assert( isSavepnt );
    assert( (pPager->doNotSpill & SPILLFLAG_ROLLBACK)==0 );
    pPager->doNotSpill |= SPILLFLAG_ROLLBACK;
    rc = sqlite3PagerGet(pPager, pgno, &pPg, 1);
    assert( (pPager->doNotSpill & SPILLFLAG_ROLLBACK)!=0 );
    pPager->doNotSpill &= ~SPILLFLAG_ROLLBACK;
    if( rc!=SQLITE_OK ) return rc;
    pPg->flags &= ~PGHDR_NEED_READ;
    sqlite3PcacheMakeDirty(pPg);
  }
  if( pPg ){
    /* No page should ever be explicitly rolled back that is in use, except
    ** for page 1 which is held in use in order to keep the lock on the
    ** database active. However such a page may be rolled back as a result
    ** of an internal error resulting in an automatic call to
................................................................................
void sqlite3PagerDontWrite(PgHdr *pPg){
  Pager *pPager = pPg->pPager;
  if( !pPager->tempFile && (pPg->flags&PGHDR_DIRTY) && pPager->nSavepoint==0 ){
    PAGERTRACE(("DONT_WRITE page %d of %d\n", pPg->pgno, PAGERID(pPager)));
    IOTRACE(("CLEAN %p %d\n", pPager, pPg->pgno))
    pPg->flags |= PGHDR_DONT_WRITE;
    pPg->flags &= ~PGHDR_WRITEABLE;

    pager_set_pagehash(pPg);
  }
}

/*
** This routine is called to increment the value of the database file 
** change-counter, stored as a 4-byte big-endian integer starting at 

    assert( isSavepnt );
    assert( (pPager->doNotSpill & SPILLFLAG_ROLLBACK)==0 );
    pPager->doNotSpill |= SPILLFLAG_ROLLBACK;
    rc = sqlite3PagerGet(pPager, pgno, &pPg, 1);
    assert( (pPager->doNotSpill & SPILLFLAG_ROLLBACK)!=0 );
    pPager->doNotSpill &= ~SPILLFLAG_ROLLBACK;
    if( rc!=SQLITE_OK ) return rc;

    sqlite3PcacheMakeDirty(pPg);
  }
  if( pPg ){
    /* No page should ever be explicitly rolled back that is in use, except
    ** for page 1 which is held in use in order to keep the lock on the
    ** database active. However such a page may be rolled back as a result
    ** of an internal error resulting in an automatic call to
................................................................................
void sqlite3PagerDontWrite(PgHdr *pPg){
  Pager *pPager = pPg->pPager;
  if( !pPager->tempFile && (pPg->flags&PGHDR_DIRTY) && pPager->nSavepoint==0 ){
    PAGERTRACE(("DONT_WRITE page %d of %d\n", pPg->pgno, PAGERID(pPager)));
    IOTRACE(("CLEAN %p %d\n", pPager, pPg->pgno))
    pPg->flags |= PGHDR_DONT_WRITE;
    pPg->flags &= ~PGHDR_WRITEABLE;
    testcase( pPg->flags & PGHDR_NEED_SYNC );
    pager_set_pagehash(pPg);
  }
}

/*
** This routine is called to increment the value of the database file 
** change-counter, stored as a 4-byte big-endian integer starting at 

  u8 bPurgeable;                      /* True if pages are on backing store */
  u8 eCreate;                         /* eCreate value for for xFetch() */
  int (*xStress)(void*,PgHdr*);       /* Call to try make a page clean */
  void *pStress;                      /* Argument to xStress */
  sqlite3_pcache *pCache;             /* Pluggable cache module */
};


/*
** Debug tracing macros





*/
#if defined(SQLITE_DEBUG) && 0
  int sqlite3PcacheTrace = 2;

# define pcacheTrace(X) if(sqlite3PcacheTrace){sqlite3DebugPrintf X;}
  void pcacheDump(PCache *pCache){
    int N;
    int i, j;
    sqlite3_pcache_page *pLower;
    PgHdr *pPg;
    unsigned char *a;
  
    if( sqlite3PcacheTrace<2 ) return;
    if( pCache->pCache==0 ) return;
    N = sqlite3PcachePagecount(pCache);
    if( N>5 ) N = 5;
    for(i=1; i<=N; i++){
       pLower = sqlite3GlobalConfig.pcache2.xFetch(pCache->pCache, i, 0);
       if( pLower==0 ) continue;
       pPg = (PgHdr*)pLower->pExtra;
       printf("%3d: nRef %2d flgs %02x data ", i, pPg->nRef, pPg->flags);
       a = (unsigned char *)pLower->pBuf;
       for(j=0; j<12; j++) printf("%02x", a[j]);
................................................................................
       }
    }
  }
  #else
# define pcacheTrace(X)
# define pcacheDump(X)
#endif









































/********************************** Linked List Management ********************/

/* Allowed values for second argument to pcacheManageDirtyList() */
#define PCACHE_DIRTYLIST_REMOVE   1    /* Remove pPage from dirty list */
#define PCACHE_DIRTYLIST_ADD      2    /* Add pPage to the dirty list */
#define PCACHE_DIRTYLIST_FRONT    3    /* Move pPage to the front of the list */
................................................................................
  pPgHdr = (PgHdr *)pPage->pExtra;

  if( !pPgHdr->pPage ){
    return pcacheFetchFinishWithInit(pCache, pgno, pPage);
  }
  pCache->nRefSum++;
  pPgHdr->nRef++;

  return pPgHdr;
}

/*
** Decrement the reference count on a page. If the page is clean and the
** reference count drops to 0, then it is made eligible for recycling.
*/
................................................................................
}

/*
** Increase the reference count of a supplied page by 1.
*/
void sqlite3PcacheRef(PgHdr *p){
  assert(p->nRef>0);

  p->nRef++;
  p->pCache->nRefSum++;
}

/*
** Drop a page from the cache. There must be exactly one reference to the
** page. This function deletes that reference, so after it returns the
** page pointed to by p is invalid.
*/
void sqlite3PcacheDrop(PgHdr *p){
  assert( p->nRef==1 );

  if( p->flags&PGHDR_DIRTY ){
    pcacheManageDirtyList(p, PCACHE_DIRTYLIST_REMOVE);
  }
  p->pCache->nRefSum--;
  sqlite3GlobalConfig.pcache2.xUnpin(p->pCache->pCache, p->pPage, 1);
}

/*
** Make sure the page is marked as dirty. If it isn't dirty already,
** make it so.
*/
void sqlite3PcacheMakeDirty(PgHdr *p){
  assert( p->nRef>0 );

  if( p->flags & (PGHDR_CLEAN|PGHDR_DONT_WRITE) ){    /*OPTIMIZATION-IF-FALSE*/
    p->flags &= ~PGHDR_DONT_WRITE;
    if( p->flags & PGHDR_CLEAN ){
      p->flags ^= (PGHDR_DIRTY|PGHDR_CLEAN);
      pcacheTrace(("%p.DIRTY %d\n",p->pCache,p->pgno));
      assert( (p->flags & (PGHDR_DIRTY|PGHDR_CLEAN))==PGHDR_DIRTY );
      pcacheManageDirtyList(p, PCACHE_DIRTYLIST_ADD);
    }

  }
}

/*
** Make sure the page is marked as clean. If it isn't clean already,
** make it so.
*/
void sqlite3PcacheMakeClean(PgHdr *p){

  if( ALWAYS((p->flags & PGHDR_DIRTY)!=0) ){
    assert( (p->flags & PGHDR_CLEAN)==0 );
    pcacheManageDirtyList(p, PCACHE_DIRTYLIST_REMOVE);
    p->flags &= ~(PGHDR_DIRTY|PGHDR_NEED_SYNC|PGHDR_WRITEABLE);
    p->flags |= PGHDR_CLEAN;
    pcacheTrace(("%p.CLEAN %d\n",p->pCache,p->pgno));

    if( p->nRef==0 ){
      pcacheUnpin(p);
    }
  }
}

/*
................................................................................
/*
** Change the page number of page p to newPgno. 
*/
void sqlite3PcacheMove(PgHdr *p, Pgno newPgno){
  PCache *pCache = p->pCache;
  assert( p->nRef>0 );
  assert( newPgno>0 );

  pcacheTrace(("%p.MOVE %d -> %d\n",pCache,p->pgno,newPgno));
  sqlite3GlobalConfig.pcache2.xRekey(pCache->pCache, p->pPage, p->pgno,newPgno);
  p->pgno = newPgno;
  if( (p->flags&PGHDR_DIRTY) && (p->flags&PGHDR_NEED_SYNC) ){
    pcacheManageDirtyList(p, PCACHE_DIRTYLIST_FRONT);
  }
}

  u8 bPurgeable;                      /* True if pages are on backing store */
  u8 eCreate;                         /* eCreate value for for xFetch() */
  int (*xStress)(void*,PgHdr*);       /* Call to try make a page clean */
  void *pStress;                      /* Argument to xStress */
  sqlite3_pcache *pCache;             /* Pluggable cache module */
};

/********************************** Test and Debug Logic **********************/
/*
** Debug tracing macros.  Enable by by changing the "0" to "1" and
** recompiling.
**
** When sqlite3PcacheTrace is 1, single line trace messages are issued.
** When sqlite3PcacheTrace is 2, a dump of the pcache showing all cache entries
** is displayed for many operations, resulting in a lot of output.
*/
#if defined(SQLITE_DEBUG) && 0
  int sqlite3PcacheTrace = 2;       /* 0: off  1: simple  2: cache dumps */
  int sqlite3PcacheMxDump = 9999;   /* Max cache entries for pcacheDump() */
# define pcacheTrace(X) if(sqlite3PcacheTrace){sqlite3DebugPrintf X;}
  void pcacheDump(PCache *pCache){
    int N;
    int i, j;
    sqlite3_pcache_page *pLower;
    PgHdr *pPg;
    unsigned char *a;
  
    if( sqlite3PcacheTrace<2 ) return;
    if( pCache->pCache==0 ) return;
    N = sqlite3PcachePagecount(pCache);
    if( N>sqlite3PcacheMxDump ) N = sqlite3PcacheMxDump;
    for(i=1; i<=N; i++){
       pLower = sqlite3GlobalConfig.pcache2.xFetch(pCache->pCache, i, 0);
       if( pLower==0 ) continue;
       pPg = (PgHdr*)pLower->pExtra;
       printf("%3d: nRef %2d flgs %02x data ", i, pPg->nRef, pPg->flags);
       a = (unsigned char *)pLower->pBuf;
       for(j=0; j<12; j++) printf("%02x", a[j]);
................................................................................
       }
    }
  }
  #else
# define pcacheTrace(X)
# define pcacheDump(X)
#endif

/*
** Check invariants on a PgHdr entry.  Return true if everything is OK.
** Return false if any invariant is violated.
**
** This routine is for use inside of assert() statements only.  For
** example:
**
**          assert( sqlite3PcachePageSanity(pPg) );
*/
#if SQLITE_DEBUG
int sqlite3PcachePageSanity(PgHdr *pPg){
  PCache *pCache;
  assert( pPg!=0 );
  assert( pPg->pgno>0 );    /* Page number is 1 or more */
  pCache = pPg->pCache;
  assert( pCache!=0 );      /* Every page has an associated PCache */
  if( pPg->flags & PGHDR_CLEAN ){
    assert( (pPg->flags & PGHDR_DIRTY)==0 );/* Cannot be both CLEAN and DIRTY */
    assert( pCache->pDirty!=pPg );          /* CLEAN pages not on dirty list */
    assert( pCache->pDirtyTail!=pPg );
  }
  /* WRITEABLE pages must also be DIRTY */
  if( pPg->flags & PGHDR_WRITEABLE ){
    assert( pPg->flags & PGHDR_DIRTY );     /* WRITEABLE implies DIRTY */
  }
  /* NEED_SYNC can be set independently of WRITEABLE.  This can happen,
  ** for example, when using the sqlite3PagerDontWrite() optimization:
  **    (1)  Page X is journalled, and gets WRITEABLE and NEED_SEEK.
  **    (2)  Page X moved to freelist, WRITEABLE is cleared
  **    (3)  Page X reused, WRITEABLE is set again
  ** If NEED_SYNC had been cleared in step 2, then it would not be reset
  ** in step 3, and page might be written into the database without first
  ** syncing the rollback journal, which might cause corruption on a power
  ** loss.
  */
  return 1;
}
#endif /* SQLITE_DEBUG */


/********************************** Linked List Management ********************/

/* Allowed values for second argument to pcacheManageDirtyList() */
#define PCACHE_DIRTYLIST_REMOVE   1    /* Remove pPage from dirty list */
#define PCACHE_DIRTYLIST_ADD      2    /* Add pPage to the dirty list */
#define PCACHE_DIRTYLIST_FRONT    3    /* Move pPage to the front of the list */
................................................................................
  pPgHdr = (PgHdr *)pPage->pExtra;

  if( !pPgHdr->pPage ){
    return pcacheFetchFinishWithInit(pCache, pgno, pPage);
  }
  pCache->nRefSum++;
  pPgHdr->nRef++;
  assert( sqlite3PcachePageSanity(pPgHdr) );
  return pPgHdr;
}

/*
** Decrement the reference count on a page. If the page is clean and the
** reference count drops to 0, then it is made eligible for recycling.
*/
................................................................................
}

/*
** Increase the reference count of a supplied page by 1.
*/
void sqlite3PcacheRef(PgHdr *p){
  assert(p->nRef>0);
  assert( sqlite3PcachePageSanity(p) );
  p->nRef++;
  p->pCache->nRefSum++;
}

/*
** Drop a page from the cache. There must be exactly one reference to the
** page. This function deletes that reference, so after it returns the
** page pointed to by p is invalid.
*/
void sqlite3PcacheDrop(PgHdr *p){
  assert( p->nRef==1 );
  assert( sqlite3PcachePageSanity(p) );
  if( p->flags&PGHDR_DIRTY ){
    pcacheManageDirtyList(p, PCACHE_DIRTYLIST_REMOVE);
  }
  p->pCache->nRefSum--;
  sqlite3GlobalConfig.pcache2.xUnpin(p->pCache->pCache, p->pPage, 1);
}

/*
** Make sure the page is marked as dirty. If it isn't dirty already,
** make it so.
*/
void sqlite3PcacheMakeDirty(PgHdr *p){
  assert( p->nRef>0 );
  assert( sqlite3PcachePageSanity(p) );
  if( p->flags & (PGHDR_CLEAN|PGHDR_DONT_WRITE) ){    /*OPTIMIZATION-IF-FALSE*/
    p->flags &= ~PGHDR_DONT_WRITE;
    if( p->flags & PGHDR_CLEAN ){
      p->flags ^= (PGHDR_DIRTY|PGHDR_CLEAN);
      pcacheTrace(("%p.DIRTY %d\n",p->pCache,p->pgno));
      assert( (p->flags & (PGHDR_DIRTY|PGHDR_CLEAN))==PGHDR_DIRTY );
      pcacheManageDirtyList(p, PCACHE_DIRTYLIST_ADD);
    }
    assert( sqlite3PcachePageSanity(p) );
  }
}

/*
** Make sure the page is marked as clean. If it isn't clean already,
** make it so.
*/
void sqlite3PcacheMakeClean(PgHdr *p){
  assert( sqlite3PcachePageSanity(p) );
  if( ALWAYS((p->flags & PGHDR_DIRTY)!=0) ){
    assert( (p->flags & PGHDR_CLEAN)==0 );
    pcacheManageDirtyList(p, PCACHE_DIRTYLIST_REMOVE);
    p->flags &= ~(PGHDR_DIRTY|PGHDR_NEED_SYNC|PGHDR_WRITEABLE);
    p->flags |= PGHDR_CLEAN;
    pcacheTrace(("%p.CLEAN %d\n",p->pCache,p->pgno));
    assert( sqlite3PcachePageSanity(p) );
    if( p->nRef==0 ){
      pcacheUnpin(p);
    }
  }
}

/*
................................................................................
/*
** Change the page number of page p to newPgno. 
*/
void sqlite3PcacheMove(PgHdr *p, Pgno newPgno){
  PCache *pCache = p->pCache;
  assert( p->nRef>0 );
  assert( newPgno>0 );
  assert( sqlite3PcachePageSanity(p) );
  pcacheTrace(("%p.MOVE %d -> %d\n",pCache,p->pgno,newPgno));
  sqlite3GlobalConfig.pcache2.xRekey(pCache->pCache, p->pPage, p->pgno,newPgno);
  p->pgno = newPgno;
  if( (p->flags&PGHDR_DIRTY) && (p->flags&PGHDR_NEED_SYNC) ){
    pcacheManageDirtyList(p, PCACHE_DIRTYLIST_FRONT);
  }
}

/* Bit values for PgHdr.flags */
#define PGHDR_CLEAN           0x001  /* Page not on the PCache.pDirty list */
#define PGHDR_DIRTY           0x002  /* Page is on the PCache.pDirty list */
#define PGHDR_WRITEABLE       0x004  /* Journaled and ready to modify */
#define PGHDR_NEED_SYNC       0x008  /* Fsync the rollback journal before
                                     ** writing this page to the database */
#define PGHDR_NEED_READ       0x010  /* Content is unread */
#define PGHDR_DONT_WRITE      0x020  /* Do not write content to disk */
#define PGHDR_MMAP            0x040  /* This is an mmap page object */

#define PGHDR_WAL_APPEND      0x080  /* Appended to wal file */

/* Initialize and shutdown the page cache subsystem */
int sqlite3PcacheInitialize(void);
void sqlite3PcacheShutdown(void);

/* Page cache buffer management:
** These routines implement SQLITE_CONFIG_PAGECACHE.
................................................................................
#if defined(SQLITE_CHECK_PAGES) || defined(SQLITE_DEBUG)
/* Iterate through all dirty pages currently stored in the cache. This
** interface is only available if SQLITE_CHECK_PAGES is defined when the 
** library is built.
*/
void sqlite3PcacheIterateDirty(PCache *pCache, void (*xIter)(PgHdr *));
#endif






/* Set and get the suggested cache-size for the specified pager-cache.
**
** If no global maximum is configured, then the system attempts to limit
** the total number of pages cached by purgeable pager-caches to the sum
** of the suggested cache-sizes.
*/

/* Bit values for PgHdr.flags */
#define PGHDR_CLEAN           0x001  /* Page not on the PCache.pDirty list */
#define PGHDR_DIRTY           0x002  /* Page is on the PCache.pDirty list */
#define PGHDR_WRITEABLE       0x004  /* Journaled and ready to modify */
#define PGHDR_NEED_SYNC       0x008  /* Fsync the rollback journal before
                                     ** writing this page to the database */

#define PGHDR_DONT_WRITE      0x010  /* Do not write content to disk */
#define PGHDR_MMAP            0x020  /* This is an mmap page object */

#define PGHDR_WAL_APPEND      0x040  /* Appended to wal file */

/* Initialize and shutdown the page cache subsystem */
int sqlite3PcacheInitialize(void);
void sqlite3PcacheShutdown(void);

/* Page cache buffer management:
** These routines implement SQLITE_CONFIG_PAGECACHE.
................................................................................
#if defined(SQLITE_CHECK_PAGES) || defined(SQLITE_DEBUG)
/* Iterate through all dirty pages currently stored in the cache. This
** interface is only available if SQLITE_CHECK_PAGES is defined when the 
** library is built.
*/
void sqlite3PcacheIterateDirty(PCache *pCache, void (*xIter)(PgHdr *));
#endif

#if defined(SQLITE_DEBUG)
/* Check invariants on a PgHdr object */
int sqlite3PcachePageSanity(PgHdr*);
#endif

/* Set and get the suggested cache-size for the specified pager-cache.
**
** If no global maximum is configured, then the system attempts to limit
** the total number of pages cached by purgeable pager-caches to the sum
** of the suggested cache-sizes.
*/