PostgreSQL 源码解读（110）- WAL#6（Insert&WAL - XLogRe...

本节简单介绍了XLogRecordAssemble函数的实现逻辑,该函数从已注册的数据和缓冲区中组装XLOG record到XLogRecData链中,为XLOG Record的插入作准备。

一、数据结构

全局静态变量
XLogRecordAssemble使用的全局变量包括hdr_rdt/hdr_scratch/rdatas等.

/* flags for the in-progress insertion */
//用于插入过程中的标记信息
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.
 * 在构建XLOG Record时通常会存储记录的头部信息.
 * 'hdr_scratch'并不是一个普通(plain)变量,而是在初始化时通过palloc初始化,
 *   因为我们希望该变量已经是MAXALIGNed并且已被0x00填充.
 *
 * For simplicity, it's allocated large enough to hold the headers for any
 * WAL record.
 * 简单起见,该变量预先会分配足够大的空间用于存储所有WAL Record的头部信息.
 */
static XLogRecData hdr_rdt;
static char *hdr_scratch = NULL;

#define SizeOfXlogOrigin    (sizeof(RepOriginId) + sizeof(char))

#define HEADER_SCRATCH_SIZE \
    (SizeOfXLogRecord + \
     MaxSizeOfXLogRecordBlockHeader * (XLR_MAX_BLOCK_ID + 1) + \
     SizeOfXLogRecordDataHeaderLong + SizeOfXlogOrigin)
/*
 * An array of XLogRecData structs, to hold registered data.
 * XLogRecData结构体数组,存储已注册的数据.
 */
static XLogRecData *rdatas;
static int  num_rdatas;         /* entries currently used */
//已分配的空间大小
static int  max_rdatas;         /* allocated size */
//是否调用XLogBeginInsert函数
static bool begininsert_called = false;

static XLogCtlData *XLogCtl = NULL;

/* flags for the in-progress insertion */
static uint8 curinsert_flags = 0;

/*
 * A chain of XLogRecDatas to hold the "main data" of a WAL record, registered
 * with XLogRegisterData(...).
 * 存储WAL Record "main data"的XLogRecDatas数据链
 */
static XLogRecData *mainrdata_head;
static XLogRecData *mainrdata_last = (XLogRecData *) &mainrdata_head;
//链中某个位置的mainrdata大小
static uint32 mainrdata_len; /* total # of bytes in chain */

/*
 * ProcLastRecPtr points to the start of the last XLOG record inserted by the
 * current backend.  It is updated for all inserts.  XactLastRecEnd points to
 * end+1 of the last record, and is reset when we end a top-level transaction,
 * or start a new one; so it can be used to tell if the current transaction has
 * created any XLOG records.
 * ProcLastRecPtr指向当前后端插入的最后一条XLOG记录的开头。
 * 它针对所有插入进行更新。
 * XactLastRecEnd指向最后一条记录的末尾位置 + 1，
 *   并在结束顶级事务或启动新事务时重置;
 *   因此，它可以用来判断当前事务是否创建了任何XLOG记录。
 *
 * While in parallel mode, this may not be fully up to date.  When committing,
 * a transaction can assume this covers all xlog records written either by the
 * user backend or by any parallel worker which was present at any point during
 * the transaction.  But when aborting, or when still in parallel mode, other
 * parallel backends may have written WAL records at later LSNs than the value
 * stored here.  The parallel leader advances its own copy, when necessary,
 * in WaitForParallelWorkersToFinish.
 * 在并行模式下，这可能不是完全是最新的。
 * 在提交时，事务可以假定覆盖了用户后台进程或在事务期间出现的并行worker进程的所有xlog记录。
 * 但是，当中止时，或者仍然处于并行模式时，其他并行后台进程可能在较晚的LSNs中写入了WAL记录，
 *   而不是存储在这里的值。
 * 当需要时，并行处理进程的leader在WaitForParallelWorkersToFinish中会推进自己的副本。
 */
XLogRecPtr  ProcLastRecPtr = InvalidXLogRecPtr;
XLogRecPtr  XactLastRecEnd = InvalidXLogRecPtr;
XLogRecPtr XactLastCommitEnd = InvalidXLogRecPtr;

/* For WALInsertLockAcquire/Release functions */
//用于WALInsertLockAcquire/Release函数
static int  MyLockNo = 0;
static bool holdingAllLocks = false;

宏定义
XLogRegisterBuffer函数使用的flags

/* flags for XLogRegisterBuffer */
//XLogRegisterBuffer函数使用的flags
#define REGBUF_FORCE_IMAGE  0x01    /* 强制执行full-page-write;force a full-page image */
#define REGBUF_NO_IMAGE     0x02    /* 不需要FPI;don't take a full-page image */
#define REGBUF_WILL_INIT    (0x04 | 0x02)   /* 在回放时重新初始化page(表示NO_IMAGE);
                                             * page will be re-initialized at
                                             * replay (implies NO_IMAGE) */
#define REGBUF_STANDARD     0x08    /* 标准的page layout(数据在pd_lower和pd_upper之间的数据会被跳过)
                                     * page follows "standard" page layout,
                                     * (data between pd_lower and pd_upper
                                     * will be skipped) */
#define REGBUF_KEEP_DATA    0x10    /* include data even if a full-page image
                                      * is taken */
/*
 * Flag bits for the record being inserted, set using XLogSetRecordFlags().
 */
#define XLOG_INCLUDE_ORIGIN     0x01    /* include the replication origin */
#define XLOG_MARK_UNIMPORTANT   0x02    /* record not important for durability */    

XLogRecData
xloginsert.c中的函数构造一个XLogRecData结构体链用于标识最后的WAL记录

/*
 * The functions in xloginsert.c construct a chain of XLogRecData structs
 * to represent the final WAL record.
 * xloginsert.c中的函数构造一个XLogRecData结构体链用于标识最后的WAL记录
 */
typedef struct XLogRecData
{
    //链中的下一个结构体,如无则为NULL
    struct XLogRecData *next;   /* next struct in chain, or NULL */
    //rmgr数据的起始地址
    char       *data;           /* start of rmgr data to include */
    //rmgr数据大小
    uint32      len;            /* length of rmgr data to include */
} XLogRecData;

registered_buffer
对于每一个使用XLogRegisterBuffer注册的每个数据块,填充到registered_buffer结构体中

/*
 * For each block reference registered with XLogRegisterBuffer, we fill in
 * a registered_buffer struct.
 * 对于每一个使用XLogRegisterBuffer注册的每个数据块,
 *   填充到registered_buffer结构体中
 */
typedef struct
{
    //slot是否在使用?
    bool        in_use;         /* is this slot in use? */
    //REGBUF_* 相关标记
    uint8       flags;          /* REGBUF_* flags */
    //定义关系和数据库的标识符
    RelFileNode rnode;          /* identifies the relation and block */
    //fork进程编号
    ForkNumber  forkno;
    //块编号
    BlockNumber block;
    //页内容
    Page        page;           /* page content */
    //rdata链中的数据总大小
    uint32      rdata_len;      /* total length of data in rdata chain */
    //使用该数据块注册的数据链头
    XLogRecData *rdata_head;    /* head of the chain of data registered with
                                 * this block */
    //使用该数据块注册的数据链尾
    XLogRecData *rdata_tail;    /* last entry in the chain, or &rdata_head if
                                 * empty */
    //临时rdatas数据引用,用于存储XLogRecordAssemble()中使用的备份块数据
    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 */
    //用于存储压缩版本的备份块镜像的缓存
    char        compressed_page[PGLZ_MAX_BLCKSZ];
} registered_buffer;
//registered_buffer指针(全局变量)
static registered_buffer *registered_buffers;
//已分配的大小
static int  max_registered_buffers; /* allocated size */
//最大块号 + 1(当前注册块)
static int  max_registered_block_id = 0;    /* highest block_id + 1 currently
                                             * registered */

二、源码解读

XLogRecordAssemble函数从已注册的数据和缓冲区中组装XLOG record到XLogRecData链中,组装完成后可以使用XLogInsertRecord()函数插入到WAL buffer中.

 /*
  * Assemble a WAL record from the registered data and buffers into an
  * XLogRecData chain, ready for insertion with XLogInsertRecord().
  * 从已注册的数据和缓冲区中组装XLOG record到XLogRecData链中,
  * 组装完成后可以使用XLogInsertRecord()函数插入.
  * 
  * The record header fields are filled in, except for the xl_prev field. The
  * calculated CRC does not include the record header yet.
  * 除了xl_prev外,XLOG Record的header域已填充完毕.
  * 计算的CRC还没有包含header信息.
  *
  * 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.
  * 如存在已注册的缓冲区,而且full-page-image没有全部包括这些数据,
  *   *fpw_lsn设置为这些页面中最小的LSN.
  * 基于RedoRecPtr和doPageWrites已更新为最新的假设,
  *   已组装的XLOG Record对在此假设上的插入是OK的.
  */
 static XLogRecData *
 XLogRecordAssemble(RmgrId rmid, uint8 info,
                    XLogRecPtr RedoRecPtr, bool doPageWrites,
                    XLogRecPtr *fpw_lsn)
{
     XLogRecData *rdt;//XLogRecData指针
     uint32      total_len = 0;//XLOG Record大小
     int         block_id;//块ID
     pg_crc32c   rdata_crc;//CRC
     registered_buffer *prev_regbuf = NULL;//已注册的buffer指针
     XLogRecData *rdt_datas_last;//
     XLogRecord *rechdr;//头部信息
     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.
      * 对于同一个XLOG Record,该函数可以被多次调用.
      * 下面我们对rdata链进行的所有更新必须处理这种情况.
      */

     /* The record begins with the fixed-size header */
     //XLOG Record的头部大小是固定的.
     rechdr = (XLogRecord *) scratch;
     scratch += SizeOfXLogRecord;//指针移动

     hdr_rdt.next = NULL;//hdr_rdt --> static XLogRecData hdr_rdt;
     rdt_datas_last = &hdr_rdt;//
     hdr_rdt.data = hdr_scratch;//rmgr数据的起始偏移

     /*
      * 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.
      * 如正在搜索此记录,则强制检查该记录的一致性.
      * 在该处理过程开始前执行此项处理,以便XLogInsert()的调用者
      *   可以直接传递XLR_CHECK_CONSISTENCY给XLOG 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.
      * 构造保存所有块参考的数据部分的rdata链.这包括FPI的数据.
      * 同时,在scratch缓冲区中为所有的块引用追加头部信息.
      */
     *fpw_lsn = InvalidXLogRecPtr;//初始化变量
     for (block_id = 0; block_id < max_registered_block_id; block_id++)//遍历已注册的block
     {
         registered_buffer *regbuf = ®istered_buffers[block_id];//获取根据block_id获取缓冲区
         bool        needs_backup;//是否需要backup block
         bool        needs_data;//是否需要data
         XLogRecordBlockHeader bkpb;//XLogRecordBlockHeader
         XLogRecordBlockImageHeader bimg;//XLogRecordBlockImageHeader
         XLogRecordBlockCompressHeader cbimg = {0};//压缩存储时需要
         bool        samerel;//是否同一个rel?
         bool        is_compressed = false;//是否压缩
         bool        include_image;//是否包括FPI

         if (!regbuf->in_use)//未在使用,继续下一个
             continue;

         /* Determine if this block needs to be backed up */
         //确定此block是否需要backup
         if (regbuf->flags & REGBUF_FORCE_IMAGE)
             needs_backup = true;//强制要求FPI
         else if (regbuf->flags & REGBUF_NO_IMAGE)
             needs_backup = false;//强制要求不要IMAGE
         else if (!doPageWrites)
             needs_backup = false;//doPageWrites标记设置为F
         else//doPageWrites = T
         {
             /*
              * 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.
              * 不管该page是否标准page layout,
              *   我们假定在每一个page中最前面的数据是page LSN,
              *   
              */
             XLogRecPtr  page_lsn = PageGetLSN(regbuf->page);//获取LSN

             needs_backup = (page_lsn <= RedoRecPtr);//是否需要backup
             if (!needs_backup)//不需要
             {
                 if (*fpw_lsn == InvalidXLogRecPtr || page_lsn < *fpw_lsn)
                     *fpw_lsn = page_lsn;//设置LSN
             }
         }

         /* Determine if the buffer data needs to included */
         //确定buffer中的data是否需要包括在其中
         if (regbuf->rdata_len == 0)//没有数据
             needs_data = false;
         else if ((regbuf->flags & REGBUF_KEEP_DATA) != 0)//需要包括data
             needs_data = true;
         else
             needs_data = !needs_backup;//needs_backup取反
         //BlockHeader设置值
         bkpb.id = block_id;//块ID
         bkpb.fork_flags = regbuf->forkno;//forkno
         bkpb.data_length = 0;//数据长度

         if ((regbuf->flags & REGBUF_WILL_INIT) == REGBUF_WILL_INIT)
             bkpb.fork_flags |= BKPBLOCK_WILL_INIT;//设置标记

         /*
          * If needs_backup is true or WAL checking is enabled for current
          * resource manager, log a full-page write for the current block.
          * 如needs_backup为T,或者当前RM的WAL检查已启用,
          *   为当前block执行full-page-write
          */
         //需要backup或者要求执行一致性检查  
         include_image = needs_backup || (info & XLR_CHECK_CONSISTENCY) != 0;

         if (include_image)
         {
             //包含块镜像
             Page        page = regbuf->page;//获取对应的page
             uint16      compressed_len = 0;//压缩后的大小

             /*
              * The page needs to be backed up, so calculate its hole length
              * and offset.
              * page需要备份,计算空闲空间大小和偏移
              */
             if (regbuf->flags & REGBUF_STANDARD)
             {
                 //如为标准的REGBUF
                 /* Assume we can omit data between pd_lower and pd_upper */
                 //假定我们可以省略pd_lower和pd_upper之间的数据
                 uint16      lower = ((PageHeader) page)->pd_lower;//获取lower
                 uint16      upper = ((PageHeader) page)->pd_upper;//获取upper

                 if (lower >= SizeOfPageHeaderData &&
                     upper > lower &&
                     upper <= BLCKSZ)
                 {
                     //lower大于Page的头部 && upper大于lower && upper小于块大小
                     bimg.hole_offset = lower;
                     cbimg.hole_length = upper - lower;
                 }
                 else
                 {
                     /* No "hole" to remove */
                     //没有空闲空间可以移除
                     bimg.hole_offset = 0;
                     cbimg.hole_length = 0;
                 }
             }
             else
             {
                 //不是标准的REGBUF
                 /* Not a standard page header, don't try to eliminate "hole" */
                 //不是标准的page header,不要尝试估算"hole"
                 bimg.hole_offset = 0;
                 cbimg.hole_length = 0;
             }

             /*
              * Try to compress a block image if wal_compression is enabled
              * 如果wal_compression启用,则尝试压缩
              */
             if (wal_compression)
             {
                 is_compressed =
                     XLogCompressBackupBlock(page, bimg.hole_offset,
                                             cbimg.hole_length,
                                             regbuf->compressed_page,
                                             &compressed_len);//调用XLogCompressBackupBlock压缩
             }

             /*
              * Fill in the remaining fields in the XLogRecordBlockHeader
              * struct
              * 填充XLogRecordBlockHeader结构体的剩余域字段
              */
             bkpb.fork_flags |= BKPBLOCK_HAS_IMAGE;

             /*
              * Construct XLogRecData entries for the page content.
              * 为page内容构造XLogRecData入口
              */
             rdt_datas_last->next = ®buf->bkp_rdatas[0];
             rdt_datas_last = rdt_datas_last->next;
             //设置标记
             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.
              * 如WAL一致性检查已启用,被更新的block已在XLOG Record中包含了FPI.
              * 在redo期间,在设置了BKPIMAGE_APPLY标记的情况下full-page才会回放.
              */
             if (needs_backup)
                 bimg.bimg_info |= BKPIMAGE_APPLY;//设置标记

             if (is_compressed)//是否压缩?
             {
                 bimg.length = compressed_len;//压缩后的空间
                 bimg.bimg_info |= BKPIMAGE_IS_COMPRESSED;//压缩标记

                 rdt_datas_last->data = regbuf->compressed_page;//放在registered_buffer中
                 rdt_datas_last->len = compressed_len;//长度
             }
             else
             {
                 //没有压缩
                 //image的大小
                 bimg.length = BLCKSZ - cbimg.hole_length;

                 if (cbimg.hole_length == 0)
                 {
                     rdt_datas_last->data = page;//数据指针直接指向page
                     rdt_datas_last->len = BLCKSZ;//大小为block size
                 }
                 else
                 {
                     /* must skip the hole */
                     //跳过hole
                     rdt_datas_last->data = page;//数据指针
                     rdt_datas_last->len = bimg.hole_offset;//获取hole的偏移

                     rdt_datas_last->next = ®buf->bkp_rdatas[1];//第2部分
                     rdt_datas_last = rdt_datas_last->next;//

                     rdt_datas_last->data =
                         page + (bimg.hole_offset + cbimg.hole_length);//指针指向第二部分
                     rdt_datas_last->len =
                         BLCKSZ - (bimg.hole_offset + cbimg.hole_length);//设置长度
                 }
             }

             total_len += bimg.length;//调整总长度
         }

         if (needs_data)//需要包含数据
         {
             /*
              * Link the caller-supplied rdata chain for this buffer to the
              * overall list.
              * 把该缓冲区链接到调用者提供的rdata链中构成一个整体的链表
              */
             bkpb.fork_flags |= BKPBLOCK_HAS_DATA;//设置标记
             bkpb.data_length = regbuf->rdata_len;//长度
             total_len += regbuf->rdata_len;//总大小

             rdt_datas_last->next = regbuf->rdata_head;//调整指针
             rdt_datas_last = regbuf->rdata_tail;
         }

         //存在上一个regbuf 而且是同一个RefFileNode(关系一样/表空间一样/block一样)
         if (prev_regbuf && RelFileNodeEquals(regbuf->rnode, prev_regbuf->rnode))
         {
             samerel = true;//设置标记
             bkpb.fork_flags |= BKPBLOCK_SAME_REL;//同一个REL
         }
         else
             samerel = false;
         prev_regbuf = regbuf;//切换为当前的regbuf

         /* Ok, copy the header to the scratch buffer */
         //已OK,拷贝头部信息到scratch缓冲区中
         memcpy(scratch, &bkpb, SizeOfXLogRecordBlockHeader);
         scratch += SizeOfXLogRecordBlockHeader;//调整偏移
         if (include_image)
         {
             //包含FPI,追加SizeOfXLogRecordBlockImageHeader
             memcpy(scratch, &bimg, SizeOfXLogRecordBlockImageHeader);
             scratch += SizeOfXLogRecordBlockImageHeader;//调整偏移
             if (cbimg.hole_length != 0 && is_compressed)
             {
                 //压缩存储,追加SizeOfXLogRecordBlockCompressHeader
                 memcpy(scratch, &cbimg,
                        SizeOfXLogRecordBlockCompressHeader);
                 scratch += SizeOfXLogRecordBlockCompressHeader;//调整偏移
             }
         }
         if (!samerel)
         {
             //不是同一个REL,追加RelFileNode
             memcpy(scratch, ®buf->rnode, sizeof(RelFileNode));
             scratch += sizeof(RelFileNode);//调整偏移
         }
         //后跟BlockNumber
         memcpy(scratch, ®buf->block, sizeof(BlockNumber));
         scratch += sizeof(BlockNumber);//调整偏移
     }

     /* followed by the record's origin, if any */
     //接下来,是XLOG Record origin
     if ((curinsert_flags & XLOG_INCLUDE_ORIGIN) &&
         replorigin_session_origin != InvalidRepOriginId)
     {
         //
         *(scratch++) = (char) XLR_BLOCK_ID_ORIGIN;
         memcpy(scratch, &replorigin_session_origin, sizeof(replorigin_session_origin));
         scratch += sizeof(replorigin_session_origin);
     }

     /* followed by main data, if any */
     //接下来是main data
     if (mainrdata_len > 0)//main data大小 > 0
     {
         if (mainrdata_len > 255)//超过255,则使用Long格式
         {
             *(scratch++) = (char) XLR_BLOCK_ID_DATA_LONG;
             memcpy(scratch, &mainrdata_len, sizeof(uint32));
             scratch += sizeof(uint32);
         }
         else//否则使用Short格式
         {
             *(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);//头部大小
     total_len += hdr_rdt.len;//总大小

     /*
      * Calculate CRC of the data
      * 计算数据的CRC
      *
      * 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.
      * 由于我们还不知道prev-link的数值,因此头部不在最初的CRC中.
      * 因此，CRC将按照以下顺序表示整个记录的CRC: rdata，然后是backup blocks，然后是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);

     /*
      * 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.
      * 填充记录头部信息的其他域字段.
      * Prev-link将在该记录插入在哪里的时候再填充.
      * CRC还不包括记录的头部信息.
      */
     rechdr->xl_xid = GetCurrentTransactionIdIfAny();
     rechdr->xl_tot_len = total_len;
     rechdr->xl_info = info;
     rechdr->xl_rmid = rmid;
     rechdr->xl_prev = InvalidXLogRecPtr;
     rechdr->xl_crc = rdata_crc;

     return &hdr_rdt;
}

三、跟踪分析

场景一:清除数据后,执行checkpoint后的第一次插入
测试脚本如下:

testdb=# truncate table t_wal_partition;
TRUNCATE TABLE
testdb=# checkpoint;
CHECKPOINT
testdb=# insert into t_wal_partition(c1,c2,c3) VALUES(1,'checkpoint','checkpoint');

设置断点,进入XLogRecordAssemble

(gdb) b XLogRecordAssemble
Breakpoint 1 at 0x565411: file xloginsert.c, line 488.
(gdb) c
Continuing.

Breakpoint 1, XLogRecordAssemble (rmid=10 '\n', info=128 '\200', RedoRecPtr=5507633240, doPageWrites=true, 
    fpw_lsn=0x7fff05cfe378) at xloginsert.c:488
488     uint32      total_len = 0;

输入参数:
rmid=10即0x0A --> Heap
RedoRecPtr=5507633240
doPageWrites=true,需要full-page-write
fpw_lsn=0x7fff05cfe378
接下来是变量赋值,
其中hdr_scratch的定义为:static char *hdr_scratch = NULL;
hdr_rdt的定义为:static XLogRecData hdr_rdt;

(gdb) n
491     registered_buffer *prev_regbuf = NULL;
(gdb) 
494     char       *scratch = hdr_scratch;
(gdb) 
502     rechdr = (XLogRecord *) scratch;
(gdb) 
503     scratch += SizeOfXLogRecord;
(gdb) 

XLOG Record的头部信息

(gdb) p *(XLogRecord *)rechdr
$11 = {xl_tot_len = 114, xl_xid = 1997, xl_prev = 5507669824, xl_info = 128 '\200', xl_rmid = 1 '\001', xl_crc = 3794462175}

scratch指针指向Header之后的地址

(gdb) p hdr_scratch
$12 = 0x18a24c0 "r"

为全局变量hdr_rdt赋值

505     hdr_rdt.next = NULL;
(gdb) 
506     rdt_datas_last = &hdr_rdt;
(gdb) 
507     hdr_rdt.data = hdr_scratch;
(gdb) p hdr_rdt
$5 = {next = 0x0, data = 0x18a24c0 "r", len = 26}
(gdb) p *(XLogRecord *)hdr_rdt.data
$7 = {xl_tot_len = 114, xl_xid = 1997, xl_prev = 5507669824, xl_info = 128 '\200', xl_rmid = 1 '\001', xl_crc = 3794462175}

不执行一致性检查

(gdb) n
515     if (wal_consistency_checking[rmid])
(gdb) 
523     *fpw_lsn = InvalidXLogRecPtr;
(gdb) 

初始化fpw_lsn,开始循环.
已注册的block id只有1个.

(gdb) n
524     for (block_id = 0; block_id < max_registered_block_id; block_id++)
(gdb) p max_registered_block_id
$13 = 1

获取已注册的buffer.
其中:
rnode->RelFilenode结构体,spcNode->表空间/dbNode->数据库/relNode->关系
block->块ID
page->数据页指针(char *)
rdata_len->rdata链中的数据总大小
rdata_head->使用该数据块注册的数据链头
rdata_tail->使用该数据块注册的数据链尾
bkp_rdatas->临时rdatas数据引用,用于存储XLogRecordAssemble()中使用的备份块数据.
bkp_rdatas用于组装block image,bkp_rdatas[0]存储空闲空间(hole)前的数据,bkp_rdatas[1]存储空闲空间后的数据.

(gdb) n
526         registered_buffer *regbuf = ®istered_buffers[block_id];
(gdb) 
531         XLogRecordBlockCompressHeader cbimg = {0};
(gdb) p *regbuf
$14 = {in_use = true, flags = 14 '\016', rnode = {spcNode = 1663, dbNode = 16402, relNode = 25258}, forkno = MAIN_FORKNUM, 
  block = 0, page = 0x7fb8539e7380 "", rdata_len = 32, rdata_head = 0x18a22c0, rdata_tail = 0x18a22d8, bkp_rdatas = {{
      next = 0x18a4230, data = 0x7fb85390f380 "\001", len = 252}, {next = 0x18a22a8, data = 0x7fb85390fe28 "\315\a", 
      len = 5464}}, compressed_page = '\000' }

注意:
在内存中,main data已由函数XLogRegisterData注册,由mainrdata_head和mainrdata_last指针维护,本例中,填充了xl_heap_insert结构体.
block data由XLogRegisterBuffer初始化,由XLogRegisterBufData填充数据,在本例中,通过XLogRegisterBufData注册了两次数据,第一次是xl_heap_header结构体,第二次是实际的数据(实质上只是数据指针,最终需要什么数据,由组装器确定).

(gdb) p *mainrdata_head
$18 = {next = 0x18a22c0, data = 0x7fff05cfe3f0 "\001", len = 3}
(gdb) p *(xl_heap_insert *)mainrdata_head->data
$20 = {offnum = 1, flags = 0 '\000'}
(gdb) p *regbuf->rdata_head
$32 = {next = 0x18a22d8, data = 0x7fff05cfe3e0 "\003", len = 5}
(gdb) p *(xl_heap_header *)regbuf->rdata_head->data
$28 = {t_infomask2 = 3, t_infomask = 2050, t_hoff = 24 '\030'}
(gdb) p *regbuf->rdata_head->next
$34 = {next = 0x18a22f0, data = 0x18edaef "", len = 27}

以字符格式显示地址0x18edaef之后的27个字节(tuple data)

(gdb) x/27bc 0x18edaef
0x18edaef:  0 '\000'    1 '\001'    0 '\000'    0 '\000'    0 '\000'    23 '\027'   99 'c'  104 'h'
0x18edaf7:  101 'e' 99 'c'  107 'k' 112 'p' 111 'o' 105 'i' 110 'n' 116 't'
0x18edaff:  23 '\027'   99 'c'  104 'h' 101 'e' 99 'c'  107 'k' 112 'p' 111 'o'
0x18edb07:  105 'i' 110 'n' 116 't'

继续往下执行,由于该记录是第一条记录,因此无需执行full-page-image

(gdb) n
533         bool        is_compressed = false;
(gdb) 
536         if (!regbuf->in_use)
(gdb) 
540         if (regbuf->flags & REGBUF_FORCE_IMAGE)
(gdb) p regbuf->flags
$36 = 14 '\016'
(gdb) n
542         else if (regbuf->flags & REGBUF_NO_IMAGE)
(gdb) 
543             needs_backup = false;

needs_data为T,事务日志中仅写入tuple data

(gdb) n
564         if (regbuf->rdata_len == 0)
(gdb) p regbuf->rdata_len
$37 = 32
(gdb) n
566         else if ((regbuf->flags & REGBUF_KEEP_DATA) != 0)
(gdb) 
569             needs_data = !needs_backup;
(gdb) 
571         bkpb.id = block_id;
(gdb) p needs_data
$38 = true

设置XLogRecordBlockHeader字段值,page中第一个tuple,标记设置为BKPBLOCK_WILL_INIT

(gdb) n
572         bkpb.fork_flags = regbuf->forkno;
(gdb) 
573         bkpb.data_length = 0;
(gdb) 
575         if ((regbuf->flags & REGBUF_WILL_INIT) == REGBUF_WILL_INIT)
(gdb) n
576             bkpb.fork_flags |= BKPBLOCK_WILL_INIT;
(gdb) 
582         include_image = needs_backup || (info & XLR_CHECK_CONSISTENCY) != 0;
(gdb) p bkpb
$40 = {id = 0 '\000', fork_flags = 64 '@', data_length = 0}
(gdb) 

不需要执行FPI

(gdb) p info
$41 = 128 '\200'
(gdb) n
584         if (include_image)
(gdb) p include_image
$42 = false
(gdb) 

需要包含数据

(gdb) n
691         if (needs_data)
(gdb) 
697             bkpb.fork_flags |= BKPBLOCK_HAS_DATA;
(gdb) 
698             bkpb.data_length = regbuf->rdata_len;
(gdb) 
699             total_len += regbuf->rdata_len;
(gdb) 
701             rdt_datas_last->next = regbuf->rdata_head;
(gdb) 
702             rdt_datas_last = regbuf->rdata_tail;
(gdb) p bkpb
$43 = {id = 0 '\000', fork_flags = 96 '`', data_length = 32}
(gdb) p total_len
$44 = 32
(gdb) p *rdt_datas_last
$45 = {next = 0x18a22c0, data = 0x18a24c0 "r", len = 26}

已OK,拷贝头部信息到scratch缓冲区中

(gdb) n
705         if (prev_regbuf && RelFileNodeEquals(regbuf->rnode, prev_regbuf->rnode))
(gdb) p prev_regbuf
$46 = (registered_buffer *) 0x0
(gdb) n
711             samerel = false;
(gdb) 
712         prev_regbuf = regbuf;
(gdb) 
715         memcpy(scratch, &bkpb, SizeOfXLogRecordBlockHeader);

后面是RefFileNode + BlockNumber

(gdb) 
716         scratch += SizeOfXLogRecordBlockHeader;
(gdb) 
717         if (include_image)
(gdb) 
728         if (!samerel)
(gdb) 
730             memcpy(scratch, ®buf->rnode, sizeof(RelFileNode));
(gdb) 
731             scratch += sizeof(RelFileNode);
(gdb) 
733         memcpy(scratch, ®buf->block, sizeof(BlockNumber));
(gdb) 
734         scratch += sizeof(BlockNumber);
(gdb) 
524     for (block_id = 0; block_id < max_registered_block_id; block_id++)

结束循环

524     for (block_id = 0; block_id < max_registered_block_id; block_id++)
(gdb) 

接下来是replorigin_session_origin(实际并不需要)

738     if ((curinsert_flags & XLOG_INCLUDE_ORIGIN) &&
(gdb) p curinsert_flags
$47 = 1 '\001'
(gdb) 
$48 = 1 '\001'
(gdb) n
739         replorigin_session_origin != InvalidRepOriginId)
(gdb) 
738     if ((curinsert_flags & XLOG_INCLUDE_ORIGIN) &&

接下来是main data

(gdb) 
747     if (mainrdata_len > 0)
(gdb) 
749         if (mainrdata_len > 255)
(gdb) 
757             *(scratch++) = (char) XLR_BLOCK_ID_DATA_SHORT;
(gdb) 
(gdb) 
758             *(scratch++) = (uint8) mainrdata_len;
(gdb) 
760         rdt_datas_last->next = mainrdata_head;
(gdb) 
761         rdt_datas_last = mainrdata_last;
(gdb) 
762         total_len += mainrdata_len;
(gdb) 

计算大小

764     rdt_datas_last->next = NULL;
(gdb) 
766     hdr_rdt.len = (scratch - hdr_scratch);
(gdb) p scratch
$49 = 0x18a24ee ""
(gdb) p hdr_scratch
$50 = 0x18a24c0 "r"
(gdb) p hdr_rdt.len
$51 = 26
(gdb) p total_len
$52 = 35
(gdb) 
(gdb) n
767     total_len += hdr_rdt.len;
(gdb) 

计算CRC

(gdb) 
777     INIT_CRC32C(rdata_crc);
(gdb) 
778     COMP_CRC32C(rdata_crc, hdr_scratch + SizeOfXLogRecord, hdr_rdt.len - SizeOfXLogRecord);
(gdb) 
779     for (rdt = hdr_rdt.next; rdt != NULL; rdt = rdt->next)
(gdb) n
780         COMP_CRC32C(rdata_crc, rdt->data, rdt->len);
(gdb) 
779     for (rdt = hdr_rdt.next; rdt != NULL; rdt = rdt->next)
(gdb) 
780         COMP_CRC32C(rdata_crc, rdt->data, rdt->len);
(gdb) 
779     for (rdt = hdr_rdt.next; rdt != NULL; rdt = rdt->next)
(gdb) 
780         COMP_CRC32C(rdata_crc, rdt->data, rdt->len);
(gdb) 
779     for (rdt = hdr_rdt.next; rdt != NULL; rdt = rdt->next)
(gdb) 
787     rechdr->xl_xid = GetCurrentTransactionIdIfAny();

填充记录头部信息的其他域字段.

(gdb) n
788     rechdr->xl_tot_len = total_len;
(gdb) 
789     rechdr->xl_info = info;
(gdb) 
790     rechdr->xl_rmid = rmid;
(gdb) 
791     rechdr->xl_prev = InvalidXLogRecPtr;
(gdb) 
792     rechdr->xl_crc = rdata_crc;
(gdb) 
794     return &hdr_rdt;
(gdb) 
795 }
(gdb) p rechdr
$62 = (XLogRecord *) 0x18a24c0
(gdb) p *rechdr
$63 = {xl_tot_len = 81, xl_xid = 1998, xl_prev = 0, xl_info = 128 '\200', xl_rmid = 10 '\n', xl_crc = 1852971194}
(gdb) 

full-page-write场景后续再行分析

四、参考资料

Write Ahead Logging — WAL
PostgreSQL 源码解读（4）- 插入数据#3（heap_insert）
PostgreSQL 事务日志WAL结构浅析
PG Source Code

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