text: /*-------------------------------------------------------------------------
   *
   * polar_csn.c
   *		various csn related function.
   *
   * Copyright (c) 2020, Alibaba Group Holding Limited
   * 
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   *
   * IDENTIFICATION
   *	  external/polar_csn/polar_csn.c
   *
   *-------------------------------------------------------------------------
   */
  
  #include "postgres.h"
  
  #include "fmgr.h"
  #include "utils/guc.h"
  #include "access/transam.h"
  
  /*
   * txid and TxidSnapshot type are
   * copied from txid.c
   */
  
  /* Use unsigned variant internally */
  typedef uint64 txid;
  
  /*
   * Snapshot containing 8byte txids.
   */
  typedef struct
  {
  	/*
  	 * 4-byte length hdr, should not be touched directly.
  	 *
  	 * Explicit embedding is ok as we want always correct alignment anyway.
  	 */
  	int32		__varsz;
  
  	uint32		nxip;			/* number of txids in xip array */
  	txid		xmin;
  	txid		xmax;
  	/* in-progress txids, xmin <= xip[i] < xmax: */
  	/* 
  	 * POLAR csn
  	 * To make txid_snapshot storage compatible, we should store csn in xip
  	 * and store an invalid xid in front of csn to differentiate csn snapshot
  	 * with xid snapshot
  	 */
  	txid		xip[FLEXIBLE_ARRAY_MEMBER];
  } TxidSnapshot;
  
  extern bool is_csn_snapshot(const TxidSnapshot *snap);
  extern CommitSeqNo txid_snapshot_get_csn(const TxidSnapshot *snap);
  
  PG_MODULE_MAGIC;
  
  /*
   * txid_snapshot_csn(txid_snapshot) returns int8
   *
   *		return snapshot's csn
   */
  PG_FUNCTION_INFO_V1(txid_snapshot_csn);
  
  Datum
  txid_snapshot_csn(PG_FUNCTION_ARGS)
  {
  	TxidSnapshot *snap = (TxidSnapshot *) PG_GETARG_VARLENA_P(0);
  	CommitSeqNo csn = InvalidCommitSeqNo;
  
  	if (is_csn_snapshot(snap))
  	{
  		csn = txid_snapshot_get_csn(snap);
  	}
  	
  	PG_RETURN_INT64(csn);
  }
  
  /*
   * txid_csn(int8) returns int8
   *
   *		return xid's csn
   */
  PG_FUNCTION_INFO_V1(txid_csn);
  
  Datum
  txid_csn(PG_FUNCTION_ARGS)
  {
  	TransactionId xid = (TransactionId) PG_GETARG_INT64(0);
  
      if (polar_csn_enable)
  	    PG_RETURN_UINT64(polar_xact_get_csn(xid, POLAR_CSN_MAX_NORMAL, false));
      else
          PG_RETURN_UINT64(InvalidCommitSeqNo);
  }
  