SQLite

** The pager is used to access a database disk file.  It implements
** atomic commit and rollback through the use of a journal file that
** is separate from the database file.  The pager also implements file
** locking to prevent two processes from writing the same database
** file simultaneously, or one process from reading the database while
** another is writing.
**
** @(#) $Id: pager.c,v 1.346 2007/06/16 11:17:46 drh Exp $
*/
#ifndef SQLITE_OMIT_DISKIO
#include "sqliteInt.h"
#include "os.h"
#include "pager.h"
#include <assert.h>
#include <string.h>

** is cleared.  The modified page cannot be written back into the original
** database file until the journal pages has been synced to disk and the
** PgHdr.needSync has been cleared.
**
** The PgHdr.dirty flag is set when sqlite3PagerWrite() is called and
** is cleared again when the page content is written back to the original
** database file.
**
** Details of important structure elements:
**
** needSync
**
**     If this is true, this means that it is not safe to write the page
**     content to the database because the original content needed
**     for rollback has not by synced to the main rollback journal.
**     The original content may have been written to the rollback journal
**     but it has not yet been synced.  So we cannot write to the database
**     file because power failure might cause the page in the journal file
**     to never reach the disk.  It is as if the write to the journal file
**     does not occur until the journal file is synced.
**     
**     This flag is false if the page content exactly matches what
**     currently exists in the database file.  The needSync flag is also
**     false if the original content has been written to the main rollback
**     journal and synced.  If the page represents a new page that has
**     been added onto the end of the database during the current
**     transaction, the needSync flag is true until the original database
**     size in the journal header has been synced to disk.
**
** inJournal
**
**     This is true if the original page has been written into the main
**     rollback journal.  This is always false for new pages added to
**     the end of the database file during the current transaction.
**     And this flag says nothing about whether or not the journal
**     has been synced to disk.  For pages that are in the original
**     database file, the following expression should always be true:
**
**       inJournal = (pPager->aInJournal[(pgno-1)/8] & (1<<((pgno-1)%8))!=0
**
**     The pPager->aInJournal[] array is only valid for the original
**     pages of the database, not new pages that are added to the end
**     of the database, so obviously the above expression cannot be
**     valid for new pages.  For new pages inJournal is always 0.
**
** dirty
**
**     When true, this means that the content of the page has been
**     modified and needs to be written back to the database file.
**     If false, it means that either the content of the page is
**     unchanged or else the content is unimportant and we do not
**     care whether or not it is preserved.
**
** alwaysRollback
**
**     This means that the sqlite3PagerDontRollback() API should be
**     ignored for this page.  The DontRollback() API attempts to say
**     that the content of the page on disk is unimportant (it is an
**     unused page on the freelist) so that it is unnecessary to 
**     rollback changes to this page because the content of the page
**     can change without changing the meaning of the database.  This
**     flag overrides any DontRollback() attempt.  This flag is set
**     when a page that originally contained valid data is added to
**     the freelist.  Later in the same transaction, this page might
**     be pulled from the freelist and reused for something different
**     and at that point the DontRollback() API will be called because
**     pages taken from the freelist do not need to be protected by
**     the rollback journal.  But this flag says that the page was
**     not originally part of the freelist so that it still needs to
**     be rolled back in spite of any subsequent DontRollback() calls.
**
** needRead 
**
**     This flag means (when true) that the content of the page has
**     not yet been loaded from disk.  The in-memory content is just
**     garbage.  (Actually, we zero the content, but you should not
**     make any assumptions about the content nevertheless.)  If the
**     content is needed in the future, it should be read from the
**     original database file.
*/
typedef struct PgHdr PgHdr;
struct PgHdr {
  Pager *pPager;                 /* The pager to which this page belongs */
  Pgno pgno;                     /* The page number for this page */
  PgHdr *pNextHash, *pPrevHash;  /* Hash collision chain for PgHdr.pgno */
  PgHdr *pNextFree, *pPrevFree;  /* Freelist of pages where nRef==0 */

    makeClean(pPgOld);
    pPg->needSync = pPgOld->needSync;
  }else{
    pPg->needSync = 0;
  }
  if( pPager->aInJournal && (int)pgno<=pPager->origDbSize ){
    pPg->inJournal =  (pPager->aInJournal[pgno/8] & (1<<(pgno&7)))!=0;


  }else{
    pPg->inJournal = 0;
    assert( pPg->needSync==0 || (int)pgno>pPager->origDbSize );
  }

  /* Change the page number for pPg and insert it into the new hash-chain. */
  assert( pgno!=0 );
  pPg->pgno = pgno;
  h = pgno & (pPager->nHash-1);
  if( pPager->aHash[h] ){