** 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;}
  static void pcachePageTrace(int i, sqlite3_pcache_page *pLower){
    PgHdr *pPg;
    unsigned char *a;
    int j;
    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]);
    printf(" ptr %p\n", pPg);
  }
  static void pcacheDump(PCache *pCache){
  void pcacheDump(PCache *pCache){
    int N;
    int i;
    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);
       pcachePageTrace(i, pLower);
       if( ((PgHdr*)pLower)->pPage==0 ){
       a = (unsigned char *)pLower->pBuf;
       for(j=0; j<12; j++) printf("%02x", a[j]);
       printf("\n");
       if( pPg->pPage==0 ){
         sqlite3GlobalConfig.pcache2.xUnpin(pCache->pCache, pLower, 0);
       }
    }
  }
#else
  #else
# define pcacheTrace(X)
# define pcachePageTrace(PGNO, X)
# define pcacheDump(X)
#endif

/*
** Return 1 if pPg is on the dirty list for pCache.  Return 0 if not.
** This routine runs inside of assert() statements only.
*/
#ifdef SQLITE_DEBUG
static int pageOnDirtyList(PCache *pCache, PgHdr *pPg){
  PgHdr *p;
  for(p=pCache->pDirty; p; p=p->pDirtyNext){
    if( p==pPg ) return 1;
  }
  return 0;
}
#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) );
*/
#ifdef SQLITE_DEBUG
int sqlite3PcachePageSanity(PgHdr *pPg){
  PCache *pCache;
  assert( pPg!=0 );
  assert( pPg->pgno>0 || pPg->pPager==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( !pageOnDirtyList(pCache, pPg) );/* CLEAN pages not on dirty list */
    assert( pCache->pDirty!=pPg );          /* CLEAN pages not on dirty list */
  }else{
    assert( (pPg->flags & PGHDR_DIRTY)!=0 );/* If not CLEAN must be DIRTY */
    assert( pPg->pDirtyNext==0 || pPg->pDirtyNext->pDirtyPrev==pPg );
    assert( pPg->pDirtyPrev==0 || pPg->pDirtyPrev->pDirtyNext==pPg );
    assert( pPg->pDirtyPrev!=0 || pCache->pDirty==pPg );
    assert( pCache->pDirtyTail!=pPg );
    assert( pageOnDirtyList(pCache, 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:

  **          (createFlag==1 AND !(bPurgeable AND pDirty)
  */
  eCreate = createFlag & pCache->eCreate;
  assert( eCreate==0 || eCreate==1 || eCreate==2 );
  assert( createFlag==0 || pCache->eCreate==eCreate );
  assert( createFlag==0 || eCreate==1+(!pCache->bPurgeable||!pCache->pDirty) );
  pRes = sqlite3GlobalConfig.pcache2.xFetch(pCache->pCache, pgno, eCreate);
  pcacheTrace(("%p.FETCH %d%s (result: %p) ",pCache,pgno,
  pcacheTrace(("%p.FETCH %d%s (result: %p)\n",pCache,pgno,
               createFlag?" create":"",pRes));
  pcachePageTrace(pgno, pRes);
  return pRes;
}

/*
** If the sqlite3PcacheFetch() routine is unable to allocate a new
** page because no clean pages are available for reuse and the cache
** size limit has been reached, then this routine can be invoked to 

  assert( p->nRef>0 );
  p->pCache->nRefSum--;
  if( (--p->nRef)==0 ){
    if( p->flags&PGHDR_CLEAN ){
      pcacheUnpin(p);
    }else{
      pcacheManageDirtyList(p, PCACHE_DIRTYLIST_FRONT);
      assert( sqlite3PcachePageSanity(p) );
    }
  }
}

/*
** Increase the reference count of a supplied page by 1.
*/

  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) );
    }
    assert( sqlite3PcachePageSanity(p) );
  }
}

/*
** Make sure the page is marked as clean. If it isn't clean already,

  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);
    assert( sqlite3PcachePageSanity(p) );
  }
}

/*
** Drop every cache entry whose page number is greater than "pgno". The
** caller must ensure that there are no outstanding references to any pages
** other than page 1 with a page number greater than pgno.

    PgHdr *p;
    PgHdr *pNext;
    pcacheTrace(("%p.TRUNCATE %d\n",pCache,pgno));
    for(p=pCache->pDirty; p; p=pNext){
      pNext = p->pDirtyNext;
      /* This routine never gets call with a positive pgno except right
      ** after sqlite3PcacheCleanAll().  So if there are dirty pages,
      ** it must be that pgno==0.
      ** it must be that pgno>0.
      */
      assert( p->pgno>0 );
      if( p->pgno>pgno ){
        assert( p->flags&PGHDR_DIRTY );
        sqlite3PcacheMakeClean(p);
      }
    }