/*
* GetLatestSnapshot
* Get a snapshot that is up-to-date as of the current instant,
* even if we are executing in transaction-snapshot mode.
*/
Snapshot
GetLatestSnapshot(void)
{
/*
* We might be able to relax this, but nothing that could otherwise work
* needs it.
*/
if (IsInParallelMode())
elog(ERROR,
"cannot update SecondarySnapshot during a parallel operation");
/*
* So far there are no cases requiring support for GetLatestSnapshot()
* during logical decoding, but it wouldn't be hard to add if required.
*/
Assert(!HistoricSnapshotActive());
/* If first call in transaction, go ahead and set the xact snapshot */
if (!FirstSnapshotSet)
return GetTransactionSnapshot();
SecondarySnapshot = GetSnapshotData(&SecondarySnapshotData);
return SecondarySnapshot;
}
/*
* GetCatalogSnapshot
* Get a snapshot that is sufficiently up-to-date for scan of the
* system catalog with the specified OID.
*/
Snapshot
GetCatalogSnapshot(Oid relid)
{
/*
* Return historic snapshot while we're doing logical decoding, so we can
* see the appropriate state of the catalog.
*
* This is the primary reason for needing to reset the system caches after
* finishing decoding.
*/
if (HistoricSnapshotActive())
return HistoricSnapshot;
return GetNonHistoricCatalogSnapshot(relid);
}
/*
* GetLatestSnapshot
* Get a snapshot that is up-to-date as of the current instant,
* even if we are executing in transaction-snapshot mode.
*/
Snapshot
GetLatestSnapshot(void)
{
/*
* So far there are no cases requiring support for GetLatestSnapshot()
* during logical decoding, but it wouldn't be hard to add if required.
*/
Assert(!HistoricSnapshotActive());
/* If first call in transaction, go ahead and set the xact snapshot */
if (!FirstSnapshotSet)
return GetTransactionSnapshot();
SecondarySnapshot = GetSnapshotData(&SecondarySnapshotData);
return SecondarySnapshot;
}
/*
* SetTransactionSnapshot
* Set the transaction's snapshot from an imported MVCC snapshot.
*
* Note that this is very closely tied to GetTransactionSnapshot --- it
* must take care of all the same considerations as the first-snapshot case
* in GetTransactionSnapshot.
*/
static void
SetTransactionSnapshot(Snapshot sourcesnap, TransactionId sourcexid,
PGPROC *sourceproc)
{
/* Caller should have checked this already */
Assert(!FirstSnapshotSet);
Assert(pairingheap_is_empty(&RegisteredSnapshots));
Assert(FirstXactSnapshot == NULL);
Assert(!HistoricSnapshotActive());
/*
* Even though we are not going to use the snapshot it computes, we must
* call GetSnapshotData, for two reasons: (1) to be sure that
* CurrentSnapshotData's XID arrays have been allocated, and (2) to update
* RecentXmin and RecentGlobalXmin. (We could alternatively include those
* two variables in exported snapshot files, but it seems better to have
* snapshot importers compute reasonably up-to-date values for them.)
*/
CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
/*
* Now copy appropriate fields from the source snapshot.
*/
CurrentSnapshot->xmin = sourcesnap->xmin;
CurrentSnapshot->xmax = sourcesnap->xmax;
CurrentSnapshot->xcnt = sourcesnap->xcnt;
Assert(sourcesnap->xcnt <= GetMaxSnapshotXidCount());
memcpy(CurrentSnapshot->xip, sourcesnap->xip,
sourcesnap->xcnt * sizeof(TransactionId));
CurrentSnapshot->subxcnt = sourcesnap->subxcnt;
Assert(sourcesnap->subxcnt <= GetMaxSnapshotSubxidCount());
memcpy(CurrentSnapshot->subxip, sourcesnap->subxip,
sourcesnap->subxcnt * sizeof(TransactionId));
CurrentSnapshot->suboverflowed = sourcesnap->suboverflowed;
CurrentSnapshot->takenDuringRecovery = sourcesnap->takenDuringRecovery;
/* NB: curcid should NOT be copied, it's a local matter */
/*
* Now we have to fix what GetSnapshotData did with MyPgXact->xmin and
* TransactionXmin. There is a race condition: to make sure we are not
* causing the global xmin to go backwards, we have to test that the
* source transaction is still running, and that has to be done
* atomically. So let procarray.c do it.
*
* Note: in serializable mode, predicate.c will do this a second time. It
* doesn't seem worth contorting the logic here to avoid two calls,
* especially since it's not clear that predicate.c *must* do this.
*/
if (sourceproc != NULL)
{
if (!ProcArrayInstallRestoredXmin(CurrentSnapshot->xmin, sourceproc))
ereport(ERROR,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("could not import the requested snapshot"),
errdetail("The source transaction is not running anymore.")));
}
else if (!ProcArrayInstallImportedXmin(CurrentSnapshot->xmin, sourcexid))
ereport(ERROR,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("could not import the requested snapshot"),
errdetail("The source transaction %u is not running anymore.",
sourcexid)));
/*
* In transaction-snapshot mode, the first snapshot must live until end of
* xact, so we must make a copy of it. Furthermore, if we're running in
* serializable mode, predicate.c needs to do its own processing.
*/
if (IsolationUsesXactSnapshot())
{
if (IsolationIsSerializable())
SetSerializableTransactionSnapshot(CurrentSnapshot, sourcexid);
/* Make a saved copy */
CurrentSnapshot = CopySnapshot(CurrentSnapshot);
FirstXactSnapshot = CurrentSnapshot;
/* Mark it as "registered" in FirstXactSnapshot */
FirstXactSnapshot->regd_count++;
pairingheap_add(&RegisteredSnapshots, &FirstXactSnapshot->ph_node);
}
FirstSnapshotSet = true;
}
/*
* GetTransactionSnapshot
* Get the appropriate snapshot for a new query in a transaction.
*
* Note that the return value may point at static storage that will be modified
* by future calls and by CommandCounterIncrement(). Callers should call
* RegisterSnapshot or PushActiveSnapshot on the returned snap if it is to be
* used very long.
*/
Snapshot
GetTransactionSnapshot(void)
{
/*
* Return historic snapshot if doing logical decoding. We'll never need a
* non-historic transaction snapshot in this (sub-)transaction, so there's
* no need to be careful to set one up for later calls to
* GetTransactionSnapshot().
*/
if (HistoricSnapshotActive())
{
Assert(!FirstSnapshotSet);
return HistoricSnapshot;
}
/* First call in transaction? */
if (!FirstSnapshotSet)
{
Assert(pairingheap_is_empty(&RegisteredSnapshots));
Assert(FirstXactSnapshot == NULL);
if (IsInParallelMode())
elog(ERROR,
"cannot take query snapshot during a parallel operation");
/*
* In transaction-snapshot mode, the first snapshot must live until
* end of xact regardless of what the caller does with it, so we must
* make a copy of it rather than returning CurrentSnapshotData
* directly. Furthermore, if we're running in serializable mode,
* predicate.c needs to wrap the snapshot fetch in its own processing.
*/
if (IsolationUsesXactSnapshot())
{
/* First, create the snapshot in CurrentSnapshotData */
if (IsolationIsSerializable())
CurrentSnapshot = GetSerializableTransactionSnapshot(&CurrentSnapshotData);
else
CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
/* Make a saved copy */
CurrentSnapshot = CopySnapshot(CurrentSnapshot);
FirstXactSnapshot = CurrentSnapshot;
/* Mark it as "registered" in FirstXactSnapshot */
FirstXactSnapshot->regd_count++;
pairingheap_add(&RegisteredSnapshots, &FirstXactSnapshot->ph_node);
}
else
CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
/* Don't allow catalog snapshot to be older than xact snapshot. */
CatalogSnapshotStale = true;
FirstSnapshotSet = true;
return CurrentSnapshot;
}
if (IsolationUsesXactSnapshot())
return CurrentSnapshot;
/* Don't allow catalog snapshot to be older than xact snapshot. */
CatalogSnapshotStale = true;
CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
return CurrentSnapshot;
}
HTAB *
HistoricSnapshotGetTupleCids(void)
{
Assert(HistoricSnapshotActive());
return tuplecid_data;
}
