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Set next multixid's offset when creating a new multixid #879

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2d6ed3c
Set next multixid's offset when creating a new multixid
hlinnaka Dec 3, 2025
ad3fcf9
Fix setting next multixid's offset at offset wraparound
hlinnaka Dec 5, 2025
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236 changes: 164 additions & 72 deletions src/backend/access/transam/multixact.c
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Original file line number Diff line number Diff line change
Expand Up @@ -84,7 +84,6 @@
#include "pg_trace.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
#include "storage/condition_variable.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procarray.h"
Expand Down Expand Up @@ -276,12 +275,6 @@ typedef struct MultiXactStateData
/* support for members anti-wraparound measures */
MultiXactOffset offsetStopLimit; /* known if oldestOffsetKnown */

/*
* This is used to sleep until a multixact offset is written when we want
* to create the next one.
*/
ConditionVariable nextoff_cv;

/*
* Per-backend data starts here. We have two arrays stored in the area
* immediately following the MultiXactStateData struct. Each is indexed by
Expand Down Expand Up @@ -386,6 +379,9 @@ static MemoryContext MXactContext = NULL;
#define debug_elog6(a,b,c,d,e,f)
#endif

/* hack to deal with WAL generated with older minor versions */
static int64 pre_initialized_offsets_page = -1;

/* internal MultiXactId management */
static void MultiXactIdSetOldestVisible(void);
static void RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
Expand Down Expand Up @@ -922,13 +918,66 @@ RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
int entryno;
int slotno;
MultiXactOffset *offptr;
int i;
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 = multi + 1;
if (next < FirstMultiXactId)
next = FirstMultiXactId;
next_pageno = MultiXactIdToOffsetPage(next);
next_entryno = MultiXactIdToOffsetEntry(next);

/*
* Older minor versions didn't set the next multixid's offset in this
* function, and therefore didn't initialize the next page until the next
* multixid was assigned. If we're replaying WAL that was generated by
* such a version, the next page might not be initialized yet. Initialize
* it now.
*/
if (InRecovery &&
next_pageno != pageno &&
pg_atomic_read_u64(&MultiXactOffsetCtl->shared->latest_page_number) == pageno)
{
elog(DEBUG1, "next offsets page is not initialized, initializing it now");

lock = SimpleLruGetBankLock(MultiXactOffsetCtl, next_pageno);
LWLockAcquire(lock, LW_EXCLUSIVE);

/* Create and zero the page */
slotno = SimpleLruZeroPage(MultiXactOffsetCtl, next_pageno);

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

LWLockRelease(lock);

/*
* Remember that we initialized the page, so that we don't zero it
* again at the XLOG_MULTIXACT_ZERO_OFF_PAGE record.
*/
pre_initialized_offsets_page = next_pageno;
}

/*
* Set the starting offset of this multixid's members.
*
* In the common case, it was already be 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);

Expand All @@ -943,22 +992,54 @@ RecordNewMultiXact(MultiXactId multi, MultiXactOffset offset,
offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
offptr += entryno;

*offptr = offset;
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;
}

MultiXactOffsetCtl->shared->page_dirty[slotno] = true;
/* 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);

/*
* If anybody was waiting to know the offset of this multixact ID we just
* wrote, they can read it now, so wake them up.
*/
ConditionVariableBroadcast(&MultiXactState->nextoff_cv);

prev_pageno = -1;

for (i = 0; i < nmembers; i++, offset++)
for (int i = 0; i < nmembers; i++, offset++)
{
TransactionId *memberptr;
uint32 *flagsptr;
Expand Down Expand Up @@ -1148,8 +1229,11 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
result = FirstMultiXactId;
}

/* Make sure there is room for the MXID in the file. */
ExtendMultiXactOffset(result);
/*
* Make sure there is room for the next MXID in the file. Assigning this
* MXID sets the next MXID's offset already.
*/
ExtendMultiXactOffset(result + 1);

/*
* Reserve the members space, similarly to above. Also, be careful not to
Expand Down Expand Up @@ -1314,7 +1398,6 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
MultiXactOffset nextOffset;
MultiXactMember *ptr;
LWLock *lock;
bool slept = false;

debug_elog3(DEBUG2, "GetMembers: asked for %u", multi);

Expand Down Expand Up @@ -1391,23 +1474,14 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
* one's. However, there are some corner cases to worry about:
*
* 1. This multixact may be the latest one created, in which case there is
* no next one to look at. In this case the nextOffset value we just
* saved is the correct endpoint.
* no next one to look at. The next multixact's offset should be set
* already, as we set it in RecordNewMultiXact(), but we used to not do
* that in older minor versions. To cope with that case, if this
* multixact is the latest one created, use the nextOffset value we read
* above as the endpoint.
*
* 2. The next multixact may still be in process of being filled in: that
* is, another process may have done GetNewMultiXactId but not yet written
* the offset entry for that ID. In that scenario, it is guaranteed that
* the offset entry for that multixact exists (because GetNewMultiXactId
* won't release MultiXactGenLock until it does) but contains zero
* (because we are careful to pre-zero offset pages). Because
* GetNewMultiXactId will never return zero as the starting offset for a
* multixact, when we read zero as the next multixact's offset, we know we
* have this case. We handle this by sleeping on the condition variable
* we have just for this; the process in charge will signal the CV as soon
* as it has finished writing the multixact offset.
*
* 3. Because GetNewMultiXactId increments offset zero to offset one to
* handle case #2, there is an ambiguity near the point of offset
* 2. Because GetNewMultiXactId skips over offset zero, to reserve zero
* for to mean "unset", there is an ambiguity near the point of offset
* wraparound. If we see next multixact's offset is one, is that our
* multixact's actual endpoint, or did it end at zero with a subsequent
* increment? We handle this using the knowledge that if the zero'th
Expand All @@ -1419,7 +1493,6 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
* cases, so it seems better than holding the MultiXactGenLock for a long
* time on every multixact creation.
*/
retry:
pageno = MultiXactIdToOffsetPage(multi);
entryno = MultiXactIdToOffsetEntry(multi);

Expand Down Expand Up @@ -1482,31 +1555,17 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,
nextMXOffset = *offptr;

if (nextMXOffset == 0)
{
/* Corner case 2: next multixact is still being filled in */
LWLockRelease(lock);
CHECK_FOR_INTERRUPTS();

INJECTION_POINT("multixact-get-members-cv-sleep", NULL);

ConditionVariableSleep(&MultiXactState->nextoff_cv,
WAIT_EVENT_MULTIXACT_CREATION);
slept = true;
goto retry;
}
ereport(ERROR,
(errcode(ERRCODE_DATA_CORRUPTED),
errmsg("MultiXact %u has invalid next offset",
multi)));

length = nextMXOffset - offset;
}

LWLockRelease(lock);
lock = NULL;

/*
* If we slept above, clean up state; it's no longer needed.
*/
if (slept)
ConditionVariableCancelSleep();

ptr = (MultiXactMember *) palloc(length * sizeof(MultiXactMember));

truelength = 0;
Expand Down Expand Up @@ -1549,7 +1608,7 @@ GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **members,

if (!TransactionIdIsValid(*xactptr))
{
/* Corner case 3: we must be looking at unused slot zero */
/* Corner case 2: we must be looking at unused slot zero */
Assert(offset == 0);
continue;
}
Expand Down Expand Up @@ -1996,7 +2055,6 @@ MultiXactShmemInit(void)

/* Make sure we zero out the per-backend state */
MemSet(MultiXactState, 0, SHARED_MULTIXACT_STATE_SIZE);
ConditionVariableInit(&MultiXactState->nextoff_cv);
}
else
Assert(found);
Expand Down Expand Up @@ -2203,26 +2261,34 @@ TrimMultiXact(void)
pageno);

/*
* Zero out the remainder of the current offsets 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. Zero those so case 2 of GetMultiXactIdMembers()
* operates normally.
* Set the offset of nextMXact on the offsets page. This is normally done
* in RecordNewMultiXact() of the previous multixact, but we used to not
* do that in older minor versions. To ensure that the next offset is set
* if the binary was just upgraded from an older minor version, do it now.
*
* 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);
if (entryno != 0)
{
int slotno;
MultiXactOffset *offptr;
LWLock *lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);

LWLockAcquire(lock, LW_EXCLUSIVE);
slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, nextMXact);
if (entryno == 0)
slotno = SimpleLruZeroPage(MultiXactOffsetCtl, pageno);
else
slotno = SimpleLruReadPage(MultiXactOffsetCtl, pageno, true, nextMXact);
offptr = (MultiXactOffset *) MultiXactOffsetCtl->shared->page_buffer[slotno];
offptr += entryno;

MemSet(offptr, 0, BLCKSZ - (entryno * sizeof(MultiXactOffset)));
*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);
Expand Down Expand Up @@ -3407,14 +3473,24 @@ multixact_redo(XLogReaderState *record)

memcpy(&pageno, XLogRecGetData(record), sizeof(pageno));

lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
LWLockAcquire(lock, LW_EXCLUSIVE);
/*
* Skip the record if we already initialized the page at the previous
* XLOG_MULTIXACT_CREATE_ID record. See RecordNewMultiXact().
*/
if (pre_initialized_offsets_page != pageno)
{
lock = SimpleLruGetBankLock(MultiXactOffsetCtl, pageno);
LWLockAcquire(lock, LW_EXCLUSIVE);

slotno = ZeroMultiXactOffsetPage(pageno, false);
SimpleLruWritePage(MultiXactOffsetCtl, slotno);
Assert(!MultiXactOffsetCtl->shared->page_dirty[slotno]);
slotno = ZeroMultiXactOffsetPage(pageno, false);
SimpleLruWritePage(MultiXactOffsetCtl, slotno);
Assert(!MultiXactOffsetCtl->shared->page_dirty[slotno]);

LWLockRelease(lock);
LWLockRelease(lock);
}
else
elog(DEBUG1, "skipping initialization of offsets page " INT64_FORMAT " because it was already initialized on multixid creation", pageno);
pre_initialized_offsets_page = -1;
}
else if (info == XLOG_MULTIXACT_ZERO_MEM_PAGE)
{
Expand All @@ -3440,6 +3516,22 @@ multixact_redo(XLogReaderState *record)
TransactionId max_xid;
int i;

if (pre_initialized_offsets_page != -1)
{
/*
* If we implicitly initialized the next offsets page while
* replaying an XLOG_MULTIXACT_CREATE_ID record that was generated
* with an older minor version, we still expect to see an
* XLOG_MULTIXACT_ZERO_OFF_PAGE record for it before any other
* XLOG_MULTIXACT_CREATE_ID records. Therefore this case should
* not happen. If it does, we'll continue with the replay, but
* log a message to note that something's funny.
*/
elog(LOG, "expected to see an XLOG_MULTIXACT_ZERO_OFF_PAGE record for page " INT64_FORMAT " that was implicitly initialized earlier",
pre_initialized_offsets_page);
pre_initialized_offsets_page = -1;
}

/* Store the data back into the SLRU files */
RecordNewMultiXact(xlrec->mid, xlrec->moff, xlrec->nmembers,
xlrec->members);
Expand Down
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