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This does four basic things. First, it provides convenience routines to coordinate the startup and shutdown of parallel workers. Second, it synchronizes various pieces of state (e.g. GUCs, combo CID mappings, transaction snapshot) from the parallel group leader to the worker processes. Third, it prohibits various operations that would result in unsafe changes to that state while parallelism is active. Finally, it propagates events that would result in an ErrorResponse, NoticeResponse, or NotifyResponse message being sent to the client from the parallel workers back to the master, from which they can then be sent on to the client. Robert Haas, Amit Kapila, Noah Misch, Rushabh Lathia, Jeevan Chalke. Suggestions and review from Andres Freund, Heikki Linnakangas, Noah Misch, Simon Riggs, Euler Taveira, and Jim Nasby.
508 lines
18 KiB
C
508 lines
18 KiB
C
/*-------------------------------------------------------------------------
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*
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* varsup.c
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* postgres OID & XID variables support routines
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*
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* Copyright (c) 2000-2015, PostgreSQL Global Development Group
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*
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* IDENTIFICATION
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* src/backend/access/transam/varsup.c
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*
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*-------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include "access/clog.h"
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#include "access/commit_ts.h"
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#include "access/subtrans.h"
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#include "access/transam.h"
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#include "access/xact.h"
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#include "access/xlog.h"
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#include "commands/dbcommands.h"
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#include "miscadmin.h"
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#include "postmaster/autovacuum.h"
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#include "storage/pmsignal.h"
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#include "storage/proc.h"
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#include "utils/syscache.h"
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/* Number of OIDs to prefetch (preallocate) per XLOG write */
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#define VAR_OID_PREFETCH 8192
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/* pointer to "variable cache" in shared memory (set up by shmem.c) */
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VariableCache ShmemVariableCache = NULL;
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/*
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* Allocate the next XID for a new transaction or subtransaction.
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*
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* The new XID is also stored into MyPgXact before returning.
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*
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* Note: when this is called, we are actually already inside a valid
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* transaction, since XIDs are now not allocated until the transaction
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* does something. So it is safe to do a database lookup if we want to
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* issue a warning about XID wrap.
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*/
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TransactionId
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GetNewTransactionId(bool isSubXact)
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{
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TransactionId xid;
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/*
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* Workers synchronize transaction state at the beginning of each parallel
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* operation, so we can't account for new XIDs after that point.
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*/
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if (IsInParallelMode())
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elog(ERROR, "cannot assign TransactionIds during a parallel operation");
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/*
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* During bootstrap initialization, we return the special bootstrap
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* transaction id.
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*/
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if (IsBootstrapProcessingMode())
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{
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Assert(!isSubXact);
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MyPgXact->xid = BootstrapTransactionId;
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return BootstrapTransactionId;
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}
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/* safety check, we should never get this far in a HS slave */
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if (RecoveryInProgress())
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elog(ERROR, "cannot assign TransactionIds during recovery");
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LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
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xid = ShmemVariableCache->nextXid;
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/*----------
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* Check to see if it's safe to assign another XID. This protects against
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* catastrophic data loss due to XID wraparound. The basic rules are:
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*
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* If we're past xidVacLimit, start trying to force autovacuum cycles.
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* If we're past xidWarnLimit, start issuing warnings.
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* If we're past xidStopLimit, refuse to execute transactions, unless
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* we are running in single-user mode (which gives an escape hatch
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* to the DBA who somehow got past the earlier defenses).
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*
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* Note that this coding also appears in GetNewMultiXactId.
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*----------
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*/
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if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))
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{
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/*
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* For safety's sake, we release XidGenLock while sending signals,
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* warnings, etc. This is not so much because we care about
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* preserving concurrency in this situation, as to avoid any
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* possibility of deadlock while doing get_database_name(). First,
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* copy all the shared values we'll need in this path.
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*/
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TransactionId xidWarnLimit = ShmemVariableCache->xidWarnLimit;
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TransactionId xidStopLimit = ShmemVariableCache->xidStopLimit;
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TransactionId xidWrapLimit = ShmemVariableCache->xidWrapLimit;
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Oid oldest_datoid = ShmemVariableCache->oldestXidDB;
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LWLockRelease(XidGenLock);
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/*
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* To avoid swamping the postmaster with signals, we issue the autovac
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* request only once per 64K transaction starts. This still gives
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* plenty of chances before we get into real trouble.
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*/
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if (IsUnderPostmaster && (xid % 65536) == 0)
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SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
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if (IsUnderPostmaster &&
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TransactionIdFollowsOrEquals(xid, xidStopLimit))
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{
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char *oldest_datname = get_database_name(oldest_datoid);
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/* complain even if that DB has disappeared */
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if (oldest_datname)
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ereport(ERROR,
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(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
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errmsg("database is not accepting commands to avoid wraparound data loss in database \"%s\"",
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oldest_datname),
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errhint("Stop the postmaster and vacuum that database in single-user mode.\n"
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"You might also need to commit or roll back old prepared transactions.")));
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else
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ereport(ERROR,
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(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
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errmsg("database is not accepting commands to avoid wraparound data loss in database with OID %u",
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oldest_datoid),
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errhint("Stop the postmaster and vacuum that database in single-user mode.\n"
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"You might also need to commit or roll back old prepared transactions.")));
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}
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else if (TransactionIdFollowsOrEquals(xid, xidWarnLimit))
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{
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char *oldest_datname = get_database_name(oldest_datoid);
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/* complain even if that DB has disappeared */
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if (oldest_datname)
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ereport(WARNING,
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(errmsg("database \"%s\" must be vacuumed within %u transactions",
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oldest_datname,
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xidWrapLimit - xid),
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errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
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"You might also need to commit or roll back old prepared transactions.")));
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else
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ereport(WARNING,
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(errmsg("database with OID %u must be vacuumed within %u transactions",
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oldest_datoid,
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xidWrapLimit - xid),
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errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
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"You might also need to commit or roll back old prepared transactions.")));
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}
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/* Re-acquire lock and start over */
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LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
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xid = ShmemVariableCache->nextXid;
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}
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/*
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* If we are allocating the first XID of a new page of the commit log,
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* zero out that commit-log page before returning. We must do this while
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* holding XidGenLock, else another xact could acquire and commit a later
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* XID before we zero the page. Fortunately, a page of the commit log
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* holds 32K or more transactions, so we don't have to do this very often.
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*
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* Extend pg_subtrans and pg_commit_ts too.
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*/
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ExtendCLOG(xid);
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ExtendCommitTs(xid);
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ExtendSUBTRANS(xid);
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/*
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* Now advance the nextXid counter. This must not happen until after we
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* have successfully completed ExtendCLOG() --- if that routine fails, we
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* want the next incoming transaction to try it again. We cannot assign
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* more XIDs until there is CLOG space for them.
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*/
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TransactionIdAdvance(ShmemVariableCache->nextXid);
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/*
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* We must store the new XID into the shared ProcArray before releasing
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* XidGenLock. This ensures that every active XID older than
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* latestCompletedXid is present in the ProcArray, which is essential for
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* correct OldestXmin tracking; see src/backend/access/transam/README.
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*
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* XXX by storing xid into MyPgXact without acquiring ProcArrayLock, we
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* are relying on fetch/store of an xid to be atomic, else other backends
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* might see a partially-set xid here. But holding both locks at once
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* would be a nasty concurrency hit. So for now, assume atomicity.
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*
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* Note that readers of PGXACT xid fields should be careful to fetch the
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* value only once, rather than assume they can read a value multiple
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* times and get the same answer each time.
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*
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* The same comments apply to the subxact xid count and overflow fields.
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*
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* A solution to the atomic-store problem would be to give each PGXACT its
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* own spinlock used only for fetching/storing that PGXACT's xid and
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* related fields.
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*
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* If there's no room to fit a subtransaction XID into PGPROC, set the
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* cache-overflowed flag instead. This forces readers to look in
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* pg_subtrans to map subtransaction XIDs up to top-level XIDs. There is a
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* race-condition window, in that the new XID will not appear as running
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* until its parent link has been placed into pg_subtrans. However, that
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* will happen before anyone could possibly have a reason to inquire about
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* the status of the XID, so it seems OK. (Snapshots taken during this
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* window *will* include the parent XID, so they will deliver the correct
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* answer later on when someone does have a reason to inquire.)
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*/
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{
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/*
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* Use volatile pointer to prevent code rearrangement; other backends
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* could be examining my subxids info concurrently, and we don't want
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* them to see an invalid intermediate state, such as incrementing
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* nxids before filling the array entry. Note we are assuming that
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* TransactionId and int fetch/store are atomic.
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*/
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volatile PGPROC *myproc = MyProc;
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volatile PGXACT *mypgxact = MyPgXact;
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if (!isSubXact)
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mypgxact->xid = xid;
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else
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{
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int nxids = mypgxact->nxids;
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if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
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{
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myproc->subxids.xids[nxids] = xid;
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mypgxact->nxids = nxids + 1;
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}
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else
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mypgxact->overflowed = true;
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}
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}
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LWLockRelease(XidGenLock);
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return xid;
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}
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/*
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* Read nextXid but don't allocate it.
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*/
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TransactionId
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ReadNewTransactionId(void)
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{
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TransactionId xid;
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LWLockAcquire(XidGenLock, LW_SHARED);
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xid = ShmemVariableCache->nextXid;
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LWLockRelease(XidGenLock);
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return xid;
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}
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/*
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* Determine the last safe XID to allocate given the currently oldest
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* datfrozenxid (ie, the oldest XID that might exist in any database
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* of our cluster), and the OID of the (or a) database with that value.
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*/
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void
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SetTransactionIdLimit(TransactionId oldest_datfrozenxid, Oid oldest_datoid)
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{
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TransactionId xidVacLimit;
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TransactionId xidWarnLimit;
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TransactionId xidStopLimit;
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TransactionId xidWrapLimit;
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TransactionId curXid;
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Assert(TransactionIdIsNormal(oldest_datfrozenxid));
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/*
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* The place where we actually get into deep trouble is halfway around
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* from the oldest potentially-existing XID. (This calculation is
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* probably off by one or two counts, because the special XIDs reduce the
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* size of the loop a little bit. But we throw in plenty of slop below,
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* so it doesn't matter.)
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*/
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xidWrapLimit = oldest_datfrozenxid + (MaxTransactionId >> 1);
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if (xidWrapLimit < FirstNormalTransactionId)
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xidWrapLimit += FirstNormalTransactionId;
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/*
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* We'll refuse to continue assigning XIDs in interactive mode once we get
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* within 1M transactions of data loss. This leaves lots of room for the
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* DBA to fool around fixing things in a standalone backend, while not
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* being significant compared to total XID space. (Note that since
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* vacuuming requires one transaction per table cleaned, we had better be
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* sure there's lots of XIDs left...)
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*/
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xidStopLimit = xidWrapLimit - 1000000;
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if (xidStopLimit < FirstNormalTransactionId)
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xidStopLimit -= FirstNormalTransactionId;
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/*
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* We'll start complaining loudly when we get within 10M transactions of
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* the stop point. This is kind of arbitrary, but if you let your gas
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* gauge get down to 1% of full, would you be looking for the next gas
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* station? We need to be fairly liberal about this number because there
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* are lots of scenarios where most transactions are done by automatic
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* clients that won't pay attention to warnings. (No, we're not gonna make
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* this configurable. If you know enough to configure it, you know enough
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* to not get in this kind of trouble in the first place.)
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*/
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xidWarnLimit = xidStopLimit - 10000000;
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if (xidWarnLimit < FirstNormalTransactionId)
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xidWarnLimit -= FirstNormalTransactionId;
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/*
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* We'll start trying to force autovacuums when oldest_datfrozenxid gets
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* to be more than autovacuum_freeze_max_age transactions old.
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*
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* Note: guc.c ensures that autovacuum_freeze_max_age is in a sane range,
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* so that xidVacLimit will be well before xidWarnLimit.
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*
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* Note: autovacuum_freeze_max_age is a PGC_POSTMASTER parameter so that
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* we don't have to worry about dealing with on-the-fly changes in its
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* value. It doesn't look practical to update shared state from a GUC
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* assign hook (too many processes would try to execute the hook,
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* resulting in race conditions as well as crashes of those not connected
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* to shared memory). Perhaps this can be improved someday. See also
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* SetMultiXactIdLimit.
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*/
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xidVacLimit = oldest_datfrozenxid + autovacuum_freeze_max_age;
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if (xidVacLimit < FirstNormalTransactionId)
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xidVacLimit += FirstNormalTransactionId;
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/* Grab lock for just long enough to set the new limit values */
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LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
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ShmemVariableCache->oldestXid = oldest_datfrozenxid;
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ShmemVariableCache->xidVacLimit = xidVacLimit;
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ShmemVariableCache->xidWarnLimit = xidWarnLimit;
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ShmemVariableCache->xidStopLimit = xidStopLimit;
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ShmemVariableCache->xidWrapLimit = xidWrapLimit;
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ShmemVariableCache->oldestXidDB = oldest_datoid;
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curXid = ShmemVariableCache->nextXid;
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LWLockRelease(XidGenLock);
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/* Log the info */
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ereport(DEBUG1,
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(errmsg("transaction ID wrap limit is %u, limited by database with OID %u",
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xidWrapLimit, oldest_datoid)));
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/*
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* If past the autovacuum force point, immediately signal an autovac
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* request. The reason for this is that autovac only processes one
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* database per invocation. Once it's finished cleaning up the oldest
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* database, it'll call here, and we'll signal the postmaster to start
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* another iteration immediately if there are still any old databases.
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*/
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if (TransactionIdFollowsOrEquals(curXid, xidVacLimit) &&
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IsUnderPostmaster && !InRecovery)
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SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
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/* Give an immediate warning if past the wrap warn point */
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if (TransactionIdFollowsOrEquals(curXid, xidWarnLimit) && !InRecovery)
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{
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char *oldest_datname;
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/*
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* We can be called when not inside a transaction, for example during
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* StartupXLOG(). In such a case we cannot do database access, so we
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* must just report the oldest DB's OID.
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*
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* Note: it's also possible that get_database_name fails and returns
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* NULL, for example because the database just got dropped. We'll
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* still warn, even though the warning might now be unnecessary.
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*/
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if (IsTransactionState())
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oldest_datname = get_database_name(oldest_datoid);
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else
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oldest_datname = NULL;
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if (oldest_datname)
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ereport(WARNING,
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(errmsg("database \"%s\" must be vacuumed within %u transactions",
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oldest_datname,
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xidWrapLimit - curXid),
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errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
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"You might also need to commit or roll back old prepared transactions.")));
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else
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ereport(WARNING,
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(errmsg("database with OID %u must be vacuumed within %u transactions",
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oldest_datoid,
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xidWrapLimit - curXid),
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errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
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"You might also need to commit or roll back old prepared transactions.")));
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}
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}
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/*
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* ForceTransactionIdLimitUpdate -- does the XID wrap-limit data need updating?
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*
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* We primarily check whether oldestXidDB is valid. The cases we have in
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* mind are that that database was dropped, or the field was reset to zero
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* by pg_resetxlog. In either case we should force recalculation of the
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* wrap limit. Also do it if oldestXid is old enough to be forcing
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* autovacuums or other actions; this ensures we update our state as soon
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* as possible once extra overhead is being incurred.
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*/
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bool
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ForceTransactionIdLimitUpdate(void)
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{
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TransactionId nextXid;
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TransactionId xidVacLimit;
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TransactionId oldestXid;
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Oid oldestXidDB;
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/* Locking is probably not really necessary, but let's be careful */
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LWLockAcquire(XidGenLock, LW_SHARED);
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nextXid = ShmemVariableCache->nextXid;
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xidVacLimit = ShmemVariableCache->xidVacLimit;
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oldestXid = ShmemVariableCache->oldestXid;
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oldestXidDB = ShmemVariableCache->oldestXidDB;
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LWLockRelease(XidGenLock);
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if (!TransactionIdIsNormal(oldestXid))
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return true; /* shouldn't happen, but just in case */
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if (!TransactionIdIsValid(xidVacLimit))
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return true; /* this shouldn't happen anymore either */
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if (TransactionIdFollowsOrEquals(nextXid, xidVacLimit))
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return true; /* past VacLimit, don't delay updating */
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if (!SearchSysCacheExists1(DATABASEOID, ObjectIdGetDatum(oldestXidDB)))
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return true; /* could happen, per comments above */
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return false;
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}
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/*
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* GetNewObjectId -- allocate a new OID
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*
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* OIDs are generated by a cluster-wide counter. Since they are only 32 bits
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* wide, counter wraparound will occur eventually, and therefore it is unwise
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* to assume they are unique unless precautions are taken to make them so.
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* Hence, this routine should generally not be used directly. The only
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* direct callers should be GetNewOid() and GetNewRelFileNode() in
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* catalog/catalog.c.
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*/
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Oid
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GetNewObjectId(void)
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{
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Oid result;
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/* safety check, we should never get this far in a HS slave */
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if (RecoveryInProgress())
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elog(ERROR, "cannot assign OIDs during recovery");
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LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
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/*
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* Check for wraparound of the OID counter. We *must* not return 0
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* (InvalidOid); and as long as we have to check that, it seems a good
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* idea to skip over everything below FirstNormalObjectId too. (This
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* basically just avoids lots of collisions with bootstrap-assigned OIDs
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* right after a wrap occurs, so as to avoid a possibly large number of
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* iterations in GetNewOid.) 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;
|
|
}
|