PostgreSQL数据库事务系统——获取事务号

左手的ㄟ右手 2022-09-11 11:24 22阅读 0赞

本节介绍了PostgreSQL获取事务号XID的逻辑,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。 */
    TransactionId
    GetNewTransactionId(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;
    BEGIN
    14:26:50 (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 GetNewTransactionId
    Breakpoint 6 at 0x545e80: file varsup.c, line 56.
    (gdb) c
    Continuing.
    Breakpoint 6, GetNewTransactionId (isSubXact=false) at varsup.c:56
    56      if (IsInParallelMode())
    (gdb)

查看调用栈

(gdb) bt
    #0 GetNewTransactionId (isSubXact=false) at varsup.c:56
    #1 0x0000000000546bd9 in AssignTransactionId (s=0xf9c720 <TopTransactionStateData>) 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 <return> to continue, or q <return> 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) n
    63      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) n
    171     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) n
    223         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) n
    243     return xid;
    (gdb) 
    244 }
    (gdb) 
    AssignTransactionId (s=0xf9c720 <TopTransactionStateData>) at xact.c:558
    558     if (!isSubXact)
    (gdb)