusive-locked (or share-exclusive
     * in case of hint bits) and marked dirty before we get here, otherwise we
     * could end up violating one of the rules in access/transam/README.
     *
     * Some callers intentionally register a clean page and never update that
     * page's LSN; in that case they can pass the flag REGBUF_NO_CHANGE to
     * bypass these checks.
     */
#ifdef USE_ASSERT_CHECKING
    if (!(flags & REGBUF_NO_CHANGE))
    {
        Assert(BufferIsDirty(buffer));
        Assert(BufferIsLockedByMeInMode(buffer, BUFFER_LOCK_EXCLUSIVE) ||
               BufferIsLockedByMeInMode(buffer, BUFFER_LOCK_SHARE_EXCLUSIVE));
    }
#endif
 
    if (block_id >= max_registered_block_id)
    {
        if (block_id >= max_registered_buffers)
            elog(ERROR, "too many registered buffers");
        max_registered_block_id = block_id + 1;
    }
 
    regbuf = &registered_buffers[block_id];
 
    BufferGetTag(buffer, &regbuf->rlocator, &regbuf->forkno, &regbuf->block);
    regbuf->page = BufferGetPage(buffer);
    regbuf->flags = flags;
    regbuf->rdata_tail = (XLogRecData *) &regbuf->rdata_head;
    regbuf->rdata_len = 0;
 
    /*
     * Check that this page hasn't already been registered with some other
     * block_id.
     */
#ifdef USE_ASSERT_CHECKING
    {
        int         i;
 
        for (i = 0; i < max_registered_block_id; i++)
        {
            registered_buffer *regbuf_old = &registered_buffers[i];
 
            if (i == block_id || !regbuf_old->in_use)
                continue;
 
            Assert(!RelFileLocatorEquals(regbuf_old->rlocator, regbuf->rlocator) ||
                   regbuf_old->forkno != regbuf->forkno ||
                   regbuf_old->block != regbuf->block);
        }
    }
#endif
 
    regbuf->in_use = true;
}
 
/*
 * Like XLogRegisterBuffer, but for registering a block that's not in the
 * shared buffer pool (i.e. when you don't have a Buffer for it).
 */
void
XLogRegisterBlock(uint8 block_id, RelFileLocator *rlocator, ForkNumber forknum,
                  BlockNumber blknum, const PageData *page, uint8 flags)
{
    registered_buffer *regbuf;
 
    Assert(begininsert_called);
 
    if (block_id >= max_registered_block_id)
        max_registered_block_id = block_id + 1;
 
    if (block_id >= max_registered_buffers)
        elog(ERROR, "too many registered buffers");
 
    regbuf = &registered_buffers[block_id];
 
    regbuf->rlocator = *rlocator;
    regbuf->forkno = forknum;
    regbuf->block = blknum;
    regbuf->page = page;
    regbuf->flags = flags;
    regbuf->rdata_tail = (XLogRecData *) &regbuf->rdata_head;
    regbuf->rdata_len = 0;
 
    /*
     * Check that this page hasn't already been registered with some other
     * block_id.
     */
#ifdef USE_ASSERT_CHECKING
    {
        int         i;
 
        for (i = 0; i < max_registered_block_id; i++)
        {
            registered_buffer *regbuf_old = &registered_buffers[i];
 
            if (i == block_id || !regbuf_old->in_use)
                continue;
 
            Assert(!RelFileLocatorEquals(regbuf_old->rlocator, regbuf->rlocator) ||
                   regbuf_old->forkno != regbuf->forkno ||
                   regbuf_old->block != regbuf->block);
        }
    }
#endif
 
    regbuf->in_use = true;
}
 
/*
 * Add data to the WAL record that's being constructed.
 *
 * The data is appended to the "main chunk", available at replay with
 * XLogRecGetData().
 */
void
XLogRegisterData(const void *data, uint32 len)
{
    XLogRecData *rdata;
 
    Assert(begininsert_called);
 
    if (num_rdatas >= max_rdatas)
        ereport(ERROR,
                (errmsg_internal("too much WAL data"),
                 errdetail_internal("%d out of %d data segments are already in use.",
                                    num_rdatas, max_rdatas)));
    rdata = &rdatas[num_rdatas++];
 
    rdata->data = data;
    rdata->len = len;
 
    /*
     * we use the mainrdata_last pointer to track the end of the chain, so no
     * need to clear 'next' here.
     */
 
    mainrdata_last->next = rdata;
    mainrdata_last = rdata;
 
    mainrdata_len += len;
}
 
/*
 * Add buffer-specific data to the WAL record that's being constructed.
 *
 * Block_id must reference a block previously registered with
 * XLogRegisterBuffer(). If this is called more than once for the same
 * block_id, the data is appended.
 *
 * The maximum amount of data that can be registered per block is 65535
 * bytes. That should be plenty; if you need more than BLCKSZ bytes to
 * reconstruct the changes to the page, you might as well just log a full
 * copy of it. (the "main data" that's not associated with a block is not
 * limited)
 */
void
XLogRegisterBufData(uint8 block_id, const void *data, uint32 len)
{
    registered_buffer *regbuf;
    XLogRecData *rdata;
 
    Assert(begininsert_called);
 
    /* find the registered buffer struct */
    regbuf = &registered_buffers[block_id];
    if (!regbuf->in_use)
        elog(ERROR, "no block with id %d registered with WAL insertion",
             block_id);
 
    /*
     * Check against max_rdatas and ensure we do not register more data per
     * buffer than can be handled by the physical data format; i.e. that
     * regbuf->rdata_len does not grow beyond what
     * XLogRecordBlockHeader->data_length can hold.
     */
    if (num_rdatas >= max_rdatas)
        ereport(ERROR,
                (errmsg_internal("too much WAL data"),
                 errdetail_internal("%d out of %d data segments are already in use.",
                                    num_rdatas, max_rdatas)));
    if (regbuf->rdata_len + len > UINT16_MAX || len > UINT16_MAX)
        ereport(ERROR,
                (errmsg_internal("too much WAL data"),
                 errdetail_internal("Registering more than maximum %u bytes allowed to block %u: current %u bytes, adding %u bytes.",
                                    UINT16_MAX, block_id, regbuf->rdata_len, len)));
 
    rdata = &rdatas[num_rdatas++];
 
    rdata->data = data;
    rdata->len = len;
 
    regbuf->rdata_tail->next = rdata;
    regbuf->rdata_tail = rdata;
    regbuf->rdata_len += len;
}
 
/*
 * Set insert status flags for the upcoming WAL record.
 *
 * The flags that can be used here are:
 * - XLOG_INCLUDE_ORIGIN, to determine if the replication origin should be
 *   included in the record.
 * - XLOG_MARK_UNIMPORTANT, to signal that the record is not important for
 *   durability, which allows to avoid triggering WAL archiving and other
 *   background activity.
 */
void
XLogSetRecordFlags(uint8 flags)
{
    Assert(begininsert_called);
    curinsert_flags |= flags;
}
 
/*
 * Insert an XLOG record having the specified RMID and info bytes, with the
 * body of the record being the data and buffer references registered earlier
 * with XLogRegister* calls.
 *
 * Returns XLOG pointer to end of record (beginning of next record).
 * This can be used as LSN for data pages affected by the logged action.
 * (LSN is the XLOG point up to which the XLOG must be flushed to disk
 * before the data page can be written out.  This implements the basic
 * WAL rule "write the log before the data".)
 */
XLogRecPtr
XLogInsert(RmgrId rmid, uint8 info)
{
    XLogRecPtr  EndPos;
 
    /* XLogBeginInsert() must have been called. */
    if (!begininsert_called)
        elog(ERROR, "XLogBeginInsert was not called");
 
    /*
     * The caller can set rmgr bits, XLR_SPECIAL_REL_UPDATE and
     * XLR_CHECK_CONSISTENCY; the rest are reserved for use by me.
     */
    if ((info & ~(XLR_RMGR_INFO_MASK |
                  XLR_SPECIAL_REL_UPDATE |
                  XLR_CHECK_CONSISTENCY)) != 0)
        elog(PANIC, "invalid xlog info mask %02X", info);
 
    TRACE_POSTGRESQL_WAL_INSERT(rmid, info);
 
    /*
     * In bootstrap mode, we don't actually log anything but XLOG resources;
     * return a phony record pointer.
     */
    if (IsBootstrapProcessingMode() && rmid != RM_XLOG_ID)
    {
        XLogResetInsertion();
        EndPos = SizeOfXLogLongPHD; /* start of 1st chkpt record */
        return EndPos;
    }
 
    do
    {
        XLogRecPtr  RedoRecPtr;
        bool        doPageWrites;
        bool        topxid_included = false;
        XLogRecPtr  fpw_lsn;
        XLogRecData *rdt;
        int         num_fpi = 0;
        uint64      fpi_bytes = 0;
 
        /*
         * Get values needed to decide whether to do full-page writes. Since
         * we don't yet have an insertion lock, these could change under us,
         * but XLogInsertRecord will recheck them once it has a lock.
         */
        GetFullPageWriteInfo(&RedoRecPtr, &doPageWrites);
 
        rdt = XLogRecordAssemble(rmid, info, RedoRecPtr, doPageWrites,
                                 &fpw_lsn, &num_fpi, &fpi_bytes,
                                 &topxid_included);
 
        EndPos = XLogInsertRecord(rdt, fpw_lsn, curinsert_flags, num_fpi,
                                  fpi_bytes, topxid_included);
    } while (!XLogRecPtrIsValid(EndPos));
 
    XLogResetInsertion();
 
    return EndPos;
}
 
/*
 * Simple wrapper to XLogInsert to insert a WAL record with elementary
 * contents (only an int64 is supported as value currently).
 */
XLogRecPtr
XLogSimpleInsertInt64(RmgrId rmid, uint8 info, int64 value)
{
    XLogBeginInsert();
    XLogRegisterData(&value, sizeof(value));
    return XLogInsert(rmid, info);
}
 
/*
 * XLogGetFakeLSN - get a fake LSN for an index page that isn't WAL-logged.
 *
 * Some index AMs use LSNs to detect concurrent page modifications, but not
 * all index pages are WAL-logged.  This function provides a sequence of fake
 * LSNs for that purpose.
 */
XLogRecPtr
XLogGetFakeLSN(Relation rel)
{
    if (rel->rd_rel->relpersistence == RELPERSISTENCE_TEMP)
    {
        /*
         * Temporary relations are only accessible in our session, so a simple
         * backend-local counter will do.
         */
        static XLogRecPtr counter = FirstNormalUnloggedLSN;
 
        return counter++;
    }
    else if (rel->rd_rel->relpersistence == RELPERSISTENCE_UNLOGGED)
    {
        /*
         * Unlogged relations are accessible from other backends, and survive
         * (clean) restarts.  GetFakeLSNForUnloggedRel() handles that for us.
         */
        return GetFakeLSNForUnloggedRel();
    }
    else
    {
        /*
         * WAL-logging on this relation will start after commit, so its LSNs
         * must be distinct numbers smaller than the LSN at the next commit.
         * Emit a dummy WAL record if insert-LSN hasn't advanced after the
         * last call.
         */
        static XLogRecPtr lastlsn = InvalidXLogRecPtr;
        XLogRecPtr  currlsn = GetXLogInsertEndRecPtr();
 
        Assert(!RelationNeedsWAL(rel));
        Assert(RelationIsPermanent(rel));
 
        /* No need for an actual record if we already have a distinct LSN */
        if (XLogRecPtrIsValid(lastlsn) && lastlsn == currlsn)
            currlsn = XLogAssignLSN();
 
        lastlsn = currlsn;
        return currlsn;
    }
}
 
/*
 * Assemble a WAL record from the registered data and buffers into an
 * XLogRecData chain, ready for insertion with XLogInsertRecord().
 *
 * The record header fields are filled in, except for the xl_prev field. The
 * calculated CRC does not include the record header yet.
 *
 * If there are any registered buffers, and a full-page image was not taken
 * of all of them, *fpw_lsn is set to the lowest LSN among such pages. This
 * signals that the assembled record is only good for insertion on the
 * assumption that the RedoRecPtr and doPageWrites values were up-to-date.
 *
 * *topxid_included is set if the topmost transaction ID is logged with the
 * current subtransaction.
 */
static XLogRecData *
XLogRecordAssemble(RmgrId rmid, uint8 info,
                   XLogRecPtr RedoRecPtr, bool doPageWrites,
                   XLogRecPtr *fpw_lsn, int *num_fpi, uint64 *fpi_bytes,
                   bool *topxid_included)
{
    XLogRecData *rdt;
    uint64      total_len = 0;
    int         block_id;
    pg_crc32c   rdata_crc;
    registered_buffer *prev_regbuf = NULL;
    XLogRecData *rdt_datas_last;
    XLogRecord *rechdr;
    char       *scratch = hdr_scratch;
 
    /*
     * Note: this function can be called multiple times for the same record.
     * All the modifications we do to the rdata chains below must handle that.
     */
 
    /* The record begins with the fixed-size header */
    rechdr = (XLogRecord *) scratch;
    scratch += SizeOfXLogRecord;
 
    hdr_rdt.next = NULL;
    rdt_datas_last = &hdr_rdt;
    hdr_rdt.data = hdr_scratch;
 
    /*
     * Enforce consistency checks for this record if user is looking for it.
     * Do this before at the beginning of this routine to give the possibility
     * for callers of XLogInsert() to pass XLR_CHECK_CONSISTENCY directly for
     * a record.
     */
    if (wal_consistency_checking[rmid])
        info |= XLR_CHECK_CONSISTENCY;
 
    /*
     * Make an rdata chain containing all the data portions of all block
     * references. This includes the data for full-page images. Also append
     * the headers for the block references in the scratch buffer.
     */
    *fpw_lsn = InvalidXLogRecPtr;
    for (block_id = 0; block_id < max_registered_block_id; block_id++)
    {
        registered_buffer *regbuf = &registered_buffers[block_id];
        bool        needs_backup;
        bool        needs_data;
        XLogRecordBlockHeader bkpb;
        XLogRecordBlockImageHeader bimg;
        XLogRecordBlockCompressHeader cbimg = {0};
        bool        samerel;
        bool        is_compressed = false;
        bool        include_image;
 
        if (!regbuf->in_use)
            continue;
 
        /* Determine if this block needs to be backed up */
        if (regbuf->flags & REGBUF_FORCE_IMAGE)
            needs_backup = true;
        else if (regbuf->flags & REGBUF_NO_IMAGE)
            needs_backup = false;
        else if (!doPageWrites)
            needs_backup = false;
        else
        {
            /*
             * We assume page LSN is first data on *every* page that can be
             * passed to XLogInsert, whether it has the standard page layout
             * or not.
             */
            XLogRecPtr  page_lsn = PageGetLSN(regbuf->page);
 
            needs_backup = (page_lsn <= RedoRecPtr);
            if (!needs_backup)
            {
                if (!XLogRecPtrIsValid(*fpw_lsn) || page_lsn < *fpw_lsn)
                    *fpw_lsn = page_lsn;
            }
        }
 
        /* Determine if the buffer data needs to included */
        if (regbuf->rdata_len == 0)
            needs_data = false;
        else if ((regbuf->flags & REGBUF_KEEP_DATA) != 0)
            needs_data = true;
        else
            needs_data = !needs_backup;
 
        bkpb.id = block_id;
        bkpb.fork_flags = regbuf->forkno;
        bkpb.data_length = 0;
 
        if ((regbuf->flags & REGBUF_WILL_INIT) == REGBUF_WILL_INIT)
            bkpb.fork_flags |= BKPBLOCK_WILL_INIT;
 
        /*
         * If needs_backup is true or WAL checking is enabled for current
         * resource manager, log a full-page write for the current block.
         */
        include_image = needs_backup || (info & XLR_CHECK_CONSISTENCY) != 0;
 
        if (include_image)
        {
            const PageData *page = regbuf->page;
            uint16      compressed_len = 0;
 
            /*
             * The page needs to be backed up, so calculate its hole length
             * and offset.
             */
            if (regbuf->flags & REGBUF_STANDARD)
            {
                /* Assume we can omit data between pd_lower and pd_upper */
                uint16      lower = ((const PageHeaderData *) page)->pd_lower;
                uint16      upper = ((const PageHeaderData *) page)->pd_upper;
 
                if (lower >= SizeOfPageHeaderData &&
                    upper > lower &&
                    upper <= BLCKSZ)
                {
                    bimg.hole_offset = lower;
                    cbimg.hole_length = upper - lower;
                }
                else
                {
                    /* No "hole" to remove */
                    bimg.hole_offset = 0;
                    cbimg.hole_length = 0;
                }
            }
            else
            {
                /* Not a standard page header, don't try to eliminate "hole" */
                bimg.hole_offset = 0;
                cbimg.hole_length = 0;
            }
 
            /*
             * Try to compress a block image if wal_compression is enabled
             */
            if (wal_compression != WAL_COMPRESSION_NONE)
            {
                is_compressed =
                    XLogCompressBackupBlock(page, bimg.hole_offset,
                                            cbimg.hole_length,
                                            regbuf->compressed_page,
                                            &compressed_len);
            }
 
            /*
             * Fill in the remaining fields in the XLogRecordBlockHeader
             * struct
             */
            bkpb.fork_flags |= BKPBLOCK_HAS_IMAGE;
 
            /* Report a full page image constructed for the WAL record */
            *num_fpi += 1;
 
            /*
             * Construct XLogRecData entries for the page content.
             */
            rdt_datas_last->next = &regbuf->bkp_rdatas[0];
            rdt_datas_last = rdt_datas_last->next;
 
            bimg.bimg_info = (cbimg.hole_length == 0) ? 0 : BKPIMAGE_HAS_HOLE;
 
            /*
             * If WAL consistency checking is enabled for the resource manager
             * of this WAL record, a full-page image is included in the record
             * for the block modified. During redo, the full-page is replayed
             * only if BKPIMAGE_APPLY is set.
             */
            if (needs_backup)
                bimg.bimg_info |= BKPIMAGE_APPLY;
 
            if (is_compressed)
            {
                /* The current compression is stored in the WAL record */
                bimg.length = compressed_len;
 
                /* Set the compression method used for this block */
                switch ((WalCompression) wal_compression)
                {
                    case WAL_COMPRESSION_PGLZ:
                        bimg.bimg_info |= BKPIMAGE_COMPRESS_PGLZ;
                        break;
 
                    case WAL_COMPRESSION_LZ4:
#ifdef USE_LZ4
                        bimg.bimg_info |= BKPIMAGE_COMPRESS_LZ4;
#else
                        elog(ERROR, "LZ4 is not supported by this build");
#endif
                        break;
 
                    case WAL_COMPRESSION_ZSTD:
#ifdef USE_ZSTD
                        bimg.bimg_info |= BKPIMAGE_COMPRESS_ZSTD;
#else
                        elog(ERROR, "zstd is not supported by this build");
#endif
                        break;
 
                    case WAL_COMPRESSION_NONE:
                        Assert(false);  /* cannot happen */
                        break;
                        /* no default case, so that compiler will warn */
                }
 
                rdt_datas_last->data = regbuf->compressed_page;
                rdt_datas_last->len = compressed_len;
            }
            else
            {
                bimg.length = BLCKSZ - cbimg.hole_length;
 
                if (cbimg.hole_length == 0)
                {
                    rdt_datas_last->data = page;
                    rdt_datas_last->len = BLCKSZ;
                }
                else
                {
                    /* must skip the hole */
                    rdt_datas_last->data = page;
                    rdt_datas_last->len = bimg.hole_offset;
 
                    rdt_datas_last->next = &regbuf->bkp_rdatas[1];
                    rdt_datas_last = rdt_datas_last->next;
 
                    rdt_datas_last->data =
                        page + (bimg.hole_offset + cbimg.hole_length);
                    rdt_datas_last->len =
                        BLCKSZ - (bimg.hole_offset + cbimg.hole_length);
                }
            }
 
            total_len += bimg.length;
 
            /* Track the WAL full page images in bytes */
            *fpi_bytes += bimg.length;
        }
 
        if (needs_data)
        {
            /*
             * When copying to XLogRecordBlockHeader, the length is narrowed
             * to an uint16.  Double-check that it is still correct.
             */
            Assert(regbuf->rdata_len <= UINT16_MAX);
 
            /*
             * Link the caller-supplied rdata chain for this buffer to the
             * overall list.
             */
            bkpb.fork_flags |= BKPBLOCK_HAS_DATA;
            bkpb.data_length = (uint16) regbuf->rdata_len;
            total_len += regbuf->rdata_len;
 
            rdt_datas_last->next = regbuf->rdata_head;
            rdt_datas_last = regbuf->rdata_tail;
        }
 
        if (prev_regbuf && RelFileLocatorEquals(regbuf->rlocator, prev_regbuf->rlocator))
        {
            samerel = true;
            bkpb.fork_flags |= BKPBLOCK_SAME_REL;
        }
        else
            samerel = false;
        prev_regbuf = regbuf;
 
        /* Ok, copy the header to the scratch buffer */
        memcpy(scratch, &bkpb, SizeOfXLogRecordBlockHeader);
        scratch += SizeOfXLogRecordBlockHeader;
        if (include_image)
        {
            memcpy(scratch, &bimg, SizeOfXLogRecordBlockImageHeader);
            scratch += SizeOfXLogRecordBlockImageHeader;
            if (cbimg.hole_length != 0 && is_compressed)
            {
                memcpy(scratch, &cbimg,
                       SizeOfXLogRecordBlockCompressHeader);
                scratch += SizeOfXLogRecordBlockCompressHeader;
            }
        }
        if (!samerel)
        {
            memcpy(scratch, &regbuf->rlocator, sizeof(RelFileLocator));
            scratch += sizeof(RelFileLocator);
        }
        memcpy(scratch, &regbuf->block, sizeof(BlockNumber));
        scratch += sizeof(BlockNumber);
    }
 
    /* followed by the record's origin, if any */
    if ((curinsert_flags & XLOG_INCLUDE_ORIGIN) &&
        replorigin_xact_state.origin != InvalidReplOriginId)
    {
        *(scratch++) = (char) XLR_BLOCK_ID_ORIGIN;
        memcpy(scratch, &replorigin_xact_state.origin, sizeof(replorigin_xact_state.origin));
        scratch += sizeof(replorigin_xact_state.origin);
    }
 
    /* followed by toplevel XID, if not already included in previous record */
    if (IsSubxactTopXidLogPending())
    {
        TransactionId xid = GetTopTransactionIdIfAny();
 
        /* Set the flag that the top xid is included in the WAL */
        *topxid_included = true;
 
        *(scratch++) = (char) XLR_BLOCK_ID_TOPLEVEL_XID;
        memcpy(scratch, &xid, sizeof(TransactionId));
        scratch += sizeof(TransactionId);
    }
 
    /* followed by main data, if any */
    if (mainrdata_len > 0)
    {
        if (mainrdata_len > 255)
        {
            uint32      mainrdata_len_4b;
 
            if (mainrdata_len > PG_UINT32_MAX)
                ereport(ERROR,
                        (errmsg_internal("too much WAL data"),
                         errdetail_internal("Main data length is %" PRIu64 " bytes for a maximum of %u bytes.",
                                            mainrdata_len,
                                            PG_UINT32_MAX)));
 
            mainrdata_len_4b = (uint32) mainrdata_len;
            *(scratch++) = (char) XLR_BLOCK_ID_DATA_LONG;
            memcpy(scratch, &mainrdata_len_4b, sizeof(uint32));
            scratch += sizeof(uint32);
        }
        else
        {
            *(scratch++) = (char) XLR_BLOCK_ID_DATA_SHORT;
            *(scratch++) = (uint8) mainrdata_len;
        }
        rdt_datas_last->next = mainrdata_head;
        rdt_datas_last = mainrdata_last;
        total_len += mainrdata_len;
    }
    rdt_datas_last->next = NULL;
 
    hdr_rdt.len = (scratch - hdr_scratch);
    total_len += hdr_rdt.len;
 
    /*
     * Calculate CRC of the data
     *
     * Note that the record header isn't added into the CRC initially since we
     * don't know the prev-link yet.  Thus, the CRC will represent the CRC of
     * the whole record in the order: rdata, then backup blocks, then record
     * header.
     */
    INIT_CRC32C(rdata_crc);
    COMP_CRC32C(rdata_crc, hdr_scratch + SizeOfXLogRecord, hdr_rdt.len - SizeOfXLogRecord);
    for (rdt = hdr_rdt.next; rdt != NULL; rdt = rdt->next)
        COMP_CRC32C(rdata_crc, rdt->data, rdt->len);
 
    /*
     * Ensure that the XLogRecord is not too large.
     *
     * XLogReader machinery is only able to handle records up to a certain
     * size (ignoring machine resource limitations), so make sure that we will
     * not emit records larger than the sizes advertised to be supported.
     */
    if (total_len > XLogRecordMaxSize)
        ereport(ERROR,
                (errmsg_internal("oversized WAL record"),
                 errdetail_internal("WAL record would be %" PRIu64 " bytes (of maximum %u bytes); rmid %u flags %u.",
                                    total_len, XLogRecordMaxSize, rmid, info)));
 
    /*
     * Fill in the fields in the record header. Prev-link is filled in later,
     * once we know where in the WAL the record will be inserted. The CRC does
     * not include the record header yet.
     */
    rechdr->xl_xid = GetCurrentTransactionIdIfAny();
    rechdr->xl_tot_len = (uint32) total_len;
    rechdr->xl_info = info;
    rechdr->xl_rmid = rmid;
    rechdr->xl_prev = InvalidXLogRecPtr;
    rechdr->xl_crc = rdata_crc;
 
    return &hdr_rdt;
}
 
/*
 * Create a compressed version of a backup block image.
 *
 * Returns false if compression fails (i.e., compressed result is actually
 * bigger than original). Otherwise, returns true and sets 'dlen' to
 * the length of compressed block image.
 */
static bool
XLogCompressBackupBlock(const PageData *page, uint16 hole_offset, uint16 hole_length,
                        void *dest, uint16 *dlen)
{
    int32       orig_len = BLCKSZ - hole_length;
    int32       len = -1;
    int32       extra_bytes = 0;
    const void *source;
    PGAlignedBlock tmp;
 
    if (hole_length != 0)
    {
        /* must skip the hole */
        memcpy(tmp.data, page, hole_offset);
        memcpy(tmp.data + hole_offset,
               page + (hole_offset + hole_length),
               BLCKSZ - (hole_length + hole_offset));
        source = tmp.data;
 
        /*
         * Extra data needs to be stored in WAL record for the compressed
         * version of block image if the hole exists.
         */
        extra_bytes = SizeOfXLogRecordBlockCompressHeader;
    }
    else
        source = page;
 
    switch ((WalCompression) wal_compression)
    {
        case WAL_COMPRESSION_PGLZ:
            len = pglz_compress(source, orig_len, dest, PGLZ_strategy_default);
            break;
 
        case WAL_COMPRESSION_LZ4:
#ifdef USE_LZ4
            len = LZ4_compress_default(source, dest, orig_len,
                                       COMPRESS_BUFSIZE);
            if (len <= 0)
                len = -1;       /* failure */
#else
            elog(ERROR, "LZ4 is not supported by this build");
#endif
            break;
 
        case WAL_COMPRESSION_ZSTD:
#ifdef USE_ZSTD
            len = ZSTD_compress(dest, COMPRESS_BUFSIZE, source, orig_len,
                                ZSTD_CLEVEL_DEFAULT);
            if (ZSTD_isError(len))
                len = -1;       /* failure */
#else
            elog(ERROR, "zstd is not supported by this build");
#endif
            break;
 
        case WAL_COMPRESSION_NONE:
            Assert(false);      /* cannot happen */
            break;
            /* no default case, so that compiler will warn */
    }
 
    /*
     * We recheck the actual size even if compression reports success and see
     * if the number of bytes saved by compression is larger than the length
     * of extra data needed for the compressed version of block image.
     */
    if (len >= 0 &&
        len + extra_bytes < orig_len)
    {
        *dlen = (uint16) len;   /* successful compression */
        return true;
    }
    return false;
}
 
/*
 * Determine whether the buffer referenced has to be backed up.
 *
 * Since we don't yet have the insert lock, fullPageWrites and runningBackups
 * (which forces full-page writes) could change later, so the result should
 * be used for optimization purposes only.
 */
bool
XLogCheckBufferNeedsBackup(Buffer buffer)
{
    XLogRecPtr  RedoRecPtr;
    bool        doPageWrites;
    Page        page;
 
    GetFullPageWriteInfo(&RedoRecPtr, &doPageWrites);
 
    page = BufferGetPage(buffer);
 
    if (doPageWrites && PageGetLSN(page) <= RedoRecPtr)
        return true;            /* buffer requires backup */
 
    return false;               /* buffer does not need to be backed up */
}
 
/*
 * Write a backup block if needed when we are setting a hint. Note that
 * this may be called for a variety of page types, not just heaps.
 *
 * Callable while holding just a share-exclusive lock on the buffer
 * content. That suffices to prevent concurrent modifications of the
 * buffer. The buffer already needs to have been marked dirty by
 * MarkBufferDirtyHint().
 *
 * We only need to do something if page has not yet been full page written in
 * this checkpoint round. The LSN of the inserted wal record is returned if we
 * had to write, InvalidXLogRecPtr otherwise.
 */
XLogRecPtr
XLogSaveBufferForHint(Buffer buffer, bool buffer_std)
{
    XLogRecPtr  recptr = InvalidXLogRecPtr;
    XLogRecPtr  lsn;
    XLogRecPtr  RedoRecPtr;
 
    /* this also verifies that we hold an appropriate lock */
    Assert(BufferIsDirty(buffer));
 
    /*
     * Update RedoRecPtr so that we can make the right decision. It's possible
     * that a new checkpoint will start just after GetRedoRecPtr(), but that
     * is ok, as the buffer is already dirty, ensuring that any BufferSync()
     * started after the buffer was marked dirty cannot complete without
     * flushing this buffer.  If a checkpoint started between marking the
     * buffer dirty and this check, we will emit an unnecessary WAL record (as
     * the buffer will be written out as part of the checkpoint), but the
     * window for that is not big.
     */
    RedoRecPtr = GetRedoRecPtr();
 
    /*
     * We assume page LSN is first data on *every* page that can be passed to
     * XLogInsert, whether it has the standard page layout or not.
     */
    lsn = PageGetLSN(BufferGetPage(buffer));
 
    if (lsn <= RedoRecPtr)
    {
        int         flags = 0;
 
        XLogBeginInsert();
 
        if (buffer_std)
            flags |= REGBUF_STANDARD;
 
        XLogRegisterBuffer(0, buffer, flags);
 
        recptr = XLogInsert(RM_XLOG_ID, XLOG_FPI_FOR_HINT);
    }
 
    return recptr;
}
 
/*
 * Write a WAL record containing a full image of a page. Caller is responsible
 * for writing the page to disk after calling this routine.
 *
 * Note: If you're using this function, you should be building pages in private
 * memory and writing them directly to smgr.  If you're using buffers, call
 * log_newpage_buffer instead.
 *
 * If the page follows the standard page layout, with a PageHeader and unused
 * space between pd_lower and pd_upper, set 'page_std' to true. That allows
 * the unused space to be left out from the WAL record, making it smaller.
 */
XLogRecPtr
log_newpage(RelFileLocator *rlocator, ForkNumber forknum, BlockNumber blkno,
            Page page, bool page_std)
{
    int         flags;
    XLogRecPtr  recptr;
 
    flags = REGBUF_FORCE_IMAGE;
    if (page_std)
        flags |= REGBUF_STANDARD;
 
    XLogBeginInsert();
    XLogRegisterBlock(0, rlocator, forknum, blkno, page, flags);
    recptr = XLogInsert(RM_XLOG_ID, XLOG_FPI);
 
    /*
     * The page may be uninitialized. If so, we can't set the LSN because that
     * would corrupt the page.
     */
    if (!PageIsNew(page))
    {
        PageSetLSN(page, recptr);
    }
 
    return recptr;
}
 
/*
 * Like log_newpage(), but allows logging multiple pages in one operation.
 * It is more efficient than calling log_newpage() for each page separately,
 * because we can write multiple pages in a single WAL record.
 */
void
log_newpages(RelFileLocator *rlocator, ForkNumber forknum, int num_pages,
             BlockNumber *blknos, Page *pages, bool page_std)
{
    int         flags;
    XLogRecPtr  recptr;
    int         i;
    int         j;
 
    flags = REGBUF_FORCE_IMAGE;
    if (page_std)
        flags |= REGBUF_STANDARD;
 
    /*
     * Iterate over all the pages. They are collected into batches of
     * XLR_MAX_BLOCK_ID pages, and a single WAL-record is written for each
     * batch.
     */
    XLogEnsureRecordSpace(XLR_MAX_BLOCK_ID - 1, 0);
 
    i = 0;
    while (i < num_pages)
    {
        int         batch_start = i;
        int         nbatch;
 
        XLogBeginInsert();
 
        nbatch = 0;
        while (nbatch < XLR_MAX_BLOCK_ID && i < num_pages)
        {
            XLogRegisterBlock(nbatch, rlocator, forknum, blknos[i], pages[i], flags);
            i++;
            nbatch++;
        }
 
        recptr = XLogInsert(RM_XLOG_ID, XLOG_FPI);
 
        for (j = batch_start; j < i; j++)
        {
            /*
             * The page may be uninitialized. If so, we can't set the LSN
             * because that would corrupt the page.
             */
            if (!PageIsNew(pages[j]))
            {
                PageSetLSN(pages[j], recptr);
            }
        }
    }
}
 
/*
 * Write a WAL record containing a full image of a page.
 *
 * Caller should initialize the buffer and mark it dirty before calling this
 * function.  This function will set the page LSN.
 *
 * If the page follows the standard page layout, with a PageHeader and unused
 * space between pd_lower and pd_upper, set 'page_std' to true. That allows
 * the unused space to be left out from the WAL record, making it smaller.
 */
XLogRecPtr
log_newpage_buffer(Buffer buffer, bool page_std)
{
    Page        page = BufferGetPage(buffer);
    RelFileLocator rlocator;
    ForkNumber  forknum;
    BlockNumber blkno;
 
    /* Shared buffers should be modified in a critical section. */
    Assert(CritSectionCount > 0);
 
    BufferGetTag(buffer, &rlocator, &forknum, &blkno);
 
    return log_newpage(&rlocator, forknum, blkno, page, page_std);
}
 
/*
 * WAL-log a range of blocks in a relation.
 *
 * An image of all pages with block numbers 'startblk' <= X < 'endblk' is
 * written to the WAL. If the range is large, this is done in multiple WAL
 * records.
 *
 * If all page follows the standard page layout, with a PageHeader and unused
 * space between pd_lower and pd_upper, set 'page_std' to true. That allows
 * the unused space to be left out from the WAL records, making them smaller.
 *
 * NOTE: This function acquires exclusive-locks on the pages. Typically, this
 * is used on a newly-built relation, and the caller is holding a
 * AccessExclusiveLock on it, so no other backend can be accessing it at the
 * same time. If that's not the case, you must ensure that this does not
 * cause a deadlock through some other means.
 */
void
log_newpage_range(Relation rel, ForkNumber forknum,
                  BlockNumber startblk, BlockNumber endblk,
                  bool page_std)
{
    int         flags;
    BlockNumber blkno;
 
    flags = REGBUF_FORCE_IMAGE;
    if (page_std)
        flags |= REGBUF_STANDARD;
 
    /*
     * Iterate over all the pages in the range. They are collected into
     * batches of XLR_MAX_BLOCK_ID pages, and a single WAL-record is written
     * for each batch.
     */
    XLogEnsureRecordSpace(XLR_MAX_BLOCK_ID - 1, 0);
 
    blkno = startblk;
    while (blkno < endblk)
    {
        Buffer      bufpack[XLR_MAX_BLOCK_ID];
        XLogRecPtr  recptr;
        int         nbufs;
        int         i;
 
        CHECK_FOR_INTERRUPTS();
 
        /* Collect a batch of blocks. */
        nbufs = 0;
        while (nbufs < XLR_MAX_BLOCK_ID && blkno < endblk)
        {
            Buffer      buf = ReadBufferExtended(rel, forknum, blkno,
                                                 RBM_NORMAL, NULL);
 
            LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
 
            /*
             * Completely empty pages are not WAL-logged. Writing a WAL record
             * would change the LSN, and we don't want that. We want the page
             * to stay empty.
             */
            if (!PageIsNew(BufferGetPage(buf)))
                bufpack[nbufs++] = buf;
            else
                UnlockReleaseBuffer(buf);
            blkno++;
        }
 
        /* Nothing more to do if all remaining blocks were empty. */
        if (nbufs == 0)
            break;
 
        /* Write WAL record for this batch. */
        XLogBeginInsert();
 
        START_CRIT_SECTION();
        for (i = 0; i < nbufs; i++)
        {
            MarkBufferDirty(bufpack[i]);
            XLogRegisterBuffer(i, bufpack[i], flags);
        }
 
        recptr = XLogInsert(RM_XLOG_ID, XLOG_FPI);
 
        for (i = 0; i < nbufs; i++)
            PageSetLSN(BufferGetPage(bufpack[i]), recptr);
 
        END_CRIT_SECTION();
 
        for (i = 0; i < nbufs; i++)
            UnlockReleaseBuffer(bufpack[i]);
    }
}
 
/*
 * Allocate working buffers needed for WAL record construction.
 */
void
InitXLogInsert(void)
{
#ifdef USE_ASSERT_CHECKING
 
    /*
     * Check that any records assembled can be decoded.  This is capped based
     * on what XLogReader would require at its maximum bound.  The XLOG_BLCKSZ
     * addend covers the larger allocate_recordbuf() demand.  This code path
     * is called once per backend, more than enough for this check.
     */
    size_t      max_required =
        DecodeXLogRecordRequiredSpace(XLogRecordMaxSize + XLOG_BLCKSZ);
 
    Assert(AllocSizeIsValid(max_required));
#endif
 
    /* Initialize the working areas */
    if (xloginsert_cxt == NULL)
    {
        xloginsert_cxt = AllocSetContextCreate(TopMemoryContext,
                                               "WAL record construction",
                                               ALLOCSET_DEFAULT_SIZES);
    }
 
    if (registered_buffers == NULL)
    {
        registered_buffers = (registered_buffer *)
            MemoryContextAllocZero(xloginsert_cxt,
                                   sizeof(registered_buffer) * (XLR_NORMAL_MAX_BLOCK_ID + 1));
        max_registered_buffers = XLR_NORMAL_MAX_BLOCK_ID + 1;
    }
    if (rdatas == NULL)
    {
        rdatas = MemoryContextAlloc(xloginsert_cxt,
                                    sizeof(XLogRecData) * XLR_NORMAL_RDATAS);
        max_rdatas = XLR_NORMAL_RDATAS;
    }
 
    /*
     * Allocate a buffer to hold the header information for a WAL record.
     */
    if (hdr_scratch == NULL)
        hdr_scratch = MemoryContextAllocZero(xloginsert_cxt,
                                             HEADER_SCRATCH_SIZE);
}