** and *pChanged is set to 1.
**
** If the checksum cannot be verified return non-zero. If the header
** is read successfully and the checksum verified, return zero.
*/
int walIndexTryHdr(Wal *pWal, int *pChanged){
  u32 aCksum[2];               /* Checksum on the header content */
  WalIndexHdr hdr1, hdr2;      /* Two copies of the header content */
  WalIndexHdr *aHdr;           /* Header in shared memory */

  assert( pWal->pWiData );
  if( pWal->szWIndex==0 ){
    /* The wal-index is of size 0 bytes. This is handled in the same way
    ** as an invalid header. The caller will run recovery to construct
    ** a valid wal-index file before accessing the database.
................................................................................
    return 1;
  }

  /* Read the header. The caller may or may not have an exclusive 
  ** (WRITE, PENDING, CHECKPOINT or RECOVER) lock on the wal-index
  ** file, meaning it is possible that an inconsistent snapshot is read
  ** from the file. If this happens, return non-zero.





  */
  aHdr = (WalIndexHdr*)pWal->pWiData;
  memcpy(&hdr1, &aHdr[0], sizeof(hdr1));
  sqlite3OsShmBarrier(pWal->pDbFd);
  memcpy(&hdr2, &aHdr[1], sizeof(hdr2));

  if( memcmp(&hdr1, &hdr2, sizeof(hdr1))!=0 ){

    /* Dirty read */
    return 1;
  }  
  walChecksumBytes((u8*)&hdr1, sizeof(hdr1)-sizeof(hdr1.aCksum), 0, aCksum);
  if( aCksum[0]!=hdr1.aCksum[0] || aCksum[1]!=hdr1.aCksum[1] ){

    /* Malformed header */
    return 1;
  }





  if( memcmp(&pWal->hdr, &hdr1, sizeof(WalIndexHdr)) ){
    *pChanged = 1;
    memcpy(&pWal->hdr, &hdr1, sizeof(WalIndexHdr));
    pWal->szPage = pWal->hdr.szPage;
  }

  /* The header was successfully read. Return zero. */
  return 0;
}

** and *pChanged is set to 1.
**
** If the checksum cannot be verified return non-zero. If the header
** is read successfully and the checksum verified, return zero.
*/
int walIndexTryHdr(Wal *pWal, int *pChanged){
  u32 aCksum[2];               /* Checksum on the header content */
  WalIndexHdr h1, h2;          /* Two copies of the header content */
  WalIndexHdr *aHdr;           /* Header in shared memory */

  assert( pWal->pWiData );
  if( pWal->szWIndex==0 ){
    /* The wal-index is of size 0 bytes. This is handled in the same way
    ** as an invalid header. The caller will run recovery to construct
    ** a valid wal-index file before accessing the database.
................................................................................
    return 1;
  }

  /* Read the header. The caller may or may not have an exclusive 
  ** (WRITE, PENDING, CHECKPOINT or RECOVER) lock on the wal-index
  ** file, meaning it is possible that an inconsistent snapshot is read
  ** from the file. If this happens, return non-zero.
  **
  ** There are two copies of the header at the beginning of the wal-index.
  ** When reading, read [0] first then [1].  Writes are in the reverse order.
  ** Memory barriers are used to prevent the compiler or the hardware from
  ** reordering the reads and writes.
  */
  aHdr = (WalIndexHdr*)pWal->pWiData;
  memcpy(&h1, &aHdr[0], sizeof(h1));
  sqlite3OsShmBarrier(pWal->pDbFd);
  memcpy(&h2, &aHdr[1], sizeof(h2));


  if( memcmp(&h1, &h2, sizeof(h1))!=0 ){
    return 1;   /* Dirty read */

  }  


  if( h1.szPage==0 ){
    return 1;   /* Malformed header - probably all zeros */

  }
  walChecksumBytes((u8*)&h1, sizeof(h1)-sizeof(h1.aCksum), 0, aCksum);
  if( aCksum[0]!=h1.aCksum[0] || aCksum[1]!=h1.aCksum[1] ){
    return 1;   /* Checksum does not match */
  }

  if( memcmp(&pWal->hdr, &h1, sizeof(WalIndexHdr)) ){
    *pChanged = 1;
    memcpy(&pWal->hdr, &h1, sizeof(WalIndexHdr));
    pWal->szPage = pWal->hdr.szPage;
  }

  /* The header was successfully read. Return zero. */
  return 0;
}