Method beginSnapshot

src/jrd/tpc.cpp:915–970

SnapshotHandle TipCache::beginSnapshot(thread_db* tdbb, AttNumber attachmentId, CommitNumber& commitNumber)
{
	// Can only be called on initialized TipCache
	fb_assert(m_tpcHeader);
	GlobalTpcHeader* header = m_tpcHeader->getHeader();

	fb_assert(attachmentId);

	// Lock mutex
	SharedMutexGuard guard(m_snapshots);

	// Remap snapshot list if it has been grown by someone else
	remapSnapshots(false);

	SnapshotList* snapshots = m_snapshots->getHeader();

	if (commitNumber != 0)
	{
		ULONG slotsUsed = snapshots->slots_used.load(std::memory_order_relaxed);
		bool found = false;

		for (SnapshotHandle slotNumber = 0; slotNumber < slotsUsed; ++slotNumber)
		{
			if (snapshots->slots[slotNumber].attachment_id.load(std::memory_order_relaxed) != 0 &&
				snapshots->slots[slotNumber].snapshot.load(std::memory_order_relaxed) == commitNumber)
			{
				found = true;
				break;
			}
		}

		if (!found)
			ERR_post(Arg::Gds(isc_tra_snapshot_does_not_exist));
	}

	SnapshotHandle slotNumber = allocateSnapshotSlot();

	// Note, that allocateSnapshotSlot might remap memory and thus invalidate pointers
	snapshots = m_snapshots->getHeader();

	// Store snapshot commit number and return handle
	SnapshotData* slot = snapshots->slots + slotNumber;

	if (commitNumber == 0)
		commitNumber = header->latest_commit_number.load(std::memory_order_acquire);

	slot->snapshot.store(commitNumber, std::memory_order_release);

	// Only assign attachment_id after we completed all other work
	slot->attachment_id.store(attachmentId, std::memory_order_release);

	// And now move allocator watermark position after we used slot for sure
	snapshots->min_free_slot = slotNumber + 1;

	return slotNumber;
}

void TipCache::deallocateSnapshotSlot(SnapshotHandle slotNumber)