/* RecordTransactionCommit
* Returns latest XID among xact and its children, or InvalidTransactionId if the xact has no XID. (We compute that here just because it's easier.)
* If you change this function, see RecordTransactionCommitPrepared also. */
static TransactionId RecordTransactionCommit(void) {
TransactionId xid = GetTopTransactionIdIfAny();
bool markXidCommitted = TransactionIdIsValid(xid);
TransactionId latestXid = InvalidTransactionId;
int nrels;
RelFileNode *rels;
int nchildren;
TransactionId *children;
int nmsgs = 0;
SharedInvalidationMessage *invalMessages = NULL;
bool RelcacheInitFileInval = false;
bool wrote_xlog;
/* Get data needed for commit record */
nrels = smgrGetPendingDeletes(true, &rels);
nchildren = xactGetCommittedChildren(&children);
if (XLogStandbyInfoActive())
nmsgs = xactGetCommittedInvalidationMessages(&invalMessages, &RelcacheInitFileInval);
wrote_xlog = (XactLastRecEnd != 0);
/* If we haven't been assigned an XID yet, we neither can, nor do we want to write a COMMIT record. */
if (!markXidCommitted) {
/*
* We expect that every smgrscheduleunlink is followed by a catalog
* update, and hence XID assignment, so we shouldn't get here with any
* pending deletes. Use a real test not just an Assert to check this,
* since it's a bit fragile.
*/
if (nrels != 0)
elog(ERROR, "cannot commit a transaction that deleted files but has no xid");
/* Can't have child XIDs either; AssignTransactionId enforces this */
Assert(nchildren == 0);
/*
* Transactions without an assigned xid can contain invalidation
* messages (e.g. explicit relcache invalidations or catcache
* invalidations for inplace updates); standbys need to process those.
* We can't emit a commit record without an xid, and we don't want to
* force assigning an xid, because that'd be problematic for e.g.
* vacuum. Hence we emit a bespoke record for the invalidations. We
* don't want to use that in case a commit record is emitted, so they
* happen synchronously with commits (besides not wanting to emit more
* WAL records).
*/
if (nmsgs != 0)
{
LogStandbyInvalidations(nmsgs, invalMessages,
RelcacheInitFileInval);
wrote_xlog = true; /* not strictly necessary */
}
/*
* If we didn't create XLOG entries, we're done here; otherwise we
* should trigger flushing those entries the same as a commit record
* would. This will primarily happen for HOT pruning and the like; we
* want these to be flushed to disk in due time.
*/
if (!wrote_xlog)
goto cleanup;
}
else
{
bool replorigin;
/*
* Are we using the replication origins feature? Or, in other words,
* are we replaying remote actions?
*/
replorigin = (replorigin_session_origin != InvalidRepOriginId &&
replorigin_session_origin != DoNotReplicateId);
/*
* Begin commit critical section and insert the commit XLOG record.
*/
/* Tell bufmgr and smgr to prepare for commit */
BufmgrCommit();
/*
* Mark ourselves as within our "commit critical section". This
* forces any concurrent checkpoint to wait until we've updated
* pg_xact. Without this, it is possible for the checkpoint to set
* REDO after the XLOG record but fail to flush the pg_xact update to
* disk, leading to loss of the transaction commit if the system
* crashes a little later.
*
* Note: we could, but don't bother to, set this flag in
* RecordTransactionAbort. That's because loss of a transaction abort
* is noncritical; the presumption would be that it aborted, anyway.
*
* It's safe to change the delayChkpt flag of our own backend without
* holding the ProcArrayLock, since we're the only one modifying it.
* This makes checkpoint's determination of which xacts are delayChkpt
* a bit fuzzy, but it doesn't matter.
*/
START_CRIT_SECTION();
MyPgXact->delayChkpt = true;
SetCurrentTransactionStopTimestamp();
XactLogCommitRecord(xactStopTimestamp,
nchildren, children, nrels, rels,
nmsgs, invalMessages,
RelcacheInitFileInval, forceSyncCommit,
MyXactFlags,
InvalidTransactionId, NULL /* plain commit */ );
if (replorigin)
/* Move LSNs forward for this replication origin */
replorigin_session_advance(replorigin_session_origin_lsn,
XactLastRecEnd);
/*
* Record commit timestamp. The value comes from plain commit
* timestamp if there's no replication origin; otherwise, the
* timestamp was already set in replorigin_session_origin_timestamp by
* replication.
*
* We don't need to WAL-log anything here, as the commit record
* written above already contains the data.
*/
if (!replorigin || replorigin_session_origin_timestamp == 0)
replorigin_session_origin_timestamp = xactStopTimestamp;
TransactionTreeSetCommitTsData(xid, nchildren, children,
replorigin_session_origin_timestamp,
replorigin_session_origin, false);
}
/* Check if we want to commit asynchronously. We can allow the XLOG flush
* to happen asynchronously if synchronous_commit=off, or if the current
* transaction has not performed any WAL-logged operation or didn't assign
* an xid. The transaction can end up not writing any WAL, even if it has
* an xid, if it only wrote to temporary and/or unlogged tables. It can
* end up having written WAL without an xid if it did HOT pruning. In
* case of a crash, the loss of such a transaction will be irrelevant;
* temp tables will be lost anyway, unlogged tables will be truncated and
* HOT pruning will be done again later. (Given the foregoing, you might
* think that it would be unnecessary to emit the XLOG record at all in
* this case, but we don't currently try to do that. It would certainly
* cause problems at least in Hot Standby mode, where the
* KnownAssignedXids machinery requires tracking every XID assignment. It
* might be OK to skip it only when wal_level < replica, but for now we
* don't.)
* However, if we're doing cleanup of any non-temp rels or committing any
* command that wanted to force sync commit, then we must flush XLOG
* immediately. (We must not allow asynchronous commit if there are any
* non-temp tables to be deleted, because we might delete the files before
* the COMMIT record is flushed to disk. We do allow asynchronous commit
* if all to-be-deleted tables are temporary though, since they are lost
* anyway if we crash.) */
// 需要将事务日志刷入磁盘，然后事务才能提交
if ((wrote_xlog && markXidCommitted && synchronous_commit > SYNCHRONOUS_COMMIT_OFF) || forceSyncCommit || nrels > 0) {
XLogFlush(XactLastRecEnd); // 刷事务日志到磁盘
/* Now we may update the CLOG, if we wrote a COMMIT record above */
if (markXidCommitted) TransactionIdCommitTree(xid, nchildren, children); // 更新事务的状态
} else {
/* Asynchronous commit case:
* This enables possible committed transaction loss in the case of a
* postmaster crash because WAL buffers are left unwritten. Ideally we
* could issue the WAL write without the fsync, but some
* wal_sync_methods do not allow separate write/fsync.
* Report the latest async commit LSN, so that the WAL writer knows to
* flush this commit. */
XLogSetAsyncXactLSN(XactLastRecEnd); // 设置异步提交的最新的LSN
// 将最新的LSN保存到异步提交的事务组中，不用等事务日志刷入磁盘即可提交事务，同时将事务状态保存到clog中
/* We must not immediately update the CLOG, since we didn't flush the
* XLOG. Instead, we store the LSN up to which the XLOG must be
* flushed before the CLOG may be updated. */
if (markXidCommitted) TransactionIdAsyncCommitTree(xid, nchildren, children, XactLastRecEnd);
}
/*
* If we entered a commit critical section, leave it now, and let
* checkpoints proceed.
*/
if (markXidCommitted)
{
MyPgXact->delayChkpt = false;
END_CRIT_SECTION();
}
/* Compute latestXid while we have the child XIDs handy */
latestXid = TransactionIdLatest(xid, nchildren, children);
/*
* Wait for synchronous replication, if required. Similar to the decision
* above about using committing asynchronously we only want to wait if
* this backend assigned an xid and wrote WAL. No need to wait if an xid
* was assigned due to temporary/unlogged tables or due to HOT pruning.
*
* Note that at this stage we have marked clog, but still show as running
* in the procarray and continue to hold locks.
*/
if (wrote_xlog && markXidCommitted)
SyncRepWaitForLSN(XactLastRecEnd, true);
/* remember end of last commit record */
XactLastCommitEnd = XactLastRecEnd;
/* Reset XactLastRecEnd until the next transaction writes something */
XactLastRecEnd = 0;
cleanup:
/* Clean up local data */
if (rels)
pfree(rels);
return latestXid;
}
