**    May you do good and not evil.
**    May you find forgiveness for yourself and forgive others.
**    May you share freely, never taking more than you give.
**
*************************************************************************
** This file implements that page cache.
**
** @(#) $Id: pcache.c,v 1.12 2008/08/25 07:12:29 danielk1977 Exp $
*/
#include "sqliteInt.h"

/*
** A complete page cache is an instance of this structure.
*/
struct PCache {
................................................................................
** Deadlock within the module is avoided by never blocking on the MEM2 
** mutex while the LRU mutex is held.
*/

#define pcacheEnterGlobal() sqlite3_mutex_enter(pcache.mutex_lru)
#define pcacheExitGlobal()  sqlite3_mutex_leave(pcache.mutex_lru)

/*
** Increment the reference count on both page p and its cache by n.
*/
static void pcacheRef(PgHdr *p, int n){
  /* This next block assert()s that the number of references to the 
  ** PCache is the sum of the number of references to all pages in
  ** the PCache. This is a bit expensive to leave turned on all the 
  ** time, even in debugging builds.
  */
#if 0
  PgHdr *pHdr;
  int nRef = 0;
  for(pHdr=p->pCache->pClean; pHdr; pHdr=pHdr->pNext) nRef += pHdr->nRef;
  for(pHdr=p->pCache->pDirty; pHdr; pHdr=pHdr->pNext) nRef += pHdr->nRef;
  assert( p->pCache->nRef==nRef );
#endif
  p->nRef += n;
  p->pCache->nRef += n;
}

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

#ifndef NDEBUG
/*
** This routine verifies that the number of entries in the hash table
** is pCache->nPage.  This routine is used within assert() statements
** only and is therefore disabled during production builds.
................................................................................
    for(pPage=pCache->apHash[h]; pPage; pPage=pPage->pNextHash){
      if( pPage->pgno==pgno ){
        if( pPage->nRef==0 /* && (pPage->flags & PGHDR_DIRTY)==0 */ ){
          pcacheEnterGlobal();
          pcacheRemoveFromLruList(pPage);
          pcacheExitGlobal();
        }
        pcacheRef(pPage, 1);


        *ppPage = pPage;
        return SQLITE_OK;
      }
    }
  }

  if( createFlag ){
................................................................................
    if( pPage==0 ){
      return SQLITE_NOMEM;
    }

    pPage->pPager = 0;
    pPage->flags = 0;
    pPage->pDirty = 0;
    pPage->nRef = 0;
    pPage->pgno = pgno;
    pPage->pCache = pCache;

    pcacheRef(pPage, 1);
    pcacheAddToList(&pCache->pClean, pPage);
    pcacheAddToHash(pPage);
  }else{
    *ppPage = 0;
  }

  return SQLITE_OK;
................................................................................
/*
** Dereference a page.  When the reference count reaches zero,
** move the page to the LRU list if it is clean.
*/
void sqlite3PcacheRelease(PgHdr *p){
  assert( p->nRef>0 );
  assert( p->pCache->iInUseDB || p->pCache->iInUseMM );
  pcacheRef(p, -1);
  if( p->nRef!=0 ) return;


  if( p->pCache->xDestroy ){
    p->pCache->xDestroy(p);
  }
#if 0
  if( (p->flags & PGHDR_DIRTY)!=0 ) return;
#endif
  pcacheEnterGlobal();
  pcacheAddToLruList(p);
  pcacheExitGlobal();

}

void sqlite3PcacheRef(PgHdr *p){
  assert(p->nRef>=0);
  pcacheRef(p, 1);
}

/*
** Drop a page from the cache.  This should be the only reference to
** the page.
*/
void sqlite3PcacheDrop(PgHdr *p){

**    May you do good and not evil.
**    May you find forgiveness for yourself and forgive others.
**    May you share freely, never taking more than you give.
**
*************************************************************************
** This file implements that page cache.
**
** @(#) $Id: pcache.c,v 1.13 2008/08/25 14:49:42 danielk1977 Exp $
*/
#include "sqliteInt.h"

/*
** A complete page cache is an instance of this structure.
*/
struct PCache {
................................................................................
** Deadlock within the module is avoided by never blocking on the MEM2 
** mutex while the LRU mutex is held.
*/

#define pcacheEnterGlobal() sqlite3_mutex_enter(pcache.mutex_lru)
#define pcacheExitGlobal()  sqlite3_mutex_leave(pcache.mutex_lru)





















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

#ifndef NDEBUG
/*
** This routine verifies that the number of entries in the hash table
** is pCache->nPage.  This routine is used within assert() statements
** only and is therefore disabled during production builds.
................................................................................
    for(pPage=pCache->apHash[h]; pPage; pPage=pPage->pNextHash){
      if( pPage->pgno==pgno ){
        if( pPage->nRef==0 /* && (pPage->flags & PGHDR_DIRTY)==0 */ ){
          pcacheEnterGlobal();
          pcacheRemoveFromLruList(pPage);
          pcacheExitGlobal();
        }
        if( (pPage->nRef++)==0 ){
          pCache->nRef++;
        }
        *ppPage = pPage;
        return SQLITE_OK;
      }
    }
  }

  if( createFlag ){
................................................................................
    if( pPage==0 ){
      return SQLITE_NOMEM;
    }

    pPage->pPager = 0;
    pPage->flags = 0;
    pPage->pDirty = 0;

    pPage->pgno = pgno;
    pPage->pCache = pCache;
    pPage->nRef = 1;
    pCache->nRef++;
    pcacheAddToList(&pCache->pClean, pPage);
    pcacheAddToHash(pPage);
  }else{
    *ppPage = 0;
  }

  return SQLITE_OK;
................................................................................
/*
** Dereference a page.  When the reference count reaches zero,
** move the page to the LRU list if it is clean.
*/
void sqlite3PcacheRelease(PgHdr *p){
  assert( p->nRef>0 );
  assert( p->pCache->iInUseDB || p->pCache->iInUseMM );
  p->nRef--;
  if( p->nRef==0 ){
    PCache *pCache = p->pCache;
    pCache->nRef--;
    if( p->pCache->xDestroy ){
      p->pCache->xDestroy(p);
    }
#if 0
    if( (p->flags & PGHDR_DIRTY)!=0 ) return;
#endif
    pcacheEnterGlobal();
    pcacheAddToLruList(p);
    pcacheExitGlobal();
  }
}

void sqlite3PcacheRef(PgHdr *p){
  assert(p->nRef>0);
  p->nRef++;
}

/*
** Drop a page from the cache.  This should be the only reference to
** the page.
*/
void sqlite3PcacheDrop(PgHdr *p){