*************************************************************************
** This file implements that page cache.
*/
#include "sqliteInt.h"

/*
** A complete page cache is an instance of this structure.
















*/
struct PCache {
  PgHdr *pDirty, *pDirtyTail;         /* List of dirty pages in LRU order */
  PgHdr *pSynced;                     /* Last synced page in dirty page list */
  int nRefSum;                        /* Sum of ref counts over all pages */
  int szCache;                        /* Configured cache size */
  int szSpill;                        /* Size before spilling occurs */
................................................................................
                pPage->pgno));
  if( addRemove & PCACHE_DIRTYLIST_REMOVE ){
    assert( pPage->pDirtyNext || pPage==p->pDirtyTail );
    assert( pPage->pDirtyPrev || pPage==p->pDirty );
  
    /* Update the PCache1.pSynced variable if necessary. */
    if( p->pSynced==pPage ){
      PgHdr *pSynced = pPage->pDirtyPrev;
      while( pSynced && (pSynced->flags&PGHDR_NEED_SYNC) ){
        pSynced = pSynced->pDirtyPrev;
      }
      p->pSynced = pSynced;
    }
  
    if( pPage->pDirtyNext ){
      pPage->pDirtyNext->pDirtyPrev = pPage->pDirtyPrev;
    }else{
      assert( pPage==p->pDirtyTail );
      p->pDirtyTail = pPage->pDirtyPrev;
................................................................................
  if( pCache->eCreate==2 ) return 0;

  if( sqlite3PcachePagecount(pCache)>pCache->szSpill ){
    /* Find a dirty page to write-out and recycle. First try to find a 
    ** page that does not require a journal-sync (one with PGHDR_NEED_SYNC
    ** cleared), but if that is not possible settle for any other 
    ** unreferenced dirty page.
    */




    for(pPg=pCache->pSynced; 
        pPg && (pPg->nRef || (pPg->flags&PGHDR_NEED_SYNC)); 
        pPg=pPg->pDirtyPrev
    );
    pCache->pSynced = pPg;
    if( !pPg ){
      for(pPg=pCache->pDirtyTail; pPg && pPg->nRef; pPg=pPg->pDirtyPrev);

*************************************************************************
** This file implements that page cache.
*/
#include "sqliteInt.h"

/*
** A complete page cache is an instance of this structure.
**
** pDirty, pDirtyTail, pSynced:
**   All dirty pages are linked into the doubly linked list using
**   PgHdr.pDirtyNext and pDirtyPrev. The list is maintained in LRU order
**   such that p was added to the list more recently than p->pDirtyNext.
**   PCache.pDirty points to the first (newest) element in the list and
**   pDirtyTail to the last (oldest).
**
**   The PCache.pSynced variable is used to optimize searching for a dirty
**   page to eject from the cache mid-transaction. It is better to eject
**   a page that does not require a journal sync than one that does. 
**   Therefore, pSynced is maintained to that it *almost* always points
**   to either the oldest page in the pDirty/pDirtyTail list that has a
**   clear PGHDR_NEED_SYNC flag or to a page that is older than this one
**   (so that the right page to eject can be found by following pDirtyPrev
**   pointers).
*/
struct PCache {
  PgHdr *pDirty, *pDirtyTail;         /* List of dirty pages in LRU order */
  PgHdr *pSynced;                     /* Last synced page in dirty page list */
  int nRefSum;                        /* Sum of ref counts over all pages */
  int szCache;                        /* Configured cache size */
  int szSpill;                        /* Size before spilling occurs */
................................................................................
                pPage->pgno));
  if( addRemove & PCACHE_DIRTYLIST_REMOVE ){
    assert( pPage->pDirtyNext || pPage==p->pDirtyTail );
    assert( pPage->pDirtyPrev || pPage==p->pDirty );
  
    /* Update the PCache1.pSynced variable if necessary. */
    if( p->pSynced==pPage ){
      p->pSynced = pPage->pDirtyPrev;




    }
  
    if( pPage->pDirtyNext ){
      pPage->pDirtyNext->pDirtyPrev = pPage->pDirtyPrev;
    }else{
      assert( pPage==p->pDirtyTail );
      p->pDirtyTail = pPage->pDirtyPrev;
................................................................................
  if( pCache->eCreate==2 ) return 0;

  if( sqlite3PcachePagecount(pCache)>pCache->szSpill ){
    /* Find a dirty page to write-out and recycle. First try to find a 
    ** page that does not require a journal-sync (one with PGHDR_NEED_SYNC
    ** cleared), but if that is not possible settle for any other 
    ** unreferenced dirty page.
    **
    ** If the LRU page in the dirty list that has a clear PGHDR_NEED_SYNC
    ** flag is currently referenced, then the following may leave pSynced
    ** set incorrectly (pointing to other than the LRU page with NEED_SYNC
    ** cleared). This is Ok, as pSynced is just an optimization.  */
    for(pPg=pCache->pSynced; 
        pPg && (pPg->nRef || (pPg->flags&PGHDR_NEED_SYNC)); 
        pPg=pPg->pDirtyPrev
    );
    pCache->pSynced = pPg;
    if( !pPg ){
      for(pPg=pCache->pDirtyTail; pPg && pPg->nRef; pPg=pPg->pDirtyPrev);