文章摘要
这篇文章主要讨论了一个事务ID管理的函数SetTransactionIdLimit，用于计算事务ID的限制值（如xidWrapLimit、xidStopLimit、xidWarnLimit和xidVacLimit）。这些限制值用于确保事务ID在正常运行中不会溢出，从而避免数据丢失或系统崩溃。代码还涉及事务ID的同步、错误报告以及警告信息的输出，特别是在事务ID超过阈值时，会触发VACUUM操作以清理旧事务ID。此外，文章还提到了如何判断事务ID是否超过设定限制（TransactionIdFollowsOrEquals函数），以及生成新的事务ID（GetNewTransactionId函数）。总体来看，文章聚焦于事务ID的管理机制及其错误处理策略。
void
SetTransactionIdLimit(TransactionId oldest_datfrozenxid, Oid oldest_datoid)
{
TransactionId xidVacLimit;
TransactionId xidWarnLimit;
TransactionId xidStopLimit;
TransactionId xidWrapLimit;
TransactionId curXid;
Assert(TransactionIdIsNormal(oldest_datfrozenxid));
xidWrapLimit=oldest_datfrozenxid + (MaxTransactionId >> 1);
if (xidWrapLimit < FirstNormalTransactionId)
xidWrapLimit +=FirstNormalTransactionId;
xidStopLimit=xidWrapLimit – 1000000;
if (xidStopLimit < FirstNormalTransactionId)
xidStopLimit -=FirstNormalTransactionId;
xidWarnLimit=xidStopLimit – 10000000;
if (xidWarnLimit < FirstNormalTransactionId)
xidWarnLimit -=FirstNormalTransactionId;
xidVacLimit=oldest_datfrozenxid + autovacuum_freeze_max_age;
if (xidVacLimit < FirstNormalTransactionId)
xidVacLimit +=FirstNormalTransactionId;
LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
ShmemVariableCache->oldestXid=oldest_datfrozenxid;
ShmemVariableCache->xidVacLimit=xidVacLimit;
ShmemVariableCache->xidWarnLimit=xidWarnLimit;
ShmemVariableCache->xidStopLimit=xidStopLimit;
ShmemVariableCache->xidWrapLimit=xidWrapLimit;
ShmemVariableCache->oldestXidDB=oldest_datoid;
curXid=XidFromFullTransactionId(ShmemVariableCache->nextFullXid);
LWLockRelease(XidGenLock);
ereport(DEBUG1,
(errmsg(“transaction ID wrap limit is %u, limited by database with OID %u”,
xidWrapLimit, oldest_datoid)));
if (TransactionIdFollowsOrEquals(curXid, xidVacLimit) &&
IsUnderPostmaster && !InRecovery)
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
if (TransactionIdFollowsOrEquals(curXid, xidWarnLimit) && !InRecovery)
{
char *oldest_datname;
if (IsTransactionState())
oldest_datname=get_database_name(oldest_datoid);
else
oldest_datname=NULL;
if (oldest_datname)
ereport(WARNING,
(errmsg(“database “%s” must be vacuumed within %u transactions”,
oldest_datname,
xidWrapLimit – curXid),
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 – curXid),
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.”)));
}
}
bool
TransactionIdFollowsOrEquals(TransactionId id1, TransactionId id2)
{
int32 diff;
if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
return (id1 >=id2);
diff=(int32) (id1 – id2);
return (diff >=0);
}
FullTransactionId
GetNewTransactionId(bool isSubXact)
{
full_xid=ShmemVariableCache->nextFullXid;
xid=XidFromFullTransactionId(full_xid);
if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))
{
TransactionId xidWarnLimit=ShmemVariableCache->xidWarnLimit;
TransactionId xidStopLimit=ShmemVariableCache->xidStopLimit;
TransactionId xidWrapLimit=ShmemVariableCache->xidWrapLimit;
Oid oldest_datoid=ShmemVariableCache->oldestXidDB;
if (IsUnderPostmaster &&
TransactionIdFollowsOrEquals(xid, xidStopLimit))
{
char *oldest_datname=get_database_name(oldest_datoid);
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.”)));
}
}
}