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：
代码语言：javascript复制-- 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 | t
virtualxid | | | | | 2/8 | | | | | 2/8 | 7444 | ExclusiveLock | t | t
virtualxid | | | | | 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 | t
virtualxid | | | | | 1/1 | | | | | 1/0 | 5833 | ExclusiveLock | t | t
relation | 13212 | 102189 | | | | | | | | 2/8 | 7444 | AccessShareLock | t | f
relation | 13212 | 102189 | | | | | | | | 1/0 | 5833 | AccessExclusiveLock | f | fstartup等待堆栈
代码语言：javascript复制StartupProcessMain
StartupXLOG
standby_redo
StandbyAcquireAccessExclusiveLock
LockAcquire
LockAcquireExtended
WaitOnLock
ProcSleep
ResolveRecoveryConflictWithLock
ProcWaitForSignalstartup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
代码语言：javascript复制ProcSleep
ResolveRecoveryConflictWithLock
if (GetCurrentTimestamp() >= ltime && ltime != 0)
VirtualTransactionId *backends;
backends = GetLockConflicts(&locktag, AccessExclusiveLock);
ResolveRecoveryConflictWithVirtualXIDs(backends, PROCSIG_RECOVERY_CONFLICT_LOCK, false)VirtualTransactionId是什么？
代码语言：javascript复制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？
代码语言：javascript复制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 ' 00', locktag_lockmethodid = 1 ' 01'}
(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
代码语言：javascript复制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
代码语言：javascript复制 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
代码语言：javascript复制(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
代码语言：javascript复制static void
ResolveRecoveryConflictWithVirtualXIDs(
VirtualTransactionId *waitlist, // 冲突的vxid
ProcSignalReason reason, // PROCSIG_RECOVERY_CONFLICT_LOCK
bool report_waiting)
{
...
/* wait until the virtual xid is gone */
while (!VirtualXactLock(*waitlist, false))
...
/* Is it time to kill it? */
if (WaitExceedsMaxStandbyDelay())
...
CancelVirtualTransaction
SignalVirtualTransaction
for (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);
