PostgreSQL如何获取事务号XID

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主要解析了函数AssignTransactionId->GetNewTransactionId的实现逻辑。

在GetNewTransactionId函数中,检查是否可以安全的分配XID.
这可以防止由于XID wraparound而导致的灾难性数据丢失.
基本规则是:
如果超过了xidVacLimit,开始尝试强制autovacuum循环.
如果超过了xidWarnLimit,开始发出警告.
如果超过了xidStopLimit,拒绝执行事务,直至以单用户模式运行.
(这为DBA提供了一个逃生通道，使他们能够通过数据库早期的安全性检测,进入数据库进行维护)

ShmemVariableCache->xidVacLimit -> 200,000,561 -> 大于等于该值,触发自动vacuum
ShmemVariableCache->xidWarnLimit -> 2,136,484,208 -> 大于等于该值,系统报警
ShmemVariableCache->xidStopLimit -> 2,146,484,208 -> 大于等于该值,系统不允许执行事务,使用单用户模式处理

一、数据结构

TransactionState
事务状态结构体

/* *  transaction states - transaction state from server perspective *  事务状态枚举 - 服务器视角的事务状态 */typedef enum TransState{    TRANS_DEFAULT,              /* idle 空闲 */    TRANS_START,                /* transaction starting 事务启动 */    TRANS_INPROGRESS,           /* inside a valid transaction 进行中 */    TRANS_COMMIT,               /* commit in progress 提交中 */    TRANS_ABORT,                /* abort in progress 回滚中 */    TRANS_PREPARE               /* prepare in progress 准备中 */} TransState;/* *  transaction block states - transaction state of client queries *  事务块状态 - 客户端查询的事务状态 * * Note: the subtransaction states are used only for non-topmost * transactions; the others appear only in the topmost transaction. * 注意:subtransaction只用于非顶层事务;其他字段用于顶层事务. */typedef enum TBlockState{    /* not-in-transaction-block states 未进入事务块状态 */    TBLOCK_DEFAULT,             /* idle 空闲  */    TBLOCK_STARTED,             /* running single-query transaction 单个查询事务 */    /* transaction block states 事务块状态 */    TBLOCK_BEGIN,               /* starting transaction block 开始事务块 */    TBLOCK_INPROGRESS,          /* live transaction 进行中 */    TBLOCK_IMPLICIT_INPROGRESS, /* live transaction after implicit BEGIN 隐式事务,进行中 */    TBLOCK_PARALLEL_INPROGRESS, /* live transaction inside parallel worker 并行worker中的事务,进行中 */    TBLOCK_END,                 /* COMMIT received 接收到COMMIT */    TBLOCK_ABORT,               /* failed xact, awaiting ROLLBACK 失败,等待ROLLBACK */    TBLOCK_ABORT_END,           /* failed xact, ROLLBACK received 失败,已接收ROLLBACK */    TBLOCK_ABORT_PENDING,       /* live xact, ROLLBACK received 进行中,接收到ROLLBACK */    TBLOCK_PREPARE,             /* live xact, PREPARE received 进行中,接收到PREPARE */    /* subtransaction states 子事务状态 */    TBLOCK_SUBBEGIN,            /* starting a subtransaction 开启 */    TBLOCK_SUBINPROGRESS,       /* live subtransaction 进行中 */    TBLOCK_SUBRELEASE,          /* RELEASE received 接收到RELEASE */    TBLOCK_SUBCOMMIT,           /* COMMIT received while TBLOCK_SUBINPROGRESS 进行中,接收到COMMIT */    TBLOCK_SUBABORT,            /* failed subxact, awaiting ROLLBACK 失败,等待ROLLBACK */    TBLOCK_SUBABORT_END,        /* failed subxact, ROLLBACK received 失败,已接收ROLLBACK */    TBLOCK_SUBABORT_PENDING,    /* live subxact, ROLLBACK received 进行中,接收到ROLLBACK */    TBLOCK_SUBRESTART,          /* live subxact, ROLLBACK TO received 进行中,接收到ROLLBACK TO */    TBLOCK_SUBABORT_RESTART     /* failed subxact, ROLLBACK TO received 失败,已接收ROLLBACK TO */} TBlockState;/* *  transaction state structure *  事务状态结构体 */typedef struct TransactionStateData{    //事务ID    TransactionId transactionId;    /* my XID, or Invalid if none */    //子事务ID    SubTransactionId subTransactionId;  /* my subxact ID */    //保存点名称    char       *name;           /* savepoint name, if any */    //保存点级别    int         savepointLevel; /* savepoint level */    //低级别的事务状态    TransState  state;          /* low-level state */    //高级别的事务状态    TBlockState blockState;     /* high-level state */    //事务嵌套深度    int         nestingLevel;   /* transaction nesting depth */    //GUC上下文嵌套深度    int         gucNestLevel;   /* GUC context nesting depth */    //事务生命周期上下文    MemoryContext curTransactionContext;    /* my xact-lifetime context */    //查询资源    ResourceOwner curTransactionOwner;  /* my query resources */    //按XID顺序保存的已提交的子事务ID    TransactionId *childXids;   /* subcommitted child XIDs, in XID order */    //childXids数组大小    int         nChildXids;     /* # of subcommitted child XIDs */    //分配的childXids数组空间    int         maxChildXids;   /* allocated size of childXids[] */    //上一个CurrentUserId    Oid         prevUser;       /* previous CurrentUserId setting */    //上一个SecurityRestrictionContext    int         prevSecContext; /* previous SecurityRestrictionContext */    //上一事务是否只读?    bool        prevXactReadOnly;   /* entry-time xact r/o state */    //是否处于Recovery?    bool        startedInRecovery;  /* did we start in recovery? */    //XID是否已保存在WAL Record中?    bool        didLogXid;      /* has xid been included in WAL record? */    //Enter/ExitParallelMode计数器    int         parallelModeLevel;  /* Enter/ExitParallelMode counter */    //父事务状态    struct TransactionStateData *parent;    /* back link to parent */} TransactionStateData;//结构体指针typedef TransactionStateData *TransactionState;

ShmemVariableCache
VariableCache是共享内存中的一种数据结构，用于跟踪OID和XID分配状态。
ShmemVariableCache->共享内存中的指针

/* * VariableCache is a data structure in shared memory that is used to track * OID and XID assignment state.  For largely historical reasons, there is * just one struct with different fields that are protected by different * LWLocks. * VariableCache是共享内存中的一种数据结构，用于跟踪OID和XID分配状态。 * 由于历史原因，这个结构体有不同的字段,由不同的LWLocks保护。 * * Note: xidWrapLimit and oldestXidDB are not "active" values, but are * used just to generate useful messages when xidWarnLimit or xidStopLimit * are exceeded. * 注意:xidWrapLimit和oldestXidDB是不"活跃"的值,在xidWarnLimit或xidStopLimit *   超出限制时用于产生有用的信息. */typedef struct VariableCacheData{    /*     * These fields are protected by OidGenLock.     * 这些域字段通过OidGenLock字段保护     */    //下一个待分配的OID    Oid         nextOid;        /* next OID to assign */    //在必须执行XLOG work前可用OIDs    uint32      oidCount;       /* OIDs available before must do XLOG work */    /*     * These fields are protected by XidGenLock.     * 这些字段通过XidGenLock锁保护.     */    //下一个待分配的事务ID    TransactionId nextXid;      /* next XID to assign */    //集群范围内最小datfrozenxid    TransactionId oldestXid;    /* cluster-wide minimum datfrozenxid */    //在该XID开始强制执行autovacuum    TransactionId xidVacLimit;  /* start forcing autovacuums here */    //在该XID开始提出警告    TransactionId xidWarnLimit; /* start complaining here */    //在该XID开外,拒绝生成下一个XID    TransactionId xidStopLimit; /* refuse to advance nextXid beyond here */    //"世界末日"XID,需回卷    TransactionId xidWrapLimit; /* where the world ends */    //持有最小datfrozenxid的DB    Oid         oldestXidDB;    /* database with minimum datfrozenxid */    /*     * These fields are protected by CommitTsLock     * 这些字段通过CommitTsLock锁保护     */    TransactionId oldestCommitTsXid;    TransactionId newestCommitTsXid;    /*     * These fields are protected by ProcArrayLock.     * 这些字段通过ProcArrayLock锁保护     */    TransactionId latestCompletedXid;   /* newest XID that has committed or                                         * aborted */    /*     * These fields are protected by CLogTruncationLock     * 这些字段通过CLogTruncationLock锁保护     */    //clog中最古老的XID    TransactionId oldestClogXid;    /* oldest it's safe to look up in clog */} VariableCacheData;//结构体指针typedef VariableCacheData *VariableCache;/* pointer to "variable cache" in shared memory (set up by shmem.c) *///共享内存中的指针(通过shmem.c设置)VariableCache ShmemVariableCache = NULL;

二、源码解读

AssignTransactionId函数,给定的TransactionState分配一个新的持久化事务号XID,在此函数调用前,不会为事务分配XIDs.GetNewTransactionId是获取事务ID实际执行的地方,该函数从共享内存变量ShmemVariableCache中获取nextXid,+1后作为新的XID.

/* * Allocate the next XID for a new transaction or subtransaction. * 为新事务或新子事务分配XID * * The new XID is also stored into MyPgXact before returning. * 在返回前,XID会存储在全局变量MyPgXact中 * * Note: when this is called, we are actually already inside a valid * transaction, since XIDs are now not allocated until the transaction * does something.  So it is safe to do a database lookup if we want to * issue a warning about XID wrap. * 注意:在该函数调用时,我们实际上已在一个有效的事务中,因为XIDs在事务不做些事情前不会分配. * 因此，如果我们想发出关于XID wrap回卷的警告，那么进行数据库查找是安全的。 */TransactionIdGetNewTransactionId(bool isSubXact){    TransactionId xid;    /*     * Workers synchronize transaction state at the beginning of each parallel     * operation, so we can't account for new XIDs after that point.     * 在每个并行操作前,Parallel Workers同步事务状态,     *   因此我们不能在这时候请求XIDs     */    if (IsInParallelMode())        elog(ERROR, "cannot assign TransactionIds during a parallel operation");    /*     * During bootstrap initialization, we return the special bootstrap     * transaction id.     * 在宇宙初启时,返回特别的bootstrap事务ID     */    if (IsBootstrapProcessingMode())    {        Assert(!isSubXact);        MyPgXact->xid = BootstrapTransactionId;        return BootstrapTransactionId;//--> 1    }    /* safety check, we should never get this far in a HS standby */    * 安全检查    if (RecoveryInProgress())        elog(ERROR, "cannot assign TransactionIds during recovery");    LWLockAcquire(XidGenLock, LW_EXCLUSIVE);    //从共享内存中获取下一个XID    xid = ShmemVariableCache->nextXid;    /*----------     * Check to see if it's safe to assign another XID.  This protects against     * catastrophic data loss due to XID wraparound.  The basic rules are:     * 检查是否可以安全的分配另外一个XID.     * 这可以防止由于XID wraparound而导致的灾难性数据丢失.     * 基本规则是:     *     * If we're past xidVacLimit, start trying to force autovacuum cycles.     * If we're past xidWarnLimit, start issuing warnings.     * If we're past xidStopLimit, refuse to execute transactions, unless     * we are running in single-user mode (which gives an escape hatch     * to the DBA who somehow got past the earlier defenses).     * 如果超过了xidVacLimit,开始尝试强制autovacuum循环.     * 如果超过了xidWarnLimit,开始发出警告.     * 如果超过了xidStopLimit,拒绝执行事务,直至以单用户模式运行.     * (这为DBA提供了一个逃生通道，使他们能够通过数据库早期的安全性检测)     *     * Note that this coding also appears in GetNewMultiXactId.     * 注意这部分代码在GetNewMultiXactId中也会出现.     *----------     */    //TransactionIdFollowsOrEquals --> is id1 logically >= id2?    if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))    {        //xid >= ShmemVariableCache->xidVacLimit        /*         * For safety's sake, we release XidGenLock while sending signals,         * warnings, etc.  This is not so much because we care about         * preserving concurrency in this situation, as to avoid any         * possibility of deadlock while doing get_database_name(). First,         * copy all the shared values we'll need in this path.         * 为了安全起见，我们在发送信号、警告等时释放XidGenLock。         * 这并不是因为我们关心在这种情况下并发性，         *   而是因为在执行get_database_name()时要避免出现死锁         */        //获取相关XID        TransactionId xidWarnLimit = ShmemVariableCache->xidWarnLimit;        TransactionId xidStopLimit = ShmemVariableCache->xidStopLimit;        TransactionId xidWrapLimit = ShmemVariableCache->xidWrapLimit;        Oid         oldest_datoid = ShmemVariableCache->oldestXidDB;        LWLockRelease(XidGenLock);        /*         * To avoid swamping the postmaster with signals, we issue the autovac         * request only once per 64K transaction starts.  This still gives         * plenty of chances before we get into real trouble.         * 为了避免信号淹没postmaster，我们每64K事务开始时只发出一次autovac请求。         * 在我们陷入真正的麻烦之前，这仍然给了我们很多解决问题的机会。         */        if (IsUnderPostmaster && (xid % 65536) == 0)            //每隔64K发一次            SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);        if (IsUnderPostmaster &&            TransactionIdFollowsOrEquals(xid, xidStopLimit))        {            //xid >= ShmemVariableCache->xidStopLimit            char       *oldest_datname = get_database_name(oldest_datoid);            /* complain even if that DB has disappeared */            //就算DB已消失,也要不停的警告:(            if (oldest_datname)                ereport(ERROR,                        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),                         errmsg("database is not accepting commands to avoid wraparound data loss in database \"%s\"",                                oldest_datname),                         errhint("Stop the postmaster and vacuum that database in single-user mode.\n"                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));            else                ereport(ERROR,                        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),                         errmsg("database is not accepting commands to avoid wraparound data loss in database with OID %u",                                oldest_datoid),                         errhint("Stop the postmaster and vacuum that database in single-user mode.\n"                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));        }        else if (TransactionIdFollowsOrEquals(xid, xidWarnLimit))        {            ////xid >= ShmemVariableCache->xidWarnLimit            char       *oldest_datname = get_database_name(oldest_datoid);            /* complain even if that DB has disappeared */            if (oldest_datname)                ereport(WARNING,                        (errmsg("database \"%s\" must be vacuumed within %u transactions",                                oldest_datname,                                xidWrapLimit - xid),                         errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));            else                ereport(WARNING,                        (errmsg("database with OID %u must be vacuumed within %u transactions",                                oldest_datoid,                                xidWrapLimit - xid),                         errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));        }        /* Re-acquire lock and start over */        //重新获取锁并启动        LWLockAcquire(XidGenLock, LW_EXCLUSIVE);        xid = ShmemVariableCache->nextXid;    }    /*     * If we are allocating the first XID of a new page of the commit log,     * zero out that commit-log page before returning. We must do this while     * holding XidGenLock, else another xact could acquire and commit a later     * XID before we zero the page.  Fortunately, a page of the commit log     * holds 32K or more transactions, so we don't have to do this very often.     * 如果在clog的新page中分配第一个XID,返回前初始化clog page.     * 必须在持有XidGenLock锁时执行这个操作,否则的话,     *   其他事务可能会请求该锁并在初始化page前提交了一个新事务.     * 幸运的是，提交日志的一个页面包含32K或更多的事务，所以我们不需要经常这样做。     *     * Extend pg_subtrans and pg_commit_ts too.     * 同时扩展pg_subtrans和pg_commit_ts     */    ExtendCLOG(xid);    ExtendCommitTs(xid);    ExtendSUBTRANS(xid);    /*     * Now advance the nextXid counter.  This must not happen until after we     * have successfully completed ExtendCLOG() --- if that routine fails, we     * want the next incoming transaction to try it again.  We cannot assign     * more XIDs until there is CLOG space for them.     * 现在可以更新nextXid计数器了.     * 这必须在我们成功完成ExtendCLOG()之后才能执行--如果该例程失败，     *   我们希望下一个进入的事务再次尝试。     * 不能分配更多的xid，除非有空闲的CLOG空间。     */    //ShmemVariableCache->nextXid ++    //if ((ShmemVariableCache->nextXid) < FirstNormalTransactionId)     //   (ShmemVariableCache->nextXid) = FirstNormalTransactionId;     TransactionIdAdvance(ShmemVariableCache->nextXid);    /*     * We must store the new XID into the shared ProcArray before releasing     * XidGenLock.  This ensures that every active XID older than     * latestCompletedXid is present in the ProcArray, which is essential for     * correct OldestXmin tracking; see src/backend/access/transam/README.     * 在释放XidGenLock前,存储新的XID到共享数据结构ProcArray中.     * 这可以确保每一个活动的比latestCompletedXid旧的XID都会出现在ProcArray中,     *   这样可以减少OldestXmin的跟踪,请查看src/backend/access/transam/README.     *      * Note that readers of PGXACT xid fields should be careful to fetch the     * value only once, rather than assume they can read a value multiple     * times and get the same answer each time.  Note we are assuming that     * TransactionId and int fetch/store are atomic.     * 要注意的是读取PGXACT.xid字段时小心只提取一次,     *   而不是假定可以多次读取该值而认为每次返回的值都一样.     * 同时我们假定TransactionId和int 提取/写入是原子的.     *      * The same comments apply to the subxact xid count and overflow fields.     * 对于subxact xid计数器和溢出字段,参见上述注释.     *     * Use of a write barrier prevents dangerous code rearrangement in this     * function; other backends could otherwise e.g. be examining my subxids     * info concurrently, and we don't want them to see an invalid     * intermediate state, such as an incremented nxids before the array entry     * is filled.     * 在该函数中,进入堡垒进行写可以防止危险的事情出现.     * 否则其他后台进程可能会比如同步检查本进程的subxids信息,但我们不希望它们看到无效的中间状态,     *   例如，在数组条目被填充之前增加nxid。     *     * Other processes that read nxids should do so before reading xids     * elements with a pg_read_barrier() in between, so that they can be sure     * not to read an uninitialized array element; see     * src/backend/storage/lmgr/README.barrier.     * 其他读取nxids的进程应在使用pg_read_barrier()函数读取xids条目前执行相关操作,     *   这样可以确保它们不会读取到未经初始化的数组条目,查看src/backend/storage/lmgr/README.barrier说明.     *     * If there's no room to fit a subtransaction XID into PGPROC, set the     * cache-overflowed flag instead.  This forces readers to look in     * pg_subtrans to map subtransaction XIDs up to top-level XIDs. There is a     * race-condition window, in that the new XID will not appear as running     * until its parent link has been placed into pg_subtrans. However, that     * will happen before anyone could possibly have a reason to inquire about     * the status of the XID, so it seems OK.  (Snapshots taken during this     * window *will* include the parent XID, so they will deliver the correct     * answer later on when someone does have a reason to inquire.)     * 如果没有空间将子事务XID放入PGPROC中，则设置cache-overflow标志。     * 这可以强制readers查看pg_subtrans以将子事务xid映射到顶层xid。     * 有一个race-condition窗口，在它的父链接被放入pg_subtrans之前，新的XID不会显示为正在运行.     * 然而，在都有可能有理由查询XID的状态之前，这种情况就会发生，所以看起来是可以的。     * (在这个窗口中拍摄的快照*将*包含父XID，因此当稍后有进程查询时，它们将提供正确的答案。)     */    if (!isSubXact)        MyPgXact->xid = xid;    /* LWLockRelease acts as barrier */    else    {        int         nxids = MyPgXact->nxids;        if (nxids < PGPROC_MAX_CACHED_SUBXIDS)        {            MyProc->subxids.xids[nxids] = xid;            pg_write_barrier();            MyPgXact->nxids = nxids + 1;        }        else            MyPgXact->overflowed = true;    }    //释放锁    LWLockRelease(XidGenLock);    return xid;}

三、跟踪分析

执行txid_current,触发函数调用

14:26:26 (xdb@[local]:5432)testdb=#  begin;BEGIN14:26:50 (xdb@[local]:5432)testdb=#* select txid_current_if_assigned(); txid_current_if_assigned --------------------------(1 row)14:26:55 (xdb@[local]:5432)testdb=#* select txid_current();

启动gdb,设置断点

(gdb) b GetNewTransactionIdBreakpoint 6 at 0x545e80: file varsup.c, line 56.(gdb) cContinuing.Breakpoint 6, GetNewTransactionId (isSubXact=false) at varsup.c:5656      if (IsInParallelMode())(gdb)

查看调用栈

(gdb) bt#0  GetNewTransactionId (isSubXact=false) at varsup.c:56#1  0x0000000000546bd9 in AssignTransactionId (s=0xf9c720 ) at xact.c:557#2  0x000000000054693d in GetTopTransactionId () at xact.c:392#3  0x00000000009fe1f3 in txid_current (fcinfo=0x254c7e0) at txid.c:443#4  0x00000000006cfebd in ExecInterpExpr (state=0x254c6f8, econtext=0x254c3e8, isnull=0x7ffe3d4a31f7)    at execExprInterp.c:654#5  0x00000000006d1ac6 in ExecInterpExprStillValid (state=0x254c6f8, econtext=0x254c3e8, isNull=0x7ffe3d4a31f7)    at execExprInterp.c:1786#6  0x00000000007140dd in ExecEvalExprSwitchContext (state=0x254c6f8, econtext=0x254c3e8, isNull=0x7ffe3d4a31f7)    at ../../../src/include/executor/executor.h:303#7  0x000000000071414b in ExecProject (projInfo=0x254c6f0) at ../../../src/include/executor/executor.h:337#8  0x0000000000714323 in ExecResult (pstate=0x254c2d0) at nodeResult.c:136#9  0x00000000006e4c30 in ExecProcNodeFirst (node=0x254c2d0) at execProcnode.c:445#10 0x00000000006d9974 in ExecProcNode (node=0x254c2d0) at ../../../src/include/executor/executor.h:237#11 0x00000000006dc22d in ExecutePlan (estate=0x254c0b8, planstate=0x254c2d0, use_parallel_mode=false,     operation=CMD_SELECT, sendTuples=true, numberTuples=0, direction=ForwardScanDirection, dest=0x24ccf10,     execute_once=true) at execMain.c:1723#12 0x00000000006d9f5c in standard_ExecutorRun (queryDesc=0x256b8e8, direction=ForwardScanDirection, count=0,     execute_once=true) at execMain.c:364#13 0x00000000006d9d7f in ExecutorRun (queryDesc=0x256b8e8, direction=ForwardScanDirection, count=0, execute_once=true)    at execMain.c:307#14 0x00000000008ccf5a in PortalRunSelect (portal=0x250c860, forward=true, count=0, dest=0x24ccf10) at pquery.c:932#15 0x00000000008ccbf3 in PortalRun (portal=0x250c860, count=9223372036854775807, isTopLevel=true, run_once=true,     dest=0x24ccf10, altdest=0x24ccf10, completionTag=0x7ffe3d4a3570 "") at pquery.c:773#16 0x00000000008c6b1e in exec_simple_query (query_string=0x24a6ec8 "select txid_current();") at postgres.c:1145#17 0x00000000008cae70 in PostgresMain (argc=1, argv=0x24d2dc8, dbname=0x24d2c30 "testdb", username=0x24a3ba8 "xdb")    at postgres.c:4182---Type  to continue, or q  to quit---#18 0x000000000082642b in BackendRun (port=0x24c8c00) at postmaster.c:4361#19 0x0000000000825b8f in BackendStartup (port=0x24c8c00) at postmaster.c:4033#20 0x0000000000821f1c in ServerLoop () at postmaster.c:1706#21 0x00000000008217b4 in PostmasterMain (argc=1, argv=0x24a1b60) at postmaster.c:1379#22 0x00000000007488ef in main (argc=1, argv=0x24a1b60) at main.c:228(gdb)

获取XidGenLock锁

(gdb) n63      if (IsBootstrapProcessingMode())(gdb) 71      if (RecoveryInProgress())(gdb) 74      LWLockAcquire(XidGenLock, LW_EXCLUSIVE);(gdb)

获取共享内存变量ShmemVariableCache->nextXid -> 2409

(gdb) 76      xid = ShmemVariableCache->nextXid;(gdb) 91      if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))(gdb) p *ShmemVariableCache$16 = {nextOid = 42628, oidCount = 8191, nextXid = 2409, oldestXid = 561, xidVacLimit = 200000561,   xidWarnLimit = 2136484208, xidStopLimit = 2146484208, xidWrapLimit = 2147484208, oldestXidDB = 16400,   oldestCommitTsXid = 0, newestCommitTsXid = 0, latestCompletedXid = 2408, oldestClogXid = 561}(gdb)

扩展clog

(gdb) n171     ExtendCLOG(xid);(gdb) 172     ExtendCommitTs(xid);(gdb) 173     ExtendSUBTRANS(xid);(gdb) 181     TransactionIdAdvance(ShmemVariableCache->nextXid);(gdb)

ShmemVariableCache->nextXid++ -> 2410

(gdb) p ShmemVariableCache->nextXid$17 = 2410

获取进程和事务信息

(gdb) n223         volatile PGXACT *mypgxact = MyPgXact;(gdb) 225         if (!isSubXact)(gdb) 226             mypgxact->xid = xid;(gdb)

释放锁XidGenLock

(gdb) 241     LWLockRelease(XidGenLock);(gdb) p *ShmemVariableCache$18 = {nextOid = 42628, oidCount = 8191, nextXid = 2410, oldestXid = 561, xidVacLimit = 200000561,   xidWarnLimit = 2136484208, xidStopLimit = 2146484208, xidWrapLimit = 2147484208, oldestXidDB = 16400,   oldestCommitTsXid = 0, newestCommitTsXid = 0, latestCompletedXid = 2408, oldestClogXid = 561}

返回xid(2409),完成调用.

(gdb) n243     return xid;(gdb) 244 }(gdb) AssignTransactionId (s=0xf9c720 ) at xact.c:558558     if (!isSubXact)(gdb)

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