multixact.c

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/*-------------------------------------------------------------------------
 *
 * multixact.c
 *      PostgreSQL multi-transaction-log manager
 *
 * The pg_multixact manager is a pg_xact-like manager that stores an array of
 * MultiXactMember for each MultiXactId.  It is a fundamental part of the
 * shared-row-lock implementation.  Each MultiXactMember is comprised of a
 * TransactionId and a set of flag bits.  The name is a bit historical:
 * originally, a MultiXactId consisted of more than one TransactionId (except
 * in rare corner cases), hence "multi".  Nowadays, however, it's perfectly
 * legitimate to have MultiXactIds that only include a single Xid.
 *
 * The meaning of the flag bits is opaque to this module, but they are mostly
 * used in heapam.c to identify lock modes that each of the member transactions
 * is holding on any given tuple.  This module just contains support to store
 * and retrieve the arrays.
 *
 * We use two SLRU areas, one for storing the offsets at which the data
 * starts for each MultiXactId in the other one.  This trick allows us to
 * store variable length arrays of TransactionIds.  (We could alternatively
 * use one area containing counts and TransactionIds, with valid MultiXactId
 * values pointing at slots containing counts; but that way seems less robust
 * since it would get completely confused if someone inquired about a bogus
 * MultiXactId that pointed to an intermediate slot containing an XID.)
 *
 * XLOG interactions: this module generates a record whenever a new OFFSETs or
 * MEMBERs page is initialized to zeroes, as well as an
 * XLOG_MULTIXACT_CREATE_ID record whenever a new MultiXactId is defined.
 * This module ignores the WAL rule "write xlog before data," because it
 * suffices that actions recording a MultiXactId in a heap xmax do follow that
 * rule.  The only way for the MXID to be referenced from any data page is for
 * heap_lock_tuple() or heap_update() to have put it there, and each generates
 * an XLOG record that must follow ours.  The normal LSN interlock between the
 * data page and that XLOG record will ensure that our XLOG record reaches
 * disk first.  If the SLRU members/offsets data reaches disk sooner than the
 * XLOG records, we do not care; after recovery, no xmax will refer to it.  On
 * the flip side, to ensure that all referenced entries _do_ reach disk, this
 * module's XLOG records completely rebuild the data entered since the last
 * checkpoint.  We flush and sync all dirty OFFSETs and MEMBERs pages to disk
 * before each checkpoint is considered complete.
 *
 * Like clog.c, and unlike subtrans.c, we have to preserve state across
 * crashes and ensure that MXID and offset numbering increases monotonically
 * across a crash.  We do this in the same way as it's done for transaction
 * IDs: the WAL record is guaranteed to contain evidence of every MXID we
 * could need to worry about, and we just make sure that at the end of
 * replay, the next-MXID and next-offset counters are at least as large as
 * anything we saw during replay.
 *
 * We are able to remove segments no longer necessary by carefully tracking
 * each table's used values: during vacuum, any multixact older than a certain
 * value is removed; the cutoff value is stored in pg_class.  The minimum value
 * across all tables in each database is stored in pg_database, and the global
 * minimum across all databases is part of pg_control and is kept in shared
 * memory.  Whenever that minimum is advanced, the SLRUs are truncated.
 *
 * When new multixactid values are to be created, care is taken that the
 * counter does not fall within the wraparound horizon considering the global
 * minimum value.
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/multixact.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/multixact.h"
#include "access/multixact_internal.h"
#include "access/slru.h"
#include "access/twophase.h"
#include "access/twophase_rmgr.h"
#include "access/xlog.h"
#include "access/xloginsert.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "utils/guc_hooks.h"
#include "utils/injection_point.h"
#include "utils/lsyscache.h"
#include "utils/memutils.h"
 
 
/*
 * Thresholds used to keep members disk usage in check when multixids have a
 * lot of members.  When MULTIXACT_MEMBER_LOW_THRESHOLD is reached, vacuum
 * starts freezing multixids more aggressively, even if the normal multixid
 * age limits haven't been reached yet.
 */
#define MULTIXACT_MEMBER_LOW_THRESHOLD      UINT64CONST(2000000000)
#define MULTIXACT_MEMBER_HIGH_THRESHOLD     UINT64CONST(4000000000)
 
static inline MultiXactId
NextMultiXactId(MultiXactId multi)
{
    return multi == MaxMultiXactId ? FirstMultiXactId : multi + 1;
}
 
static inline MultiXactId
PreviousMultiXactId(MultiXactId multi)
{
    return multi == FirstMultiXactId ? MaxMultiXactId : multi - 1;
}
 
/*
 * Links to shared-memory data structures for MultiXact control
 */
static SlruCtlData MultiXactOffsetCtlData;
static SlruCtlData MultiXactMemberCtlData;
 
#define MultiXactOffsetCtl  (&MultiXactOffsetCtlData)
#define MultiXactMemberCtl  (&MultiXactMemberCtlData)
 
/*
 * MultiXact state shared across all backends.  All this state is protected
 * by MultiXactGenLock.  (We also use SLRU bank's lock of MultiXactOffset and
 * MultiXactMember to guard accesses to the two sets of SLRU buffers.  For
 * concurrency's sake, we avoid holding more than one of these locks at a
 * time.)
 */
typedef struct MultiXactStateData
{
    /* next-to-be-assigned MultiXactId */
    MultiXactId nextMXact;
 
    /* next-to-be-assigned offset */
    MultiXactOffset nextOffset;
 
    /* Have we completed multixact startup? */
    bool        finishedStartup;
 
    /*
     * Oldest multixact that is still potentially referenced by a relation.
     * Anything older than this should not be consulted.  These values are
     * updated by vacuum.
     */
    MultiXactId oldestMultiXactId;
    Oid         oldestMultiXactDB;
 
    /*
     * Oldest multixact offset that is potentially referenced by a multixact
     * referenced by a relation.
     */
    MultiXactOffset oldestOffset;
 
    /* support for anti-wraparound measures */
    MultiXactId multiVacLimit;
    MultiXactId multiWarnLimit;
    MultiXactId multiStopLimit;
    MultiXactId multiWrapLimit;
 
    /*
     * Per-backend data starts here.  We have two arrays stored in the area
     * immediately following the MultiXactStateData struct:
     *
     * OldestMemberMXactId[k] is the oldest MultiXactId each backend's current
     * transaction(s) could possibly be a member of, or InvalidMultiXactId
     * when the backend has no live transaction that could possibly be a
     * member of a MultiXact.  Each backend sets its entry to the current
     * nextMXact counter just before first acquiring a shared lock in a given
     * transaction, and clears it at transaction end. (This works because only
     * during or after acquiring a shared lock could an XID possibly become a
     * member of a MultiXact, and that MultiXact would have to be created
     * during or after the lock acquisition.)
     *
     * In the OldestMemberMXactId array, there's a slot for all normal
     * backends (0..MaxBackends-1) followed by a slot for max_prepared_xacts
     * prepared transactions.
     *
     * OldestVisibleMXactId[k] is the oldest MultiXactId each backend's
     * current transaction(s) think is potentially live, or InvalidMultiXactId
     * when not in a transaction or not in a transaction that's paid any
     * attention to MultiXacts yet.  This is computed when first needed in a
     * given transaction, and cleared at transaction end.  We can compute it
     * as the minimum of the valid OldestMemberMXactId[] entries at the time
     * we compute it (using nextMXact if none are valid).  Each backend is
     * required not to attempt to access any SLRU data for MultiXactIds older
     * than its own OldestVisibleMXactId[] setting; this is necessary because
     * the relevant SLRU data can be concurrently truncated away.
     *
     * In the OldestVisibleMXactId array, there's a slot for all normal
     * backends (0..MaxBackends-1) only. No slots for prepared transactions.
     *
     * The oldest valid value among all of the OldestMemberMXactId[] and
     * OldestVisibleMXactId[] entries is considered by vacuum as the earliest
     * possible value still having any live member transaction -- OldestMxact.
     * Any value older than that is typically removed from tuple headers, or
     * "frozen" via being replaced with a new xmax.  VACUUM can sometimes even
     * remove an individual MultiXact xmax whose value is >= its OldestMxact
     * cutoff, though typically only when no individual member XID is still
     * running.  See FreezeMultiXactId for full details.
     *
     * Whenever VACUUM advances relminmxid, then either its OldestMxact cutoff
     * or the oldest extant Multi remaining in the table is used as the new
     * pg_class.relminmxid value (whichever is earlier).  The minimum of all
     * relminmxid values in each database is stored in pg_database.datminmxid.
     * In turn, the minimum of all of those values is stored in pg_control.
     * This is used as the truncation point for pg_multixact when unneeded
     * segments get removed by vac_truncate_clog() during vacuuming.
     */
    MultiXactId perBackendXactIds[FLEXIBLE_ARRAY_MEMBER];
} MultiXactStateData;
 
/*
 * Sizes of OldestMemberMXactId and OldestVisibleMXactId arrays.
 */
#define NumMemberSlots      (MaxBackends + max_prepared_xacts)
#define NumVisibleSlots     MaxBackends
 
/* Pointers to the state data in shared memory */
static MultiXactStateData *MultiXactState;
static MultiXactId *OldestMemberMXactId;
static MultiXactId *OldestVisibleMXactId;
 
 
static inline MultiXactId *
MyOldestMemberMXactIdSlot(void)
{
    /*
     * The first MaxBackends entries in the OldestMemberMXactId array are
     * reserved for regular backends.  MyProcNumber should index into one of
     * them.
     */
    Assert(MyProcNumber >= 0 && MyProcNumber < MaxBackends);
    return &OldestMemberMXactId[MyProcNumber];
}
 
static inline MultiXactId *
PreparedXactOldestMemberMXactIdSlot(ProcNumber procno)
{
    int         prepared_xact_idx;
 
    Assert(procno >= FIRST_PREPARED_XACT_PROC_NUMBER);
    prepared_xact_idx = procno - FIRST_PREPARED_XACT_PROC_NUMBER;
 
    /*
     * The first MaxBackends entries in the OldestMemberMXactId array are
     * reserved for regular backends.  Prepared xacts come after them.
     */
    Assert(MaxBackends + prepared_xact_idx < NumMemberSlots);
    return &OldestMemberMXactId[MaxBackends + prepared_xact_idx];
}
 
static inline MultiXactId *
MyOldestVisibleMXactIdSlot(void)
{
    Assert(MyProcNumber >= 0 && MyProcNumber < NumVisibleSlots);
    return &OldestVisibleMXactId[MyProcNumber];
}
 
/*
 * Definitions for the backend-local MultiXactId cache.
 *
 * We use this cache to store known MultiXacts, so we don't need to go to
 * SLRU areas every time.
 *
 * The cache lasts for the duration of a single transaction, the rationale
 * for this being that most entries will contain our own TransactionId and
 * so they will be uninteresting by the time our next transaction starts.
 * (XXX not clear that this is correct --- other members of the MultiXact
 * could hang around longer than we did.  However, it's not clear what a
 * better policy for flushing old cache entries would be.)  FIXME actually
 * this is plain wrong now that multixact's may contain update Xids.
 *
 * We allocate the cache entries in a memory context that is deleted at
 * transaction end, so we don't need to do retail freeing of entries.
 */
typedef struct mXactCacheEnt
{
    MultiXactId multi;
    int         nmembers;
    dlist_node  node;
    MultiXactMember members[FLEXIBLE_ARRAY_MEMBER];
} mXactCacheEnt;
 
#define MAX_CACHE_ENTRIES   256
static dclist_head MXactCache = DCLIST_STATIC_INIT(MXactCache);
static MemoryContext MXactContext = NULL;
 
#ifdef MULTIXACT_DEBUG
#define debug_elog2(a,b) elog(a,b)
#define debug_elog3(a,b,c) elog(a,b,c)
#define debug_elog4(a,b,c,d) elog(a,b,c,d)
#define debug_elog5(a,b,c,d,e) elog(a,b,c,d,e)
#define debug_elog6(a,b,c,d,e,f) elog(a,b,c,d,e,f)
#else
#define debug_elog2(a,b)
#define debug_elog3(a,b,c)
#define debug_elog4(a,b,c,d)
#define debug_elog5(a,b,c,d,e)
#define debug_elog6(a,b,c,d,e,f)
#endif
 
/* internal MultiXactId management */
static void MultiXactIdSetOldestVisible(void);
static void RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
                               int nmembers, MultiXactMember *members);
static MultiXactId GetNewMultiXactId(int nmembers, MultiXactOffset *offset);
 
/* MultiXact cache management */
static int  mxactMemberComparator(const void *arg1, const void *arg2);
static MultiXactId mXactCacheGetBySet(int nmembers, MultiXactMember *members);
static int  mXactCacheGetById(MultiXactId multi, MultiXactMember **members);
static void mXactCachePut(MultiXactId multi, int nmembers,
                          MultiXactMember *members);
 
/* management of SLRU infrastructure */
 
/* opaque_data type for MultiXactMemberIoErrorDetail */
typedef struct MultiXactMemberSlruReadContext
{
    MultiXactId multi;
    MultiXactOffset offset;
} MultiXactMemberSlruReadContext;
 
static bool MultiXactOffsetPagePrecedes(int64 page1, int64 page2);
static bool MultiXactMemberPagePrecedes(int64 page1, int64 page2);
static int  MultiXactOffsetIoErrorDetail(const void *opaque_data);
static int  MultiXactMemberIoErrorDetail(const void *opaque_data);
static void ExtendMultiXactOffset(MultiXactId multi);
static void ExtendMultiXactMember(MultiXactOffset offset, int nmembers);
static void SetOldestOffset(void);
static bool find_multixact_start(MultiXactId multi, MultiXactOffset *result);
static void WriteMTruncateXlogRec(Oid oldestMultiDB,
                                  MultiXactId endTruncOff,
                                  MultiXactOffset endTruncMemb);
 
 
/*
 * MultiXactIdCreate
 *      Construct a MultiXactId representing two TransactionIds.
 *
 * The two XIDs must be different, or be requesting different statuses.
 *
 * NB - we don't worry about our local MultiXactId cache here, because that
 * is handled by the lower-level routines.
 */
MultiXactId
MultiXactIdCreate(TransactionId xid1, MultiXactStatus status1,
                  TransactionId xid2, MultiXactStatus status2)
{
    MultiXactId newMulti;
    MultiXactMember members[2];
 
    Assert(TransactionIdIsValid(xid1));
    Assert(TransactionIdIsValid(xid2));
 
    Assert(!TransactionIdEquals(xid1, xid2) || (status1 != status2));
 
    /* MultiXactIdSetOldestMember() must have been called already. */
    Assert(MultiXactIdIsValid(*MyOldestMemberMXactIdSlot()));
 
    /*
     * Note: unlike MultiXactIdExpand, we don't bother to check that both XIDs
     * are still running.  In typical usage, xid2 will be our own XID and the
     * caller just did a check on xid1, so it'd be wasted effort.
     */
 
    members[0].xid = xid1;
    members[0].status = status1;
    members[1].xid = xid2;
    members[1].status = status2;
 
    newMulti = MultiXactIdCreateFromMembers(2, members);
 
    debug_elog3(DEBUG2, "Create: %s",
                mxid_to_string(newMulti, 2, members));
 
    return newMulti;
}
 
/*
 * MultiXactIdExpand
 *      Add a TransactionId to a pre-existing MultiXactId.
 *
 * If the TransactionId is already a member of the passed MultiXactId with the
 * same status, just return it as-is.
 *
 * Note that we do NOT actually modify the membership of a pre-existing
 * MultiXactId; instead we create a new one.  This is necessary to avoid
 * a race condition against code trying to wait for one MultiXactId to finish;
 * see notes in heapam.c.
 *
 * NB - we don't worry about our local MultiXactId cache here, because that
 * is handled by the lower-level routines.
 *
 * Note: It is critical that MultiXactIds that come from an old cluster (i.e.
 * one upgraded by pg_upgrade from a cluster older than this feature) are not
 * passed in.
 */
MultiXactId
MultiXactIdExpand(MultiXactId multi, TransactionId xid, MultiXactStatus status)
{
    MultiXactId newMulti;
    MultiXactMember *members;
    MultiXactMember *newMembers;
    int         nmembers;
    int         i;
    int         j;
 
    Assert(MultiXactIdIsValid(multi));
    Assert(TransactionIdIsValid(xid));
 
    /* MultiXactIdSetOldestMember() must have been called already. */
    Assert(MultiXactIdIsValid(*MyOldestMemberMXactIdSlot()));
 
    debug_elog5(DEBUG2, "Expand: received multi %u, xid %u status %s",
                multi, xid, mxstatus_to_string(status));
 
    /*
     * Note: we don't allow for old multis here.  The reason is that the only
     * caller of this function does a check that the multixact is no longer
     * running.
     */
    nmembers = GetMultiXactIdMembers(multi, &members, false, false);
 
    if (nmembers < 0)
    {
        MultiXactMember member;
 
        /*
         * The MultiXactId is obsolete.  This can only happen if all the
         * MultiXactId members stop running between the caller checking and
         * passing it to us.  It would be better to return that fact to the
         * caller, but it would complicate the API and it's unlikely to happen
         * too often, so just deal with it by creating a singleton MultiXact.
         */
        member.xid = xid;
        member.status = status;
        newMulti = MultiXactIdCreateFromMembers(1, &member);
 
        debug_elog4(DEBUG2, "Expand: %u has no members, create singleton %u",
                    multi, newMulti);
        return newMulti;
    }
 
    /*
     * If the TransactionId is already a member of the MultiXactId with the
     * same status, just return the existing MultiXactId.
     */
    for (i = 0; i < nmembers; i++)
    {
        if (TransactionIdEquals(members[i].xid, xid) &&
            (members[i].status == status))
        {
            debug_elog4(DEBUG2, "Expand: %u is already a member of %u",
                        xid, multi);
            pfree(members);
            return multi;
        }
    }
 
    /*
     * Determine which of the members of the MultiXactId are still of
     * interest. This is any running transaction, and also any transaction
     * that grabbed something stronger than just a lock and was committed. (An
     * update that aborted is of no interest here; and having more than one
     * update Xid in a multixact would cause errors elsewhere.)
     *
     * Removing dead members is not just an optimization: freezing of tuples
     * whose Xmax are multis depends on this behavior.
     *
     * Note we have the same race condition here as above: j could be 0 at the
     * end of the loop.
     */
    newMembers = palloc_array(MultiXactMember, nmembers + 1);
 
    for (i = 0, j = 0; i < nmembers; i++)
    {
        if (TransactionIdIsInProgress(members[i].xid) ||
            (ISUPDATE_from_mxstatus(members[i].status) &&
             TransactionIdDidCommit(members[i].xid)))
        {
            newMembers[j].xid = members[i].xid;
            newMembers[j++].status = members[i].status;
        }
    }
 
    newMembers[j].xid = xid;
    newMembers[j++].status = status;
    newMulti = MultiXactIdCreateFromMembers(j, newMembers);
 
    pfree(members);
    pfree(newMembers);
 
    debug_elog3(DEBUG2, "Expand: returning new multi %u", newMulti);
 
    return newMulti;
}
 
/*
 * MultiXactIdIsRunning
 *      Returns whether a MultiXactId is "running".
 *
 * We return true if at least one member of the given MultiXactId is still
 * running.  Note that a "false" result is certain not to change,
 * because it is not legal to add members to an existing MultiXactId.
 *
 * Caller is expected to have verified that the multixact does not come from
 * a pg_upgraded share-locked tuple.
 */
bool
MultiXactIdIsRunning(MultiXactId multi, bool isLockOnly)
{
    MultiXactMember *members;
    int         nmembers;
    int         i;
 
    debug_elog3(DEBUG2, "IsRunning %u?", multi);
 
    /*
     * "false" here means we assume our callers have checked that the given
     * multi cannot possibly come from a pg_upgraded database.
     */
    nmembers = GetMultiXactIdMembers(multi, &members, false, isLockOnly);
 
    if (nmembers <= 0)
    {
        debug_elog2(DEBUG2, "IsRunning: no members");
        return false;
    }
 
    /*
     * Checking for myself is cheap compared to looking in shared memory;
     * return true if any live subtransaction of the current top-level
     * transaction is a member.
     *
     * This is not needed for correctness, it's just a fast path.
     */
    for (i = 0; i < nmembers; i++)
    {
        if (TransactionIdIsCurrentTransactionId(members[i].xid))
        {
            debug_elog3(DEBUG2, "IsRunning: I (%d) am running!", i);
            pfree(members);
            return true;
        }
    }
 
    /*
     * This could be made faster by having another entry point in procarray.c,
     * walking the PGPROC array only once for all the members.  But in most
     * cases nmembers should be small enough that it doesn't much matter.
     */
    for (i = 0; i < nmembers; i++)
    {
        if (TransactionIdIsInProgress(members[i].xid))
        {
            debug_elog4(DEBUG2, "IsRunning: member %d (%u) is running",
                        i, members[i].xid);
            pfree(members);
            return true;
        }
    }
 
    pfree(members);
 
    debug_elog3(DEBUG2, "IsRunning: %u is not running", multi);
 
    return false;
}
 
/*
 * MultiXactIdSetOldestMember
 *      Save the oldest MultiXactId this transaction could be a member of.
 *
 * We set the OldestMemberMXactId for a given transaction the first time it's
 * going to do some operation that might require a MultiXactId (tuple lock,
 * update or delete).  We need to do this even if we end up using a
 * TransactionId instead of a MultiXactId, because there is a chance that
 * another transaction would add our XID to a MultiXactId.
 *
 * The value to set is the next-to-be-assigned MultiXactId, so this is meant to
 * be called just before doing any such possibly-MultiXactId-able operation.
 */
void
MultiXactIdSetOldestMember(void)
{
    if (!MultiXactIdIsValid(*MyOldestMemberMXactIdSlot()))
    {
        MultiXactId nextMXact;
 
        /*
         * You might think we don't need to acquire a lock here, since
         * fetching and storing of TransactionIds is probably atomic, but in
         * fact we do: suppose we pick up nextMXact and then lose the CPU for
         * a long time.  Someone else could advance nextMXact, and then
         * another someone else could compute an OldestVisibleMXactId that
         * would be after the value we are going to store when we get control
         * back.  Which would be wrong.
         *
         * Note that a shared lock is sufficient, because it's enough to stop
         * someone from advancing nextMXact; and nobody else could be trying
         * to write to our OldestMember entry, only reading (and we assume
         * storing it is atomic.)
         */
        LWLockAcquire(MultiXactGenLock, LW_SHARED);
 
        nextMXact = MultiXactState->nextMXact;
 
        *MyOldestMemberMXactIdSlot() = nextMXact;
 
        LWLockRelease(MultiXactGenLock);
 
        debug_elog4(DEBUG2, "MultiXact: setting OldestMember[%d] = %u",
                    MyProcNumber, nextMXact);
    }
}
 
/*
 * MultiXactIdSetOldestVisible
 *      Save the oldest MultiXactId this transaction considers possibly live.
 *
 * We set the OldestVisibleMXactId for a given transaction the first time
 * it's going to inspect any MultiXactId.  Once we have set this, we are
 * guaranteed that SLRU data for MultiXactIds >= our own OldestVisibleMXactId
 * won't be truncated away.
 *
 * The value to set is the oldest of nextMXact and all the valid per-backend
 * OldestMemberMXactId[] entries.  Because of the locking we do, we can be
 * certain that no subsequent call to MultiXactIdSetOldestMember can set
 * an OldestMemberMXactId[] entry older than what we compute here.  Therefore
 * there is no live transaction, now or later, that can be a member of any
 * MultiXactId older than the OldestVisibleMXactId we compute here.
 */
static void
MultiXactIdSetOldestVisible(void)
{
    if (!MultiXactIdIsValid(*MyOldestVisibleMXactIdSlot()))
    {
        MultiXactId oldestMXact;
        int         i;
 
        LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
 
        oldestMXact = MultiXactState->nextMXact;
        for (i = 0; i < NumMemberSlots; i++)
        {
            MultiXactId thisoldest = OldestMemberMXactId[i];
 
            if (MultiXactIdIsValid(thisoldest) &&
                MultiXactIdPrecedes(thisoldest, oldestMXact))
                oldestMXact = thisoldest;
        }
 
        *MyOldestVisibleMXactIdSlot() = oldestMXact;
 
        LWLockRelease(MultiXactGenLock);
 
        debug_elog4(DEBUG2, "MultiXact: setting OldestVisible[%d] = %u",
                    MyProcNumber, oldestMXact);
    }
}
 
/*
 * ReadNextMultiXactId
 *      Return the next MultiXactId to be assigned, but don't allocate it
 */
MultiXactId
ReadNextMultiXactId(void)
{
    MultiXactId mxid;
 
    /* XXX we could presumably do this without a lock. */
    LWLockAcquire(MultiXactGenLock, LW_SHARED);
    mxid = MultiXactState->nextMXact;
    LWLockRelease(MultiXactGenLock);
 
    return mxid;
}
 
/*
 * ReadMultiXactIdRange
 *      Get the range of IDs that may still be referenced by a relation.
 */
void
ReadMultiXactIdRange(MultiXactId *oldest, MultiXactId *next)
{
    LWLockAcquire(MultiXactGenLock, LW_SHARED);
    *oldest = MultiXactState->oldestMultiXactId;
    *next = MultiXactState->nextMXact;
    LWLockRelease(MultiXactGenLock);
}
 
 
/*
 * MultiXactIdCreateFromMembers
 *      Make a new MultiXactId from the specified set of members
 *
 * Make XLOG, SLRU and cache entries for a new MultiXactId, recording the
 * given TransactionIds as members.  Returns the newly created MultiXactId.
 *
 * NB: the passed members[] array will be sorted in-place.
 */
MultiXactId
MultiXactIdCreateFromMembers(int nmembers, MultiXactMember *members)
{
    MultiXactId multi;
    MultiXactOffset offset;
    xl_multixact_create xlrec;
 
    debug_elog3(DEBUG2, "Create: %s",
                mxid_to_string(InvalidMultiXactId, nmembers, members));
 
    /*
     * See if the same set of members already exists in our cache; if so, just
     * re-use that MultiXactId.  (Note: it might seem that looking in our
     * cache is insufficient, and we ought to search disk to see if a
     * duplicate definition already exists.  But since we only ever create
     * MultiXacts containing our own XID, in most cases any such MultiXacts
     * were in fact created by us, and so will be in our cache.  There are
     * corner cases where someone else added us to a MultiXact without our
     * knowledge, but it's not worth checking for.)
     */
    multi = mXactCacheGetBySet(nmembers, members);
    if (MultiXactIdIsValid(multi))
    {
        debug_elog2(DEBUG2, "Create: in cache!");
        return multi;
    }
 
    /* Verify that there is a single update Xid among the given members. */
    {
        int         i;
        bool        has_update = false;
 
        for (i = 0; i < nmembers; i++)
        {
            if (ISUPDATE_from_mxstatus(members[i].status))
            {
                if (has_update)
                    elog(ERROR, "new multixact has more than one updating member: %s",
                         mxid_to_string(InvalidMultiXactId, nmembers, members));
                has_update = true;
            }
        }
    }
 
    /* Load the injection point before entering the critical section */
    INJECTION_POINT_LOAD("multixact-create-from-members");
 
    /*
     * Assign the MXID and offsets range to use, and make sure there is space
     * in the OFFSETs and MEMBERs files.  NB: this routine does
     * START_CRIT_SECTION().
     *
     * Note: unlike MultiXactIdCreate and MultiXactIdExpand, we do not check
     * that we've called MultiXactIdSetOldestMember here.  This is because
     * this routine is used in some places to create new MultiXactIds of which
     * the current backend is not a member, notably during freezing of multis
     * in vacuum.  During vacuum, in particular, it would be unacceptable to
     * keep OldestMulti set, in case it runs for long.
     */
    multi = GetNewMultiXactId(nmembers, &offset);
 
    INJECTION_POINT_CACHED("multixact-create-from-members", NULL);
 
    /* Make an XLOG entry describing the new MXID. */
    xlrec.mid = multi;
    xlrec.moff = offset;
    xlrec.nmembers = nmembers;
 
    /*
     * XXX Note: there's a lot of padding space in MultiXactMember.  We could
     * find a more compact representation of this Xlog record -- perhaps all
     * the status flags in one XLogRecData, then all the xids in another one?
     * Not clear that it's worth the trouble though.
     */
    XLogBeginInsert();
    XLogRegisterData(&xlrec, SizeOfMultiXactCreate);
    XLogRegisterData(members, nmembers * sizeof(MultiXactMember));
 
    (void) XLogInsert(RM_MULTIXACT_ID, XLOG_MULTIXACT_CREATE_ID);
 
    /* Now enter the information into the OFFSETs and MEMBERs logs */
    RecordNewMultiXact(multi, offset, nmembers, members);
 
    /* Done with critical section */
    END_CRIT_SECTION();
 
    /* Store the new MultiXactId in the local cache, too */
    mXactCachePut(multi, nmembers, members);
 
    debug_elog2(DEBUG2, "Create: all done");
 
    return multi;
}
 
/*
 * RecordNewMultiXact
 *      Write info about a new multixact into the offsets and members files
 *
 * This is broken out of MultiXactIdCreateFromMembers so that xlog replay can
 * use it.
 */
static void
RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
                   int nmembers, MultiXactMember *members)
{
    int64       pageno;
    int64       prev_pageno;
    int         entryno;
    int         slotno;
    MultiXactOffset *offptr;
    MultiXactId next;
    int64       next_pageno;
    int         next_entryno;
    MultiXactOffset *next_offptr;
    MultiXactOffset next_offset;
    LWLock     *lock;
    LWLock     *prevlock = NULL;
 
    /* position of this multixid in the offsets SLRU area  */
    pageno = MultiXactIdToOffsetPage(multi);
    entryno = MultiXactIdToOffsetEntry(multi);
 
    /* position of the next multixid */
    next = NextMultiXactId(multi);
    next_pageno = MultiXactIdToOffsetPage(next);
    next_entryno = MultiXactIdToOffsetEntry(next);
 
    /*
     * Set the starting offset of this multixid's members.
     *
     * In the common case, it was already set by the previous
     * RecordNewMultiXact call, as this was the next multixid of the previous
     * multixid.  But if multiple backends are generating multixids
     * concurrently, we might race ahead and get called before the previous
     * multixid.
     */
    lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
    LWLockAcquire(lock, LW_EXCLUSIVE);
 
    slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, &multi);
    offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
    offptr += entryno;
 
    if (*offptr != offset)
    {
        /* should already be set to the correct value, or not at all */
        Assert(*offptr == 0);
        *offptr = offset;
        MultiXactOffsetCtl->shared->page_dirty[slotno] = true;
    }
 
    /*
     * Set the next multixid's offset to the end of this multixid's members.
     */
    if (next_pageno == pageno)
    {
        next_offptr = offptr + 1;
    }
    else
    {
        /* must be the first entry on the page */
        Assert(next_entryno == 0 || next == FirstMultiXactId);
 
        /* Swap the lock for a lock on the next page */
        LWLockRelease(lock);
        lock = SimpleLruGetBankLock(MultiXactOffsetCtl, next_pageno);
        LWLockAcquire(lock, LW_EXCLUSIVE);
 
        slotno = SimpleLruReadPage(MultiXactOffsetCtl, next_pageno, true, &next);
        next_offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
        next_offptr += next_entryno;
    }
 
    /* Like in GetNewMultiXactId(), skip over offset 0 */
    next_offset = offset + nmembers;
    if (next_offset == 0)
        next_offset = 1;
    if (*next_offptr != next_offset)
    {
        /* should already be set to the correct value, or not at all */
        Assert(*next_offptr == 0);
        *next_offptr = next_offset;
        MultiXactOffsetCtl->shared->page_dirty[slotno] = true;
    }
 
    /* Release MultiXactOffset SLRU lock. */
    LWLockRelease(lock);
 
    prev_pageno = -1;
 
    for (int i = 0; i < nmembers; i++, offset++)
    {
        TransactionId *memberptr;
        uint32     *flagsptr;
        uint32      flagsval;
        int         bshift;
        int         flagsoff;
        int         memberoff;
 
        Assert(members[i].status <= MultiXactStatusUpdate);
 
        pageno = MXOffsetToMemberPage(offset);
        memberoff = MXOffsetToMemberOffset(offset);
        flagsoff = MXOffsetToFlagsOffset(offset);
        bshift = MXOffsetToFlagsBitShift(offset);
 
        if (pageno != prev_pageno)
        {
            MultiXactMemberSlruReadContext slru_read_context = {multi, offset};
 
            /*
             * MultiXactMember SLRU page is changed so check if this new page
             * fall into the different SLRU bank then release the old bank's
             * lock and acquire lock on the new bank.
             */
            lock = SimpleLruGetBankLock(MultiXactMemberCtl, pageno);
            if (lock != prevlock)
            {
                if (prevlock != NULL)
                    LWLockRelease(prevlock);
 
                LWLockAcquire(lock, LW_EXCLUSIVE);
                prevlock = lock;
            }
            slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true,
                                       &slru_read_context);
            prev_pageno = pageno;
        }
 
        memberptr = (TransactionId *)
            (MultiXactMemberCtl->shared->page_buffer[slotno] + memberoff);
 
        *memberptr = members[i].xid;
 
        flagsptr = (uint32 *)
            (MultiXactMemberCtl->shared->page_buffer[slotno] + flagsoff);
 
        flagsval = *flagsptr;
        flagsval &= ~(((1 << MXACT_MEMBER_BITS_PER_XACT) - 1) << bshift);
        flagsval |= (members[i].status << bshift);
        *flagsptr = flagsval;
 
        MultiXactMemberCtl->shared->page_dirty[slotno] = true;
    }
 
    if (prevlock != NULL)
        LWLockRelease(prevlock);
}
 
/*
 * GetNewMultiXactId
 *      Get the next MultiXactId.
 *
 * Also, reserve the needed amount of space in the "members" area.  The
 * starting offset of the reserved space is returned in *offset.
 *
 * This may generate XLOG records for expansion of the offsets and/or members
 * files.  Unfortunately, we have to do that while holding MultiXactGenLock
 * to avoid race conditions --- the XLOG record for zeroing a page must appear
 * before any backend can possibly try to store data in that page!
 *
 * We start a critical section before advancing the shared counters.  The
 * caller must end the critical section after writing SLRU data.
 */
static MultiXactId
GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
{
    MultiXactId result;
    MultiXactOffset nextOffset;
 
    debug_elog3(DEBUG2, "GetNew: for %d xids", nmembers);
 
    /* safety check, we should never get this far in a HS standby */
    if (RecoveryInProgress())
        elog(ERROR, "cannot assign MultiXactIds during recovery");
 
    LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
 
    /* Assign the MXID */
    result = MultiXactState->nextMXact;
 
    /*----------
     * Check to see if it's safe to assign another MultiXactId.  This protects
     * against catastrophic data loss due to multixact wraparound.  The basic
     * rules are:
     *
     * If we're past multiVacLimit or the safe threshold for member storage
     * space, or we don't know what the safe threshold for member storage is,
     * start trying to force autovacuum cycles.
     * If we're past multiWarnLimit, start issuing warnings.
     * If we're past multiStopLimit, refuse to create new MultiXactIds.
     *
     * Note these are pretty much the same protections in GetNewTransactionId.
     *----------
     */
    if (!MultiXactIdPrecedes(result, MultiXactState->multiVacLimit))
    {
        /*
         * For safety's sake, we release MultiXactGenLock while sending
         * signals, warnings, etc.  This is not so much because we care about
         * preserving concurrency in this situation, as to avoid any
         * possibility of deadlock while doing get_database_name(). First,
         * copy all the shared values we'll need in this path.
         */
        MultiXactId multiWarnLimit = MultiXactState->multiWarnLimit;
        MultiXactId multiStopLimit = MultiXactState->multiStopLimit;
        MultiXactId multiWrapLimit = MultiXactState->multiWrapLimit;
        Oid         oldest_datoid = MultiXactState->oldestMultiXactDB;
 
        LWLockRelease(MultiXactGenLock);
 
        if (IsUnderPostmaster &&
            !MultiXactIdPrecedes(result, multiStopLimit))
        {
            char       *oldest_datname = get_database_name(oldest_datoid);
 
            /*
             * Immediately kick autovacuum into action as we're already in
             * ERROR territory.
             */
            SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
 
            /* complain even if that DB has disappeared */
            if (oldest_datname)
                ereport(ERROR,
                        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                         errmsg("database is not accepting commands that assign new MultiXactIds to avoid wraparound data loss in database \"%s\"",
                                oldest_datname),
                         errhint("Execute a database-wide VACUUM in that database.\n"
                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
            else
                ereport(ERROR,
                        (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                         errmsg("database is not accepting commands that assign new MultiXactIds to avoid wraparound data loss in database with OID %u",
                                oldest_datoid),
                         errhint("Execute a database-wide VACUUM in that database.\n"
                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
        }
 
        /*
         * To avoid swamping the postmaster with signals, we issue the autovac
         * request only once per 64K multis generated.  This still gives
         * plenty of chances before we get into real trouble.
         */
        if (IsUnderPostmaster && ((result % 65536) == 0 || result == FirstMultiXactId))
            SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
 
        if (!MultiXactIdPrecedes(result, multiWarnLimit))
        {
            char       *oldest_datname = get_database_name(oldest_datoid);
 
            /* complain even if that DB has disappeared */
            if (oldest_datname)
                ereport(WARNING,
                        (errmsg_plural("database \"%s\" must be vacuumed before %u more MultiXactId is used",
                                       "database \"%s\" must be vacuumed before %u more MultiXactIds are used",
                                       multiWrapLimit - result,
                                       oldest_datname,
                                       multiWrapLimit - result),
                         errdetail("Approximately %.2f%% of MultiXactIds are available for use.",
                                   (double) (multiWrapLimit - result) / (MaxMultiXactId / 2) * 100),
                         errhint("Execute a database-wide VACUUM in that database.\n"
                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
            else
                ereport(WARNING,
                        (errmsg_plural("database with OID %u must be vacuumed before %u more MultiXactId is used",
                                       "database with OID %u must be vacuumed before %u more MultiXactIds are used",
                                       multiWrapLimit - result,
                                       oldest_datoid,
                                       multiWrapLimit - result),
                         errdetail("Approximately %.2f%% of MultiXactIds are available for use.",
                                   (double) (multiWrapLimit - result) / (MaxMultiXactId / 2) * 100),
                         errhint("Execute a database-wide VACUUM in that database.\n"
                                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
        }
 
        /* Re-acquire lock and start over */
        LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
        result = MultiXactState->nextMXact;
    }
 
    /*
     * Make sure there is room for the next MXID in the file.  Assigning this
     * MXID sets the next MXID's offset already.
     */
    ExtendMultiXactOffset(NextMultiXactId(result));
 
    /*
     * Reserve the members space, similarly to above.
     */
    nextOffset = MultiXactState->nextOffset;
 
    /*
     * Offsets are 64-bit integers and will never wrap around.  Firstly, it
     * would take an unrealistic amount of time and resources to consume 2^64
     * offsets.  Secondly, multixid creation is WAL-logged, so you would run
     * out of LSNs before reaching offset wraparound.  Nevertheless, check for
     * wraparound as a sanity check.
     */
    if (nextOffset + nmembers < nextOffset)
        ereport(ERROR,
                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                 errmsg("MultiXact members would wrap around")));
    *offset = nextOffset;
 
    ExtendMultiXactMember(nextOffset, nmembers);
 
    /*
     * Critical section from here until caller has written the data into the
     * just-reserved SLRU space; we don't want to error out with a partly
     * written MultiXact structure.  (In particular, failing to write our
     * start offset after advancing nextMXact would effectively corrupt the
     * previous MultiXact.)
     */
    START_CRIT_SECTION();
 
    /*
     * Advance counters.  As in GetNewTransactionId(), this must not happen
     * until after file extension has succeeded!
     */
    MultiXactState->nextMXact = NextMultiXactId(result);
    MultiXactState->nextOffset += nmembers;
 
    LWLockRelease(MultiXactGenLock);
 
    debug_elog4(DEBUG2, "GetNew: returning %u offset %" PRIu64,
                result, *offset);
    return result;
}
 
/*
 * GetMultiXactIdMembers
 *      Return the set of MultiXactMembers that make up a MultiXactId
 *
 * Return value is the number of members found, or -1 if there are none,
 * and *members is set to a newly palloc'ed array of members.  It's the
 * caller's responsibility to free it when done with it.
 *
 * from_pgupgrade must be passed as true if and only if only the multixact
 * corresponds to a value from a tuple that was locked in a 9.2-or-older
 * installation and later pg_upgrade'd (that is, the infomask is
 * HEAP_LOCKED_UPGRADED).  In this case, we know for certain that no members
 * can still be running, so we return -1 just like for an empty multixact
 * without any further checking.  It would be wrong to try to resolve such a
 * multixact: either the multixact is within the current valid multixact
 * range, in which case the returned result would be bogus, or outside that
 * range, in which case an error would be raised.
 *
 * In all other cases, the passed multixact must be within the known valid
 * range, that is, greater than or equal to oldestMultiXactId, and less than
 * nextMXact.  Otherwise, an error is raised.
 *
 * isLockOnly must be set to true if caller is certain that the given multi
 * is used only to lock tuples; can be false without loss of correctness,
 * but passing a true means we can return quickly without checking for
 * old updates.
 */
int
GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
                      bool from_pgupgrade, bool isLockOnly)
{
    int64       pageno;
    int64       prev_pageno;
    int         entryno;
    int         slotno;
    MultiXactOffset *offptr;
    MultiXactOffset offset;
    MultiXactOffset nextMXOffset;
    int         length;
    MultiXactId oldestMXact;
    MultiXactId nextMXact;
    MultiXactMember *ptr;
    LWLock     *lock;
 
    debug_elog3(DEBUG2, "GetMembers: asked for %u", multi);
 
    if (!MultiXactIdIsValid(multi) || from_pgupgrade)
    {
        *members = NULL;
        return -1;
    }
 
    /* See if the MultiXactId is in the local cache */
    length = mXactCacheGetById(multi, members);
    if (length >= 0)
    {
        debug_elog3(DEBUG2, "GetMembers: found %s in the cache",
                    mxid_to_string(multi, length, *members));
        return length;
    }
 
    /* Set our OldestVisibleMXactId[] entry if we didn't already */
    MultiXactIdSetOldestVisible();
 
    /*
     * If we know the multi is used only for locking and not for updates, then
     * we can skip checking if the value is older than our oldest visible
     * multi.  It cannot possibly still be running.
     */
    if (isLockOnly &&
        MultiXactIdPrecedes(multi, *MyOldestVisibleMXactIdSlot()))
    {
        debug_elog2(DEBUG2, "GetMembers: a locker-only multi is too old");
        *members = NULL;
        return -1;
    }
 
    /*
     * We check known limits on MultiXact before resorting to the SLRU area.
     *
     * An ID older than MultiXactState->oldestMultiXactId cannot possibly be
     * useful; it has already been removed, or will be removed shortly, by
     * truncation.  If one is passed, an error is raised.
     *
     * Also, an ID >= nextMXact shouldn't ever be seen here; if it is seen, it
     * implies undetected ID wraparound has occurred.  This raises a hard
     * error.
     *
     * Shared lock is enough here since we aren't modifying any global state.
     * Acquire it just long enough to grab the current counter values.
     */
    LWLockAcquire(MultiXactGenLock, LW_SHARED);
 
    oldestMXact = MultiXactState->oldestMultiXactId;
    nextMXact = MultiXactState->nextMXact;
 
    LWLockRelease(MultiXactGenLock);
 
    if (MultiXactIdPrecedes(multi, oldestMXact))
        ereport(ERROR,
                (errcode(ERRCODE_INTERNAL_ERROR),
                 errmsg("MultiXactId %u does no longer exist -- apparent wraparound",
                        multi)));
 
    if (!MultiXactIdPrecedes(multi, nextMXact))
        ereport(ERROR,
                (errcode(ERRCODE_INTERNAL_ERROR),
                 errmsg("MultiXactId %u has not been created yet -- apparent wraparound",
                        multi)));
 
    /*
     * Find out the offset at which we need to start reading MultiXactMembers
     * and the number of members in the multixact.  We determine the latter as
     * the difference between this multixact's starting offset and the next
     * one's.
     */
    pageno = MultiXactIdToOffsetPage(multi);
    entryno = MultiXactIdToOffsetEntry(multi);
 
    /* Acquire the bank lock for the page we need. */
    lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
    LWLockAcquire(lock, LW_EXCLUSIVE);
 
    /* read this multi's offset */
    slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, &multi);
    offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
    offptr += entryno;
    offset = *offptr;
 
    if (offset == 0)
        ereport(ERROR,
                (errcode(ERRCODE_DATA_CORRUPTED),
                 errmsg("MultiXact %u has invalid offset", multi)));
 
    /* read next multi's offset */
    {
        MultiXactId tmpMXact;
 
        /* handle wraparound if needed */
        tmpMXact = NextMultiXactId(multi);
 
        prev_pageno = pageno;
 
        pageno = MultiXactIdToOffsetPage(tmpMXact);
        entryno = MultiXactIdToOffsetEntry(tmpMXact);
 
        if (pageno != prev_pageno)
        {
            LWLock     *newlock;
 
            /*
             * Since we're going to access a different SLRU page, if this page
             * falls under a different bank, release the old bank's lock and
             * acquire the lock of the new bank.
             */
            newlock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
            if (newlock != lock)
            {
                LWLockRelease(lock);
                LWLockAcquire(newlock, LW_EXCLUSIVE);
                lock = newlock;
            }
            slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, &tmpMXact);
        }
 
        offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
        offptr += entryno;
        nextMXOffset = *offptr;
    }
 
    LWLockRelease(lock);
    lock = NULL;
 
    /* Sanity check the next offset */
    if (nextMXOffset == 0)
        ereport(ERROR,
                (errcode(ERRCODE_DATA_CORRUPTED),
                 errmsg("MultiXact %u has invalid next offset", multi)));
    if (nextMXOffset == offset)
        ereport(ERROR,
                (errcode(ERRCODE_DATA_CORRUPTED),
                 errmsg("MultiXact %u with offset (%" PRIu64 ") has zero members",
                        multi, offset)));
    if (nextMXOffset < offset)
        ereport(ERROR,
                (errcode(ERRCODE_DATA_CORRUPTED),
                 errmsg("MultiXact %u has offset (%" PRIu64 ") greater than its next offset (%" PRIu64 ")",
                        multi, offset, nextMXOffset)));
    if (nextMXOffset - offset > INT32_MAX)
        ereport(ERROR,
                (errcode(ERRCODE_DATA_CORRUPTED),
                 errmsg("MultiXact %u has too many members (%" PRIu64 ")",
                        multi, nextMXOffset - offset)));
    length = nextMXOffset - offset;
 
    /* read the members */
    ptr = (MultiXactMember *) palloc(length * sizeof(MultiXactMember));
    prev_pageno = -1;
    for (int i = 0; i < length; i++, offset++)
    {
        TransactionId *xactptr;
        uint32     *flagsptr;
        int         flagsoff;
        int         bshift;
        int         memberoff;
 
        pageno = MXOffsetToMemberPage(offset);
        memberoff = MXOffsetToMemberOffset(offset);
 
        if (pageno != prev_pageno)
        {
            MultiXactMemberSlruReadContext slru_read_context = {multi, offset};
            LWLock     *newlock;
 
            /*
             * Since we're going to access a different SLRU page, if this page
             * falls under a different bank, release the old bank's lock and
             * acquire the lock of the new bank.
             */
            newlock = SimpleLruGetBankLock(MultiXactMemberCtl, pageno);
            if (newlock != lock)
            {
                if (lock)
                    LWLockRelease(lock);
                LWLockAcquire(newlock, LW_EXCLUSIVE);
                lock = newlock;
            }
            slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true,
                                       &slru_read_context);
            prev_pageno = pageno;
        }
 
        xactptr = (TransactionId *)
            (MultiXactMemberCtl->shared->page_buffer[slotno] + memberoff);
        Assert(TransactionIdIsValid(*xactptr));
 
        flagsoff = MXOffsetToFlagsOffset(offset);
        bshift = MXOffsetToFlagsBitShift(offset);
        flagsptr = (uint32 *) (MultiXactMemberCtl->shared->page_buffer[slotno] + flagsoff);
 
        ptr[i].xid = *xactptr;
        ptr[i].status = (*flagsptr >> bshift) & MXACT_MEMBER_XACT_BITMASK;
    }
 
    LWLockRelease(lock);
 
    /*
     * Copy the result into the local cache.
     */
    mXactCachePut(multi, length, ptr);
 
    debug_elog3(DEBUG2, "GetMembers: no cache for %s",
                mxid_to_string(multi, length, ptr));
    *members = ptr;
    return length;
}
 
/*
 * mxactMemberComparator
 *      qsort comparison function for MultiXactMember
 *
 * We can't use wraparound comparison for XIDs because that does not respect
 * the triangle inequality!  Any old sort order will do.
 */
static int
mxactMemberComparator(const void *arg1, const void *arg2)
{
    MultiXactMember member1 = *(const MultiXactMember *) arg1;
    MultiXactMember member2 = *(const MultiXactMember *) arg2;
 
    if (member1.xid > member2.xid)
        return 1;
    if (member1.xid < member2.xid)
        return -1;
    if (member1.status > member2.status)
        return 1;
    if (member1.status < member2.status)
        return -1;
    return 0;
}
 
/*
 * mXactCacheGetBySet
 *      returns a MultiXactId from the cache based on the set of
 *      TransactionIds that compose it, or InvalidMultiXactId if
 *      none matches.
 *
 * This is helpful, for example, if two transactions want to lock a huge
 * table.  By using the cache, the second will use the same MultiXactId
 * for the majority of tuples, thus keeping MultiXactId usage low (saving
 * both I/O and wraparound issues).
 *
 * NB: the passed members array will be sorted in-place.
 */
static MultiXactId
mXactCacheGetBySet(int nmembers, MultiXactMember *members)
{
    dlist_iter  iter;
 
    debug_elog3(DEBUG2, "CacheGet: looking for %s",
                mxid_to_string(InvalidMultiXactId, nmembers, members));
 
    /* sort the array so comparison is easy */
    qsort(members, nmembers, sizeof(MultiXactMember), mxactMemberComparator);
 
    dclist_foreach(iter, &MXactCache)
    {
        mXactCacheEnt *entry = dclist_container(mXactCacheEnt, node,
                                                iter.cur);
 
        if (entry->nmembers != nmembers)
            continue;
 
        /*
         * We assume the cache entries are sorted, and that the unused bits in
         * "status" are zeroed.
         */
        if (memcmp(members, entry->members, nmembers * sizeof(MultiXactMember)) == 0)
        {
            debug_elog3(DEBUG2, "CacheGet: found %u", entry->multi);
            dclist_move_head(&MXactCache, iter.cur);
            return entry->multi;
        }
    }
 
    debug_elog2(DEBUG2, "CacheGet: not found :-(");
    return InvalidMultiXactId;
}
 
/*
 * mXactCacheGetById
 *      returns the composing MultiXactMember set from the cache for a
 *      given MultiXactId, if present.
 *
 * If successful, *xids is set to the address of a palloc'd copy of the
 * MultiXactMember set.  Return value is number of members, or -1 on failure.
 */
static int
mXactCacheGetById(MultiXactId multi, MultiXactMember **members)
{
    dlist_iter  iter;
 
    debug_elog3(DEBUG2, "CacheGet: looking for %u", multi);
 
    dclist_foreach(iter, &MXactCache)
    {
        mXactCacheEnt *entry = dclist_container(mXactCacheEnt, node,
                                                iter.cur);
 
        if (entry->multi == multi)
        {
            MultiXactMember *ptr;
            Size        size;
 
            size = sizeof(MultiXactMember) * entry->nmembers;
            ptr = (MultiXactMember *) palloc(size);
 
            memcpy(ptr, entry->members, size);
 
            debug_elog3(DEBUG2, "CacheGet: found %s",
                        mxid_to_string(multi,
                                       entry->nmembers,
                                       entry->members));
 
            /*
             * Note we modify the list while not using a modifiable iterator.
             * This is acceptable only because we exit the iteration
             * immediately afterwards.
             */
            dclist_move_head(&MXactCache, iter.cur);
 
            *members = ptr;
            return entry->nmembers;
        }
    }
 
    debug_elog2(DEBUG2, "CacheGet: not found");
    return -1;
}
 
/*
 * mXactCachePut
 *      Add a new MultiXactId and its composing set into the local cache.
 */
static void
mXactCachePut(MultiXactId multi, int nmembers, MultiXactMember *members)
{
    mXactCacheEnt *entry;
 
    debug_elog3(DEBUG2, "CachePut: storing %s",
                mxid_to_string(multi, nmembers, members));
 
    if (MXactContext == NULL)
    {
        /* The cache only lives as long as the current transaction */
        debug_elog2(DEBUG2, "CachePut: initializing memory context");
        MXactContext = AllocSetContextCreate(TopTransactionContext,
                                             "MultiXact cache context",
                                             ALLOCSET_SMALL_SIZES);
    }
 
    entry = (mXactCacheEnt *)
        MemoryContextAlloc(MXactContext,
                           offsetof(mXactCacheEnt, members) +
                           nmembers * sizeof(MultiXactMember));
 
    entry->multi = multi;
    entry->nmembers = nmembers;
    memcpy(entry->members, members, nmembers * sizeof(MultiXactMember));
 
    /* mXactCacheGetBySet assumes the entries are sorted, so sort them */
    qsort(entry->members, nmembers, sizeof(MultiXactMember), mxactMemberComparator);
 
    dclist_push_head(&MXactCache, &entry->node);
    if (dclist_count(&MXactCache) > MAX_CACHE_ENTRIES)
    {
        dlist_node *node;
 
        node = dclist_tail_node(&MXactCache);
        dclist_delete_from(&MXactCache, node);
 
        entry = dclist_container(mXactCacheEnt, node, node);
        debug_elog3(DEBUG2, "CachePut: pruning cached multi %u",
                    entry->multi);
 
        pfree(entry);
    }
}
 
char *
mxstatus_to_string(MultiXactStatus status)
{
    switch (status)
    {
        case MultiXactStatusForKeyShare:
            return "keysh";
        case MultiXactStatusForShare:
            return "sh";
        case MultiXactStatusForNoKeyUpdate:
            return "fornokeyupd";
        case MultiXactStatusForUpdate:
            return "forupd";
        case MultiXactStatusNoKeyUpdate:
            return "nokeyupd";
        case MultiXactStatusUpdate:
            return "upd";
        default:
            elog(ERROR, "unrecognized multixact status %d", status);
            return "";
    }
}
 
char *
mxid_to_string(MultiXactId multi, int nmembers, MultiXactMember *members)
{
    static char *str = NULL;
    StringInfoData buf;
    int         i;
 
    if (str != NULL)
        pfree(str);
 
    initStringInfo(&buf);
 
    appendStringInfo(&buf, "%u %d[%u (%s)", multi, nmembers, members[0].xid,
                     mxstatus_to_string(members[0].status));
 
    for (i = 1; i < nmembers; i++)
        appendStringInfo(&buf, ", %u (%s)", members[i].xid,
                         mxstatus_to_string(members[i].status));
 
    appendStringInfoChar(&buf, ']');
    str = MemoryContextStrdup(TopMemoryContext, buf.data);
    pfree(buf.data);
    return str;
}
 
/*
 * AtEOXact_MultiXact
 *      Handle transaction end for MultiXact
 *
 * This is called at top transaction commit or abort (we don't care which).
 */
void
AtEOXact_MultiXact(void)
{
    /*
     * Reset our OldestMemberMXactId and OldestVisibleMXactId values, both of
     * which should only be valid while within a transaction.
     *
     * We assume that storing a MultiXactId is atomic and so we need not take
     * MultiXactGenLock to do this.
     */
    *MyOldestMemberMXactIdSlot() = InvalidMultiXactId;
    *MyOldestVisibleMXactIdSlot() = InvalidMultiXactId;
 
    /*
     * Discard the local MultiXactId cache.  Since MXactContext was created as
     * a child of TopTransactionContext, we needn't delete it explicitly.
     */
    MXactContext = NULL;
    dclist_init(&MXactCache);
}
 
/*
 * AtPrepare_MultiXact
 *      Save multixact state at 2PC transaction prepare
 *
 * In this phase, we only store our OldestMemberMXactId value in the two-phase
 * state file.
 */
void
AtPrepare_MultiXact(void)
{
    MultiXactId myOldestMember = *MyOldestMemberMXactIdSlot();
 
    if (MultiXactIdIsValid(myOldestMember))
        RegisterTwoPhaseRecord(TWOPHASE_RM_MULTIXACT_ID, 0,
                               &myOldestMember, sizeof(MultiXactId));
}
 
/*
 * PostPrepare_MultiXact
 *      Clean up after successful PREPARE TRANSACTION
 */
void
PostPrepare_MultiXact(FullTransactionId fxid)
{
    MultiXactId myOldestMember;
 
    /*
     * Transfer our OldestMemberMXactId value to the slot reserved for the
     * prepared transaction.
     */
    myOldestMember = *MyOldestMemberMXactIdSlot();
    if (MultiXactIdIsValid(myOldestMember))
    {
        ProcNumber  dummyProcNumber = TwoPhaseGetDummyProcNumber(fxid, false);
 
        /*
         * Even though storing MultiXactId is atomic, acquire lock to make
         * sure others see both changes, not just the reset of the slot of the
         * current backend. Using a volatile pointer might suffice, but this
         * isn't a hot spot.
         */
        LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
 
        *PreparedXactOldestMemberMXactIdSlot(dummyProcNumber) = myOldestMember;
        *MyOldestMemberMXactIdSlot() = InvalidMultiXactId;
 
        LWLockRelease(MultiXactGenLock);
    }
 
    /*
     * We don't need to transfer OldestVisibleMXactId value, because the
     * transaction is not going to be looking at any more multixacts once it's
     * prepared.
     *
     * We assume that storing a MultiXactId is atomic and so we need not take
     * MultiXactGenLock to do this.
     */
    *MyOldestVisibleMXactIdSlot() = InvalidMultiXactId;
 
    /*
     * Discard the local MultiXactId cache like in AtEOXact_MultiXact.
     */
    MXactContext = NULL;
    dclist_init(&MXactCache);
}
 
/*
 * multixact_twophase_recover
 *      Recover the state of a prepared transaction at startup
 */
void
multixact_twophase_recover(FullTransactionId fxid, uint16 info,
                           void *recdata, uint32 len)
{
    ProcNumber  dummyProcNumber = TwoPhaseGetDummyProcNumber(fxid, false);
    MultiXactId oldestMember;
 
    /*
     * Get the oldest member XID from the state file record, and set it in the
     * OldestMemberMXactId slot reserved for this prepared transaction.
     */
    Assert(len == sizeof(MultiXactId));
    oldestMember = *((MultiXactId *) recdata);
 
    *PreparedXactOldestMemberMXactIdSlot(dummyProcNumber) = oldestMember;
}
 
/*
 * multixact_twophase_postcommit
 *      Similar to AtEOXact_MultiXact but for COMMIT PREPARED
 */
void
multixact_twophase_postcommit(FullTransactionId fxid, uint16 info,
                              void *recdata, uint32 len)
{
    ProcNumber  dummyProcNumber = TwoPhaseGetDummyProcNumber(fxid, true);
 
    Assert(len == sizeof(MultiXactId));
 
    *PreparedXactOldestMemberMXactIdSlot(dummyProcNumber) = InvalidMultiXactId;
}
 
/*
 * multixact_twophase_postabort
 *      This is actually just the same as the COMMIT case.
 */
void
multixact_twophase_postabort(FullTransactionId fxid, uint16 info,
                             void *recdata, uint32 len)
{
    multixact_twophase_postcommit(fxid, info, recdata, len);
}
 
/*
 * Initialization of shared memory for MultiXact.
 *
 * MultiXactSharedStateShmemSize() calculates the size of the MultiXactState
 * struct, and the two per-backend MultiXactId arrays.  They are carved out of
 * the same allocation.  MultiXactShmemSize() additionally includes the memory
 * needed for the two SLRU areas.
 */
static Size
MultiXactSharedStateShmemSize(void)
{
    Size        size;
 
    size = offsetof(MultiXactStateData, perBackendXactIds);
    size = add_size(size,
                    mul_size(sizeof(MultiXactId), NumMemberSlots));
    size = add_size(size,
                    mul_size(sizeof(MultiXactId), NumVisibleSlots));
    return size;
}
 
Size
MultiXactShmemSize(void)
{
    Size        size;
 
    size = MultiXactSharedStateShmemSize();
    size = add_size(size, SimpleLruShmemSize(multixact_offset_buffers, 0));
    size = add_size(size, SimpleLruShmemSize(multixact_member_buffers, 0));
 
    return size;
}
 
void
MultiXactShmemInit(void)
{
    bool        found;
 
    debug_elog2(DEBUG2, "Shared Memory Init for MultiXact");
 
    MultiXactOffsetCtl->PagePrecedes = MultiXactOffsetPagePrecedes;
    MultiXactMemberCtl->PagePrecedes = MultiXactMemberPagePrecedes;
    MultiXactOffsetCtl->errdetail_for_io_error = MultiXactOffsetIoErrorDetail;
    MultiXactMemberCtl->errdetail_for_io_error = MultiXactMemberIoErrorDetail;
 
    SimpleLruInit(MultiXactOffsetCtl,
                  "multixact_offset", multixact_offset_buffers, 0,
                  "pg_multixact/offsets", LWTRANCHE_MULTIXACTOFFSET_BUFFER,
                  LWTRANCHE_MULTIXACTOFFSET_SLRU,
                  SYNC_HANDLER_MULTIXACT_OFFSET,
                  false);
    SlruPagePrecedesUnitTests(MultiXactOffsetCtl, MULTIXACT_OFFSETS_PER_PAGE);
    SimpleLruInit(MultiXactMemberCtl,
                  "multixact_member", multixact_member_buffers, 0,
                  "pg_multixact/members", LWTRANCHE_MULTIXACTMEMBER_BUFFER,
                  LWTRANCHE_MULTIXACTMEMBER_SLRU,
                  SYNC_HANDLER_MULTIXACT_MEMBER,
                  true);
    /* doesn't call SimpleLruTruncate() or meet criteria for unit tests */
 
    /* Initialize our shared state struct */
    MultiXactState = ShmemInitStruct("Shared MultiXact State",
                                     MultiXactSharedStateShmemSize(),
                                     &found);
    if (!IsUnderPostmaster)
    {
        Assert(!found);
 
        /* Make sure we zero out the per-backend state */
        MemSet(MultiXactState, 0, MultiXactSharedStateShmemSize());
    }
    else
        Assert(found);
 
    /*
     * Set up array pointers.
     */
    OldestMemberMXactId = MultiXactState->perBackendXactIds;
    OldestVisibleMXactId = OldestMemberMXactId + NumMemberSlots;
}
 
/*
 * GUC check_hook for multixact_offset_buffers
 */
bool
check_multixact_offset_buffers(int *newval, void **extra, GucSource source)
{
    return check_slru_buffers("multixact_offset_buffers", newval);
}
 
/*
 * GUC check_hook for multixact_member_buffers
 */
bool
check_multixact_member_buffers(int *newval, void **extra, GucSource source)
{
    return check_slru_buffers("multixact_member_buffers", newval);
}
 
/*
 * This func must be called ONCE on system install.  It creates the initial
 * MultiXact segments.  (The MultiXacts directories are assumed to have been
 * created by initdb, and MultiXactShmemInit must have been called already.)
 */
void
BootStrapMultiXact(void)
{
    /* Zero the initial pages and flush them to disk */
    SimpleLruZeroAndWritePage(MultiXactOffsetCtl, 0);
    SimpleLruZeroAndWritePage(MultiXactMemberCtl, 0);
}
 
/*
 * This must be called ONCE during postmaster or standalone-backend startup.
 *
 * StartupXLOG has already established nextMXact/nextOffset by calling
 * MultiXactSetNextMXact and/or MultiXactAdvanceNextMXact, and the oldestMulti
 * info from pg_control and/or MultiXactAdvanceOldest, but we haven't yet
 * replayed WAL.
 */
void
StartupMultiXact(void)
{
    MultiXactId multi = MultiXactState->nextMXact;
    MultiXactOffset offset = MultiXactState->nextOffset;
    int64       pageno;
 
    /*
     * Initialize offset's idea of the latest page number.
     */
    pageno = MultiXactIdToOffsetPage(multi);
    pg_atomic_write_u64(&MultiXactOffsetCtl->shared->latest_page_number,
                        pageno);
 
    /*
     * Initialize member's idea of the latest page number.
     */
    pageno = MXOffsetToMemberPage(offset);
    pg_atomic_write_u64(&MultiXactMemberCtl->shared->latest_page_number,
                        pageno);
}
 
/*
 * This must be called ONCE at the end of startup/recovery.
 */
void
TrimMultiXact(void)
{
    MultiXactId nextMXact;
    MultiXactOffset offset;
    MultiXactId oldestMXact;
    Oid         oldestMXactDB;
    int64       pageno;
    int         entryno;
    int         flagsoff;
 
    LWLockAcquire(MultiXactGenLock, LW_SHARED);
    nextMXact = MultiXactState->nextMXact;
    offset = MultiXactState->nextOffset;
    oldestMXact = MultiXactState->oldestMultiXactId;
    oldestMXactDB = MultiXactState->oldestMultiXactDB;
    LWLockRelease(MultiXactGenLock);
 
    /* Clean up offsets state */
 
    /*
     * (Re-)Initialize our idea of the latest page number for offsets.
     */
    pageno = MultiXactIdToOffsetPage(nextMXact);
    pg_atomic_write_u64(&MultiXactOffsetCtl->shared->latest_page_number,
                        pageno);
 
    /*
     * Set the offset of nextMXact on the offsets page.  This is normally done
     * in RecordNewMultiXact() of the previous multixact, but let's be sure
     * the next page exists, if the nextMXact was reset with pg_resetwal for
     * example.
     *
     * Zero out the remainder of the page.  See notes in TrimCLOG() for
     * background.  Unlike CLOG, some WAL record covers every pg_multixact
     * SLRU mutation.  Since, also unlike CLOG, we ignore the WAL rule "write
     * xlog before data," nextMXact successors may carry obsolete, nonzero
     * offset values.
     */
    entryno = MultiXactIdToOffsetEntry(nextMXact);
    {
        int         slotno;
        MultiXactOffset *offptr;
        LWLock     *lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
 
        LWLockAcquire(lock, LW_EXCLUSIVE);
        if (entryno == 0 || nextMXact == FirstMultiXactId)
            slotno = SimpleLruZeroPage(MultiXactOffsetCtl, pageno);
        else
            slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, &nextMXact);
        offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
        offptr += entryno;
 
        *offptr = offset;
        if (entryno != 0 && (entryno + 1) * sizeof(MultiXactOffset) != BLCKSZ)
            MemSet(offptr + 1, 0, BLCKSZ - (entryno + 1) * sizeof(MultiXactOffset));
 
        MultiXactOffsetCtl->shared->page_dirty[slotno] = true;
        LWLockRelease(lock);
    }
 
    /*
     * And the same for members.
     *
     * (Re-)Initialize our idea of the latest page number for members.
     */
    pageno = MXOffsetToMemberPage(offset);
    pg_atomic_write_u64(&MultiXactMemberCtl->shared->latest_page_number,
                        pageno);
 
    /*
     * Zero out the remainder of the current members page.  See notes in
     * TrimCLOG() for motivation.
     */
    flagsoff = MXOffsetToFlagsOffset(offset);
    if (flagsoff != 0)
    {
        MultiXactMemberSlruReadContext slru_read_context = {InvalidMultiXactId, offset};
        int         slotno;
        TransactionId *xidptr;
        int         memberoff;
        LWLock     *lock = SimpleLruGetBankLock(MultiXactMemberCtl, pageno);
 
        LWLockAcquire(lock, LW_EXCLUSIVE);
        memberoff = MXOffsetToMemberOffset(offset);
        slotno = SimpleLruReadPage(MultiXactMemberCtl, pageno, true, &slru_read_context);
        xidptr = (TransactionId *)
            (MultiXactMemberCtl->shared->page_buffer[slotno] + memberoff);
 
        MemSet(xidptr, 0, BLCKSZ - memberoff);
 
        /*
         * Note: we don't need to zero out the flag bits in the remaining
         * members of the current group, because they are always reset before
         * writing.
         */
 
        MultiXactMemberCtl->shared->page_dirty[slotno] = true;
        LWLockRelease(lock);
    }
 
    /* signal that we're officially up */
    LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
    MultiXactState->finishedStartup = true;
    LWLockRelease(MultiXactGenLock);
 
    /* Now compute how far away the next multixid wraparound is. */
    SetMultiXactIdLimit(oldestMXact, oldestMXactDB);
}
 
/*
 * Get the MultiXact data to save in a checkpoint record
 */
void
MultiXactGetCheckptMulti(bool is_shutdown,
                         MultiXactId *nextMulti,
                         MultiXactOffset *nextMultiOffset,
                         MultiXactId *oldestMulti,
                         Oid *oldestMultiDB)
{
    LWLockAcquire(MultiXactGenLock, LW_SHARED);
    *nextMulti = MultiXactState->nextMXact;
    *nextMultiOffset = MultiXactState->nextOffset;
    *oldestMulti = MultiXactState->oldestMultiXactId;
    *oldestMultiDB = MultiXactState->oldestMultiXactDB;
    LWLockRelease(MultiXactGenLock);
 
    debug_elog6(DEBUG2,
                "MultiXact: checkpoint is nextMulti %u, nextOffset %" PRIu64 ", oldestMulti %u in DB %u",
                *nextMulti, *nextMultiOffset, *oldestMulti, *oldestMultiDB);
}
 
/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 */
void
CheckPointMultiXact(void)
{
    TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_START(true);
 
    /*
     * Write dirty MultiXact pages to disk.  This may result in sync requests
     * queued for later handling by ProcessSyncRequests(), as part of the
     * checkpoint.
     */
    SimpleLruWriteAll(MultiXactOffsetCtl, true);
    SimpleLruWriteAll(MultiXactMemberCtl, true);
 
    TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_DONE(true);
}
 
/*
 * Set the next-to-be-assigned MultiXactId and offset
 *
 * This is used when we can determine the correct next ID/offset exactly
 * from a checkpoint record.  Although this is only called during bootstrap
 * and XLog replay, we take the lock in case any hot-standby backends are
 * examining the values.
 */
void
MultiXactSetNextMXact(MultiXactId nextMulti,
                      MultiXactOffset nextMultiOffset)
{
    Assert(MultiXactIdIsValid(nextMulti));
    debug_elog4(DEBUG2, "MultiXact: setting next multi to %u offset %" PRIu64,
                nextMulti, nextMultiOffset);
 
    LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
    MultiXactState->nextMXact = nextMulti;
    MultiXactState->nextOffset = nextMultiOffset;
    LWLockRelease(MultiXactGenLock);
}
 
/*
 * Determine the last safe MultiXactId to allocate given the currently oldest
 * datminmxid (ie, the oldest MultiXactId that might exist in any database
 * of our cluster), and the OID of the (or a) database with that value.
 *
 * This also updates MultiXactState->oldestOffset, by looking up the offset of
 * MultiXactState->oldestMultiXactId.
 */
void
SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid)
{
    MultiXactId multiVacLimit;
    MultiXactId multiWarnLimit;
    MultiXactId multiStopLimit;
    MultiXactId multiWrapLimit;
    MultiXactId curMulti;
 
    Assert(MultiXactIdIsValid(oldest_datminmxid));
 
    /*
     * We pretend that a wrap will happen halfway through the multixact ID
     * space, but that's not really true, because multixacts wrap differently
     * from transaction IDs.
     */
    multiWrapLimit = oldest_datminmxid + (MaxMultiXactId >> 1);
    if (multiWrapLimit < FirstMultiXactId)
        multiWrapLimit += FirstMultiXactId;
 
    /*
     * We'll refuse to continue assigning MultiXactIds once we get within 3M
     * multi of data loss.  See SetTransactionIdLimit.
     */
    multiStopLimit = multiWrapLimit - 3000000;
    if (multiStopLimit < FirstMultiXactId)
        multiStopLimit -= FirstMultiXactId;
 
    /*
     * We'll start complaining loudly when we get within 100M multis of data
     * loss.  This is kind of arbitrary, but if you let your gas gauge get
     * down to 5% of full, would you be looking for the next gas station?  We
     * need to be fairly liberal about this number because there are lots of
     * scenarios where most transactions are done by automatic clients that
     * won't pay attention to warnings.  (No, we're not gonna make this
     * configurable.  If you know enough to configure it, you know enough to
     * not get in this kind of trouble in the first place.)
     */
    multiWarnLimit = multiWrapLimit - 100000000;
    if (multiWarnLimit < FirstMultiXactId)
        multiWarnLimit -= FirstMultiXactId;
 
    /*
     * We'll start trying to force autovacuums when oldest_datminmxid gets to
     * be more than autovacuum_multixact_freeze_max_age mxids old.
     *
     * Note: autovacuum_multixact_freeze_max_age is a PGC_POSTMASTER parameter
     * so that we don't have to worry about dealing with on-the-fly changes in
     * its value.  See SetTransactionIdLimit.
     */
    multiVacLimit = oldest_datminmxid + autovacuum_multixact_freeze_max_age;
    if (multiVacLimit < FirstMultiXactId)
        multiVacLimit += FirstMultiXactId;
 
    /* Grab lock for just long enough to set the new limit values */
    LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
    MultiXactState->oldestMultiXactId = oldest_datminmxid;
    MultiXactState->oldestMultiXactDB = oldest_datoid;
    MultiXactState->multiVacLimit = multiVacLimit;
    MultiXactState->multiWarnLimit = multiWarnLimit;
    MultiXactState->multiStopLimit = multiStopLimit;
    MultiXactState->multiWrapLimit = multiWrapLimit;
    curMulti = MultiXactState->nextMXact;
    LWLockRelease(MultiXactGenLock);
 
    /* Log the info */
    ereport(DEBUG1,
            (errmsg_internal("MultiXactId wrap limit is %u, limited by database with OID %u",
                             multiWrapLimit, oldest_datoid)));
 
    /*
     * Computing the actual limits is only possible once the data directory is
     * in a consistent state. There's no need to compute the limits while
     * still replaying WAL - no decisions about new multis are made even
     * though multixact creations might be replayed. So we'll only do further
     * checks after TrimMultiXact() has been called.
     */
    if (!MultiXactState->finishedStartup)
        return;
 
    Assert(!InRecovery);
 
    /*
     * Offsets are 64-bits wide and never wrap around, so we don't need to
     * consider them for emergency autovacuum purposes.  But now that we're in
     * a consistent state, determine MultiXactState->oldestOffset.  It will be
     * used to adjust the freezing cutoff, to keep the offsets disk usage in
     * check.
     */
    SetOldestOffset();
 
    /*
     * If past the autovacuum force point, immediately signal an autovac
     * request.  The reason for this is that autovac only processes one
     * database per invocation.  Once it's finished cleaning up the oldest
     * database, it'll call here, and we'll signal the postmaster to start
     * another iteration immediately if there are still any old databases.
     */
    if (MultiXactIdPrecedes(multiVacLimit, curMulti) && IsUnderPostmaster)
        SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
 
    /* Give an immediate warning if past the wrap warn point */
    if (MultiXactIdPrecedes(multiWarnLimit, curMulti))
    {
        char       *oldest_datname;
 
        /*
         * We can be called when not inside a transaction, for example during
         * StartupXLOG().  In such a case we cannot do database access, so we
         * must just report the oldest DB's OID.
         *
         * Note: it's also possible that get_database_name fails and returns
         * NULL, for example because the database just got dropped.  We'll
         * still warn, even though the warning might now be unnecessary.
         */
        if (IsTransactionState())
            oldest_datname = get_database_name(oldest_datoid);
        else
            oldest_datname = NULL;
 
        if (oldest_datname)
            ereport(WARNING,
                    (errmsg_plural("database \"%s\" must be vacuumed before %u more MultiXactId is used",
                                   "database \"%s\" must be vacuumed before %u more MultiXactIds are used",
                                   multiWrapLimit - curMulti,
                                   oldest_datname,
                                   multiWrapLimit - curMulti),
                     errdetail("Approximately %.2f%% of MultiXactIds are available for use.",
                               (double) (multiWrapLimit - curMulti) / (MaxMultiXactId / 2) * 100),
                     errhint("To avoid MultiXactId assignment failures, execute a database-wide VACUUM in that database.\n"
                             "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
        else
            ereport(WARNING,
                    (errmsg_plural("database with OID %u must be vacuumed before %u more MultiXactId is used",
                                   "database with OID %u must be vacuumed before %u more MultiXactIds are used",
                                   multiWrapLimit - curMulti,
                                   oldest_datoid,
                                   multiWrapLimit - curMulti),
                     errdetail("Approximately %.2f%% of MultiXactIds are available for use.",
                               (double) (multiWrapLimit - curMulti) / (MaxMultiXactId / 2) * 100),
                     errhint("To avoid MultiXactId assignment failures, execute a database-wide VACUUM in that database.\n"
                             "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
    }
}
 
/*
 * Ensure the next-to-be-assigned MultiXactId is at least minMulti,
 * and similarly nextOffset is at least minMultiOffset.
 *
 * This is used when we can determine minimum safe values from an XLog
 * record (either an on-line checkpoint or an mxact creation log entry).
 * Although this is only called during XLog replay, we take the lock in case
 * any hot-standby backends are examining the values.
 */
void
MultiXactAdvanceNextMXact(MultiXactId minMulti,
                          MultiXactOffset minMultiOffset)
{
    Assert(MultiXactIdIsValid(minMulti));
 
    LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
    if (MultiXactIdPrecedes(MultiXactState->nextMXact, minMulti))
    {
        debug_elog3(DEBUG2, "MultiXact: setting next multi to %u", minMulti);
        MultiXactState->nextMXact = minMulti;
    }
    if (MultiXactState->nextOffset < minMultiOffset)
    {
        debug_elog3(DEBUG2, "MultiXact: setting next offset to %" PRIu64,
                    minMultiOffset);
        MultiXactState->nextOffset = minMultiOffset;
    }
    LWLockRelease(MultiXactGenLock);
}
 
/*
 * Update our oldestMultiXactId value, but only if it's more recent than what
 * we had.
 *
 * This may only be called during WAL replay.
 */
void
MultiXactAdvanceOldest(MultiXactId oldestMulti, Oid oldestMultiDB)
{
    Assert(InRecovery);
 
    if (MultiXactIdPrecedes(MultiXactState->oldestMultiXactId, oldestMulti))
        SetMultiXactIdLimit(oldestMulti, oldestMultiDB);
}
 
/*
 * Make sure that MultiXactOffset has room for a newly-allocated MultiXactId.
 *
 * NB: this is called while holding MultiXactGenLock.  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 log or xlog page to make
 * room in shared memory.
 */
static void
ExtendMultiXactOffset(MultiXactId multi)
{
    int64       pageno;
    LWLock     *lock;
 
    /*
     * No work except at first MultiXactId of a page.  But beware: just after
     * wraparound, the first MultiXactId of page zero is FirstMultiXactId.
     */
    if (MultiXactIdToOffsetEntry(multi) != 0 &&
        multi != FirstMultiXactId)
        return;
 
    pageno = MultiXactIdToOffsetPage(multi);
    lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
 
    LWLockAcquire(lock, LW_EXCLUSIVE);
 
    /* Zero the page and make a WAL entry about it */
    SimpleLruZeroPage(MultiXactOffsetCtl, pageno);
    XLogSimpleInsertInt64(RM_MULTIXACT_ID, XLOG_MULTIXACT_ZERO_OFF_PAGE,
                          pageno);
 
    LWLockRelease(lock);
}
 
/*
 * Make sure that MultiXactMember has room for the members of a newly-
 * allocated MultiXactId.
 *
 * Like the above routine, this is called while holding MultiXactGenLock;
 * same comments apply.
 */
static void
ExtendMultiXactMember(MultiXactOffset offset, int nmembers)
{
    /*
     * It's possible that the members span more than one page of the members
     * file, so we loop to ensure we consider each page.  The coding is not
     * optimal if the members span several pages, but that seems unusual
     * enough to not worry much about.
     */
    while (nmembers > 0)
    {
        int         flagsoff;
        int         flagsbit;
        uint32      difference;
 
        /*
         * Only zero when at first entry of a page.
         */
        flagsoff = MXOffsetToFlagsOffset(offset);
        flagsbit = MXOffsetToFlagsBitShift(offset);
        if (flagsoff == 0 && flagsbit == 0)
        {
            int64       pageno;
            LWLock     *lock;
 
            pageno = MXOffsetToMemberPage(offset);
            lock = SimpleLruGetBankLock(MultiXactMemberCtl, pageno);
 
            LWLockAcquire(lock, LW_EXCLUSIVE);
 
            /* Zero the page and make a WAL entry about it */
            SimpleLruZeroPage(MultiXactMemberCtl, pageno);
            XLogSimpleInsertInt64(RM_MULTIXACT_ID,
                                  XLOG_MULTIXACT_ZERO_MEM_PAGE, pageno);
 
            LWLockRelease(lock);
        }
 
        /* Compute the number of items till end of current page. */
        difference = MULTIXACT_MEMBERS_PER_PAGE - offset % MULTIXACT_MEMBERS_PER_PAGE;
 
        /*
         * Advance to next page.  OK if nmembers goes negative.
         */
        nmembers -= difference;
        offset += difference;
    }
}
 
/*
 * GetOldestMultiXactId
 *
 * Return the oldest MultiXactId that's still possibly still seen as live by
 * any running transaction.  Older ones might still exist on disk, but they no
 * longer have any running member transaction.
 *
 * It's not safe to truncate MultiXact SLRU segments on the value returned by
 * this function; however, it can be set as the new relminmxid for any table
 * that VACUUM knows has no remaining MXIDs < the same value.  It is only safe
 * to truncate SLRUs when no table can possibly still have a referencing MXID.
 */
MultiXactId
GetOldestMultiXactId(void)
{
    MultiXactId oldestMXact;
 
    /*
     * This is the oldest valid value among all the OldestMemberMXactId[] and
     * OldestVisibleMXactId[] entries, or nextMXact if none are valid.
     */
    LWLockAcquire(MultiXactGenLock, LW_SHARED);
    oldestMXact = MultiXactState->nextMXact;
    for (int i = 0; i < NumMemberSlots; i++)
    {
        MultiXactId thisoldest;
 
        thisoldest = OldestMemberMXactId[i];
        if (MultiXactIdIsValid(thisoldest) &&
            MultiXactIdPrecedes(thisoldest, oldestMXact))
            oldestMXact = thisoldest;
    }
    for (int i = 0; i < NumVisibleSlots; i++)
    {
        MultiXactId thisoldest;
 
        thisoldest = OldestVisibleMXactId[i];
        if (MultiXactIdIsValid(thisoldest) &&
            MultiXactIdPrecedes(thisoldest, oldestMXact))
            oldestMXact = thisoldest;
    }
 
    LWLockRelease(MultiXactGenLock);
 
    return oldestMXact;
}
 
/*
 * Calculate the oldest member offset and install it in MultiXactState, where
 * it can be used to adjust multixid freezing cutoffs.
 */
static void
SetOldestOffset(void)
{
    MultiXactId oldestMultiXactId;
    MultiXactId nextMXact;
    MultiXactOffset oldestOffset = 0;   /* placate compiler */
    MultiXactOffset nextOffset;
    bool        oldestOffsetKnown = false;
 
    /*
     * NB: Have to prevent concurrent truncation, we might otherwise try to
     * lookup an oldestMulti that's concurrently getting truncated away.
     */
    LWLockAcquire(MultiXactTruncationLock, LW_SHARED);
 
    /* Read relevant fields from shared memory. */
    LWLockAcquire(MultiXactGenLock, LW_SHARED);
    oldestMultiXactId = MultiXactState->oldestMultiXactId;
    nextMXact = MultiXactState->nextMXact;
    nextOffset = MultiXactState->nextOffset;
    Assert(MultiXactState->finishedStartup);
    LWLockRelease(MultiXactGenLock);
 
    /*
     * Determine the offset of the oldest multixact.  Normally, we can read
     * the offset from the multixact itself, but there's an important special
     * case: if there are no multixacts in existence at all, oldestMXact
     * obviously can't point to one.  It will instead point to the multixact
     * ID that will be assigned the next time one is needed.
     */
    if (oldestMultiXactId == nextMXact)
    {
        /*
         * When the next multixact gets created, it will be stored at the next
         * offset.
         */
        oldestOffset = nextOffset;
        oldestOffsetKnown = true;
    }
    else
    {
        /*
         * Look up the offset at which the oldest existing multixact's members
         * are stored.  If we cannot find it, be careful not to fail, and
         * leave oldestOffset unchanged.  oldestOffset is initialized to zero
         * at system startup, which prevents truncating members until a proper
         * value is calculated.
         *
         * (We had bugs in early releases of PostgreSQL 9.3.X and 9.4.X where
         * the supposedly-earliest multixact might not really exist.  Those
         * should be long gone by now, so this should not fail, but let's
         * still be defensive.)
         */
        oldestOffsetKnown =
            find_multixact_start(oldestMultiXactId, &oldestOffset);
 
        if (oldestOffsetKnown)
            ereport(DEBUG1,
                    (errmsg_internal("oldest MultiXactId member is at offset %" PRIu64,
                                     oldestOffset)));
        else
            ereport(LOG,
                    (errmsg("MultiXact member truncation is disabled because oldest checkpointed MultiXact %u does not exist on disk",
                            oldestMultiXactId)));
    }
 
    LWLockRelease(MultiXactTruncationLock);
 
    /* Install the computed value */
    if (oldestOffsetKnown)
    {
        LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
        MultiXactState->oldestOffset = oldestOffset;
        LWLockRelease(MultiXactGenLock);
    }
}
 
/*
 * Find the starting offset of the given MultiXactId.
 *
 * Returns false if the file containing the multi does not exist on disk.
 * Otherwise, returns true and sets *result to the starting member offset.
 *
 * This function does not prevent concurrent truncation, so if that's
 * required, the caller has to protect against that.
 */
static bool
find_multixact_start(MultiXactId multi, MultiXactOffset *result)
{
    MultiXactOffset offset;
    int64       pageno;
    int         entryno;
    int         slotno;
    MultiXactOffset *offptr;
 
    Assert(MultiXactState->finishedStartup);
 
    pageno = MultiXactIdToOffsetPage(multi);
    entryno = MultiXactIdToOffsetEntry(multi);
 
    /*
     * Write out dirty data, so PhysicalPageExists can work correctly.
     */
    SimpleLruWriteAll(MultiXactOffsetCtl, true);
    SimpleLruWriteAll(MultiXactMemberCtl, true);
 
    if (!SimpleLruDoesPhysicalPageExist(MultiXactOffsetCtl, pageno))
        return false;
 
    /* lock is acquired by SimpleLruReadPage_ReadOnly */
    slotno = SimpleLruReadPage_ReadOnly(MultiXactOffsetCtl, pageno, &multi);
    offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
    offptr += entryno;
    offset = *offptr;
    LWLockRelease(SimpleLruGetBankLock(MultiXactOffsetCtl, pageno));
 
    *result = offset;
    return true;
}
 
/*
 * GetMultiXactInfo
 *
 * Returns information about the current MultiXact state, as of:
 * multixacts: Number of MultiXacts (nextMultiXactId - oldestMultiXactId)
 * nextOffset: Next-to-be-assigned offset
 * oldestMultiXactId: Oldest MultiXact ID still in use
 * oldestOffset: Oldest offset still in use
 */
void
GetMultiXactInfo(uint32 *multixacts, MultiXactOffset *nextOffset,
                 MultiXactId *oldestMultiXactId, MultiXactOffset *oldestOffset)
{
    MultiXactId nextMultiXactId;
 
    LWLockAcquire(MultiXactGenLock, LW_SHARED);
    *nextOffset = MultiXactState->nextOffset;
    *oldestMultiXactId = MultiXactState->oldestMultiXactId;
    nextMultiXactId = MultiXactState->nextMXact;
    *oldestOffset = MultiXactState->oldestOffset;
    LWLockRelease(MultiXactGenLock);
 
    *multixacts = nextMultiXactId - *oldestMultiXactId;
}
 
/*
 * Multixact members can be removed once the multixacts that refer to them
 * are older than every datminmxid.  autovacuum_multixact_freeze_max_age and
 * vacuum_multixact_freeze_table_age work together to make sure we never have
 * too many multixacts; we hope that, at least under normal circumstances,
 * this will also be sufficient to keep us from using too many offsets.
 * However, if the average multixact has many members, we might accumulate a
 * large amount of members, consuming disk space, while still using few enough
 * multixids that the multixid limits fail to trigger relminmxid advancement
 * by VACUUM.
 *
 * To prevent that, if the members space usage exceeds a threshold
 * (MULTIXACT_MEMBER_LOW_THRESHOLD), we effectively reduce
 * autovacuum_multixact_freeze_max_age to a value just less than the number of
 * multixacts in use.  We hope that this will quickly trigger autovacuuming on
 * the table or tables with the oldest relminmxid, thus allowing datminmxid
 * values to advance and removing some members.
 *
 * As the amount of the member space in use grows, we become more aggressive
 * in clamping this value.  That not only causes autovacuum to ramp up, but
 * also makes any manual vacuums the user issues more aggressive.  This
 * happens because vacuum_get_cutoffs() will clamp the freeze table and the
 * minimum freeze age cutoffs based on the effective
 * autovacuum_multixact_freeze_max_age this function returns.  At the extreme,
 * when the members usage reaches MULTIXACT_MEMBER_HIGH_THRESHOLD, we clamp
 * freeze_max_age to zero, and every vacuum of any table will freeze every
 * multixact.
 */
int
MultiXactMemberFreezeThreshold(void)
{
    uint32      multixacts;
    uint32      victim_multixacts;
    double      fraction;
    int         result;
    MultiXactId oldestMultiXactId;
    MultiXactOffset oldestOffset;
    MultiXactOffset nextOffset;
    uint64      members;
 
    /* Read the current offsets and multixact usage. */
    GetMultiXactInfo(&multixacts, &nextOffset, &oldestMultiXactId, &oldestOffset);
    members = nextOffset - oldestOffset;
 
    /* If member space utilization is low, no special action is required. */
    if (members <= MULTIXACT_MEMBER_LOW_THRESHOLD)
        return autovacuum_multixact_freeze_max_age;
 
    /*
     * Compute a target for relminmxid advancement.  The number of multixacts
     * we try to eliminate from the system is based on how far we are past
     * MULTIXACT_MEMBER_LOW_THRESHOLD.
     *
     * The way this formula works is that when members is exactly at the low
     * threshold, fraction = 0.0, and we set freeze_max_age equal to
     * mxid_age(oldestMultiXactId).  As members grows further, towards the
     * high threshold, fraction grows linearly from 0.0 to 1.0, and the result
     * shrinks from mxid_age(oldestMultiXactId) to 0.  Beyond the high
     * threshold, fraction > 1.0 and the result is clamped to 0.
     */
    fraction = (double) (members - MULTIXACT_MEMBER_LOW_THRESHOLD) /
        (MULTIXACT_MEMBER_HIGH_THRESHOLD - MULTIXACT_MEMBER_LOW_THRESHOLD);
 
    /* fraction could be > 1.0, but lowest possible freeze age is zero */
    if (fraction >= 1.0)
        return 0;
 
    victim_multixacts = multixacts * fraction;
    result = multixacts - victim_multixacts;
 
    /*
     * Clamp to autovacuum_multixact_freeze_max_age, so that we never make
     * autovacuum less aggressive than it would otherwise be.
     */
    return Min(result, autovacuum_multixact_freeze_max_age);
}
 
 
/*
 * Delete members segments older than newOldestOffset
 */
static void
PerformMembersTruncation(MultiXactOffset newOldestOffset)
{
    SimpleLruTruncate(MultiXactMemberCtl,
                      MXOffsetToMemberPage(newOldestOffset));
}
 
/*
 * Delete offsets segments older than newOldestMulti
 */
static void
PerformOffsetsTruncation(MultiXactId newOldestMulti)
{
    /*
     * We step back one multixact to avoid passing a cutoff page that hasn't
     * been created yet in the rare case that oldestMulti would be the first
     * item on a page and oldestMulti == nextMulti.  In that case, if we
     * didn't subtract one, we'd trigger SimpleLruTruncate's wraparound
     * detection.
     */
    SimpleLruTruncate(MultiXactOffsetCtl,
                      MultiXactIdToOffsetPage(PreviousMultiXactId(newOldestMulti)));
}
 
/*
 * Remove all MultiXactOffset and MultiXactMember segments before the oldest
 * ones still of interest.
 *
 * This is only called on a primary as part of vacuum (via
 * vac_truncate_clog()). During recovery truncation is done by replaying
 * truncation WAL records logged here.
 *
 * newOldestMulti is the oldest currently required multixact, newOldestMultiDB
 * is one of the databases preventing newOldestMulti from increasing.
 */
void
TruncateMultiXact(MultiXactId newOldestMulti, Oid newOldestMultiDB)
{
    MultiXactId oldestMulti;
    MultiXactId nextMulti;
    MultiXactOffset newOldestOffset;
    MultiXactOffset nextOffset;
 
    Assert(!RecoveryInProgress());
    Assert(MultiXactState->finishedStartup);
    Assert(MultiXactIdIsValid(newOldestMulti));
 
    /*
     * We can only allow one truncation to happen at once. Otherwise parts of
     * members might vanish while we're doing lookups or similar. There's no
     * need to have an interlock with creating new multis or such, since those
     * are constrained by the limits (which only grow, never shrink).
     */
    LWLockAcquire(MultiXactTruncationLock, LW_EXCLUSIVE);
 
    LWLockAcquire(MultiXactGenLock, LW_SHARED);
    nextMulti = MultiXactState->nextMXact;
    nextOffset = MultiXactState->nextOffset;
    oldestMulti = MultiXactState->oldestMultiXactId;
    LWLockRelease(MultiXactGenLock);
 
    /*
     * Make sure to only attempt truncation if there's values to truncate
     * away. In normal processing values shouldn't go backwards, but there's
     * some corner cases (due to bugs) where that's possible.
     */
    if (MultiXactIdPrecedesOrEquals(newOldestMulti, oldestMulti))
    {
        LWLockRelease(MultiXactTruncationLock);
        return;
    }
 
    /*
     * Compute up to where to truncate MultiXactMember. Lookup the
     * corresponding member offset for newOldestMulti for that.
     */
    if (newOldestMulti == nextMulti)
    {
        /* there are NO MultiXacts */
        newOldestOffset = nextOffset;
    }
    else if (!find_multixact_start(newOldestMulti, &newOldestOffset))
    {
        ereport(LOG,
                (errmsg("cannot truncate up to MultiXact %u because it does not exist on disk, skipping truncation",
                        newOldestMulti)));
        LWLockRelease(MultiXactTruncationLock);
        return;
    }
 
    /*
     * On crash, MultiXactIdCreateFromMembers() can leave behind multixids
     * that were not yet written out and hence have zero offset on disk. If
     * such a multixid becomes oldestMulti, we won't be able to look up its
     * offset. That should be rare, so we don't try to do anything smart about
     * it. Just skip the truncation, and hope that by the next truncation
     * attempt, oldestMulti has advanced to a valid multixid.
     */
    if (newOldestOffset == 0)
    {
        ereport(LOG,
                (errmsg("cannot truncate up to MultiXact %u because it has invalid offset, skipping truncation",
                        newOldestMulti)));
        LWLockRelease(MultiXactTruncationLock);
        return;
    }
 
    elog(DEBUG1, "performing multixact truncation: "
         "oldestMulti %u (offsets segment %" PRIx64 "), "
         "oldestOffset %" PRIu64 " (members segment %" PRIx64 ")",
         newOldestMulti,
         MultiXactIdToOffsetSegment(newOldestMulti),
         newOldestOffset,
         MXOffsetToMemberSegment(newOldestOffset));
 
    /*
     * Do truncation, and the WAL logging of the truncation, in a critical
     * section. That way offsets/members cannot get out of sync anymore, i.e.
     * once consistent the newOldestMulti will always exist in members, even
     * if we crashed in the wrong moment.
     */
    START_CRIT_SECTION();
 
    /*
     * Prevent checkpoints from being scheduled concurrently. This is critical
     * because otherwise a truncation record might not be replayed after a
     * crash/basebackup, even though the state of the data directory would
     * require it.
     */
    Assert((MyProc->delayChkptFlags & DELAY_CHKPT_START) == 0);
    MyProc->delayChkptFlags |= DELAY_CHKPT_START;
 
    /* WAL log truncation */
    WriteMTruncateXlogRec(newOldestMultiDB, newOldestMulti, newOldestOffset);
 
    /*
     * Update in-memory limits before performing the truncation, while inside
     * the critical section: Have to do it before truncation, to prevent
     * concurrent lookups of those values. Has to be inside the critical
     * section as otherwise a future call to this function would error out,
     * while looking up the oldest member in offsets, if our caller crashes
     * before updating the limits.
     */
    LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
    MultiXactState->oldestMultiXactId = newOldestMulti;
    MultiXactState->oldestMultiXactDB = newOldestMultiDB;
    MultiXactState->oldestOffset = newOldestOffset;
    LWLockRelease(MultiXactGenLock);
 
    /* First truncate members */
    PerformMembersTruncation(newOldestOffset);
 
    /* Then offsets */
    PerformOffsetsTruncation(newOldestMulti);
 
    MyProc->delayChkptFlags &= ~DELAY_CHKPT_START;
 
    END_CRIT_SECTION();
    LWLockRelease(MultiXactTruncationLock);
}
 
/*
 * Decide whether a MultiXactOffset page number is "older" for truncation
 * purposes.  Analogous to CLOGPagePrecedes().
 *
 * Offsetting the values is optional, because MultiXactIdPrecedes() has
 * translational symmetry.
 */
static bool
MultiXactOffsetPagePrecedes(int64 page1, int64 page2)
{
    MultiXactId multi1;
    MultiXactId multi2;
 
    multi1 = ((MultiXactId) page1) * MULTIXACT_OFFSETS_PER_PAGE;
    multi1 += FirstMultiXactId + 1;
    multi2 = ((MultiXactId) page2) * MULTIXACT_OFFSETS_PER_PAGE;
    multi2 += FirstMultiXactId + 1;
 
    return (MultiXactIdPrecedes(multi1, multi2) &&
            MultiXactIdPrecedes(multi1,
                                multi2 + MULTIXACT_OFFSETS_PER_PAGE - 1));
}
 
/*
 * Decide whether a MultiXactMember page number is "older" for truncation
 * purposes.  There is no "invalid offset number" and members never wrap
 * around, so use the numbers verbatim.
 */
static bool
MultiXactMemberPagePrecedes(int64 page1, int64 page2)
{
    return page1 < page2;
}
 
static int
MultiXactOffsetIoErrorDetail(const void *opaque_data)
{
    MultiXactId multixid = *(const MultiXactId *) opaque_data;
 
    return errdetail("Could not access offset of multixact %u.", multixid);
}
 
static int
MultiXactMemberIoErrorDetail(const void *opaque_data)
{
    const MultiXactMemberSlruReadContext *context = opaque_data;
 
    if (MultiXactIdIsValid(context->multi))
        return errdetail("Could not access member of multixact %u at offset %" PRIu64 ".",
                         context->multi, context->offset);
    else
        return errdetail("Could not access multixact member at offset %" PRIu64 ".",
                         context->offset);
}
 
/*
 * Decide which of two MultiXactIds is earlier.
 *
 * XXX do we need to do something special for InvalidMultiXactId?
 * (Doesn't look like it.)
 */
bool
MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2)
{
    int32       diff = (int32) (multi1 - multi2);
 
    return (diff < 0);
}
 
/*
 * MultiXactIdPrecedesOrEquals -- is multi1 logically <= multi2?
 *
 * XXX do we need to do something special for InvalidMultiXactId?
 * (Doesn't look like it.)
 */
bool
MultiXactIdPrecedesOrEquals(MultiXactId multi1, MultiXactId multi2)
{
    int32       diff = (int32) (multi1 - multi2);
 
    return (diff <= 0);
}
 
 
/*
 * Write a TRUNCATE xlog record
 *
 * We must flush the xlog record to disk before returning --- see notes in
 * TruncateCLOG().
 */
static void
WriteMTruncateXlogRec(Oid oldestMultiDB,
                      MultiXactId oldestMulti,
                      MultiXactOffset oldestOffset)
{
    XLogRecPtr  recptr;
    xl_multixact_truncate xlrec;
 
    xlrec.oldestMultiDB = oldestMultiDB;
    xlrec.oldestMulti = oldestMulti;
    xlrec.oldestOffset = oldestOffset;
 
    XLogBeginInsert();
    XLogRegisterData(&xlrec, SizeOfMultiXactTruncate);
    recptr = XLogInsert(RM_MULTIXACT_ID, XLOG_MULTIXACT_TRUNCATE_ID);
    XLogFlush(recptr);
}
 
/*
 * MULTIXACT resource manager's routines
 */
void
multixact_redo(XLogReaderState *record)
{
    uint8       info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
 
    /* Backup blocks are not used in multixact records */
    Assert(!XLogRecHasAnyBlockRefs(record));
 
    if (info == XLOG_MULTIXACT_ZERO_OFF_PAGE)
    {
        int64       pageno;
 
        memcpy(&pageno, XLogRecGetData(record), sizeof(pageno));
        SimpleLruZeroAndWritePage(MultiXactOffsetCtl, pageno);
    }
    else if (info == XLOG_MULTIXACT_ZERO_MEM_PAGE)
    {
        int64       pageno;
 
        memcpy(&pageno, XLogRecGetData(record), sizeof(pageno));
        SimpleLruZeroAndWritePage(MultiXactMemberCtl, pageno);
    }
    else if (info == XLOG_MULTIXACT_CREATE_ID)
    {
        xl_multixact_create *xlrec =
            (xl_multixact_create *) XLogRecGetData(record);
        TransactionId max_xid;
        int         i;
 
        /* Store the data back into the SLRU files */
        RecordNewMultiXact(xlrec->mid, xlrec->moff, xlrec->nmembers,
                           xlrec->members);
 
        /* Make sure nextMXact/nextOffset are beyond what this record has */
        MultiXactAdvanceNextMXact(NextMultiXactId(xlrec->mid),
                                  xlrec->moff + xlrec->nmembers);
 
        /*
         * Make sure nextXid is beyond any XID mentioned in the record. This
         * should be unnecessary, since any XID found here ought to have other
         * evidence in the XLOG, but let's be safe.
         */
        max_xid = XLogRecGetXid(record);
        for (i = 0; i < xlrec->nmembers; i++)
        {
            if (TransactionIdPrecedes(max_xid, xlrec->members[i].xid))
                max_xid = xlrec->members[i].xid;
        }
 
        AdvanceNextFullTransactionIdPastXid(max_xid);
    }
    else if (info == XLOG_MULTIXACT_TRUNCATE_ID)
    {
        xl_multixact_truncate xlrec;
        int64       pageno;
 
        memcpy(&xlrec, XLogRecGetData(record),
               SizeOfMultiXactTruncate);
 
        elog(DEBUG1, "replaying multixact truncation: "
             "oldestMulti %u (offsets segment %" PRIx64 "), "
             "oldestOffset %" PRIu64 " (members segment %" PRIx64 ")",
             xlrec.oldestMulti,
             MultiXactIdToOffsetSegment(xlrec.oldestMulti),
             xlrec.oldestOffset,
             MXOffsetToMemberSegment(xlrec.oldestOffset));
 
        /* should not be required, but more than cheap enough */
        LWLockAcquire(MultiXactTruncationLock, LW_EXCLUSIVE);
 
        /*
         * Advance the horizon values, so they're current at the end of
         * recovery.
         */
        SetMultiXactIdLimit(xlrec.oldestMulti, xlrec.oldestMultiDB);
 
        PerformMembersTruncation(xlrec.oldestOffset);
 
        /*
         * During XLOG replay, latest_page_number isn't necessarily set up
         * yet; insert a suitable value to bypass the sanity test in
         * SimpleLruTruncate.
         */
        pageno = MultiXactIdToOffsetPage(xlrec.oldestMulti);
        pg_atomic_write_u64(&MultiXactOffsetCtl->shared->latest_page_number,
                            pageno);
        PerformOffsetsTruncation(xlrec.oldestMulti);
 
        LWLockRelease(MultiXactTruncationLock);
    }
    else
        elog(PANIC, "multixact_redo: unknown op code %u", info);
}
 
/*
 * Entrypoint for sync.c to sync offsets files.
 */
int
multixactoffsetssyncfiletag(const FileTag *ftag, char *path)
{
    return SlruSyncFileTag(MultiXactOffsetCtl, ftag, path);
}
 
/*
 * Entrypoint for sync.c to sync members files.
 */
int
multixactmemberssyncfiletag(const FileTag *ftag, char *path)
{
    return SlruSyncFileTag(MultiXactMemberCtl, ftag, path);
}