/*-------------------------------------------------------------------------
 *
 * varsup.c--
 *    postgres variable relation support routines
 *
 * Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    $Header: /cvsroot/pgsql/src/backend/access/transam/varsup.c,v 1.6 1996/11/05 11:12:31 scrappy Exp $
 *
 *-------------------------------------------------------------------------
 */

#include <postgres.h>

#include <access/transam.h>
#include <storage/spin.h>
#include <access/xact.h>
#include <access/heapam.h>
#include <catalog/catname.h>

/* ---------------------
 *	spin lock for oid generation
 * ---------------------
 */
int OidGenLockId;

/* ----------------------------------------------------------------
 *	      variable relation query/update routines
 * ----------------------------------------------------------------
 */

/* --------------------------------
 *	VariableRelationGetNextXid
 * --------------------------------
 */
void
VariableRelationGetNextXid(TransactionId *xidP)
{
    Buffer buf;
    VariableRelationContents var;
    
    /* ----------------
     * We assume that a spinlock has been acquire to guarantee
     * exclusive access to the variable relation.
     * ----------------
     */
    
    /* ----------------
     *	do nothing before things are initialized
     * ----------------
     */
    if (! RelationIsValid(VariableRelation))
	return;
    
    /* ----------------
     *	read the variable page, get the the nextXid field and
     *  release the buffer
     * ----------------
     */
    buf = ReadBuffer(VariableRelation, 0);
    
    if (! BufferIsValid(buf))
	{
	    SpinRelease(OidGenLockId);
	    elog(WARN, "VariableRelationGetNextXid: ReadBuffer failed");
	}
    
    var = (VariableRelationContents) BufferGetBlock(buf);
    
    TransactionIdStore(var->nextXidData, xidP);
    ReleaseBuffer(buf);
}

/* --------------------------------
 *	VariableRelationGetLastXid
 * --------------------------------
 */
void
VariableRelationGetLastXid(TransactionId *xidP)
{
    Buffer buf;
    VariableRelationContents var;
    
    /* ----------------
     * We assume that a spinlock has been acquire to guarantee
     * exclusive access to the variable relation.
     * ----------------
     */
    
    /* ----------------
     *	do nothing before things are initialized
     * ----------------
     */
    if (! RelationIsValid(VariableRelation))
	return;
    
    /* ----------------
     *	read the variable page, get the the lastXid field and
     *  release the buffer
     * ----------------
     */
    buf = ReadBuffer(VariableRelation, 0);
    
    if (! BufferIsValid(buf))
	{
	    SpinRelease(OidGenLockId);
	    elog(WARN, "VariableRelationGetNextXid: ReadBuffer failed");
	}
    
    var = (VariableRelationContents) BufferGetBlock(buf);
    
    TransactionIdStore(var->lastXidData, xidP);
    
    ReleaseBuffer(buf);
}

/* --------------------------------
 *	VariableRelationPutNextXid
 * --------------------------------
 */
void
VariableRelationPutNextXid(TransactionId xid)
{
    Buffer buf;
    VariableRelationContents var;
    
    /* ----------------
     * We assume that a spinlock has been acquire to guarantee
     * exclusive access to the variable relation.
     * ----------------
     */
    
    /* ----------------
     *	do nothing before things are initialized
     * ----------------
     */
    if (! RelationIsValid(VariableRelation))
	return;
    
    /* ----------------
     *	read the variable page, update the nextXid field and
     *  write the page back out to disk.
     * ----------------
     */
    buf = ReadBuffer(VariableRelation, 0);
    
    if (! BufferIsValid(buf))
	{
	    SpinRelease(OidGenLockId);
	    elog(WARN, "VariableRelationPutNextXid: ReadBuffer failed");
	}
    
    var = (VariableRelationContents) BufferGetBlock(buf);
    
    TransactionIdStore(xid, &(var->nextXidData));
    
    WriteBuffer(buf);
}

/* --------------------------------
 *	VariableRelationPutLastXid
 * --------------------------------
 */
void
VariableRelationPutLastXid(TransactionId xid)
{
    Buffer buf;
    VariableRelationContents var;
    
    /* ----------------
     * We assume that a spinlock has been acquire to guarantee
     * exclusive access to the variable relation.
     * ----------------
     */
    
    /* ----------------
     *	do nothing before things are initialized
     * ----------------
     */
    if (! RelationIsValid(VariableRelation))
	return;
    
    /* ----------------
     *	read the variable page, update the lastXid field and
     *  force the page back out to disk.
     * ----------------
     */
    buf = ReadBuffer(VariableRelation, 0);
    
    if (! BufferIsValid(buf))
	{
	    SpinRelease(OidGenLockId);
	    elog(WARN, "VariableRelationPutLastXid: ReadBuffer failed");
	}
    
    var = (VariableRelationContents) BufferGetBlock(buf);
    
    TransactionIdStore(xid, &(var->lastXidData));
    
    WriteBuffer(buf);
}

/* --------------------------------
 *	VariableRelationGetNextOid
 * --------------------------------
 */
void
VariableRelationGetNextOid(Oid *oid_return)
{
    Buffer buf;
    VariableRelationContents var;
    
    /* ----------------
     * We assume that a spinlock has been acquire to guarantee
     * exclusive access to the variable relation.
     * ----------------
     */
    
    /* ----------------
     *	if the variable relation is not initialized, then we
     *  assume we are running at bootstrap time and so we return
     *  an invalid object id -- during this time GetNextBootstrapObjectId
     *  should be called instead..
     * ----------------
     */
    if (! RelationIsValid(VariableRelation)) {
	if (PointerIsValid(oid_return))
	    (*oid_return) = InvalidOid;
	return;
    }
    
    /* ----------------
     *	read the variable page, get the the nextOid field and
     *  release the buffer
     * ----------------
     */
    buf = ReadBuffer(VariableRelation, 0);
    
    if (! BufferIsValid(buf))
	{
	    SpinRelease(OidGenLockId);
	    elog(WARN, "VariableRelationGetNextXid: ReadBuffer failed");
	}
    
    var = (VariableRelationContents) BufferGetBlock(buf);
    
    if (PointerIsValid(oid_return)) {
	
        /* ----------------
         * nothing up my sleeve...  what's going on here is that this code
	 * is guaranteed never to be called until all files in data/base/
	 * are created, and the template database exists.  at that point,
	 * we want to append a pg_database tuple.  the first time we do
	 * this, the oid stored in pg_variable will be bogus, so we use
	 * a bootstrap value defined at the top of this file.
	 *
	 * this comment no longer holds true.  This code is called before
	 * all of the files in data/base are created and you can't rely
	 * on system oid's to be less than BootstrapObjectIdData. mer 9/18/91
         * ----------------
         */
	if (OidIsValid(var->nextOid))
	    (*oid_return) = var->nextOid;
	else
	    (*oid_return) = BootstrapObjectIdData;
    }
    
    ReleaseBuffer(buf);
}

/* --------------------------------
 *	VariableRelationPutNextOid
 * --------------------------------
 */
void
VariableRelationPutNextOid(Oid *oidP)
{
    Buffer buf;
    VariableRelationContents var;
    
    /* ----------------
     * We assume that a spinlock has been acquire to guarantee
     * exclusive access to the variable relation.
     * ----------------
     */
    
    /* ----------------
     *	do nothing before things are initialized
     * ----------------
     */
    if (! RelationIsValid(VariableRelation))
	return;
    
    /* ----------------
     *	sanity check
     * ----------------
     */
    if (! PointerIsValid(oidP))
	{
	    SpinRelease(OidGenLockId);
	    elog(WARN, "VariableRelationPutNextOid: invalid oid pointer");
	}
    
    /* ----------------
     *	read the variable page, update the nextXid field and
     *  write the page back out to disk.
     * ----------------
     */
    buf = ReadBuffer(VariableRelation, 0);
    
    if (! BufferIsValid(buf))
	{
	    SpinRelease(OidGenLockId);
	    elog(WARN, "VariableRelationPutNextXid: ReadBuffer failed");
	}
    
    var = (VariableRelationContents) BufferGetBlock(buf);
    
    var->nextOid = (*oidP);
    
    WriteBuffer(buf);
}

/* ----------------------------------------------------------------
 *		transaction id generation support
 * ----------------------------------------------------------------
 */

/* ----------------
 *	GetNewTransactionId
 *
 *	In the version 2 transaction system, transaction id's are
 *	restricted in several ways.
 *
 *	First, all transaction id's are even numbers (4, 88, 121342, etc).
 *	This means the binary representation of the number will never
 *	have the least significent bit set.  This bit is reserved to
 *	indicate that the transaction id does not in fact hold an XID,
 *	but rather a commit time.  This makes it possible for the
 *	vaccuum daemon to disgard information from the log and time
 *	relations for committed tuples.  This is important when archiving
 *	tuples to an optical disk because tuples with commit times
 *	stored in their xid fields will not need to consult the log
 *	and time relations.
 *
 *	Second, since we may someday preform compression of the data
 *	in the log and time relations, we cause the numbering of the
 *	transaction ids to begin at 512.  This means that some space
 *	on the page of the log and time relations corresponding to
 *	transaction id's 0 - 510 will never be used.  This space is
 *	in fact used to store the version number of the postgres
 *	transaction log and will someday store compression information
 *	about the log.
 *
 *	Lastly, rather then access the variable relation each time
 *	a backend requests a new transction id, we "prefetch" 32
 *	transaction id's by incrementing the nextXid stored in the
 *	var relation by 64 (remember only even xid's are legal) and then
 *	returning these id's one at a time until they are exhausted.
 *  	This means we reduce the number of accesses to the variable
 *	relation by 32 for each backend.
 *
 *  	Note:  32 has no special significance.  We don't want the
 *	       number to be too large because if when the backend
 *	       terminates, we lose the xid's we cached.
 *
 * ----------------
 */

#define VAR_XID_PREFETCH	32

static int prefetched_xid_count = 0;
static TransactionId next_prefetched_xid;

void
GetNewTransactionId(TransactionId *xid)
{
    TransactionId nextid;
    
    /* ----------------
     *	during bootstrap initialization, we return the special
     *  bootstrap transaction id.
     * ----------------
     */
    if (AMI_OVERRIDE) {	
	TransactionIdStore(AmiTransactionId, xid);
	return;
    }
    
    /* ----------------
     *  if we run out of prefetched xids, then we get some
     *  more before handing them out to the caller.
     * ----------------
     */
    
    if (prefetched_xid_count == 0) {
	/* ----------------
	 *	obtain exclusive access to the variable relation page
	 *
	 *	get the "next" xid from the variable relation
	 *	and save it in the prefetched id.
	 * ----------------
	 */
	SpinAcquire(OidGenLockId);
	VariableRelationGetNextXid(&nextid);
	TransactionIdStore(nextid, &next_prefetched_xid);
	
	/* ----------------
	 *	now increment the variable relation's next xid
	 *	and reset the prefetched_xid_count.  We multiply
	 *	the id by two because our xid's are always even.
	 * ----------------
	 */
	prefetched_xid_count = VAR_XID_PREFETCH;
	TransactionIdAdd(&nextid, prefetched_xid_count);
	VariableRelationPutNextXid(nextid);
	SpinRelease(OidGenLockId);
    }
    
    /* ----------------
     *	return the next prefetched xid in the pointer passed by
     *  the user and decrement the prefetch count.  We add two
     *  to id we return the next time this is called because our
     *	transaction ids are always even.
     *
     *  XXX Transaction Ids used to be even as the low order bit was
     *      used to determine commit status.  This is no long true so
     *      we now use even and odd transaction ids. -mer 5/26/92
     * ----------------
     */
    TransactionIdStore(next_prefetched_xid, xid);
    TransactionIdAdd(&next_prefetched_xid, 1);
    prefetched_xid_count--;
}

/* ----------------
 *	UpdateLastCommittedXid
 * ----------------
 */

void
UpdateLastCommittedXid(TransactionId xid)
{
    TransactionId lastid;
    
    
    /* we assume that spinlock OidGenLockId has been acquired
     * prior to entering this function
     */
    
    /* ----------------
     *	get the "last committed" transaction id from
     *  the variable relation page.
     * ----------------
     */
    VariableRelationGetLastXid(&lastid);
    
    /* ----------------
     *	if the transaction id is greater than the last committed
     *  transaction then we update the last committed transaction
     *  in the variable relation.
     * ----------------
     */
    if (TransactionIdIsLessThan(lastid, xid))
	VariableRelationPutLastXid(xid);
    
}

/* ----------------------------------------------------------------
 *		    object id generation support
 * ----------------------------------------------------------------
 */

/* ----------------
 *	GetNewObjectIdBlock
 *
 *	This support function is used to allocate a block of object ids
 *	of the given size.  applications wishing to do their own object
 *	id assignments should use this 
 * ----------------
 */
void
GetNewObjectIdBlock(Oid *oid_return, /* place to return the new object id */
		    int oid_block_size)	/* number of oids desired */
{
    Oid nextoid;		
    
    /* ----------------
     *	SOMEDAY obtain exclusive access to the variable relation page
     *  That someday is today -mer 6 Aug 1992
     * ----------------
     */
    SpinAcquire(OidGenLockId);
    
    /* ----------------
     *	get the "next" oid from the variable relation
     *	and give it to the caller.
     * ----------------
     */
    VariableRelationGetNextOid(&nextoid);
    if (PointerIsValid(oid_return))
	(*oid_return) = nextoid;
    
    /* ----------------
     *	now increment the variable relation's next oid
     *	field by the size of the oid block requested.
     * ----------------
     */
    nextoid += oid_block_size;
    VariableRelationPutNextOid(&nextoid);
    
    /* ----------------
     *	SOMEDAY relinquish our lock on the variable relation page
     *  That someday is today -mer 6 Apr 1992
     * ----------------
     */
    SpinRelease(OidGenLockId);
}

/* ----------------
 *	GetNewObjectId
 *
 *	This function allocates and parses out object ids.  Like
 *	GetNewTransactionId(), it "prefetches" 32 object ids by
 *	incrementing the nextOid stored in the var relation by 32 and then
 *	returning these id's one at a time until they are exhausted.
 *  	This means we reduce the number of accesses to the variable
 *	relation by 32 for each backend.
 *
 *  	Note:  32 has no special significance.  We don't want the
 *	       number to be too large because if when the backend
 *	       terminates, we lose the oids we cached.
 *
 * ----------------
 */

#define VAR_OID_PREFETCH	32

static int prefetched_oid_count = 0;
static Oid next_prefetched_oid;

void
GetNewObjectId(Oid *oid_return)	/* place to return the new object id */
{
    /* ----------------
     *  if we run out of prefetched oids, then we get some
     *  more before handing them out to the caller.
     * ----------------
     */
    
    if (prefetched_oid_count == 0) {
	int oid_block_size = VAR_OID_PREFETCH;
	
	/* ----------------
	 *	during bootstrap time, we want to allocate oids
	 *	one at a time.  Otherwise there might be some
	 *      bootstrap oid's left in the block we prefetch which
	 *	would be passed out after the variable relation was
	 *	initialized.  This would be bad.
	 * ----------------
	 */
	if (! RelationIsValid(VariableRelation))
	    VariableRelation = heap_openr(VariableRelationName);
	
	/* ----------------
	 *	get a new block of prefetched object ids.
	 * ----------------
	 */
	GetNewObjectIdBlock(&next_prefetched_oid, oid_block_size);
	
	/* ----------------
	 *	now reset the prefetched_oid_count.
	 * ----------------
	 */
	prefetched_oid_count = oid_block_size;
    }
    
    /* ----------------
     *	return the next prefetched oid in the pointer passed by
     *  the user and decrement the prefetch count.
     * ----------------
     */
    if (PointerIsValid(oid_return))
	(*oid_return) = next_prefetched_oid;
    
    next_prefetched_oid++;
    prefetched_oid_count--;
}

void
CheckMaxObjectId(Oid assigned_oid)
{
Oid	pass_oid;


    if (prefetched_oid_count == 0)   /* make sure next/max is set, or reload */
	GetNewObjectId(&pass_oid);

    /* ----------------
     *  If we are below prefetched limits, do nothing
     * ----------------
     */
    
    if (assigned_oid < next_prefetched_oid)
	return;

    /* ----------------
     *  If we are here, we are coming from a 'copy from' with oid's
     *
     *  If we are in the prefetched oid range, just bump it up
     *
     * ----------------
     */
    
    if (assigned_oid <= next_prefetched_oid + prefetched_oid_count - 1)
    {
	prefetched_oid_count -= assigned_oid - next_prefetched_oid + 1;
	next_prefetched_oid = assigned_oid + 1;
	return;
    }

    /* ----------------
     *  We have exceeded the prefetch 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
     *
     *
     * We now:
     *    set the variable relation with the new max oid
     *    force the backend to reload its oid cache
     *
     * We use the oid cache so we don't have to update the variable
     * relation every time
     *
     * ----------------
     */

    pass_oid = assigned_oid;
    VariableRelationPutNextOid(&pass_oid); /* not modified */
    prefetched_oid_count = 0;	/* force reload */
    pass_oid = assigned_oid;
    GetNewObjectId(&pass_oid);	/* throw away returned oid */

}

