Function read_seq_tuple

src/backend/commands/sequence.c:1353–1398

* Given an opened sequence relation, lock the page buffer and find the tuple * * *buf receives the reference to the pinned-and-ex-locked buffer * *seqdatatuple receives the reference to the sequence tuple proper * (this arg should point to a local variable of type HeapTupleData) * * Function's return value points to the data payload of the tuple */

 * Function's return value points to the data payload of the tuple
 */
static Form_pg_sequence_data
read_seq_tuple(Relation rel, Buffer *buf, HeapTuple seqdatatuple)
{
	Page		page;
	ItemId		lp;
	sequence_magic *sm;
	Form_pg_sequence_data seq;

	*buf = ReadBuffer(rel, 0);
	LockBuffer(*buf, BUFFER_LOCK_EXCLUSIVE);

	page = BufferGetPage(*buf);
	sm = (sequence_magic *) PageGetSpecialPointer(page);

	if (sm->magic != SEQ_MAGIC)
		elog(ERROR, "bad magic number in sequence \"%s\": %08X",
			 RelationGetRelationName(rel), sm->magic);

	lp = PageGetItemId(page, FirstOffsetNumber);
	Assert(ItemIdIsNormal(lp));

	/* Note we currently only bother to set these two fields of *seqdatatuple */
	seqdatatuple->t_data = (HeapTupleHeader) PageGetItem(page, lp);
	seqdatatuple->t_len = ItemIdGetLength(lp);

	/*
	 * Previous releases of Postgres neglected to prevent SELECT FOR UPDATE on
	 * a sequence, which would leave a non-frozen XID in the sequence tuple's
	 * xmax, which eventually leads to clog access failures or worse. If we
	 * see this has happened, clean up after it.  We treat this like a hint
	 * bit update, ie, don't bother to WAL-log it, since we can certainly do
	 * this again if the update gets lost.
	 */
	Assert(!(seqdatatuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI));
	if (HeapTupleHeaderGetRawXmax(seqdatatuple->t_data) != InvalidTransactionId)
	{
		HeapTupleHeaderSetXmax(seqdatatuple->t_data, InvalidTransactionId);
		seqdatatuple->t_data->t_infomask &= ~HEAP_XMAX_COMMITTED;
		seqdatatuple->t_data->t_infomask |= HEAP_XMAX_INVALID;
		MarkBufferDirtyHint(*buf, true);
	}

	seq = (Form_pg_sequence_data) GETSTRUCT(seqdatatuple);

	return seq;
}

/*
 * init_params: process the options list of CREATE or ALTER SEQUENCE, and