/*------------------------------------------------------------------------- * * varsup.c * postgres OID & XID variables support routines * * Copyright (c) 2000-2025, 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 variables struct in shared memory */ TransamVariablesData *TransamVariables = NULL; /* * Initialization of shared memory for TransamVariables. */ Size VarsupShmemSize(void) { return sizeof(TransamVariablesData); } void VarsupShmemInit(void) { bool found; /* Initialize our shared state struct */ TransamVariables = ShmemInitStruct("TransamVariables", sizeof(TransamVariablesData), &found); if (!IsUnderPostmaster) { Assert(!found); memset(TransamVariables, 0, sizeof(TransamVariablesData)); } else Assert(found); } /* * 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 = TransamVariables->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, TransamVariables->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 = TransamVariables->xidWarnLimit; TransactionId xidStopLimit = TransamVariables->xidStopLimit; TransactionId xidWrapLimit = TransamVariables->xidWrapLimit; Oid oldest_datoid = TransamVariables->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 transaction IDs 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 transaction IDs 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 transaction ID 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 = TransamVariables->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(&TransamVariables->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 = TransamVariables->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(TransamVariables->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(TransamVariables->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); TransamVariables->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(TransamVariables->oldestClogXid, oldest_datfrozenxid)) { TransamVariables->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); TransamVariables->oldestXid = oldest_datfrozenxid; TransamVariables->xidVacLimit = xidVacLimit; TransamVariables->xidWarnLimit = xidWarnLimit; TransamVariables->xidStopLimit = xidStopLimit; TransamVariables->xidWrapLimit = xidWrapLimit; TransamVariables->oldestXidDB = oldest_datoid; curXid = XidFromFullTransactionId(TransamVariables->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(TransamVariables->nextXid); xidVacLimit = TransamVariables->xidVacLimit; oldestXid = TransamVariables->oldestXid; oldestXidDB = TransamVariables->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 (TransamVariables->nextOid < ((Oid) FirstNormalObjectId)) { if (IsPostmasterEnvironment) { /* wraparound, or first post-initdb assignment, in normal mode */ TransamVariables->nextOid = FirstNormalObjectId; TransamVariables->oidCount = 0; } else { /* we may be bootstrapping, so don't enforce the full range */ if (TransamVariables->nextOid < ((Oid) FirstGenbkiObjectId)) { /* wraparound in standalone mode (unlikely but possible) */ TransamVariables->nextOid = FirstNormalObjectId; TransamVariables->oidCount = 0; } } } /* If we run out of logged for use oids then we must log more */ if (TransamVariables->oidCount == 0) { XLogPutNextOid(TransamVariables->nextOid + VAR_OID_PREFETCH); TransamVariables->oidCount = VAR_OID_PREFETCH; } result = TransamVariables->nextOid; (TransamVariables->nextOid)++; (TransamVariables->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 (TransamVariables->nextOid > nextOid) elog(ERROR, "too late to advance OID counter to %u, it is now %u", nextOid, TransamVariables->nextOid); TransamVariables->nextOid = nextOid; TransamVariables->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 TransamVariables->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 = TransamVariables->oldestXid; next_xid = XidFromFullTransactionId(TransamVariables->nextXid); Assert(TransactionIdFollowsOrEquals(xid, oldest_xid) || TransactionIdPrecedesOrEquals(xid, next_xid)); } #endif