复制冲突发生

备库起事务在读，主库truncate表，备库复制冲突发生：

-- 发生前， 5833是startup，7444是持锁的后台进程。后台进程拿表的读锁和虚拟事务ID的ex锁。
postgres=# select * from pg_locks where pid != pg_backend_pid();locktype  | database | relation | page | tuple | virtualxid | transactionid | classid | objid | objsubid | virtualtransaction | pid  |      mode       | granted | fastpath 
------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+------+-----------------+---------+----------relation   |    13212 |   102189 |      |       |            |               |         |       |          | 2/8                | 7444 | AccessShareLock | t       | tvirtualxid |          |          |      |       | 2/8        |               |         |       |          | 2/8                | 7444 | ExclusiveLock   | t       | tvirtualxid |          |          |      |       | 1/1        |               |         |       |          | 1/0                | 5833 | ExclusiveLock   | t       | t-- 发生后，stantup进程需要拿8锁AccessExclusiveLock，拿不到开始等锁。
postgres=# select * from pg_locks where pid != pg_backend_pid();locktype  | database | relation | page | tuple | virtualxid | transactionid | classid | objid | objsubid | virtualtransaction | pid  |        mode         | granted | fastpath 
------------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+------+---------------------+---------+----------virtualxid |          |          |      |       | 2/8        |               |         |       |          | 2/8                | 7444 | ExclusiveLock       | t       | tvirtualxid |          |          |      |       | 1/1        |               |         |       |          | 1/0                | 5833 | ExclusiveLock       | t       | trelation   |    13212 |   102189 |      |       |            |               |         |       |          | 2/8                | 7444 | AccessShareLock     | t       | frelation   |    13212 |   102189 |      |       |            |               |         |       |          | 1/0                | 5833 | AccessExclusiveLock | f       | f

startup等待堆栈

StartupProcessMainStartupXLOGstandby_redoStandbyAcquireAccessExclusiveLockLockAcquireLockAcquireExtendedWaitOnLockProcSleepResolveRecoveryConflictWithLockProcWaitForSignal

startup超时唤醒堆栈

等待时间超过max_standby_streaming_delay后，跳出ProcWaitForSignal重新进入ResolveRecoveryConflictWithLock，走第一个分支

ProcSleepResolveRecoveryConflictWithLockif (GetCurrentTimestamp() >= ltime && ltime != 0)VirtualTransactionId *backends;backends = GetLockConflicts(&locktag, AccessExclusiveLock);ResolveRecoveryConflictWithVirtualXIDs(backends, PROCSIG_RECOVERY_CONFLICT_LOCK, false)

VirtualTransactionId是什么？

typedef struct
{BackendId   backendId;                  /* backendId from PGPROC */LocalTransactionId localTransactionId;  /* lxid from PGPROC */
} VirtualTransactionId;

backendid和当前会话私有的一个事务ID（类似事务ID自增，从0开始，当前会话结束后清空）

GetLockConflicts怎么拿到和传入locktag冲突的vxid？

VirtualTransactionId *
GetLockConflicts(const LOCKTAG *locktag, LOCKMODE lockmode)
{ ... }(gdb) p *locktag //其中 13212 是库oid  102189 是表oid
$5 = {locktag_field1 = 13212, locktag_field2 = 102189, locktag_field3 = 0, locktag_field4 = 0, locktag_type = 0 '\000', locktag_lockmethodid = 1 '\001'}(gdb) p lockmode
$6 = 8#define NoLock					         0
#define AccessShareLock			     1	/* SELECT */
#define RowShareLock			       2	/* SELECT FOR UPDATE/FOR SHARE */
#define RowExclusiveLock		     3	/* INSERT, UPDATE, DELETE */
#define ShareUpdateExclusiveLock 4	/* VACUUM (non-FULL),ANALYZE, CREATE INDEX * CONCURRENTLY */
#define ShareLock				         5	/* CREATE INDEX (WITHOUT CONCURRENTLY) */
#define ShareRowExclusiveLock	   6	/* like EXCLUSIVE MODE, but allows ROW * SHARE */
#define ExclusiveLock		         7	/* blocks ROW SHARE/SELECT...FOR UPDATE */
#define AccessExclusiveLock		   8	/* ALTER TABLE, DROP TABLE, VACUUM FULL,* and unqualified LOCK TABLE */

fastpath

if (ConflictsWithRelationFastPath(locktag, lockmode))for (i = 0; i < ProcGlobal->allProcCount; i++).../* 排除当前PROC、排除非目标库、排除非目标表 */.../* 16个FASTPATH槽位 */for (f = 0; f < FP_LOCK_SLOTS_PER_BACKEND; f++)/* proc->fpRelId中可以记录16个表OID，遍历一遍是否命中 */if (relid != proc->fpRelId[f])continue;/* 冲突发生了，从proc中拿到vxid信息 */GET_VXID_FROM_PGPROC(vxid, *proc);if (VirtualTransactionIdIsValid(vxid))vxids[count++] = vxid;break;

遍历常规锁

	while (proclock){if (conflictMask & proclock->holdMask){PGPROC	   *proc = proclock->tag.myProc;/* A backend never blocks itself */if (proc != MyProc){VirtualTransactionId vxid;GET_VXID_FROM_PGPROC(vxid, *proc);if (VirtualTransactionIdIsValid(vxid)){int			i;/* Avoid duplicate entries. */for (i = 0; i < fast_count; ++i)if (VirtualTransactionIdEquals(vxids[i], vxid))break;if (i >= fast_count)vxids[count++] = vxid;}/* else, xact already committed or aborted */}}proclock = (PROCLOCK *) SHMQueueNext(procLocks, &proclock->lockLink,offsetof(PROCLOCK, lockLink));}

记录结果

(gdb) p * vxids
$5 = {backendId = 2, localTransactionId = 17}

拿到txid后如何处理？

超时后会 kill冲突的backend，注意 kill的信号是sig_usr1，kill的pid是持锁的那个backend

static void
ResolveRecoveryConflictWithVirtualXIDs(VirtualTransactionId *waitlist,    // 冲突的vxidProcSignalReason reason,           // PROCSIG_RECOVERY_CONFLICT_LOCKbool report_waiting)               
{ .../* wait until the virtual xid is gone */while (!VirtualXactLock(*waitlist, false)).../* Is it time to kill it? */if (WaitExceedsMaxStandbyDelay())...CancelVirtualTransactionSignalVirtualTransactionfor (index = 0; index < arrayP->numProcs; index++)if (procvxid.backendId == vxid.backendId && procvxid.localTransactionId == vxid.localTransactionId)SendProcSignal...slot = &ProcSignalSlots[backendId - 1];slot->pss_signalFlags[reason] = true;...kill(pid, SIGUSR1);