501 lines
18 KiB
C
501 lines
18 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) 2000-2014, 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/xlog.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 XID for a new transaction or subtransaction.
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*
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* The new XID is also stored into MyPgXact before 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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TransactionId
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GetNewTransactionId(bool isSubXact)
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{
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TransactionId xid;
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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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MyPgXact->xid = BootstrapTransactionId;
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return BootstrapTransactionId;
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}
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/* safety check, we should never get this far in a HS slave */
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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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xid = ShmemVariableCache->nextXid;
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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.")));
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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.")));
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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.")));
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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.")));
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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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xid = ShmemVariableCache->nextXid;
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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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TransactionIdAdvance(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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* XXX by storing xid into MyPgXact without acquiring ProcArrayLock, we
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* are relying on fetch/store of an xid to be atomic, else other backends
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* might see a partially-set xid here. But holding both locks at once
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* would be a nasty concurrency hit. So for now, assume atomicity.
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*
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* Note that readers of PGXACT xid fields should be careful to fetch the
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* value only once, rather than assume they can read a value multiple
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* times and get the same answer each time.
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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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* A solution to the atomic-store problem would be to give each PGXACT its
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* own spinlock used only for fetching/storing that PGXACT's xid and
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* related fields.
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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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{
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/*
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* Use volatile pointer to prevent code rearrangement; other backends
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* could be examining my subxids info concurrently, and we don't want
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* them to see an invalid intermediate state, such as incrementing
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* nxids before filling the array entry. Note we are assuming that
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* TransactionId and int fetch/store are atomic.
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*/
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volatile PGPROC *myproc = MyProc;
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volatile PGXACT *mypgxact = MyPgXact;
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if (!isSubXact)
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mypgxact->xid = xid;
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else
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{
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int nxids = mypgxact->nxids;
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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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mypgxact->nxids = nxids + 1;
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}
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else
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mypgxact->overflowed = true;
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}
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}
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LWLockRelease(XidGenLock);
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return 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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TransactionId
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ReadNewTransactionId(void)
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{
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TransactionId xid;
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LWLockAcquire(XidGenLock, LW_SHARED);
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xid = ShmemVariableCache->nextXid;
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LWLockRelease(XidGenLock);
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return xid;
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}
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/*
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* Determine the last safe XID to allocate given 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 1M 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. (Note that since
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* vacuuming requires one transaction per table cleaned, we had better be
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* sure there's lots of XIDs left...)
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*/
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xidStopLimit = xidWrapLimit - 1000000;
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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 10M transactions of
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* the stop point. This is kind of arbitrary, but if you let your gas
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* gauge get down to 1% of full, would you be looking for the next gas
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* station? We need to be fairly liberal about this number because there
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* are lots of scenarios where most transactions are done by automatic
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* clients that won't pay attention to warnings. (No, we're not gonna make
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* this configurable. If you know enough to configure it, you know enough
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* to not get in this kind of trouble in the first place.)
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*/
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xidWarnLimit = xidStopLimit - 10000000;
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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 = 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("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),
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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.")));
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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 - curXid),
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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.")));
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}
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}
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/*
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* ForceTransactionIdLimitUpdate -- does the XID wrap-limit data need updating?
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*
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* We primarily check whether oldestXidDB is valid. The cases we have in
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* mind are that that database was dropped, or the field was reset to zero
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* by pg_resetxlog. In either case we should force recalculation of the
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* wrap limit. Also do it if oldestXid is old enough to be forcing
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* autovacuums or other actions; this ensures we update our state as soon
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* as possible once extra overhead is being incurred.
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*/
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bool
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ForceTransactionIdLimitUpdate(void)
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{
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TransactionId nextXid;
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TransactionId xidVacLimit;
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TransactionId oldestXid;
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Oid oldestXidDB;
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/* Locking is probably not really necessary, but let's be careful */
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LWLockAcquire(XidGenLock, LW_SHARED);
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nextXid = ShmemVariableCache->nextXid;
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xidVacLimit = ShmemVariableCache->xidVacLimit;
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oldestXid = ShmemVariableCache->oldestXid;
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oldestXidDB = ShmemVariableCache->oldestXidDB;
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LWLockRelease(XidGenLock);
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if (!TransactionIdIsNormal(oldestXid))
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return true; /* shouldn't happen, but just in case */
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if (!TransactionIdIsValid(xidVacLimit))
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return true; /* this shouldn't happen anymore either */
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if (TransactionIdFollowsOrEquals(nextXid, xidVacLimit))
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return true; /* past VacLimit, don't delay updating */
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if (!SearchSysCacheExists1(DATABASEOID, ObjectIdGetDatum(oldestXidDB)))
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return true; /* could happen, per comments above */
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return false;
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}
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/*
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* GetNewObjectId -- allocate a new OID
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*
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* OIDs are generated by a cluster-wide counter. Since they are only 32 bits
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* wide, counter wraparound will occur eventually, and therefore it is unwise
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* to assume they are unique unless precautions are taken to make them so.
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* Hence, this routine should generally not be used directly. The only
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* direct callers should be GetNewOid() and GetNewRelFileNode() in
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* catalog/catalog.c.
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*/
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Oid
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GetNewObjectId(void)
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{
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Oid result;
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/* safety check, we should never get this far in a HS slave */
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if (RecoveryInProgress())
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elog(ERROR, "cannot assign OIDs during recovery");
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LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
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/*
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* Check for wraparound of the OID counter. We *must* not return 0
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* (InvalidOid); and as long as we have to check that, it seems a good
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* idea to skip over everything below FirstNormalObjectId too. (This
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* basically just avoids lots of collisions with bootstrap-assigned OIDs
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* right after a wrap occurs, so as to avoid a possibly large number of
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* iterations in GetNewOid.) Note we are relying on unsigned comparison.
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*
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* During initdb, we start the OID generator at FirstBootstrapObjectId, so
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* we only wrap if before that point when in bootstrap or standalone mode.
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* The first time through this routine after normal postmaster start, the
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* counter will be forced up to FirstNormalObjectId. This mechanism
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* leaves the OIDs between FirstBootstrapObjectId and FirstNormalObjectId
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* available for automatic assignment during initdb, while ensuring they
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* will never conflict with user-assigned OIDs.
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*/
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if (ShmemVariableCache->nextOid < ((Oid) FirstNormalObjectId))
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{
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if (IsPostmasterEnvironment)
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{
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/* wraparound, or first post-initdb assignment, in normal mode */
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ShmemVariableCache->nextOid = FirstNormalObjectId;
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ShmemVariableCache->oidCount = 0;
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}
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else
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{
|
|
/* we may be bootstrapping, so don't enforce the full range */
|
|
if (ShmemVariableCache->nextOid < ((Oid) FirstBootstrapObjectId))
|
|
{
|
|
/* 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;
|
|
}
|