diff options
Diffstat (limited to '')
| -rw-r--r-- | src/backend/access/transam/varsup.c | 637 |
1 files changed, 637 insertions, 0 deletions
diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c new file mode 100644 index 0000000..a22bf37 --- /dev/null +++ b/src/backend/access/transam/varsup.c @@ -0,0 +1,637 @@ +/*------------------------------------------------------------------------- + * + * varsup.c + * postgres OID & XID variables support routines + * + * Copyright (c) 2000-2021, 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/xlog.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 to avoid wraparound data loss in database \"%s\"", + oldest_datname), + errhint("Stop the postmaster and vacuum that database in single-user mode.\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 to avoid wraparound data loss in database with OID %u", + oldest_datoid), + errhint("Stop the postmaster and vacuum that database in single-user mode.\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 a database shutdown, 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 a database shutdown, 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 a database shutdown, 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 a database shutdown, 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 GetNewRelFileNode() 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 FirstBootstrapObjectId, 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 FirstBootstrapObjectId 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) FirstBootstrapObjectId)) + { + /* 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; +} + + +#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 |