/*
** 2008 August 05
**
** The author disclaims copyright to this source code. In place of
** a legal notice, here is a blessing:
**
** 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.
*/
#include "sqliteInt.h"

/*
** A complete page cache is an instance of this structure. Every
** entry in the cache holds a single page of the database file. The
** btree layer only operates on the cached copy of the database pages.
**
** A page cache entry is "clean" if it exactly matches what is currently
** on disk. A page is "dirty" if it has been modified and needs to be
** persisted to disk.
**
** 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 so 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 */
 i64 nRefSum; /* Sum of ref counts over all pages */
 int szCache; /* Configured cache size */
 int szSpill; /* Size before spilling occurs */
 int szPage; /* Size of every page in this cache */
 int szExtra; /* Size of extra space for each page */
 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;}
 static void pcachePageTrace(int i, sqlite3_pcache_page *pLower){
 PgHdr *pPg;
 unsigned char *a;
 int j;
 if( pLower==0 ){
 printf("%3d: NULL\n", i);
 }else{
 pPg = (PgHdr*)pLower->pExtra;
 printf("%3d: nRef %2lld 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){
 int N;
 int i;
 sqlite3_pcache_page *pLower;
 
 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);
 pcachePageTrace(i, pLower);
 if( pLower && ((PgHdr*)pLower)->pPage==0 ){
 sqlite3GlobalConfig.pcache2.xUnpin(pCache->pCache, pLower, 0);
 }
 }
 }
#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.
*/
#if defined(SQLITE_ENABLE_EXPENSIVE_ASSERT)
static int pageOnDirtyList(PCache *pCache, PgHdr *pPg){
 PgHdr *p;
 for(p=pCache->pDirty; p; p=p->pDirtyNext){
 if( p==pPg ) return 1;
 }
 return 0;
}
static int pageNotOnDirtyList(PCache *pCache, PgHdr *pPg){
 PgHdr *p;
 for(p=pCache->pDirty; p; p=p->pDirtyNext){
 if( p==pPg ) return 0;
 }
 return 1;
}
#else
# define pageOnDirtyList(A,B) 1
# define pageNotOnDirtyList(A,B) 1
#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( pageNotOnDirtyList(pCache, pPg) );/* CLEAN pages not on dirtylist */
 }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( 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:
 ** (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.
 **
 ** Another example is when the database page size is smaller than the
 ** disk sector size. When any page of a sector is journalled, all pages
 ** in that sector are marked NEED_SYNC even if they are still CLEAN, just
 ** in case they are later modified, since all pages in the same sector
 ** must be journalled and synced before any of those pages can be safely
 ** written.
 */
 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 */

/*
** Manage pPage's participation on the dirty list. Bits of the addRemove
** argument determines what operation to do. The 0x01 bit means first
** remove pPage from the dirty list. The 0x02 means add pPage back to
** the dirty list. Doing both moves pPage to the front of the dirty list.
*/
static void pcacheManageDirtyList(PgHdr *pPage, u8 addRemove){
 PCache *p = pPage->pCache;

 pcacheTrace(("%p.DIRTYLIST.%s %d\n", p,
 addRemove==1 ? "REMOVE" : addRemove==2 ? "ADD" : "FRONT",
 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( pPage->pDirtyPrev ){
 pPage->pDirtyPrev->pDirtyNext = pPage->pDirtyNext;
 }else{
 /* If there are now no dirty pages in the cache, set eCreate to 2. 
 ** This is an optimization that allows sqlite3PcacheFetch() to skip
 ** searching for a dirty page to eject from the cache when it might
 ** otherwise have to. */
 assert( pPage==p->pDirty );
 p->pDirty = pPage->pDirtyNext;
 assert( p->bPurgeable || p->eCreate==2 );
 if( p->pDirty==0 ){ /*OPTIMIZATION-IF-TRUE*/
 assert( p->bPurgeable==0 || p->eCreate==1 );
 p->eCreate = 2;
 }
 }
 }
 if( addRemove & PCACHE_DIRTYLIST_ADD ){
 pPage->pDirtyPrev = 0;
 pPage->pDirtyNext = p->pDirty;
 if( pPage->pDirtyNext ){
 assert( pPage->pDirtyNext->pDirtyPrev==0 );
 pPage->pDirtyNext->pDirtyPrev = pPage;
 }else{
 p->pDirtyTail = pPage;
 if( p->bPurgeable ){
 assert( p->eCreate==2 );
 p->eCreate = 1;
 }
 }
 p->pDirty = pPage;

 /* If pSynced is NULL and this page has a clear NEED_SYNC flag, set
 ** pSynced to point to it. Checking the NEED_SYNC flag is an 
 ** optimization, as if pSynced points to a page with the NEED_SYNC
 ** flag set sqlite3PcacheFetchStress() searches through all newer 
 ** entries of the dirty-list for a page with NEED_SYNC clear anyway. */
 if( !p->pSynced 
 && 0==(pPage->flags&PGHDR_NEED_SYNC) /*OPTIMIZATION-IF-FALSE*/
 ){
 p->pSynced = pPage;
 }
 }
 pcacheDump(p);
}

/*
** Wrapper around the pluggable caches xUnpin method. If the cache is
** being used for an in-memory database, this function is a no-op.
*/
static void pcacheUnpin(PgHdr *p){
 if( p->pCache->bPurgeable ){
 pcacheTrace(("%p.UNPIN %d\n", p->pCache, p->pgno));
 sqlite3GlobalConfig.pcache2.xUnpin(p->pCache->pCache, p->pPage, 0);
 pcacheDump(p->pCache);
 }
}

/*
** Compute the number of pages of cache requested. p->szCache is the
** cache size requested by the "PRAGMA cache_size" statement.
*/
static int numberOfCachePages(PCache *p){
 if( p->szCache>=0 ){
 /* IMPLEMENTATION-OF: R-42059-47211 If the argument N is positive then the
 ** suggested cache size is set to N. */
 return p->szCache;
 }else{
 i64 n;
 /* IMPLEMENTATION-OF: R-59858-46238 If the argument N is negative, then the
 ** number of cache pages is adjusted to be a number of pages that would
 ** use approximately abs(N*1024) bytes of memory based on the current
 ** page size. */
 n = ((-1024*(i64)p->szCache)/(p->szPage+p->szExtra));
 if( n>1000000000 ) n = 1000000000;
 return (int)n;
 }
}

/*************************************************** General Interfaces ******
**
** Initialize and shutdown the page cache subsystem. Neither of these 
** functions are threadsafe.
*/
int sqlite3PcacheInitialize(void){
 if( sqlite3GlobalConfig.pcache2.xInit==0 ){
 /* IMPLEMENTATION-OF: R-26801-64137 If the xInit() method is NULL, then the
 ** built-in default page cache is used instead of the application defined
 ** page cache. */
 sqlite3PCacheSetDefault();
 assert( sqlite3GlobalConfig.pcache2.xInit!=0 );
 }
 return sqlite3GlobalConfig.pcache2.xInit(sqlite3GlobalConfig.pcache2.pArg);
}
void sqlite3PcacheShutdown(void){
 if( sqlite3GlobalConfig.pcache2.xShutdown ){
 /* IMPLEMENTATION-OF: R-26000-56589 The xShutdown() method may be NULL. */
 sqlite3GlobalConfig.pcache2.xShutdown(sqlite3GlobalConfig.pcache2.pArg);
 }
}

/*
** Return the size in bytes of a PCache object.
*/
int sqlite3PcacheSize(void){ return sizeof(PCache); }

/*
** Create a new PCache object. Storage space to hold the object
** has already been allocated and is passed in as the p pointer. 
** The caller discovers how much space needs to be allocated by 
** calling sqlite3PcacheSize().
**
** szExtra is some extra space allocated for each page. The first
** 8 bytes of the extra space will be zeroed as the page is allocated,
** but remaining content will be uninitialized. Though it is opaque
** to this module, the extra space really ends up being the MemPage
** structure in the pager.
*/
int sqlite3PcacheOpen(
 int szPage, /* Size of every page */
 int szExtra, /* Extra space associated with each page */
 int bPurgeable, /* True if pages are on backing store */
 int (*xStress)(void*,PgHdr*),/* Call to try to make pages clean */
 void *pStress, /* Argument to xStress */
 PCache *p /* Preallocated space for the PCache */
){
 memset(p, 0, sizeof(PCache));
 p->szPage = 1;
 p->szExtra = szExtra;
 assert( szExtra>=8 ); /* First 8 bytes will be zeroed */
 p->bPurgeable = bPurgeable;
 p->eCreate = 2;
 p->xStress = xStress;
 p->pStress = pStress;
 p->szCache = 100;
 p->szSpill = 1;
 pcacheTrace(("%p.OPEN szPage %d bPurgeable %d\n",p,szPage,bPurgeable));
 return sqlite3PcacheSetPageSize(p, szPage);
}

/*
** Change the page size for PCache object. The caller must ensure that there
** are no outstanding page references when this function is called.
*/
int sqlite3PcacheSetPageSize(PCache *pCache, int szPage){
 assert( pCache->nRefSum==0 && pCache->pDirty==0 );
 if( pCache->szPage ){
 sqlite3_pcache *pNew;
 pNew = sqlite3GlobalConfig.pcache2.xCreate(
 szPage, pCache->szExtra + ROUND8(sizeof(PgHdr)),
 pCache->bPurgeable
 );
 if( pNew==0 ) return SQLITE_NOMEM_BKPT;
 sqlite3GlobalConfig.pcache2.xCachesize(pNew, numberOfCachePages(pCache));
 if( pCache->pCache ){
 sqlite3GlobalConfig.pcache2.xDestroy(pCache->pCache);
 }
 pCache->pCache = pNew;
 pCache->szPage = szPage;
 pcacheTrace(("%p.PAGESIZE %d\n",pCache,szPage));
 }
 return SQLITE_OK;
}

/*
** Try to obtain a page from the cache.
**
** This routine returns a pointer to an sqlite3_pcache_page object if
** such an object is already in cache, or if a new one is created.
** This routine returns a NULL pointer if the object was not in cache
** and could not be created.
**
** The createFlags should be 0 to check for existing pages and should
** be 3 (not 1, but 3) to try to create a new page.
**
** If the createFlag is 0, then NULL is always returned if the page
** is not already in the cache. If createFlag is 1, then a new page
** is created only if that can be done without spilling dirty pages
** and without exceeding the cache size limit.
**
** The caller needs to invoke sqlite3PcacheFetchFinish() to properly
** initialize the sqlite3_pcache_page object and convert it into a
** PgHdr object. The sqlite3PcacheFetch() and sqlite3PcacheFetchFinish()
** routines are split this way for performance reasons. When separated
** they can both (usually) operate without having to push values to
** the stack on entry and pop them back off on exit, which saves a
** lot of pushing and popping.
*/
sqlite3_pcache_page *sqlite3PcacheFetch(
 PCache *pCache, /* Obtain the page from this cache */
 Pgno pgno, /* Page number to obtain */
 int createFlag /* If true, create page if it does not exist already */
){
 int eCreate;
 sqlite3_pcache_page *pRes;

 assert( pCache!=0 );
 assert( pCache->pCache!=0 );
 assert( createFlag==3 || createFlag==0 );
 assert( pCache->eCreate==((pCache->bPurgeable && pCache->pDirty) ? 1 : 2) );

 /* eCreate defines what to do if the page does not exist.
 ** 0 Do not allocate a new page. (createFlag==0)
 ** 1 Allocate a new page if doing so is inexpensive.
 ** (createFlag==1 AND bPurgeable AND pDirty)
 ** 2 Allocate a new page even it doing so is difficult.
 ** (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,
 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 
** try harder to allocate a page. This routine might invoke the stress
** callback to spill dirty pages to the journal. It will then try to
** allocate the new page and will only fail to allocate a new page on
** an OOM error.
**
** This routine should be invoked only after sqlite3PcacheFetch() fails.
*/
int sqlite3PcacheFetchStress(
 PCache *pCache, /* Obtain the page from this cache */
 Pgno pgno, /* Page number to obtain */
 sqlite3_pcache_page **ppPage /* Write result here */
){
 PgHdr *pPg;
 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);
 }
 if( pPg ){
 int rc;
#ifdef SQLITE_LOG_CACHE_SPILL
 sqlite3_log(SQLITE_FULL, 
 "spill page %d making room for %d - cache used: %d/%d",
 pPg->pgno, pgno,
 sqlite3GlobalConfig.pcache2.xPagecount(pCache->pCache),
 numberOfCachePages(pCache));
#endif
 pcacheTrace(("%p.SPILL %d\n",pCache,pPg->pgno));
 rc = pCache->xStress(pCache->pStress, pPg);
 pcacheDump(pCache);
 if( rc!=SQLITE_OK && rc!=SQLITE_BUSY ){
 return rc;
 }
 }
 }
 *ppPage = sqlite3GlobalConfig.pcache2.xFetch(pCache->pCache, pgno, 2);
 return *ppPage==0 ? SQLITE_NOMEM_BKPT : SQLITE_OK;
}

/*
** This is a helper routine for sqlite3PcacheFetchFinish()
**
** In the uncommon case where the page being fetched has not been
** initialized, this routine is invoked to do the initialization.
** This routine is broken out into a separate function since it
** requires extra stack manipulation that can be avoided in the common
** case.
*/
static SQLITE_NOINLINE PgHdr *pcacheFetchFinishWithInit(
 PCache *pCache, /* Obtain the page from this cache */
 Pgno pgno, /* Page number obtained */
 sqlite3_pcache_page *pPage /* Page obtained by prior PcacheFetch() call */
){
 PgHdr *pPgHdr;
 assert( pPage!=0 );
 pPgHdr = (PgHdr*)pPage->pExtra;
 assert( pPgHdr->pPage==0 );
 memset(&pPgHdr->pDirty, 0, sizeof(PgHdr) - offsetof(PgHdr,pDirty));
 pPgHdr->pPage = pPage;
 pPgHdr->pData = pPage->pBuf;
 pPgHdr->pExtra = (void *)&pPgHdr[1];
 memset(pPgHdr->pExtra, 0, 8);
 pPgHdr->pCache = pCache;
 pPgHdr->pgno = pgno;
 pPgHdr->flags = PGHDR_CLEAN;
 return sqlite3PcacheFetchFinish(pCache,pgno,pPage);
}

/*
** This routine converts the sqlite3_pcache_page object returned by
** sqlite3PcacheFetch() into an initialized PgHdr object. This routine
** must be called after sqlite3PcacheFetch() in order to get a usable
** result.
*/
PgHdr *sqlite3PcacheFetchFinish(
 PCache *pCache, /* Obtain the page from this cache */
 Pgno pgno, /* Page number obtained */
 sqlite3_pcache_page *pPage /* Page obtained by prior PcacheFetch() call */
){
 PgHdr *pPgHdr;

 assert( pPage!=0 );
 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.
*/
void SQLITE_NOINLINE sqlite3PcacheRelease(PgHdr *p){
 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.
*/
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) );
 }
 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) );
 assert( (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);
 }
}

/*
** Make every page in the cache clean.
*/
void sqlite3PcacheCleanAll(PCache *pCache){
 PgHdr *p;
 pcacheTrace(("%p.CLEAN-ALL\n",pCache));
 while( (p = pCache->pDirty)!=0 ){
 sqlite3PcacheMakeClean(p);
 }
}

/*
** Clear the PGHDR_NEED_SYNC and PGHDR_WRITEABLE flag from all dirty pages.
*/
void sqlite3PcacheClearWritable(PCache *pCache){
 PgHdr *p;
 pcacheTrace(("%p.CLEAR-WRITEABLE\n",pCache));
 for(p=pCache->pDirty; p; p=p->pDirtyNext){
 p->flags &= ~(PGHDR_NEED_SYNC|PGHDR_WRITEABLE);
 }
 pCache->pSynced = pCache->pDirtyTail;
}

/*
** Clear the PGHDR_NEED_SYNC flag from all dirty pages.
*/
void sqlite3PcacheClearSyncFlags(PCache *pCache){
 PgHdr *p;
 for(p=pCache->pDirty; p; p=p->pDirtyNext){
 p->flags &= ~PGHDR_NEED_SYNC;
 }
 pCache->pSynced = pCache->pDirtyTail;
}

/*
** Change the page number of page p to newPgno. 
*/
void sqlite3PcacheMove(PgHdr *p, Pgno newPgno){
 PCache *pCache = p->pCache;
 sqlite3_pcache_page *pOther;
 assert( p->nRef>0 );
 assert( newPgno>0 );
 assert( sqlite3PcachePageSanity(p) );
 pcacheTrace(("%p.MOVE %d -> %d\n",pCache,p->pgno,newPgno));
 pOther = sqlite3GlobalConfig.pcache2.xFetch(pCache->pCache, newPgno, 0);
 if( pOther ){
 PgHdr *pXPage = (PgHdr*)pOther->pExtra;
 assert( pXPage->nRef==0 );
 pXPage->nRef++;
 pCache->nRefSum++;
 sqlite3PcacheDrop(pXPage);
 }
 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.
**
** If there is a reference to page 1 and the pgno parameter passed to this
** function is 0, then the data area associated with page 1 is zeroed, but
** the page object is not dropped.
*/
void sqlite3PcacheTruncate(PCache *pCache, Pgno pgno){
 if( pCache->pCache ){
 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.
 */
 assert( p->pgno>0 );
 if( p->pgno>pgno ){
 assert( p->flags&PGHDR_DIRTY );
 sqlite3PcacheMakeClean(p);
 }
 }
 if( pgno==0 && pCache->nRefSum ){
 sqlite3_pcache_page *pPage1;
 pPage1 = sqlite3GlobalConfig.pcache2.xFetch(pCache->pCache,1,0);
 if( ALWAYS(pPage1) ){ /* Page 1 is always available in cache, because
 ** pCache->nRefSum>0 */
 memset(pPage1->pBuf, 0, pCache->szPage);
 pgno = 1;
 }
 }
 sqlite3GlobalConfig.pcache2.xTruncate(pCache->pCache, pgno+1);
 }
}

/*
** Close a cache.
*/
void sqlite3PcacheClose(PCache *pCache){
 assert( pCache->pCache!=0 );
 pcacheTrace(("%p.CLOSE\n",pCache));
 sqlite3GlobalConfig.pcache2.xDestroy(pCache->pCache);
}

/* 
** Discard the contents of the cache.
*/
void sqlite3PcacheClear(PCache *pCache){
 sqlite3PcacheTruncate(pCache, 0);
}

/*
** Merge two lists of pages connected by pDirty and in pgno order.
** Do not bother fixing the pDirtyPrev pointers.
*/
static PgHdr *pcacheMergeDirtyList(PgHdr *pA, PgHdr *pB){
 PgHdr result, *pTail;
 pTail = &result;
 assert( pA!=0 && pB!=0 );
 for(;;){
 if( pA->pgno<pB->pgno ){
 pTail->pDirty = pA;
 pTail = pA;
 pA = pA->pDirty;
 if( pA==0 ){
 pTail->pDirty = pB;
 break;
 }
 }else{
 pTail->pDirty = pB;
 pTail = pB;
 pB = pB->pDirty;
 if( pB==0 ){
 pTail->pDirty = pA;
 break;
 }
 }
 }
 return result.pDirty;
}

/*
** Sort the list of pages in ascending order by pgno. Pages are
** connected by pDirty pointers. The pDirtyPrev pointers are
** corrupted by this sort.
**
** Since there cannot be more than 2^31 distinct pages in a database,
** there cannot be more than 31 buckets required by the merge sorter.
** One extra bucket is added to catch overflow in case something
** ever changes to make the previous sentence incorrect.
*/
#define N_SORT_BUCKET 32
static PgHdr *pcacheSortDirtyList(PgHdr *pIn){
 PgHdr *a[N_SORT_BUCKET], *p;
 int i;
 memset(a, 0, sizeof(a));
 while( pIn ){
 p = pIn;
 pIn = p->pDirty;
 p->pDirty = 0;
 for(i=0; ALWAYS(i<N_SORT_BUCKET-1); i++){
 if( a[i]==0 ){
 a[i] = p;
 break;
 }else{
 p = pcacheMergeDirtyList(a[i], p);
 a[i] = 0;
 }
 }
 if( NEVER(i==N_SORT_BUCKET-1) ){
 /* To get here, there need to be 2^(N_SORT_BUCKET) elements in
 ** the input list. But that is impossible.
 */
 a[i] = pcacheMergeDirtyList(a[i], p);
 }
 }
 p = a[0];
 for(i=1; i<N_SORT_BUCKET; i++){
 if( a[i]==0 ) continue;
 p = p ? pcacheMergeDirtyList(p, a[i]) : a[i];
 }
 return p;
}

/*
** Return a list of all dirty pages in the cache, sorted by page number.
*/
PgHdr *sqlite3PcacheDirtyList(PCache *pCache){
 PgHdr *p;
 for(p=pCache->pDirty; p; p=p->pDirtyNext){
 p->pDirty = p->pDirtyNext;
 }
 return pcacheSortDirtyList(pCache->pDirty);
}

/* 
** Return the total number of references to all pages held by the cache.
**
** This is not the total number of pages referenced, but the sum of the
** reference count for all pages.
*/
i64 sqlite3PcacheRefCount(PCache *pCache){
 return pCache->nRefSum;
}

/*
** Return the number of references to the page supplied as an argument.
*/
i64 sqlite3PcachePageRefcount(PgHdr *p){
 return p->nRef;
}

/* 
** Return the total number of pages in the cache.
*/
int sqlite3PcachePagecount(PCache *pCache){
 assert( pCache->pCache!=0 );
 return sqlite3GlobalConfig.pcache2.xPagecount(pCache->pCache);
}

#ifdef SQLITE_TEST
/*
** Get the suggested cache-size value.
*/
int sqlite3PcacheGetCachesize(PCache *pCache){
 return numberOfCachePages(pCache);
}
#endif

/*
** Set the suggested cache-size value.
*/
void sqlite3PcacheSetCachesize(PCache *pCache, int mxPage){
 assert( pCache->pCache!=0 );
 pCache->szCache = mxPage;
 sqlite3GlobalConfig.pcache2.xCachesize(pCache->pCache,
 numberOfCachePages(pCache));
}

/*
** Set the suggested cache-spill value. Make no changes if if the
** argument is zero. Return the effective cache-spill size, which will
** be the larger of the szSpill and szCache.
*/
int sqlite3PcacheSetSpillsize(PCache *p, int mxPage){
 int res;
 assert( p->pCache!=0 );
 if( mxPage ){
 if( mxPage<0 ){
 mxPage = (int)((-1024*(i64)mxPage)/(p->szPage+p->szExtra));
 }
 p->szSpill = mxPage;
 }
 res = numberOfCachePages(p);
 if( res<p->szSpill ) res = p->szSpill; 
 return res;
}

/*
** Free up as much memory as possible from the page cache.
*/
void sqlite3PcacheShrink(PCache *pCache){
 assert( pCache->pCache!=0 );
 sqlite3GlobalConfig.pcache2.xShrink(pCache->pCache);
}

/*
** Return the size of the header added by this middleware layer
** in the page-cache hierarchy.
*/
int sqlite3HeaderSizePcache(void){ return ROUND8(sizeof(PgHdr)); }

/*
** Return the number of dirty pages currently in the cache, as a percentage
** of the configured cache size.
*/
int sqlite3PCachePercentDirty(PCache *pCache){
 PgHdr *pDirty;
 int nDirty = 0;
 int nCache = numberOfCachePages(pCache);
 for(pDirty=pCache->pDirty; pDirty; pDirty=pDirty->pDirtyNext) nDirty++;
 return nCache ? (int)(((i64)nDirty * 100) / nCache) : 0;
}

#ifdef SQLITE_DIRECT_OVERFLOW_READ
/* 
** Return true if there are one or more dirty pages in the cache. Else false.
*/
int sqlite3PCacheIsDirty(PCache *pCache){
 return (pCache->pDirty!=0);
}
#endif

#if defined(SQLITE_CHECK_PAGES) || defined(SQLITE_DEBUG)
/*
** For all dirty pages currently in the cache, invoke the specified
** callback. This is only used if the SQLITE_CHECK_PAGES macro is
** defined.
*/
void sqlite3PcacheIterateDirty(PCache *pCache, void (*xIter)(PgHdr *)){
 PgHdr *pDirty;
 for(pDirty=pCache->pDirty; pDirty; pDirty=pDirty->pDirtyNext){
 xIter(pDirty);
 }
}
#endif