twophase.c

postgresql8.3.4源码,开源数据库

/*-------------------------------------------------------------------------
 *
 * twophase.c
 *		Two-phase commit support functions.
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *		$PostgreSQL: pgsql/src/backend/access/transam/twophase.c,v 1.39.2.2 2008/05/19 18:16:46 heikki Exp $
 *
 * NOTES
 *		Each global transaction is associated with a global transaction
 *		identifier (GID). The client assigns a GID to a postgres
 *		transaction with the PREPARE TRANSACTION command.
 *
 *		We keep all active global transactions in a shared memory array.
 *		When the PREPARE TRANSACTION command is issued, the GID is
 *		reserved for the transaction in the array. This is done before
 *		a WAL entry is made, because the reservation checks for duplicate
 *		GIDs and aborts the transaction if there already is a global
 *		transaction in prepared state with the same GID.
 *
 *		A global transaction (gxact) also has a dummy PGPROC that is entered
 *		into the ProcArray array; this is what keeps the XID considered
 *		running by TransactionIdIsInProgress.  It is also convenient as a
 *		PGPROC to hook the gxact's locks to.
 *
 *		In order to survive crashes and shutdowns, all prepared
 *		transactions must be stored in permanent storage. This includes
 *		locking information, pending notifications etc. All that state
 *		information is written to the per-transaction state file in
 *		the pg_twophase directory.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <time.h>
#include <unistd.h>

#include "access/heapam.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "access/twophase.h"
#include "access/twophase_rmgr.h"
#include "access/xact.h"
#include "catalog/pg_type.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/fd.h"
#include "storage/procarray.h"
#include "storage/smgr.h"
#include "utils/builtins.h"
#include "utils/memutils.h"


/*
 * Directory where Two-phase commit files reside within PGDATA
 */
#define TWOPHASE_DIR "pg_twophase"

/* GUC variable, can't be changed after startup */
int			max_prepared_xacts = 5;

/*
 * This struct describes one global transaction that is in prepared state
 * or attempting to become prepared.
 *
 * The first component of the struct is a dummy PGPROC that is inserted
 * into the global ProcArray so that the transaction appears to still be
 * running and holding locks.  It must be first because we cast pointers
 * to PGPROC and pointers to GlobalTransactionData back and forth.
 *
 * The lifecycle of a global transaction is:
 *
 * 1. After checking that the requested GID is not in use, set up an
 * entry in the TwoPhaseState->prepXacts array with the correct XID and GID,
 * with locking_xid = my own XID and valid = false.
 *
 * 2. After successfully completing prepare, set valid = true and enter the
 * contained PGPROC into the global ProcArray.
 *
 * 3. To begin COMMIT PREPARED or ROLLBACK PREPARED, check that the entry
 * is valid and its locking_xid is no longer active, then store my current
 * XID into locking_xid.  This prevents concurrent attempts to commit or
 * rollback the same prepared xact.
 *
 * 4. On completion of COMMIT PREPARED or ROLLBACK PREPARED, remove the entry
 * from the ProcArray and the TwoPhaseState->prepXacts array and return it to
 * the freelist.
 *
 * Note that if the preparing transaction fails between steps 1 and 2, the
 * entry will remain in prepXacts until recycled.  We can detect recyclable
 * entries by checking for valid = false and locking_xid no longer active.
 *
 * typedef struct GlobalTransactionData *GlobalTransaction appears in
 * twophase.h
 */
#define GIDSIZE 200

typedef struct GlobalTransactionData
{
	PGPROC		proc;			/* dummy proc */
	TimestampTz prepared_at;	/* time of preparation */
	XLogRecPtr	prepare_lsn;	/* XLOG offset of prepare record */
	Oid			owner;			/* ID of user that executed the xact */
	TransactionId locking_xid;	/* top-level XID of backend working on xact */
	bool		valid;			/* TRUE if fully prepared */
	char		gid[GIDSIZE];	/* The GID assigned to the prepared xact */
} GlobalTransactionData;

/*
 * Two Phase Commit shared state.  Access to this struct is protected
 * by TwoPhaseStateLock.
 */
typedef struct TwoPhaseStateData
{
	/* Head of linked list of free GlobalTransactionData structs */
	SHMEM_OFFSET freeGXacts;

	/* Number of valid prepXacts entries. */
	int			numPrepXacts;

	/*
	 * There are max_prepared_xacts items in this array, but C wants a
	 * fixed-size array.
	 */
	GlobalTransaction prepXacts[1];		/* VARIABLE LENGTH ARRAY */
} TwoPhaseStateData;			/* VARIABLE LENGTH STRUCT */

static TwoPhaseStateData *TwoPhaseState;


static void RecordTransactionCommitPrepared(TransactionId xid,
								int nchildren,
								TransactionId *children,
								int nrels,
								RelFileNode *rels);
static void RecordTransactionAbortPrepared(TransactionId xid,
							   int nchildren,
							   TransactionId *children,
							   int nrels,
							   RelFileNode *rels);
static void ProcessRecords(char *bufptr, TransactionId xid,
			   const TwoPhaseCallback callbacks[]);


/*
 * Initialization of shared memory
 */
Size
TwoPhaseShmemSize(void)
{
	Size		size;

	/* Need the fixed struct, the array of pointers, and the GTD structs */
	size = offsetof(TwoPhaseStateData, prepXacts);
	size = add_size(size, mul_size(max_prepared_xacts,
								   sizeof(GlobalTransaction)));
	size = MAXALIGN(size);
	size = add_size(size, mul_size(max_prepared_xacts,
								   sizeof(GlobalTransactionData)));

	return size;
}

void
TwoPhaseShmemInit(void)
{
	bool		found;

	TwoPhaseState = ShmemInitStruct("Prepared Transaction Table",
									TwoPhaseShmemSize(),
									&found);
	if (!IsUnderPostmaster)
	{
		GlobalTransaction gxacts;
		int			i;

		Assert(!found);
		TwoPhaseState->freeGXacts = INVALID_OFFSET;
		TwoPhaseState->numPrepXacts = 0;

		/*
		 * Initialize the linked list of free GlobalTransactionData structs
		 */
		gxacts = (GlobalTransaction)
			((char *) TwoPhaseState +
			 MAXALIGN(offsetof(TwoPhaseStateData, prepXacts) +
					  sizeof(GlobalTransaction) * max_prepared_xacts));
		for (i = 0; i < max_prepared_xacts; i++)
		{
			gxacts[i].proc.links.next = TwoPhaseState->freeGXacts;
			TwoPhaseState->freeGXacts = MAKE_OFFSET(&gxacts[i]);
		}
	}
	else
		Assert(found);
}


/*
 * MarkAsPreparing
 *		Reserve the GID for the given transaction.
 *
 * Internally, this creates a gxact struct and puts it into the active array.
 * NOTE: this is also used when reloading a gxact after a crash; so avoid
 * assuming that we can use very much backend context.
 */
GlobalTransaction
MarkAsPreparing(TransactionId xid, const char *gid,
				TimestampTz prepared_at, Oid owner, Oid databaseid)
{
	GlobalTransaction gxact;
	int			i;

	if (strlen(gid) >= GIDSIZE)
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
				 errmsg("transaction identifier \"%s\" is too long",
						gid)));

	LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);

	/*
	 * First, find and recycle any gxacts that failed during prepare. We do
	 * this partly to ensure we don't mistakenly say their GIDs are still
	 * reserved, and partly so we don't fail on out-of-slots unnecessarily.
	 */
	for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
	{
		gxact = TwoPhaseState->prepXacts[i];
		if (!gxact->valid && !TransactionIdIsActive(gxact->locking_xid))
		{
			/* It's dead Jim ... remove from the active array */
			TwoPhaseState->numPrepXacts--;
			TwoPhaseState->prepXacts[i] = TwoPhaseState->prepXacts[TwoPhaseState->numPrepXacts];
			/* and put it back in the freelist */
			gxact->proc.links.next = TwoPhaseState->freeGXacts;
			TwoPhaseState->freeGXacts = MAKE_OFFSET(gxact);
			/* Back up index count too, so we don't miss scanning one */
			i--;
		}
	}

	/* Check for conflicting GID */
	for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
	{
		gxact = TwoPhaseState->prepXacts[i];
		if (strcmp(gxact->gid, gid) == 0)
		{
			ereport(ERROR,
					(errcode(ERRCODE_DUPLICATE_OBJECT),
					 errmsg("transaction identifier \"%s\" is already in use",
							gid)));
		}
	}

	/* Get a free gxact from the freelist */
	if (TwoPhaseState->freeGXacts == INVALID_OFFSET)
		ereport(ERROR,
				(errcode(ERRCODE_OUT_OF_MEMORY),
				 errmsg("maximum number of prepared transactions reached"),
				 errhint("Increase max_prepared_transactions (currently %d).",
						 max_prepared_xacts)));
	gxact = (GlobalTransaction) MAKE_PTR(TwoPhaseState->freeGXacts);
	TwoPhaseState->freeGXacts = gxact->proc.links.next;

	/* Initialize it */
	MemSet(&gxact->proc, 0, sizeof(PGPROC));
	SHMQueueElemInit(&(gxact->proc.links));
	gxact->proc.waitStatus = STATUS_OK;
	/* We set up the gxact's VXID as InvalidBackendId/XID */
	gxact->proc.lxid = (LocalTransactionId) xid;
	gxact->proc.xid = xid;
	gxact->proc.xmin = InvalidTransactionId;
	gxact->proc.pid = 0;
	gxact->proc.backendId = InvalidBackendId;
	gxact->proc.databaseId = databaseid;
	gxact->proc.roleId = owner;
	gxact->proc.inCommit = false;
	gxact->proc.vacuumFlags = 0;
	gxact->proc.lwWaiting = false;
	gxact->proc.lwExclusive = false;
	gxact->proc.lwWaitLink = NULL;
	gxact->proc.waitLock = NULL;
	gxact->proc.waitProcLock = NULL;
	for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
		SHMQueueInit(&(gxact->proc.myProcLocks[i]));
	/* subxid data must be filled later by GXactLoadSubxactData */
	gxact->proc.subxids.overflowed = false;
	gxact->proc.subxids.nxids = 0;

	gxact->prepared_at = prepared_at;
	/* initialize LSN to 0 (start of WAL) */
	gxact->prepare_lsn.xlogid = 0;
	gxact->prepare_lsn.xrecoff = 0;
	gxact->owner = owner;
	gxact->locking_xid = xid;
	gxact->valid = false;
	strcpy(gxact->gid, gid);

	/* And insert it into the active array */
	Assert(TwoPhaseState->numPrepXacts < max_prepared_xacts);
	TwoPhaseState->prepXacts[TwoPhaseState->numPrepXacts++] = gxact;

	LWLockRelease(TwoPhaseStateLock);

	return gxact;
}

/*
 * GXactLoadSubxactData
 *
 * If the transaction being persisted had any subtransactions, this must
 * be called before MarkAsPrepared() to load information into the dummy
 * PGPROC.
 */
static void
GXactLoadSubxactData(GlobalTransaction gxact, int nsubxacts,
					 TransactionId *children)
{
	/* We need no extra lock since the GXACT isn't valid yet */
	if (nsubxacts > PGPROC_MAX_CACHED_SUBXIDS)
	{
		gxact->proc.subxids.overflowed = true;
		nsubxacts = PGPROC_MAX_CACHED_SUBXIDS;
	}
	if (nsubxacts > 0)
	{
		memcpy(gxact->proc.subxids.xids, children,
			   nsubxacts * sizeof(TransactionId));
		gxact->proc.subxids.nxids = nsubxacts;
	}
}

/*
 * MarkAsPrepared
 *		Mark the GXACT as fully valid, and enter it into the global ProcArray.
 */
static void
MarkAsPrepared(GlobalTransaction gxact)
{
	/* Lock here may be overkill, but I'm not convinced of that ... */
	LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
	Assert(!gxact->valid);
	gxact->valid = true;
	LWLockRelease(TwoPhaseStateLock);

	/*
	 * Put it into the global ProcArray so TransactionIdIsInProgress considers
	 * the XID as still running.
	 */
	ProcArrayAdd(&gxact->proc);
}

/*
 * LockGXact
 *		Locate the prepared transaction and mark it busy for COMMIT or PREPARE.
 */
static GlobalTransaction
LockGXact(const char *gid, Oid user)
{
	int			i;

	LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);

	for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
	{
		GlobalTransaction gxact = TwoPhaseState->prepXacts[i];

		/* Ignore not-yet-valid GIDs */
		if (!gxact->valid)
			continue;
		if (strcmp(gxact->gid, gid) != 0)
			continue;

		/* Found it, but has someone else got it locked? */
		if (TransactionIdIsValid(gxact->locking_xid))
		{
			if (TransactionIdIsActive(gxact->locking_xid))
				ereport(ERROR,
						(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				errmsg("prepared transaction with identifier \"%s\" is busy",
					   gid)));
			gxact->locking_xid = InvalidTransactionId;
		}

		if (user != gxact->owner && !superuser_arg(user))
			ereport(ERROR,
					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
				  errmsg("permission denied to finish prepared transaction"),
					 errhint("Must be superuser or the user that prepared the transaction.")));

		/*
		 * Note: it probably would be possible to allow committing from
		 * another database; but at the moment NOTIFY is known not to work and
		 * there may be some other issues as well.	Hence disallow until
		 * someone gets motivated to make it work.
		 */
		if (MyDatabaseId != gxact->proc.databaseId)
			ereport(ERROR,
					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				  errmsg("prepared transaction belongs to another database"),
					 errhint("Connect to the database where the transaction was prepared to finish it.")));

		/* OK for me to lock it */
		gxact->locking_xid = GetTopTransactionId();

		LWLockRelease(TwoPhaseStateLock);

		return gxact;
	}

	LWLockRelease(TwoPhaseStateLock);

	ereport(ERROR,
			(errcode(ERRCODE_UNDEFINED_OBJECT),
		 errmsg("prepared transaction with identifier \"%s\" does not exist",
				gid)));

	/* NOTREACHED */
	return NULL;
}

/*
 * RemoveGXact
 *		Remove the prepared transaction from the shared memory array.
 *
 * NB: caller should have already removed it from ProcArray
 */
static void
RemoveGXact(GlobalTransaction gxact)
{
	int			i;

	LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);

	for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
	{
		if (gxact == TwoPhaseState->prepXacts[i])
		{
			/* remove from the active array */
			TwoPhaseState->numPrepXacts--;
			TwoPhaseState->prepXacts[i] = TwoPhaseState->prepXacts[TwoPhaseState->numPrepXacts];

			/* and put it back in the freelist */
			gxact->proc.links.next = TwoPhaseState->freeGXacts;
			TwoPhaseState->freeGXacts = MAKE_OFFSET(gxact);