679 lines
24 KiB
C
679 lines
24 KiB
C
/*-------------------------------------------------------------------------
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*
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* varsup.c
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* postgres OID & XID variables support routines
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*
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* Copyright (c) 2000-2022, PostgreSQL Global Development Group
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*
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* IDENTIFICATION
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* src/backend/access/transam/varsup.c
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*
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*-------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include "access/clog.h"
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#include "access/commit_ts.h"
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#include "access/subtrans.h"
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#include "access/transam.h"
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#include "access/xact.h"
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#include "access/xlogutils.h"
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#include "commands/dbcommands.h"
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#include "miscadmin.h"
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#include "postmaster/autovacuum.h"
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#include "storage/pmsignal.h"
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#include "storage/proc.h"
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#include "utils/syscache.h"
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/* Number of OIDs to prefetch (preallocate) per XLOG write */
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#define VAR_OID_PREFETCH 8192
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/* pointer to "variable cache" in shared memory (set up by shmem.c) */
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VariableCache ShmemVariableCache = NULL;
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/*
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* Allocate the next FullTransactionId for a new transaction or
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* subtransaction.
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*
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* The new XID is also stored into MyProc->xid/ProcGlobal->xids[] before
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* returning.
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*
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* Note: when this is called, we are actually already inside a valid
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* transaction, since XIDs are now not allocated until the transaction
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* does something. So it is safe to do a database lookup if we want to
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* issue a warning about XID wrap.
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*/
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FullTransactionId
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GetNewTransactionId(bool isSubXact)
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{
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FullTransactionId full_xid;
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TransactionId xid;
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/*
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* Workers synchronize transaction state at the beginning of each parallel
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* operation, so we can't account for new XIDs after that point.
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*/
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if (IsInParallelMode())
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elog(ERROR, "cannot assign TransactionIds during a parallel operation");
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/*
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* During bootstrap initialization, we return the special bootstrap
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* transaction id.
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*/
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if (IsBootstrapProcessingMode())
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{
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Assert(!isSubXact);
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MyProc->xid = BootstrapTransactionId;
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ProcGlobal->xids[MyProc->pgxactoff] = BootstrapTransactionId;
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return FullTransactionIdFromEpochAndXid(0, BootstrapTransactionId);
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}
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/* safety check, we should never get this far in a HS standby */
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if (RecoveryInProgress())
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elog(ERROR, "cannot assign TransactionIds during recovery");
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LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
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full_xid = ShmemVariableCache->nextXid;
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xid = XidFromFullTransactionId(full_xid);
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/*----------
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* Check to see if it's safe to assign another XID. This protects against
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* catastrophic data loss due to XID wraparound. The basic rules are:
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*
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* If we're past xidVacLimit, start trying to force autovacuum cycles.
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* If we're past xidWarnLimit, start issuing warnings.
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* If we're past xidStopLimit, refuse to execute transactions, unless
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* we are running in single-user mode (which gives an escape hatch
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* to the DBA who somehow got past the earlier defenses).
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*
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* Note that this coding also appears in GetNewMultiXactId.
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*----------
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*/
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if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))
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{
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/*
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* For safety's sake, we release XidGenLock while sending signals,
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* warnings, etc. This is not so much because we care about
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* preserving concurrency in this situation, as to avoid any
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* possibility of deadlock while doing get_database_name(). First,
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* copy all the shared values we'll need in this path.
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*/
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TransactionId xidWarnLimit = ShmemVariableCache->xidWarnLimit;
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TransactionId xidStopLimit = ShmemVariableCache->xidStopLimit;
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TransactionId xidWrapLimit = ShmemVariableCache->xidWrapLimit;
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Oid oldest_datoid = ShmemVariableCache->oldestXidDB;
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LWLockRelease(XidGenLock);
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/*
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* To avoid swamping the postmaster with signals, we issue the autovac
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* request only once per 64K transaction starts. This still gives
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* plenty of chances before we get into real trouble.
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*/
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if (IsUnderPostmaster && (xid % 65536) == 0)
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SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
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if (IsUnderPostmaster &&
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TransactionIdFollowsOrEquals(xid, xidStopLimit))
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{
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char *oldest_datname = get_database_name(oldest_datoid);
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/* complain even if that DB has disappeared */
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if (oldest_datname)
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ereport(ERROR,
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(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
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errmsg("database is not accepting commands to avoid wraparound data loss in database \"%s\"",
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oldest_datname),
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errhint("Stop the postmaster and vacuum that database in single-user mode.\n"
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"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
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else
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ereport(ERROR,
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(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
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errmsg("database is not accepting commands to avoid wraparound data loss in database with OID %u",
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oldest_datoid),
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errhint("Stop the postmaster and vacuum that database in single-user mode.\n"
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"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
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}
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else if (TransactionIdFollowsOrEquals(xid, xidWarnLimit))
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{
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char *oldest_datname = get_database_name(oldest_datoid);
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/* complain even if that DB has disappeared */
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if (oldest_datname)
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ereport(WARNING,
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(errmsg("database \"%s\" must be vacuumed within %u transactions",
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oldest_datname,
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xidWrapLimit - xid),
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errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
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"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
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else
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ereport(WARNING,
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(errmsg("database with OID %u must be vacuumed within %u transactions",
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oldest_datoid,
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xidWrapLimit - xid),
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errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
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"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
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}
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/* Re-acquire lock and start over */
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LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
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full_xid = ShmemVariableCache->nextXid;
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xid = XidFromFullTransactionId(full_xid);
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}
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/*
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* If we are allocating the first XID of a new page of the commit log,
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* zero out that commit-log page before returning. We must do this while
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* holding XidGenLock, else another xact could acquire and commit a later
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* XID before we zero the page. Fortunately, a page of the commit log
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* holds 32K or more transactions, so we don't have to do this very often.
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*
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* Extend pg_subtrans and pg_commit_ts too.
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*/
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ExtendCLOG(xid);
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ExtendCommitTs(xid);
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ExtendSUBTRANS(xid);
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/*
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* Now advance the nextXid counter. This must not happen until after we
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* have successfully completed ExtendCLOG() --- if that routine fails, we
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* want the next incoming transaction to try it again. We cannot assign
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* more XIDs until there is CLOG space for them.
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*/
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FullTransactionIdAdvance(&ShmemVariableCache->nextXid);
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/*
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* We must store the new XID into the shared ProcArray before releasing
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* XidGenLock. This ensures that every active XID older than
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* latestCompletedXid is present in the ProcArray, which is essential for
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* correct OldestXmin tracking; see src/backend/access/transam/README.
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*
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* Note that readers of ProcGlobal->xids/PGPROC->xid should be careful to
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* fetch the value for each proc only once, rather than assume they can
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* read a value multiple times and get the same answer each time. Note we
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* are assuming that TransactionId and int fetch/store are atomic.
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*
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* The same comments apply to the subxact xid count and overflow fields.
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*
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* Use of a write barrier prevents dangerous code rearrangement in this
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* function; other backends could otherwise e.g. be examining my subxids
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* info concurrently, and we don't want them to see an invalid
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* intermediate state, such as an incremented nxids before the array entry
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* is filled.
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*
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* Other processes that read nxids should do so before reading xids
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* elements with a pg_read_barrier() in between, so that they can be sure
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* not to read an uninitialized array element; see
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* src/backend/storage/lmgr/README.barrier.
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*
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* If there's no room to fit a subtransaction XID into PGPROC, set the
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* cache-overflowed flag instead. This forces readers to look in
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* pg_subtrans to map subtransaction XIDs up to top-level XIDs. There is a
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* race-condition window, in that the new XID will not appear as running
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* until its parent link has been placed into pg_subtrans. However, that
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* will happen before anyone could possibly have a reason to inquire about
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* the status of the XID, so it seems OK. (Snapshots taken during this
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* window *will* include the parent XID, so they will deliver the correct
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* answer later on when someone does have a reason to inquire.)
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*/
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if (!isSubXact)
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{
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Assert(ProcGlobal->subxidStates[MyProc->pgxactoff].count == 0);
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Assert(!ProcGlobal->subxidStates[MyProc->pgxactoff].overflowed);
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Assert(MyProc->subxidStatus.count == 0);
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Assert(!MyProc->subxidStatus.overflowed);
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/* LWLockRelease acts as barrier */
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MyProc->xid = xid;
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ProcGlobal->xids[MyProc->pgxactoff] = xid;
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}
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else
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{
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XidCacheStatus *substat = &ProcGlobal->subxidStates[MyProc->pgxactoff];
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int nxids = MyProc->subxidStatus.count;
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Assert(substat->count == MyProc->subxidStatus.count);
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Assert(substat->overflowed == MyProc->subxidStatus.overflowed);
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if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
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{
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MyProc->subxids.xids[nxids] = xid;
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pg_write_barrier();
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MyProc->subxidStatus.count = substat->count = nxids + 1;
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}
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else
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MyProc->subxidStatus.overflowed = substat->overflowed = true;
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}
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LWLockRelease(XidGenLock);
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return full_xid;
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}
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/*
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* Read nextXid but don't allocate it.
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*/
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FullTransactionId
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ReadNextFullTransactionId(void)
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{
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FullTransactionId fullXid;
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LWLockAcquire(XidGenLock, LW_SHARED);
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fullXid = ShmemVariableCache->nextXid;
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LWLockRelease(XidGenLock);
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return fullXid;
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}
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/*
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* Advance nextXid to the value after a given xid. The epoch is inferred.
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* This must only be called during recovery or from two-phase start-up code.
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*/
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void
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AdvanceNextFullTransactionIdPastXid(TransactionId xid)
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{
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FullTransactionId newNextFullXid;
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TransactionId next_xid;
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uint32 epoch;
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/*
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* It is safe to read nextXid without a lock, because this is only called
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* from the startup process or single-process mode, meaning that no other
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* process can modify it.
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*/
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Assert(AmStartupProcess() || !IsUnderPostmaster);
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/* Fast return if this isn't an xid high enough to move the needle. */
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next_xid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
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if (!TransactionIdFollowsOrEquals(xid, next_xid))
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return;
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/*
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* Compute the FullTransactionId that comes after the given xid. To do
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* this, we preserve the existing epoch, but detect when we've wrapped
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* into a new epoch. This is necessary because WAL records and 2PC state
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* currently contain 32 bit xids. The wrap logic is safe in those cases
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* because the span of active xids cannot exceed one epoch at any given
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* point in the WAL stream.
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*/
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TransactionIdAdvance(xid);
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epoch = EpochFromFullTransactionId(ShmemVariableCache->nextXid);
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if (unlikely(xid < next_xid))
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++epoch;
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newNextFullXid = FullTransactionIdFromEpochAndXid(epoch, xid);
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/*
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* We still need to take a lock to modify the value when there are
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* concurrent readers.
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*/
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LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
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ShmemVariableCache->nextXid = newNextFullXid;
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LWLockRelease(XidGenLock);
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}
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/*
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* Advance the cluster-wide value for the oldest valid clog entry.
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*
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* We must acquire XactTruncationLock to advance the oldestClogXid. It's not
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* necessary to hold the lock during the actual clog truncation, only when we
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* advance the limit, as code looking up arbitrary xids is required to hold
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* XactTruncationLock from when it tests oldestClogXid through to when it
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* completes the clog lookup.
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*/
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void
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AdvanceOldestClogXid(TransactionId oldest_datfrozenxid)
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{
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LWLockAcquire(XactTruncationLock, LW_EXCLUSIVE);
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if (TransactionIdPrecedes(ShmemVariableCache->oldestClogXid,
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oldest_datfrozenxid))
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{
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ShmemVariableCache->oldestClogXid = oldest_datfrozenxid;
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}
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LWLockRelease(XactTruncationLock);
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}
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/*
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* Determine the last safe XID to allocate using the currently oldest
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* datfrozenxid (ie, the oldest XID that might exist in any database
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* of our cluster), and the OID of the (or a) database with that value.
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*/
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void
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SetTransactionIdLimit(TransactionId oldest_datfrozenxid, Oid oldest_datoid)
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{
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TransactionId xidVacLimit;
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TransactionId xidWarnLimit;
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TransactionId xidStopLimit;
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TransactionId xidWrapLimit;
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TransactionId curXid;
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Assert(TransactionIdIsNormal(oldest_datfrozenxid));
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/*
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* The place where we actually get into deep trouble is halfway around
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* from the oldest potentially-existing XID. (This calculation is
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* probably off by one or two counts, because the special XIDs reduce the
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* size of the loop a little bit. But we throw in plenty of slop below,
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* so it doesn't matter.)
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*/
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xidWrapLimit = oldest_datfrozenxid + (MaxTransactionId >> 1);
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if (xidWrapLimit < FirstNormalTransactionId)
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xidWrapLimit += FirstNormalTransactionId;
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/*
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* We'll refuse to continue assigning XIDs in interactive mode once we get
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* within 3M transactions of data loss. This leaves lots of room for the
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* DBA to fool around fixing things in a standalone backend, while not
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* being significant compared to total XID space. (VACUUM requires an XID
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* if it truncates at wal_level!=minimal. "VACUUM (ANALYZE)", which a DBA
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* might do by reflex, assigns an XID. Hence, we had better be sure
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* there's lots of XIDs left...) Also, at default BLCKSZ, this leaves two
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* completely-idle segments. In the event of edge-case bugs involving
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* page or segment arithmetic, idle segments render the bugs unreachable
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* outside of single-user mode.
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*/
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xidStopLimit = xidWrapLimit - 3000000;
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if (xidStopLimit < FirstNormalTransactionId)
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xidStopLimit -= FirstNormalTransactionId;
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/*
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* We'll start complaining loudly when we get within 40M transactions of
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* data loss. This is kind of arbitrary, but if you let your gas gauge
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* get down to 2% of full, would you be looking for the next gas station?
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* We need to be fairly liberal about this number because there are lots
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* of scenarios where most transactions are done by automatic clients that
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* won't pay attention to warnings. (No, we're not gonna make this
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* configurable. If you know enough to configure it, you know enough to
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* not get in this kind of trouble in the first place.)
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*/
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xidWarnLimit = xidWrapLimit - 40000000;
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if (xidWarnLimit < FirstNormalTransactionId)
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xidWarnLimit -= FirstNormalTransactionId;
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/*
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* We'll start trying to force autovacuums when oldest_datfrozenxid gets
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* to be more than autovacuum_freeze_max_age transactions old.
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*
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* Note: guc.c ensures that autovacuum_freeze_max_age is in a sane range,
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* so that xidVacLimit will be well before xidWarnLimit.
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*
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* Note: autovacuum_freeze_max_age is a PGC_POSTMASTER parameter so that
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* we don't have to worry about dealing with on-the-fly changes in its
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* value. It doesn't look practical to update shared state from a GUC
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* assign hook (too many processes would try to execute the hook,
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* resulting in race conditions as well as crashes of those not connected
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* to shared memory). Perhaps this can be improved someday. See also
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* SetMultiXactIdLimit.
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*/
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xidVacLimit = oldest_datfrozenxid + autovacuum_freeze_max_age;
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if (xidVacLimit < FirstNormalTransactionId)
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xidVacLimit += FirstNormalTransactionId;
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/* Grab lock for just long enough to set the new limit values */
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LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
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ShmemVariableCache->oldestXid = oldest_datfrozenxid;
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ShmemVariableCache->xidVacLimit = xidVacLimit;
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ShmemVariableCache->xidWarnLimit = xidWarnLimit;
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ShmemVariableCache->xidStopLimit = xidStopLimit;
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ShmemVariableCache->xidWrapLimit = xidWrapLimit;
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ShmemVariableCache->oldestXidDB = oldest_datoid;
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curXid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
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LWLockRelease(XidGenLock);
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/* Log the info */
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ereport(DEBUG1,
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(errmsg_internal("transaction ID wrap limit is %u, limited by database with OID %u",
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xidWrapLimit, oldest_datoid)));
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/*
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* If past the autovacuum force point, immediately signal an autovac
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* request. The reason for this is that autovac only processes one
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* database per invocation. Once it's finished cleaning up the oldest
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* database, it'll call here, and we'll signal the postmaster to start
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* another iteration immediately if there are still any old databases.
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*/
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if (TransactionIdFollowsOrEquals(curXid, xidVacLimit) &&
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IsUnderPostmaster && !InRecovery)
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SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
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/* Give an immediate warning if past the wrap warn point */
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if (TransactionIdFollowsOrEquals(curXid, xidWarnLimit) && !InRecovery)
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{
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char *oldest_datname;
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/*
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* We can be called when not inside a transaction, for example during
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* StartupXLOG(). In such a case we cannot do database access, so we
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* must just report the oldest DB's OID.
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*
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* Note: it's also possible that get_database_name fails and returns
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* NULL, for example because the database just got dropped. We'll
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* still warn, even though the warning might now be unnecessary.
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*/
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if (IsTransactionState())
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oldest_datname = get_database_name(oldest_datoid);
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else
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oldest_datname = NULL;
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if (oldest_datname)
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ereport(WARNING,
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(errmsg("database \"%s\" must be vacuumed within %u transactions",
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oldest_datname,
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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.")));
|
|
}
|
|
}
|
|
|
|
|
|
/*
|
|
* ForceTransactionIdLimitUpdate -- does the XID wrap-limit data need updating?
|
|
*
|
|
* We primarily check whether oldestXidDB is valid. The cases we have in
|
|
* mind are that that database was dropped, or the field was reset to zero
|
|
* by pg_resetwal. In either case we should force recalculation of the
|
|
* wrap limit. Also do it if oldestXid is old enough to be forcing
|
|
* autovacuums or other actions; this ensures we update our state as soon
|
|
* as possible once extra overhead is being incurred.
|
|
*/
|
|
bool
|
|
ForceTransactionIdLimitUpdate(void)
|
|
{
|
|
TransactionId nextXid;
|
|
TransactionId xidVacLimit;
|
|
TransactionId oldestXid;
|
|
Oid oldestXidDB;
|
|
|
|
/* Locking is probably not really necessary, but let's be careful */
|
|
LWLockAcquire(XidGenLock, LW_SHARED);
|
|
nextXid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
|
|
xidVacLimit = ShmemVariableCache->xidVacLimit;
|
|
oldestXid = ShmemVariableCache->oldestXid;
|
|
oldestXidDB = ShmemVariableCache->oldestXidDB;
|
|
LWLockRelease(XidGenLock);
|
|
|
|
if (!TransactionIdIsNormal(oldestXid))
|
|
return true; /* shouldn't happen, but just in case */
|
|
if (!TransactionIdIsValid(xidVacLimit))
|
|
return true; /* this shouldn't happen anymore either */
|
|
if (TransactionIdFollowsOrEquals(nextXid, xidVacLimit))
|
|
return true; /* past xidVacLimit, don't delay updating */
|
|
if (!SearchSysCacheExists1(DATABASEOID, ObjectIdGetDatum(oldestXidDB)))
|
|
return true; /* could happen, per comments above */
|
|
return false;
|
|
}
|
|
|
|
|
|
/*
|
|
* GetNewObjectId -- allocate a new OID
|
|
*
|
|
* OIDs are generated by a cluster-wide counter. Since they are only 32 bits
|
|
* wide, counter wraparound will occur eventually, and therefore it is unwise
|
|
* to assume they are unique unless precautions are taken to make them so.
|
|
* Hence, this routine should generally not be used directly. The only direct
|
|
* callers should be GetNewOidWithIndex() and GetNewRelFileNumber() in
|
|
* catalog/catalog.c.
|
|
*/
|
|
Oid
|
|
GetNewObjectId(void)
|
|
{
|
|
Oid result;
|
|
|
|
/* safety check, we should never get this far in a HS standby */
|
|
if (RecoveryInProgress())
|
|
elog(ERROR, "cannot assign OIDs during recovery");
|
|
|
|
LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
|
|
|
|
/*
|
|
* Check for wraparound of the OID counter. We *must* not return 0
|
|
* (InvalidOid), and in normal operation we mustn't return anything below
|
|
* FirstNormalObjectId since that range is reserved for initdb (see
|
|
* IsCatalogRelationOid()). Note we are relying on unsigned comparison.
|
|
*
|
|
* During initdb, we start the OID generator at FirstGenbkiObjectId, so we
|
|
* only wrap if before that point when in bootstrap or standalone mode.
|
|
* The first time through this routine after normal postmaster start, the
|
|
* counter will be forced up to FirstNormalObjectId. This mechanism
|
|
* leaves the OIDs between FirstGenbkiObjectId and FirstNormalObjectId
|
|
* available for automatic assignment during initdb, while ensuring they
|
|
* will never conflict with user-assigned OIDs.
|
|
*/
|
|
if (ShmemVariableCache->nextOid < ((Oid) FirstNormalObjectId))
|
|
{
|
|
if (IsPostmasterEnvironment)
|
|
{
|
|
/* wraparound, or first post-initdb assignment, in normal mode */
|
|
ShmemVariableCache->nextOid = FirstNormalObjectId;
|
|
ShmemVariableCache->oidCount = 0;
|
|
}
|
|
else
|
|
{
|
|
/* we may be bootstrapping, so don't enforce the full range */
|
|
if (ShmemVariableCache->nextOid < ((Oid) FirstGenbkiObjectId))
|
|
{
|
|
/* wraparound in standalone mode (unlikely but possible) */
|
|
ShmemVariableCache->nextOid = FirstNormalObjectId;
|
|
ShmemVariableCache->oidCount = 0;
|
|
}
|
|
}
|
|
}
|
|
|
|
/* If we run out of logged for use oids then we must log more */
|
|
if (ShmemVariableCache->oidCount == 0)
|
|
{
|
|
XLogPutNextOid(ShmemVariableCache->nextOid + VAR_OID_PREFETCH);
|
|
ShmemVariableCache->oidCount = VAR_OID_PREFETCH;
|
|
}
|
|
|
|
result = ShmemVariableCache->nextOid;
|
|
|
|
(ShmemVariableCache->nextOid)++;
|
|
(ShmemVariableCache->oidCount)--;
|
|
|
|
LWLockRelease(OidGenLock);
|
|
|
|
return result;
|
|
}
|
|
|
|
/*
|
|
* SetNextObjectId
|
|
*
|
|
* This may only be called during initdb; it advances the OID counter
|
|
* to the specified value.
|
|
*/
|
|
static void
|
|
SetNextObjectId(Oid nextOid)
|
|
{
|
|
/* Safety check, this is only allowable during initdb */
|
|
if (IsPostmasterEnvironment)
|
|
elog(ERROR, "cannot advance OID counter anymore");
|
|
|
|
/* Taking the lock is, therefore, just pro forma; but do it anyway */
|
|
LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
|
|
|
|
if (ShmemVariableCache->nextOid > nextOid)
|
|
elog(ERROR, "too late to advance OID counter to %u, it is now %u",
|
|
nextOid, ShmemVariableCache->nextOid);
|
|
|
|
ShmemVariableCache->nextOid = nextOid;
|
|
ShmemVariableCache->oidCount = 0;
|
|
|
|
LWLockRelease(OidGenLock);
|
|
}
|
|
|
|
/*
|
|
* StopGeneratingPinnedObjectIds
|
|
*
|
|
* This is called once during initdb to force the OID counter up to
|
|
* FirstUnpinnedObjectId. This supports letting initdb's post-bootstrap
|
|
* processing create some pinned objects early on. Once it's done doing
|
|
* so, it calls this (via pg_stop_making_pinned_objects()) so that the
|
|
* remaining objects it makes will be considered un-pinned.
|
|
*/
|
|
void
|
|
StopGeneratingPinnedObjectIds(void)
|
|
{
|
|
SetNextObjectId(FirstUnpinnedObjectId);
|
|
}
|
|
|
|
|
|
#ifdef USE_ASSERT_CHECKING
|
|
|
|
/*
|
|
* Assert that xid is between [oldestXid, nextXid], which is the range we
|
|
* expect XIDs coming from tables etc to be in.
|
|
*
|
|
* As ShmemVariableCache->oldestXid could change just after this call without
|
|
* further precautions, and as a wrapped-around xid could again fall within
|
|
* the valid range, this assertion can only detect if something is definitely
|
|
* wrong, but not establish correctness.
|
|
*
|
|
* This intentionally does not expose a return value, to avoid code being
|
|
* introduced that depends on the return value.
|
|
*/
|
|
void
|
|
AssertTransactionIdInAllowableRange(TransactionId xid)
|
|
{
|
|
TransactionId oldest_xid;
|
|
TransactionId next_xid;
|
|
|
|
Assert(TransactionIdIsValid(xid));
|
|
|
|
/* we may see bootstrap / frozen */
|
|
if (!TransactionIdIsNormal(xid))
|
|
return;
|
|
|
|
/*
|
|
* We can't acquire XidGenLock, as this may be called with XidGenLock
|
|
* already held (or with other locks that don't allow XidGenLock to be
|
|
* nested). That's ok for our purposes though, since we already rely on
|
|
* 32bit reads to be atomic. While nextXid is 64 bit, we only look at the
|
|
* lower 32bit, so a skewed read doesn't hurt.
|
|
*
|
|
* There's no increased danger of falling outside [oldest, next] by
|
|
* accessing them without a lock. xid needs to have been created with
|
|
* GetNewTransactionId() in the originating session, and the locks there
|
|
* pair with the memory barrier below. We do however accept xid to be <=
|
|
* to next_xid, instead of just <, as xid could be from the procarray,
|
|
* before we see the updated nextXid value.
|
|
*/
|
|
pg_memory_barrier();
|
|
oldest_xid = ShmemVariableCache->oldestXid;
|
|
next_xid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
|
|
|
|
Assert(TransactionIdFollowsOrEquals(xid, oldest_xid) ||
|
|
TransactionIdPrecedesOrEquals(xid, next_xid));
|
|
}
|
|
#endif
|