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postgresql/src/backend/access/transam/varsup.c

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/*-------------------------------------------------------------------------
*
* varsup.c
* postgres OID & XID variables support routines
*
* Copyright (c) 2000-2023, PostgreSQL Global Development Group
*
* IDENTIFICATION
* src/backend/access/transam/varsup.c
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/clog.h"
#include "access/commit_ts.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "access/xact.h"
#include "access/xlogutils.h"
#include "commands/dbcommands.h"
#include "miscadmin.h"
#include "postmaster/autovacuum.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "utils/syscache.h"
/* Number of OIDs to prefetch (preallocate) per XLOG write */
#define VAR_OID_PREFETCH 8192
/* pointer to "variable cache" in shared memory (set up by shmem.c) */
VariableCache ShmemVariableCache = NULL;
/*
* Allocate the next FullTransactionId for a new transaction or
* subtransaction.
*
* The new XID is also stored into MyProc->xid/ProcGlobal->xids[] before
* returning.
*
* Note: when this is called, we are actually already inside a valid
* transaction, since XIDs are now not allocated until the transaction
* does something. So it is safe to do a database lookup if we want to
* issue a warning about XID wrap.
*/
FullTransactionId
GetNewTransactionId(bool isSubXact)
{
FullTransactionId full_xid;
TransactionId xid;
/*
* Workers synchronize transaction state at the beginning of each parallel
* operation, so we can't account for new XIDs after that point.
*/
if (IsInParallelMode())
elog(ERROR, "cannot assign TransactionIds during a parallel operation");
/*
* During bootstrap initialization, we return the special bootstrap
* transaction id.
*/
if (IsBootstrapProcessingMode())
{
Assert(!isSubXact);
MyProc->xid = BootstrapTransactionId;
ProcGlobal->xids[MyProc->pgxactoff] = BootstrapTransactionId;
return FullTransactionIdFromEpochAndXid(0, BootstrapTransactionId);
}
/* safety check, we should never get this far in a HS standby */
if (RecoveryInProgress())
elog(ERROR, "cannot assign TransactionIds during recovery");
LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
full_xid = ShmemVariableCache->nextXid;
xid = XidFromFullTransactionId(full_xid);
/*----------
* Check to see if it's safe to assign another XID. This protects against
* catastrophic data loss due to XID wraparound. The basic rules are:
*
* If we're past xidVacLimit, start trying to force autovacuum cycles.
* If we're past xidWarnLimit, start issuing warnings.
* If we're past xidStopLimit, refuse to execute transactions, unless
* we are running in single-user mode (which gives an escape hatch
* to the DBA who somehow got past the earlier defenses).
*
* Note that this coding also appears in GetNewMultiXactId.
*----------
*/
if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))
{
/*
* For safety's sake, we release XidGenLock 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.
*/
TransactionId xidWarnLimit = ShmemVariableCache->xidWarnLimit;
TransactionId xidStopLimit = ShmemVariableCache->xidStopLimit;
TransactionId xidWrapLimit = ShmemVariableCache->xidWrapLimit;
Oid oldest_datoid = ShmemVariableCache->oldestXidDB;
LWLockRelease(XidGenLock);
/*
* To avoid swamping the postmaster with signals, we issue the autovac
* request only once per 64K transaction starts. This still gives
* plenty of chances before we get into real trouble.
*/
if (IsUnderPostmaster && (xid % 65536) == 0)
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
if (IsUnderPostmaster &&
TransactionIdFollowsOrEquals(xid, xidStopLimit))
{
char *oldest_datname = get_database_name(oldest_datoid);
/* 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 XIDs 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 XIDs 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.")));
}
else if (TransactionIdFollowsOrEquals(xid, xidWarnLimit))
{
char *oldest_datname = get_database_name(oldest_datoid);
/* complain even if that DB has disappeared */
if (oldest_datname)
ereport(WARNING,
(errmsg("database \"%s\" must be vacuumed within %u transactions",
oldest_datname,
xidWrapLimit - xid),
errhint("To avoid XID 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("database with OID %u must be vacuumed within %u transactions",
oldest_datoid,
xidWrapLimit - xid),
errhint("To avoid XID 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.")));
}
/* Re-acquire lock and start over */
LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
full_xid = ShmemVariableCache->nextXid;
xid = XidFromFullTransactionId(full_xid);
}
/*
* If we are allocating the first XID of a new page of the commit log,
* zero out that commit-log page before returning. We must do this while
* holding XidGenLock, else another xact could acquire and commit a later
* XID before we zero the page. Fortunately, a page of the commit log
* holds 32K or more transactions, so we don't have to do this very often.
*
* Extend pg_subtrans and pg_commit_ts too.
*/
ExtendCLOG(xid);
ExtendCommitTs(xid);
ExtendSUBTRANS(xid);
/*
* Now advance the nextXid counter. This must not happen until after we
* have successfully completed ExtendCLOG() --- if that routine fails, we
* want the next incoming transaction to try it again. We cannot assign
* more XIDs until there is CLOG space for them.
*/
FullTransactionIdAdvance(&ShmemVariableCache->nextXid);
/*
* We must store the new XID into the shared ProcArray before releasing
* XidGenLock. This ensures that every active XID older than
* latestCompletedXid is present in the ProcArray, which is essential for
* correct OldestXmin tracking; see src/backend/access/transam/README.
*
* Note that readers of ProcGlobal->xids/PGPROC->xid should be careful to
* fetch the value for each proc only once, rather than assume they can
* read a value multiple times and get the same answer each time. Note we
* are assuming that TransactionId and int fetch/store are atomic.
*
* The same comments apply to the subxact xid count and overflow fields.
*
* Use of a write barrier prevents dangerous code rearrangement in this
* function; other backends could otherwise e.g. be examining my subxids
* info concurrently, and we don't want them to see an invalid
* intermediate state, such as an incremented nxids before the array entry
* is filled.
*
* Other processes that read nxids should do so before reading xids
* elements with a pg_read_barrier() in between, so that they can be sure
* not to read an uninitialized array element; see
* src/backend/storage/lmgr/README.barrier.
*
* If there's no room to fit a subtransaction XID into PGPROC, set the
* cache-overflowed flag instead. This forces readers to look in
* pg_subtrans to map subtransaction XIDs up to top-level XIDs. There is a
* race-condition window, in that the new XID will not appear as running
* until its parent link has been placed into pg_subtrans. However, that
* will happen before anyone could possibly have a reason to inquire about
* the status of the XID, so it seems OK. (Snapshots taken during this
* window *will* include the parent XID, so they will deliver the correct
* answer later on when someone does have a reason to inquire.)
*/
if (!isSubXact)
{
Assert(ProcGlobal->subxidStates[MyProc->pgxactoff].count == 0);
Assert(!ProcGlobal->subxidStates[MyProc->pgxactoff].overflowed);
Assert(MyProc->subxidStatus.count == 0);
Assert(!MyProc->subxidStatus.overflowed);
/* LWLockRelease acts as barrier */
MyProc->xid = xid;
ProcGlobal->xids[MyProc->pgxactoff] = xid;
}
else
{
XidCacheStatus *substat = &ProcGlobal->subxidStates[MyProc->pgxactoff];
int nxids = MyProc->subxidStatus.count;
Assert(substat->count == MyProc->subxidStatus.count);
Assert(substat->overflowed == MyProc->subxidStatus.overflowed);
if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
{
MyProc->subxids.xids[nxids] = xid;
pg_write_barrier();
MyProc->subxidStatus.count = substat->count = nxids + 1;
}
else
MyProc->subxidStatus.overflowed = substat->overflowed = true;
}
LWLockRelease(XidGenLock);
return full_xid;
}
/*
* Read nextXid but don't allocate it.
*/
FullTransactionId
ReadNextFullTransactionId(void)
{
FullTransactionId fullXid;
LWLockAcquire(XidGenLock, LW_SHARED);
fullXid = ShmemVariableCache->nextXid;
LWLockRelease(XidGenLock);
return fullXid;
}
/*
* Advance nextXid to the value after a given xid. The epoch is inferred.
* This must only be called during recovery or from two-phase start-up code.
*/
void
AdvanceNextFullTransactionIdPastXid(TransactionId xid)
{
FullTransactionId newNextFullXid;
TransactionId next_xid;
uint32 epoch;
/*
* It is safe to read nextXid without a lock, because this is only called
* from the startup process or single-process mode, meaning that no other
* process can modify it.
*/
Assert(AmStartupProcess() || !IsUnderPostmaster);
/* Fast return if this isn't an xid high enough to move the needle. */
next_xid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
if (!TransactionIdFollowsOrEquals(xid, next_xid))
return;
/*
* Compute the FullTransactionId that comes after the given xid. To do
* this, we preserve the existing epoch, but detect when we've wrapped
* into a new epoch. This is necessary because WAL records and 2PC state
* currently contain 32 bit xids. The wrap logic is safe in those cases
* because the span of active xids cannot exceed one epoch at any given
* point in the WAL stream.
*/
TransactionIdAdvance(xid);
epoch = EpochFromFullTransactionId(ShmemVariableCache->nextXid);
if (unlikely(xid < next_xid))
++epoch;
newNextFullXid = FullTransactionIdFromEpochAndXid(epoch, xid);
/*
* We still need to take a lock to modify the value when there are
* concurrent readers.
*/
LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
ShmemVariableCache->nextXid = newNextFullXid;
LWLockRelease(XidGenLock);
}
/*
* Advance the cluster-wide value for the oldest valid clog entry.
*
* We must acquire XactTruncationLock to advance the oldestClogXid. It's not
* necessary to hold the lock during the actual clog truncation, only when we
* advance the limit, as code looking up arbitrary xids is required to hold
* XactTruncationLock from when it tests oldestClogXid through to when it
* completes the clog lookup.
*/
void
AdvanceOldestClogXid(TransactionId oldest_datfrozenxid)
{
LWLockAcquire(XactTruncationLock, LW_EXCLUSIVE);
if (TransactionIdPrecedes(ShmemVariableCache->oldestClogXid,
oldest_datfrozenxid))
{
ShmemVariableCache->oldestClogXid = oldest_datfrozenxid;
}
LWLockRelease(XactTruncationLock);
}
/*
* Determine the last safe XID to allocate using the currently oldest
* datfrozenxid (ie, the oldest XID that might exist in any database
* of our cluster), and the OID of the (or a) database with that value.
*/
void
SetTransactionIdLimit(TransactionId oldest_datfrozenxid, Oid oldest_datoid)
{
TransactionId xidVacLimit;
TransactionId xidWarnLimit;
TransactionId xidStopLimit;
TransactionId xidWrapLimit;
TransactionId curXid;
Assert(TransactionIdIsNormal(oldest_datfrozenxid));
/*
* The place where we actually get into deep trouble is halfway around
* from the oldest potentially-existing XID. (This calculation is
* probably off by one or two counts, because the special XIDs reduce the
* size of the loop a little bit. But we throw in plenty of slop below,
* so it doesn't matter.)
*/
xidWrapLimit = oldest_datfrozenxid + (MaxTransactionId >> 1);
if (xidWrapLimit < FirstNormalTransactionId)
xidWrapLimit += FirstNormalTransactionId;
/*
* We'll refuse to continue assigning XIDs in interactive mode once we get
* within 3M transactions of data loss. This leaves lots of room for the
* DBA to fool around fixing things in a standalone backend, while not
* being significant compared to total XID space. (VACUUM requires an XID
* if it truncates at wal_level!=minimal. "VACUUM (ANALYZE)", which a DBA
* might do by reflex, assigns an XID. Hence, we had better be sure
* there's lots of XIDs left...) Also, at default BLCKSZ, this leaves two
* completely-idle segments. In the event of edge-case bugs involving
* page or segment arithmetic, idle segments render the bugs unreachable
* outside of single-user mode.
*/
xidStopLimit = xidWrapLimit - 3000000;
if (xidStopLimit < FirstNormalTransactionId)
xidStopLimit -= FirstNormalTransactionId;
/*
* We'll start complaining loudly when we get within 40M transactions of
* data loss. This is kind of arbitrary, but if you let your gas gauge
* get down to 2% 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.)
*/
xidWarnLimit = xidWrapLimit - 40000000;
if (xidWarnLimit < FirstNormalTransactionId)
xidWarnLimit -= FirstNormalTransactionId;
/*
* We'll start trying to force autovacuums when oldest_datfrozenxid gets
* to be more than autovacuum_freeze_max_age transactions old.
*
* Note: guc.c ensures that autovacuum_freeze_max_age is in a sane range,
* so that xidVacLimit will be well before xidWarnLimit.
*
* Note: autovacuum_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. It doesn't look practical to update shared state from a GUC
* assign hook (too many processes would try to execute the hook,
* resulting in race conditions as well as crashes of those not connected
* to shared memory). Perhaps this can be improved someday. See also
* SetMultiXactIdLimit.
*/
xidVacLimit = oldest_datfrozenxid + autovacuum_freeze_max_age;
if (xidVacLimit < FirstNormalTransactionId)
xidVacLimit += FirstNormalTransactionId;
/* Grab lock for just long enough to set the new limit values */
LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
ShmemVariableCache->oldestXid = oldest_datfrozenxid;
ShmemVariableCache->xidVacLimit = xidVacLimit;
ShmemVariableCache->xidWarnLimit = xidWarnLimit;
ShmemVariableCache->xidStopLimit = xidStopLimit;
ShmemVariableCache->xidWrapLimit = xidWrapLimit;
ShmemVariableCache->oldestXidDB = oldest_datoid;
curXid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
LWLockRelease(XidGenLock);
/* Log the info */
ereport(DEBUG1,
(errmsg_internal("transaction ID wrap limit is %u, limited by database with OID %u",
xidWrapLimit, oldest_datoid)));
/*
* 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 (TransactionIdFollowsOrEquals(curXid, xidVacLimit) &&
IsUnderPostmaster && !InRecovery)
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
/* Give an immediate warning if past the wrap warn point */
if (TransactionIdFollowsOrEquals(curXid, xidWarnLimit) && !InRecovery)
{
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("database \"%s\" must be vacuumed within %u transactions",
oldest_datname,
xidWrapLimit - curXid),
errhint("To avoid XID 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("database with OID %u must be vacuumed within %u transactions",
oldest_datoid,
xidWrapLimit - curXid),
errhint("To avoid XID 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.")));
}
}
/*
* ForceTransactionIdLimitUpdate -- does the XID wrap-limit data need updating?
*
* We primarily check whether oldestXidDB is valid. The cases we have in
* mind are that that database was dropped, or the field was reset to zero
* by pg_resetwal. In either case we should force recalculation of the
* wrap limit. Also do it if oldestXid is old enough to be forcing
* autovacuums or other actions; this ensures we update our state as soon
* as possible once extra overhead is being incurred.
*/
bool
ForceTransactionIdLimitUpdate(void)
{
TransactionId nextXid;
TransactionId xidVacLimit;
TransactionId oldestXid;
Oid oldestXidDB;
/* Locking is probably not really necessary, but let's be careful */
LWLockAcquire(XidGenLock, LW_SHARED);
nextXid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
xidVacLimit = ShmemVariableCache->xidVacLimit;
oldestXid = ShmemVariableCache->oldestXid;
oldestXidDB = ShmemVariableCache->oldestXidDB;
LWLockRelease(XidGenLock);
if (!TransactionIdIsNormal(oldestXid))
return true; /* shouldn't happen, but just in case */
if (!TransactionIdIsValid(xidVacLimit))
return true; /* this shouldn't happen anymore either */
if (TransactionIdFollowsOrEquals(nextXid, xidVacLimit))
return true; /* past xidVacLimit, don't delay updating */
if (!SearchSysCacheExists1(DATABASEOID, ObjectIdGetDatum(oldestXidDB)))
return true; /* could happen, per comments above */
return false;
}
/*
* GetNewObjectId -- allocate a new OID
*
* OIDs are generated by a cluster-wide counter. Since they are only 32 bits
* wide, counter wraparound will occur eventually, and therefore it is unwise
* to assume they are unique unless precautions are taken to make them so.
* Hence, this routine should generally not be used directly. The only direct
* callers should be GetNewOidWithIndex() and GetNewRelFileNumber() in
* catalog/catalog.c.
*/
Oid
GetNewObjectId(void)
{
Oid result;
/* safety check, we should never get this far in a HS standby */
if (RecoveryInProgress())
elog(ERROR, "cannot assign OIDs during recovery");
LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
/*
* Check for wraparound of the OID counter. We *must* not return 0
* (InvalidOid), and in normal operation we mustn't return anything below
* FirstNormalObjectId since that range is reserved for initdb (see
* IsCatalogRelationOid()). Note we are relying on unsigned comparison.
*
* During initdb, we start the OID generator at FirstGenbkiObjectId, so we
* only wrap if before that point when in bootstrap or standalone mode.
* The first time through this routine after normal postmaster start, the
* counter will be forced up to FirstNormalObjectId. This mechanism
* leaves the OIDs between FirstGenbkiObjectId and FirstNormalObjectId
* available for automatic assignment during initdb, while ensuring they
* will never conflict with user-assigned OIDs.
*/
if (ShmemVariableCache->nextOid < ((Oid) FirstNormalObjectId))
{
if (IsPostmasterEnvironment)
{
/* wraparound, or first post-initdb assignment, in normal mode */
ShmemVariableCache->nextOid = FirstNormalObjectId;
ShmemVariableCache->oidCount = 0;
}
else
{
/* we may be bootstrapping, so don't enforce the full range */
if (ShmemVariableCache->nextOid < ((Oid) FirstGenbkiObjectId))
{
/* wraparound in standalone mode (unlikely but possible) */
ShmemVariableCache->nextOid = FirstNormalObjectId;
ShmemVariableCache->oidCount = 0;
}
}
}
/* If we run out of logged for use oids then we must log more */
if (ShmemVariableCache->oidCount == 0)
{
XLogPutNextOid(ShmemVariableCache->nextOid + VAR_OID_PREFETCH);
ShmemVariableCache->oidCount = VAR_OID_PREFETCH;
}
result = ShmemVariableCache->nextOid;
(ShmemVariableCache->nextOid)++;
(ShmemVariableCache->oidCount)--;
LWLockRelease(OidGenLock);
return result;
}
/*
* SetNextObjectId
*
* This may only be called during initdb; it advances the OID counter
* to the specified value.
*/
static void
SetNextObjectId(Oid nextOid)
{
/* Safety check, this is only allowable during initdb */
if (IsPostmasterEnvironment)
elog(ERROR, "cannot advance OID counter anymore");
/* Taking the lock is, therefore, just pro forma; but do it anyway */
LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
if (ShmemVariableCache->nextOid > nextOid)
elog(ERROR, "too late to advance OID counter to %u, it is now %u",
nextOid, ShmemVariableCache->nextOid);
ShmemVariableCache->nextOid = nextOid;
ShmemVariableCache->oidCount = 0;
LWLockRelease(OidGenLock);
}
/*
* StopGeneratingPinnedObjectIds
*
* This is called once during initdb to force the OID counter up to
* FirstUnpinnedObjectId. This supports letting initdb's post-bootstrap
* processing create some pinned objects early on. Once it's done doing
* so, it calls this (via pg_stop_making_pinned_objects()) so that the
* remaining objects it makes will be considered un-pinned.
*/
void
StopGeneratingPinnedObjectIds(void)
{
SetNextObjectId(FirstUnpinnedObjectId);
}
#ifdef USE_ASSERT_CHECKING
/*
* Assert that xid is between [oldestXid, nextXid], which is the range we
* expect XIDs coming from tables etc to be in.
*
* As ShmemVariableCache->oldestXid could change just after this call without
* further precautions, and as a wrapped-around xid could again fall within
* the valid range, this assertion can only detect if something is definitely
* wrong, but not establish correctness.
*
* This intentionally does not expose a return value, to avoid code being
* introduced that depends on the return value.
*/
void
AssertTransactionIdInAllowableRange(TransactionId xid)
{
TransactionId oldest_xid;
TransactionId next_xid;
Assert(TransactionIdIsValid(xid));
/* we may see bootstrap / frozen */
if (!TransactionIdIsNormal(xid))
return;
/*
* We can't acquire XidGenLock, as this may be called with XidGenLock
* already held (or with other locks that don't allow XidGenLock to be
* nested). That's ok for our purposes though, since we already rely on
* 32bit reads to be atomic. While nextXid is 64 bit, we only look at the
* lower 32bit, so a skewed read doesn't hurt.
*
* There's no increased danger of falling outside [oldest, next] by
* accessing them without a lock. xid needs to have been created with
* GetNewTransactionId() in the originating session, and the locks there
* pair with the memory barrier below. We do however accept xid to be <=
* to next_xid, instead of just <, as xid could be from the procarray,
* before we see the updated nextXid value.
*/
pg_memory_barrier();
oldest_xid = ShmemVariableCache->oldestXid;
next_xid = XidFromFullTransactionId(ShmemVariableCache->nextXid);
Assert(TransactionIdFollowsOrEquals(xid, oldest_xid) ||
TransactionIdPrecedesOrEquals(xid, next_xid));
}
#endif
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