src/backend/access/transam/subtrans.c - hawq - Git at Google

 /*-------------------------------------------------------------------------
  *
  * subtrans.c
  *		PostgreSQL subtransaction-log manager
  *
  * The pg_subtrans manager is a pg_clog-like manager that stores the parent
  * transaction Id for each transaction.  It is a fundamental part of the
  * nested transactions implementation.	A main transaction has a parent
  * of InvalidTransactionId, and each subtransaction has its immediate parent.
  * The tree can easily be walked from child to parent, but not in the
  * opposite direction.
  *
  * This code is based on clog.c, but the robustness requirements
  * are completely different from pg_clog, because we only need to remember
  * pg_subtrans information for currently-open transactions.  Thus, there is
  * no need to preserve data over a crash and restart.
  *
  * There are no XLOG interactions since we do not care about preserving
  * data across crashes.  During database startup, we simply force the
  * currently-active page of SUBTRANS to zeroes.
  *
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * $PostgreSQL: pgsql/src/backend/access/transam/subtrans.c,v 1.17 2006/07/13 16:49:13 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"

 #include "access/slru.h"
 #include "access/subtrans.h"
 #include "access/transam.h"
 #include "utils/tqual.h"
 #include "cdb/cdbpersistentstore.h"


 /*
  * Defines for SubTrans page sizes.  A page is the same BLCKSZ as is used
  * everywhere else in Postgres.
  *
  * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
  * SubTrans page numbering also wraps around at
  * 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE, and segment numbering at
  * 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE/SLRU_SEGMENTS_PER_PAGE.  We need take no
  * explicit notice of that fact in this module, except when comparing segment
  * and page numbers in TruncateSUBTRANS (see SubTransPagePrecedes).
  */

 /* We need eight bytes per xact */
 #define SUBTRANS_XACTS_PER_PAGE (BLCKSZ / sizeof(SubTransData))

 #define TransactionIdToPage(xid) ((xid) / (uint32) SUBTRANS_XACTS_PER_PAGE)
 #define TransactionIdToEntry(xid) ((xid) % (uint32) SUBTRANS_XACTS_PER_PAGE)


 /*
  * Link to shared-memory data structures for SUBTRANS control
  */
 static SlruCtlData SubTransCtlData;

 #define SubTransCtl  (&SubTransCtlData)


 static int	ZeroSUBTRANSPage(int pageno);
 static bool SubTransPagePrecedes(int page1, int page2);

 static void
 SubTransGetData(TransactionId xid, SubTransData* subData)
 {
 	int			pageno = TransactionIdToPage(xid);
 	int			entryno = TransactionIdToEntry(xid);
 	int			slotno;
 	SubTransData *ptr;

 	/* Can't ask about stuff that might not be around anymore */
 	Assert(TransactionIdFollowsOrEquals(xid, TransactionXmin));

 	/* Bootstrap and frozen XIDs have no parent and itself as topMostParent */
 	if (!TransactionIdIsNormal(xid))
 	{
 		subData->parent = InvalidTransactionId;
 		subData->topMostParent = xid;
 		return;
 	}

 	/* lock is acquired by SimpleLruReadPage_ReadOnly */

 	slotno = SimpleLruReadPage_ReadOnly(SubTransCtl, pageno, xid, NULL);
 	ptr = (SubTransData *) SubTransCtl->shared->page_buffer[slotno];
 	ptr += entryno;

 	subData->parent = ptr->parent;
 	subData->topMostParent = ptr->topMostParent;
 	if ( subData->topMostParent == InvalidTransactionId )
 	{
 		/* Here means parent is Main XID, hence set parent itself as topMostParent */
 		subData->topMostParent = xid;
 	}

 	LWLockRelease(SubtransControlLock);

 	return;
 }

 /*
  * Record the parent of a subtransaction in the subtrans log.
  */
 void
 SubTransSetParent(TransactionId xid, TransactionId parent)
 {
 	int			pageno = TransactionIdToPage(xid);
 	int			entryno = TransactionIdToEntry(xid);
 	int			slotno;
 	SubTransData *ptr;
 	SubTransData subData;

 	/*
 	 * Main Xact has parent and topMostParent as InvalidTransactionId
 	 */
 	if ( parent != InvalidTransactionId )
 	{
 		/* Get the topMostParent for Parent */
 		SubTransGetData(parent, &subData);
 	}
 	else
 	{
 		subData.topMostParent = InvalidTransactionId;
 	}

 	LWLockAcquire(SubtransControlLock, LW_EXCLUSIVE);

 	slotno = SimpleLruReadPage(SubTransCtl, pageno, xid);
 	ptr = (SubTransData *) SubTransCtl->shared->page_buffer[slotno];
 	ptr += entryno;

 	/* Current state should be 0 */
 	Assert(ptr->parent == InvalidTransactionId);
 	Assert(ptr->topMostParent == InvalidTransactionId);

 	ptr->parent = parent;
 	ptr->topMostParent = subData.topMostParent;

 	SubTransCtl->shared->page_dirty[slotno] = true;

 	LWLockRelease(SubtransControlLock);
 }

 /*
  * Interrogate the parent of a transaction in the subtrans log.
  */
 TransactionId
 SubTransGetParent(TransactionId xid)
 {
 	SubTransData subData;
 	SubTransGetData(xid, &subData);

 	return subData.parent;
 }

 /*
  * SubTransGetTopmostTransaction
  *
  * Returns the topmost transaction of the given transaction id.
  */
 TransactionId
 SubTransGetTopmostTransaction(TransactionId xid)
 {
 	Assert(TransactionIdIsValid(xid));
 	SubTransData subData;
 	SubTransGetData(xid, &subData);

 	Assert(TransactionIdIsValid(subData.topMostParent));
 	return subData.topMostParent;
 }

 /*
  * Initialization of shared memory for SUBTRANS
  */
 Size
 SUBTRANSShmemSize(void)
 {
 	return SimpleLruShmemSize(NUM_SUBTRANS_BUFFERS);
 }

 void
 SUBTRANSShmemInit(void)
 {
 	SubTransCtl->PagePrecedes = SubTransPagePrecedes;
 	SimpleLruInit(SubTransCtl, "SUBTRANS Ctl", NUM_SUBTRANS_BUFFERS,
 				  SubtransControlLock, SUBTRANS_DIR);
 	/* Override default assumption that writes should be fsync'd */
 	SubTransCtl->do_fsync = false;
 }

 /*
  * This func must be called ONCE on system install.  It creates
  * the initial SUBTRANS segment.  (The SUBTRANS directory is assumed to
  * have been created by the initdb shell script, and SUBTRANSShmemInit
  * must have been called already.)
  *
  * Note: it's not really necessary to create the initial segment now,
  * since slru.c would create it on first write anyway.	But we may as well
  * do it to be sure the directory is set up correctly.
  */
 void
 BootStrapSUBTRANS(void)
 {
 	int			slotno;

 	LWLockAcquire(SubtransControlLock, LW_EXCLUSIVE);

 	/* Create and zero the first page of the subtrans log */
 	slotno = ZeroSUBTRANSPage(0);

 	/* Make sure it's written out */
 	SimpleLruWritePage(SubTransCtl, slotno, NULL);
 	Assert(!SubTransCtl->shared->page_dirty[slotno]);

 	LWLockRelease(SubtransControlLock);
 }

 /*
  * Initialize (or reinitialize) a page of SUBTRANS to zeroes.
  *
  * The page is not actually written, just set up in shared memory.
  * The slot number of the new page is returned.
  *
  * Control lock must be held at entry, and will be held at exit.
  */
 static int
 ZeroSUBTRANSPage(int pageno)
 {
 	int result;

 	result = SimpleLruZeroPage(SubTransCtl, pageno);

 	return result;
 }

 /*
  * This must be called ONCE during postmaster or standalone-backend startup,
  * after StartupXLOG has initialized ShmemVariableCache->nextXid.
  *
  * oldestActiveXID is the oldest XID of any prepared transaction, or nextXid
  * if there are none.
  */
 void
 StartupSUBTRANS(TransactionId oldestActiveXID)
 {
 	int			startPage;
 	int			endPage;

 	/*
 	 * Since we don't expect pg_subtrans to be valid across crashes, we
 	 * initialize the currently-active page(s) to zeroes during startup.
 	 * Whenever we advance into a new page, ExtendSUBTRANS will likewise zero
 	 * the new page without regard to whatever was previously on disk.
 	 */

 	LWLockAcquire(SubtransControlLock, LW_EXCLUSIVE);

 	startPage = TransactionIdToPage(oldestActiveXID);
 	endPage = TransactionIdToPage(ShmemVariableCache->nextXid);

 	while (startPage != endPage)
 	{
 		(void) ZeroSUBTRANSPage(startPage);
 		startPage++;
 	}
 	(void) ZeroSUBTRANSPage(startPage);

 	LWLockRelease(SubtransControlLock);
 }

 /*
  * This must be called ONCE during postmaster or standalone-backend shutdown
  */
 void
 ShutdownSUBTRANS(void)
 {
 	/*
 	 * Flush dirty SUBTRANS pages to disk
 	 *
 	 * This is not actually necessary from a correctness point of view. We do
 	 * it merely as a debugging aid.
 	 */
 	SimpleLruFlush(SubTransCtl, false);
 }

 /*
  * Perform a checkpoint --- either during shutdown, or on-the-fly
  */
 void
 CheckPointSUBTRANS(void)
 {
 	/*
 	 * Flush dirty SUBTRANS pages to disk
 	 *
 	 * This is not actually necessary from a correctness point of view. We do
 	 * it merely to improve the odds that writing of dirty pages is done by
 	 * the checkpoint process and not by backends.
 	 */
 	SimpleLruFlush(SubTransCtl, true);
 }


 /*
  * Make sure that SUBTRANS has room for a newly-allocated XID.
  *
  * NB: this is called while holding XidGenLock.  We want it to be very fast
  * most of the time; even when it's not so fast, no actual I/O need happen
  * unless we're forced to write out a dirty subtrans page to make room
  * in shared memory.
  */
 void
 ExtendSUBTRANS(TransactionId newestXact)
 {
 	int			pageno;

 	/*
 	 * Caller must have already taken mirrored lock shared.
 	 */

 	/*
 	 * No work except at first XID of a page.  But beware: just after
 	 * wraparound, the first XID of page zero is FirstNormalTransactionId.
 	 */
 	if (TransactionIdToEntry(newestXact) != 0 &&
 		!TransactionIdEquals(newestXact, FirstNormalTransactionId))
 		return;

 	pageno = TransactionIdToPage(newestXact);

 	LWLockAcquire(SubtransControlLock, LW_EXCLUSIVE);

 	/* Zero the page */
 	ZeroSUBTRANSPage(pageno);

 	LWLockRelease(SubtransControlLock);

 }


 /*
  * Remove all SUBTRANS segments before the one holding the passed transaction ID
  *
  * This is normally called during checkpoint, with oldestXact being the
  * oldest TransactionXmin of any running transaction.
  */
 void
 TruncateSUBTRANS(TransactionId oldestXact)
 {
 	int			cutoffPage;

 	/*
 	 * The cutoff point is the start of the segment containing oldestXact. We
 	 * pass the *page* containing oldestXact to SimpleLruTruncate.
 	 */
 	cutoffPage = TransactionIdToPage(oldestXact);

 	SimpleLruTruncate(SubTransCtl, cutoffPage, false);
 }


 /*
  * Decide which of two SUBTRANS page numbers is "older" for truncation purposes.
  *
  * We need to use comparison of TransactionIds here in order to do the right
  * thing with wraparound XID arithmetic.  However, if we are asked about
  * page number zero, we don't want to hand InvalidTransactionId to
  * TransactionIdPrecedes: it'll get weird about permanent xact IDs.  So,
  * offset both xids by FirstNormalTransactionId to avoid that.
  */
 static bool
 SubTransPagePrecedes(int page1, int page2)
 {
 	TransactionId xid1;
 	TransactionId xid2;

 	xid1 = ((uint32) page1) * SUBTRANS_XACTS_PER_PAGE;
 	xid1 += FirstNormalTransactionId;
 	xid2 = ((uint32) page2) * SUBTRANS_XACTS_PER_PAGE;
 	xid2 += FirstNormalTransactionId;

 	return TransactionIdPrecedes(xid1, xid2);
 }
	/*-------------------------------------------------------------------------
	*
	* subtrans.c
	* PostgreSQL subtransaction-log manager
	*
	* The pg_subtrans manager is a pg_clog-like manager that stores the parent
	* transaction Id for each transaction. It is a fundamental part of the
	* nested transactions implementation. A main transaction has a parent
	* of InvalidTransactionId, and each subtransaction has its immediate parent.
	* The tree can easily be walked from child to parent, but not in the
	* opposite direction.
	*
	* This code is based on clog.c, but the robustness requirements
	* are completely different from pg_clog, because we only need to remember
	* pg_subtrans information for currently-open transactions. Thus, there is
	* no need to preserve data over a crash and restart.
	*
	* There are no XLOG interactions since we do not care about preserving
	* data across crashes. During database startup, we simply force the
	* currently-active page of SUBTRANS to zeroes.
	*
	* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
	* Portions Copyright (c) 1994, Regents of the University of California
	*
	* $PostgreSQL: pgsql/src/backend/access/transam/subtrans.c,v 1.17 2006/07/13 16:49:13 momjian Exp $
	*
	*-------------------------------------------------------------------------
	*/
	#include "postgres.h"

	#include "access/slru.h"
	#include "access/subtrans.h"
	#include "access/transam.h"
	#include "utils/tqual.h"
	#include "cdb/cdbpersistentstore.h"


	/*
	* Defines for SubTrans page sizes. A page is the same BLCKSZ as is used
	* everywhere else in Postgres.
	*
	* Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
	* SubTrans page numbering also wraps around at
	* 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE, and segment numbering at
	* 0xFFFFFFFF/SUBTRANS_XACTS_PER_PAGE/SLRU_SEGMENTS_PER_PAGE. We need take no
	* explicit notice of that fact in this module, except when comparing segment
	* and page numbers in TruncateSUBTRANS (see SubTransPagePrecedes).
	*/

	/* We need eight bytes per xact */
	#define SUBTRANS_XACTS_PER_PAGE (BLCKSZ / sizeof(SubTransData))

	#define TransactionIdToPage(xid) ((xid) / (uint32) SUBTRANS_XACTS_PER_PAGE)
	#define TransactionIdToEntry(xid) ((xid) % (uint32) SUBTRANS_XACTS_PER_PAGE)


	/*
	* Link to shared-memory data structures for SUBTRANS control
	*/
	static SlruCtlData SubTransCtlData;

	#define SubTransCtl (&SubTransCtlData)


	static int ZeroSUBTRANSPage(int pageno);
	static bool SubTransPagePrecedes(int page1, int page2);

	static void
	SubTransGetData(TransactionId xid, SubTransData* subData)
	{
	int pageno = TransactionIdToPage(xid);
	int entryno = TransactionIdToEntry(xid);
	int slotno;
	SubTransData *ptr;

	/* Can't ask about stuff that might not be around anymore */
	Assert(TransactionIdFollowsOrEquals(xid, TransactionXmin));

	/* Bootstrap and frozen XIDs have no parent and itself as topMostParent */
	if (!TransactionIdIsNormal(xid))
	{
	subData->parent = InvalidTransactionId;
	subData->topMostParent = xid;
	return;
	}

	/* lock is acquired by SimpleLruReadPage_ReadOnly */

	slotno = SimpleLruReadPage_ReadOnly(SubTransCtl, pageno, xid, NULL);
	ptr = (SubTransData *) SubTransCtl->shared->page_buffer[slotno];
	ptr += entryno;

	subData->parent = ptr->parent;
	subData->topMostParent = ptr->topMostParent;
	if ( subData->topMostParent == InvalidTransactionId )
	{
	/* Here means parent is Main XID, hence set parent itself as topMostParent */
	subData->topMostParent = xid;
	}

	LWLockRelease(SubtransControlLock);

	return;
	}

	/*
	* Record the parent of a subtransaction in the subtrans log.
	*/
	void
	SubTransSetParent(TransactionId xid, TransactionId parent)
	{
	int pageno = TransactionIdToPage(xid);
	int entryno = TransactionIdToEntry(xid);
	int slotno;
	SubTransData *ptr;
	SubTransData subData;

	/*
	* Main Xact has parent and topMostParent as InvalidTransactionId
	*/
	if ( parent != InvalidTransactionId )
	{
	/* Get the topMostParent for Parent */
	SubTransGetData(parent, &subData);
	}
	else
	{
	subData.topMostParent = InvalidTransactionId;
	}

	LWLockAcquire(SubtransControlLock, LW_EXCLUSIVE);

	slotno = SimpleLruReadPage(SubTransCtl, pageno, xid);
	ptr = (SubTransData *) SubTransCtl->shared->page_buffer[slotno];
	ptr += entryno;

	/* Current state should be 0 */
	Assert(ptr->parent == InvalidTransactionId);
	Assert(ptr->topMostParent == InvalidTransactionId);

	ptr->parent = parent;
	ptr->topMostParent = subData.topMostParent;

	SubTransCtl->shared->page_dirty[slotno] = true;

	LWLockRelease(SubtransControlLock);
	}

	/*
	* Interrogate the parent of a transaction in the subtrans log.
	*/
	TransactionId
	SubTransGetParent(TransactionId xid)
	{
	SubTransData subData;
	SubTransGetData(xid, &subData);

	return subData.parent;
	}

	/*
	* SubTransGetTopmostTransaction
	*
	* Returns the topmost transaction of the given transaction id.
	*/
	TransactionId
	SubTransGetTopmostTransaction(TransactionId xid)
	{
	Assert(TransactionIdIsValid(xid));
	SubTransData subData;
	SubTransGetData(xid, &subData);

	Assert(TransactionIdIsValid(subData.topMostParent));
	return subData.topMostParent;
	}

	/*
	* Initialization of shared memory for SUBTRANS
	*/
	Size
	SUBTRANSShmemSize(void)
	{
	return SimpleLruShmemSize(NUM_SUBTRANS_BUFFERS);
	}

	void
	SUBTRANSShmemInit(void)
	{
	SubTransCtl->PagePrecedes = SubTransPagePrecedes;
	SimpleLruInit(SubTransCtl, "SUBTRANS Ctl", NUM_SUBTRANS_BUFFERS,
	SubtransControlLock, SUBTRANS_DIR);
	/* Override default assumption that writes should be fsync'd */
	SubTransCtl->do_fsync = false;
	}

	/*
	* This func must be called ONCE on system install. It creates
	* the initial SUBTRANS segment. (The SUBTRANS directory is assumed to
	* have been created by the initdb shell script, and SUBTRANSShmemInit
	* must have been called already.)
	*
	* Note: it's not really necessary to create the initial segment now,
	* since slru.c would create it on first write anyway. But we may as well
	* do it to be sure the directory is set up correctly.
	*/
	void
	BootStrapSUBTRANS(void)
	{
	int slotno;

	LWLockAcquire(SubtransControlLock, LW_EXCLUSIVE);

	/* Create and zero the first page of the subtrans log */
	slotno = ZeroSUBTRANSPage(0);

	/* Make sure it's written out */
	SimpleLruWritePage(SubTransCtl, slotno, NULL);
	Assert(!SubTransCtl->shared->page_dirty[slotno]);

	LWLockRelease(SubtransControlLock);
	}

	/*
	* Initialize (or reinitialize) a page of SUBTRANS to zeroes.
	*
	* The page is not actually written, just set up in shared memory.
	* The slot number of the new page is returned.
	*
	* Control lock must be held at entry, and will be held at exit.
	*/
	static int
	ZeroSUBTRANSPage(int pageno)
	{
	int result;

	result = SimpleLruZeroPage(SubTransCtl, pageno);

	return result;
	}

	/*
	* This must be called ONCE during postmaster or standalone-backend startup,
	* after StartupXLOG has initialized ShmemVariableCache->nextXid.
	*
	* oldestActiveXID is the oldest XID of any prepared transaction, or nextXid
	* if there are none.
	*/
	void
	StartupSUBTRANS(TransactionId oldestActiveXID)
	{
	int startPage;
	int endPage;

	/*
	* Since we don't expect pg_subtrans to be valid across crashes, we
	* initialize the currently-active page(s) to zeroes during startup.
	* Whenever we advance into a new page, ExtendSUBTRANS will likewise zero
	* the new page without regard to whatever was previously on disk.
	*/

	LWLockAcquire(SubtransControlLock, LW_EXCLUSIVE);

	startPage = TransactionIdToPage(oldestActiveXID);
	endPage = TransactionIdToPage(ShmemVariableCache->nextXid);

	while (startPage != endPage)
	{
	(void) ZeroSUBTRANSPage(startPage);
	startPage++;
	}
	(void) ZeroSUBTRANSPage(startPage);

	LWLockRelease(SubtransControlLock);
	}

	/*
	* This must be called ONCE during postmaster or standalone-backend shutdown
	*/
	void
	ShutdownSUBTRANS(void)
	{
	/*
	* Flush dirty SUBTRANS pages to disk
	*
	* This is not actually necessary from a correctness point of view. We do
	* it merely as a debugging aid.
	*/
	SimpleLruFlush(SubTransCtl, false);
	}

	/*
	* Perform a checkpoint --- either during shutdown, or on-the-fly
	*/
	void
	CheckPointSUBTRANS(void)
	{
	/*
	* Flush dirty SUBTRANS pages to disk
	*
	* This is not actually necessary from a correctness point of view. We do
	* it merely to improve the odds that writing of dirty pages is done by
	* the checkpoint process and not by backends.
	*/
	SimpleLruFlush(SubTransCtl, true);
	}


	/*
	* Make sure that SUBTRANS has room for a newly-allocated XID.
	*
	* NB: this is called while holding XidGenLock. We want it to be very fast
	* most of the time; even when it's not so fast, no actual I/O need happen
	* unless we're forced to write out a dirty subtrans page to make room
	* in shared memory.
	*/
	void
	ExtendSUBTRANS(TransactionId newestXact)
	{
	int pageno;

	/*
	* Caller must have already taken mirrored lock shared.
	*/

	/*
	* No work except at first XID of a page. But beware: just after
	* wraparound, the first XID of page zero is FirstNormalTransactionId.
	*/
	if (TransactionIdToEntry(newestXact) != 0 &&
	!TransactionIdEquals(newestXact, FirstNormalTransactionId))
	return;

	pageno = TransactionIdToPage(newestXact);

	LWLockAcquire(SubtransControlLock, LW_EXCLUSIVE);

	/* Zero the page */
	ZeroSUBTRANSPage(pageno);

	LWLockRelease(SubtransControlLock);

	}


	/*
	* Remove all SUBTRANS segments before the one holding the passed transaction ID
	*
	* This is normally called during checkpoint, with oldestXact being the
	* oldest TransactionXmin of any running transaction.
	*/
	void
	TruncateSUBTRANS(TransactionId oldestXact)
	{
	int cutoffPage;

	/*
	* The cutoff point is the start of the segment containing oldestXact. We
	* pass the page containing oldestXact to SimpleLruTruncate.
	*/
	cutoffPage = TransactionIdToPage(oldestXact);

	SimpleLruTruncate(SubTransCtl, cutoffPage, false);
	}


	/*
	* Decide which of two SUBTRANS page numbers is "older" for truncation purposes.
	*
	* We need to use comparison of TransactionIds here in order to do the right
	* thing with wraparound XID arithmetic. However, if we are asked about
	* page number zero, we don't want to hand InvalidTransactionId to
	* TransactionIdPrecedes: it'll get weird about permanent xact IDs. So,
	* offset both xids by FirstNormalTransactionId to avoid that.
	*/
	static bool
	SubTransPagePrecedes(int page1, int page2)
	{
	TransactionId xid1;
	TransactionId xid2;

	xid1 = ((uint32) page1) * SUBTRANS_XACTS_PER_PAGE;
	xid1 += FirstNormalTransactionId;
	xid2 = ((uint32) page2) * SUBTRANS_XACTS_PER_PAGE;
	xid2 += FirstNormalTransactionId;

	return TransactionIdPrecedes(xid1, xid2);
	}