** checkpoint.
*/
static int ckptChecksumOk(u32 *aCkpt){
  u32 nCkpt = aCkpt[CKPT_HDR_NCKPT];
  u32 cksum1;
  u32 cksum2;

  if( nCkpt<CKPT_HDR_NCKPT ) return 0;
  ckptChecksum(aCkpt, nCkpt, &cksum1, &cksum2);
  return (cksum1==aCkpt[nCkpt-2] && cksum2==aCkpt[nCkpt-1]);
}

/*
** Attempt to load a checkpoint from meta page iMeta.
**
................................................................................
  memcpy(pShm->aWorker, p, n);
  lsmShmBarrier(pDb);
  memcpy(pShm->aClient, p, n);
  lsmFree(pDb->pEnv, p);

  return LSM_OK;
}







































/*
** Return the checkpoint-id of the checkpoint array passed as the first
** argument to this function. If the second argument is true, then assume
** that the checkpoint is made up of 32-bit big-endian integers. If it
** is false, assume that the integers are in machine byte order.
*/

** checkpoint.
*/
static int ckptChecksumOk(u32 *aCkpt){
  u32 nCkpt = aCkpt[CKPT_HDR_NCKPT];
  u32 cksum1;
  u32 cksum2;

  if( nCkpt<CKPT_HDR_NCKPT || nCkpt>(LSM_META_PAGE_SIZE)/sizeof(u32) ) return 0;
  ckptChecksum(aCkpt, nCkpt, &cksum1, &cksum2);
  return (cksum1==aCkpt[nCkpt-2] && cksum2==aCkpt[nCkpt-1]);
}

/*
** Attempt to load a checkpoint from meta page iMeta.
**
................................................................................
  memcpy(pShm->aWorker, p, n);
  lsmShmBarrier(pDb);
  memcpy(pShm->aClient, p, n);
  lsmFree(pDb->pEnv, p);

  return LSM_OK;
}

int lsmCheckpointSynced(lsm_db *pDb, i64 *piId){
  int rc = LSM_OK;
  const int nAttempt = 3;
  int i;
  for(i=0; i<nAttempt; i++){
    MetaPage *pPg;
    u32 iMeta;

    iMeta = pDb->pShmhdr->iMetaPage;
    rc = lsmFsMetaPageGet(pDb->pFS, 0, iMeta, &pPg);
    if( rc==LSM_OK ){
      int nCkpt;
      int nData;
      u8 *aData; 

      aData = lsmFsMetaPageData(pPg, &nData);
      assert( nData==LSM_META_PAGE_SIZE );
      nCkpt = lsmGetU32(&aData[CKPT_HDR_NCKPT*sizeof(u32)]);

      if( nCkpt<(LSM_META_PAGE_SIZE/sizeof(u32)) ){
        u32 *aCopy = lsmMallocRc(pDb->pEnv, sizeof(u32) * nCkpt, &rc);
        if( aCopy ){
          memcpy(aCopy, aData, nCkpt*sizeof(u32));
          ckptChangeEndianness(aCopy, nCkpt);
          if( ckptChecksumOk(aCopy) ){
            *piId = lsmCheckpointId(aCopy, 0);
          }
          lsmFree(pDb->pEnv, aCopy);
        }
      }
      lsmFsMetaPageRelease(pPg);
    }
    if( rc!=LSM_OK || pDb->pShmhdr->iMetaPage==iMeta ) break;
  }

  return (rc==LSM_OK && i==3) ? LSM_BUSY : LSM_OK;
}

/*
** Return the checkpoint-id of the checkpoint array passed as the first
** argument to this function. If the second argument is true, then assume
** that the checkpoint is made up of 32-bit big-endian integers. If it
** is false, assume that the integers are in machine byte order.
*/

    /* The first block on the free list was freed as part of the work done
    ** to create the snapshot with id iFree. So, we can reuse this block if
    ** snapshot iFree or later has been checkpointed and all currently 
    ** active clients are reading from snapshot iFree or later.  */
    i64 iFree = pFree->aEntry[0].iId;
    int bInUse = 0;




    /* TODO: The "has been checkpointed" bit */






    rc = lsmLsmInUse(pDb, iFree, &bInUse);

    if( rc==LSM_OK && bInUse==0 ){
      iRet = pFree->aEntry[0].iBlk;
      flRemoveEntry0(pFree);
      assert( iRet!=0 );
    }
  }

    /* The first block on the free list was freed as part of the work done
    ** to create the snapshot with id iFree. So, we can reuse this block if
    ** snapshot iFree or later has been checkpointed and all currently 
    ** active clients are reading from snapshot iFree or later.  */
    i64 iFree = pFree->aEntry[0].iId;
    int bInUse = 0;

    /* The "is in use" bit */
    rc = lsmLsmInUse(pDb, iFree, &bInUse);

    /* The "has been checkpointed" bit */
    if( rc==LSM_OK && bInUse==0 ){
      i64 iId = 0;
      rc = lsmCheckpointSynced(pDb, &iId);
      if( rc==LSM_OK && iId<iFree ) bInUse = 1;
      if( rc==LSM_BUSY ) rc = LSM_OK;
    }


    if( rc==LSM_OK && bInUse==0 ){
      iRet = pFree->aEntry[0].iBlk;
      flRemoveEntry0(pFree);
      assert( iRet!=0 );
    }
  }