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/*-------------------------------------------------------------------------
*
* varsup.c
* postgres OID & XID variables support routines
*
* Copyright (c) 2000, PostgreSQL Global Development Group
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/access/transam/varsup.c,v 1.41 2001/07/12 04:11:13 tgl Exp $
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/transam.h"
#include "access/xlog.h"
#include "storage/proc.h"
/* Number of OIDs to prefetch (preallocate) per XLOG write */
#define VAR_OID_PREFETCH 8192
/* Spinlocks for serializing generation of XIDs and OIDs, respectively */
SPINLOCK XidGenLockId;
SPINLOCK OidGenLockId;
/* pointer to "variable cache" in shared memory (set up by shmem.c) */
VariableCache ShmemVariableCache = NULL;
/*
* Allocate the next XID for my new transaction.
*/
void
GetNewTransactionId(TransactionId *xid)
{
/*
* During bootstrap initialization, we return the special bootstrap
* transaction id.
*/
if (AMI_OVERRIDE)
{
*xid = AmiTransactionId;
return;
}
SpinAcquire(XidGenLockId);
*xid = ShmemVariableCache->nextXid;
(ShmemVariableCache->nextXid)++;
/*
* Must set MyProc->xid before releasing XidGenLock. This ensures that
* when GetSnapshotData calls ReadNewTransactionId, all active XIDs
* before the returned value of nextXid are already present in the shared
* PROC array. Else we have a race condition.
*
* XXX by storing xid into MyProc without acquiring SInvalLock, we are
* relying on fetch/store of an xid to be atomic, else other backends
* might see a partially-set xid here. But holding both locks at once
* would be a nasty concurrency hit (and at this writing, could cause a
* deadlock against GetSnapshotData). So for now, assume atomicity.
*/
if (MyProc != (PROC *) NULL)
MyProc->xid = *xid;
SpinRelease(XidGenLockId);
}
/*
* Read nextXid but don't allocate it.
*/
void
ReadNewTransactionId(TransactionId *xid)
{
/*
* During bootstrap initialization, we return the special bootstrap
* transaction id.
*/
if (AMI_OVERRIDE)
{
*xid = AmiTransactionId;
return;
}
SpinAcquire(XidGenLockId);
*xid = ShmemVariableCache->nextXid;
SpinRelease(XidGenLockId);
}
/* ----------------------------------------------------------------
* object id generation support
* ----------------------------------------------------------------
*/
static Oid lastSeenOid = InvalidOid;
void
GetNewObjectId(Oid *oid_return)
{
SpinAcquire(OidGenLockId);
/* 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;
}
if (PointerIsValid(oid_return))
lastSeenOid = (*oid_return) = ShmemVariableCache->nextOid;
(ShmemVariableCache->nextOid)++;
(ShmemVariableCache->oidCount)--;
SpinRelease(OidGenLockId);
}
void
CheckMaxObjectId(Oid assigned_oid)
{
if (lastSeenOid != InvalidOid && assigned_oid < lastSeenOid)
return;
SpinAcquire(OidGenLockId);
if (assigned_oid < ShmemVariableCache->nextOid)
{
lastSeenOid = ShmemVariableCache->nextOid - 1;
SpinRelease(OidGenLockId);
return;
}
/* If we are in the logged oid range, just bump nextOid up */
if (assigned_oid <= ShmemVariableCache->nextOid +
ShmemVariableCache->oidCount - 1)
{
ShmemVariableCache->oidCount -=
assigned_oid - ShmemVariableCache->nextOid + 1;
ShmemVariableCache->nextOid = assigned_oid + 1;
SpinRelease(OidGenLockId);
return;
}
/*
* We have exceeded the logged oid range. We should lock the database
* and kill all other backends but we are loading oid's that we can
* not guarantee are unique anyway, so we must rely on the user.
*/
XLogPutNextOid(assigned_oid + VAR_OID_PREFETCH);
ShmemVariableCache->oidCount = VAR_OID_PREFETCH - 1;
ShmemVariableCache->nextOid = assigned_oid + 1;
SpinRelease(OidGenLockId);
}
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