SQLite

** The pager is used to access a database disk file.  It implements
** atomic commit and rollback through the use of a journal file that
** is separate from the database file.  The pager also implements file
** locking to prevent two processes from writing the same database
** file simultaneously, or one process from reading the database while
** another is writing.
**
** @(#) $Id: pager.c,v 1.57 2002/11/10 23:32:57 drh Exp $
*/
#include "os.h"         /* Must be first to enable large file support */
#include "sqliteInt.h"
#include "pager.h"
#include <assert.h>
#include <string.h>

struct PgHdr {
  Pager *pPager;                 /* The pager to which this page belongs */
  Pgno pgno;                     /* The page number for this page */
  PgHdr *pNextHash, *pPrevHash;  /* Hash collision chain for PgHdr.pgno */
  int nRef;                      /* Number of users of this page */
  PgHdr *pNextFree, *pPrevFree;  /* Freelist of pages where nRef==0 */
  PgHdr *pNextAll, *pPrevAll;    /* A list of all pages */
  PgHdr *pNextCkpt, *pPrevCkpt;  /* List of pages in the checkpoint journal */
  PgHdr *pSort;                  /* Next in list of pages to be written */
  u8 inJournal;                  /* TRUE if has been written to journal */
  u8 inCkpt;                     /* TRUE if written to the checkpoint journal */
  u8 dirty;                      /* TRUE if we need to write back changes */
  u8 alwaysRollback;             /* Disable dont_rollback() for this page */
  /* SQLITE_PAGE_SIZE bytes of page data follow this header */
  /* Pager.nExtra bytes of local data follow the page data */
};

  u8 dirtyFile;               /* True if database file has changed in any way */
  u8 alwaysRollback;          /* Disable dont_rollback() for all pages */
  u8 journalFormat;           /* Version number of the journal file */
  u8 *aInJournal;             /* One bit for each page in the database file */
  u8 *aInCkpt;                /* One bit for each page in the database */
  PgHdr *pFirst, *pLast;      /* List of free pages */
  PgHdr *pAll;                /* List of all pages */
  PgHdr *pCkpt;               /* List of pages in the checkpoint journal */
  PgHdr *aHash[N_PG_HASH];    /* Hash table to map page number of PgHdr */
};

/*
** These are bits that can be set in Pager.errMask.
*/
#define PAGER_ERR_FULL     0x01  /* a write() failed */

  if( pPager->errMask & PAGER_ERR_LOCK )    rc = SQLITE_PROTOCOL;
  if( pPager->errMask & PAGER_ERR_DISK )    rc = SQLITE_IOERR;
  if( pPager->errMask & PAGER_ERR_FULL )    rc = SQLITE_FULL;
  if( pPager->errMask & PAGER_ERR_MEM )     rc = SQLITE_NOMEM;
  if( pPager->errMask & PAGER_ERR_CORRUPT ) rc = SQLITE_CORRUPT;
  return rc;
}

/*
** Add or remove a page from the list of all pages that are in the
** checkpoint journal.
**
** The Pager keeps a separate list of pages that are currently in
** the checkpoint journal.  This helps the sqlitepager_ckpt_commit()
** routine run MUCH faster for the common case where there are many
** pages in memory but only a few are in the checkpoint journal.
*/
static void page_add_to_ckpt_list(PgHdr *pPg){
  Pager *pPager = pPg->pPager;
  if( pPg->inCkpt ) return;
  assert( pPg->pPrevCkpt==0 && pPg->pNextCkpt==0 );
  pPg->pPrevCkpt = 0;
  if( pPager->pCkpt ){
    pPager->pCkpt->pPrevCkpt = pPg;
  }
  pPg->pNextCkpt = pPager->pCkpt;
  pPager->pCkpt = pPg;
  pPg->inCkpt = 1;
}
static void page_remove_from_ckpt_list(PgHdr *pPg){
  if( !pPg->inCkpt ) return;
  if( pPg->pPrevCkpt ){
    assert( pPg->pPrevCkpt->pNextCkpt==pPg );
    pPg->pPrevCkpt->pNextCkpt = pPg->pNextCkpt;
  }else{
    assert( pPg->pPager->pCkpt==pPg );
    pPg->pPager->pCkpt = pPg->pNextCkpt;
  }
  if( pPg->pNextCkpt ){
    assert( pPg->pNextCkpt->pPrevCkpt==pPg );
    pPg->pNextCkpt->pPrevCkpt = pPg->pPrevCkpt;
  }
  pPg->pNextCkpt = 0;
  pPg->pPrevCkpt = 0;
  pPg->inCkpt = 0;
}

/*
** Find a page in the hash table given its page number.  Return
** a pointer to the page or NULL if not found.
*/
static PgHdr *pager_lookup(Pager *pPager, Pgno pgno){
  PgHdr *p = pPager->aHash[pgno % N_PG_HASH];

** a reference to the page data.
*/
int sqlitepager_ref(void *pData){
  PgHdr *pPg = DATA_TO_PGHDR(pData);
  page_ref(pPg);
  return SQLITE_OK;
}

/*
** The parameters are pointers to the head of two sorted lists
** of page headers.  Merge these two lists together and return
** a single sorted list.  This routine forms the core of the 
** merge-sort algorithm that sorts dirty pages into accending
** order prior to writing them back to the disk.
**
** In the case of a tie, left sorts in front of right.
**
** Headers are sorted in order of ascending page number.
*/
static PgHdr *page_merge(PgHdr *pLeft, PgHdr *pRight){
  PgHdr sHead;
  PgHdr *pTail;
  pTail = &sHead;
  pTail->pSort = 0;
  while( pLeft && pRight ){
    if( pLeft->pgno<=pRight->pgno ){
      pTail->pSort = pLeft;
      pLeft = pLeft->pSort;
    }else{
      pTail->pSort = pRight;
      pRight = pRight->pSort;
    }
    pTail = pTail->pSort;
  }
  if( pLeft ){
    pTail->pSort = pLeft;
  }else if( pRight ){
    pTail->pSort = pRight;
  }
  return sHead.pSort;
}


/*
** Sync the journal and then write all free dirty pages to the database
** file.
**
** Writing all free dirty pages to the database after the sync is a
** non-obvious optimization.  fsync() is an expensive operation so we
** want to minimize the number ot times it is called. After an fsync() call,
** we are free to write dirty pages back to the database.  It is best
** to go ahead and write as many dirty pages as possible to minimize 
** the risk of having to do another fsync() later on.  Writing dirty
** free pages in this way was observed to make database operations go
** up to 10 times faster.
**
** If we are writing to temporary database, there is no need to preserve
** the integrity of the journal file, so we can save time and skip the
** fsync().
**
** This routine goes to the extra trouble of sorting all the dirty
** pages by their page number prior to writing them.  Tests show that
** writing pages in order by page number gives a modest speed improvement
** under Linux.  
*/
static int syncAllPages(Pager *pPager){
  PgHdr *pPg;
  PgHdr *pToWrite;
# define NSORT 28
  Pgno lastPgno;
  int i;
  PgHdr *apSorter[NSORT];
  int rc = SQLITE_OK;

  /* Sync the journal before modifying the main database
  ** (assuming there is a journal and it needs to be synced.)
  */
  if( pPager->needSync ){
    if( !pPager->tempFile ){
      rc = sqliteOsSync(&pPager->jfd);
      if( rc!=0 ) return rc;
    }
    pPager->needSync = 0;
  }

  /* Create a list of all dirty pages
  */
  pToWrite = 0;
  for(pPg=pPager->pFirst; pPg; pPg=pPg->pNextFree){
    if( pPg->dirty ){
      pPg->pSort = pToWrite;
      pToWrite = pPg;
    }
  }

  /* Sort the list of dirty pages into accending order by
  ** page number
  */
  for(i=0; i<NSORT; i++){
    apSorter[i] = 0;
  }
  while( pToWrite ){
    pPg = pToWrite;
    pToWrite = pPg->pSort;
    pPg->pSort = 0;
    for(i=0; i<NSORT-1; i++){
      if( apSorter[i]==0 ){
        apSorter[i] = pPg;
        break;
      }else{
        pPg = page_merge(apSorter[i], pPg);
        apSorter[i] = 0;
      }
    }
    if( i>=NSORT-1 ){
      apSorter[NSORT-1] = page_merge(apSorter[NSORT-1],pPg);
    }
  }
  pToWrite = 0;
  for(i=0; i<NSORT; i++){
    pToWrite = page_merge(apSorter[i], pToWrite);
  }

  /* Write all dirty pages back to the database and mark
  ** them all clean.
  */
  lastPgno = 0;
  for(pPg=pToWrite; pPg; pPg=pPg->pSort){
    if( lastPgno==0 || pPg->pgno!=lastPgno-1 ){
      sqliteOsSeek(&pPager->fd, (pPg->pgno-1)*SQLITE_PAGE_SIZE);
    }
    rc = sqliteOsWrite(&pPager->fd, PGHDR_TO_DATA(pPg), SQLITE_PAGE_SIZE);
    if( rc!=SQLITE_OK ) break;
    pPg->dirty = 0;

    lastPgno = pPg->pgno;
  }
  return rc;
}

/*
** Acquire a page.
**

    }
    pPg->pgno = pgno;
    if( pPager->aInJournal && (int)pgno<=pPager->origDbSize ){
      pPg->inJournal = (pPager->aInJournal[pgno/8] & (1<<(pgno&7)))!=0;
    }else{
      pPg->inJournal = 0;
    }
    if( pPager->aInCkpt && (int)pgno<=pPager->ckptSize
             && (pPager->aInCkpt[pgno/8] & (1<<(pgno&7)))!=0 ){
      page_add_to_ckpt_list(pPg);
    }else{
      page_remove_from_ckpt_list(pPg);
    }
    pPg->dirty = 0;
    pPg->nRef = 1;
    REFINFO(pPg);
    pPager->nRef++;
    h = pager_hash(pgno);
    pPg->pNextHash = pPager->aHash[h];

    }
    assert( pPager->aInJournal!=0 );
    pPager->aInJournal[pPg->pgno/8] |= 1<<(pPg->pgno&7);
    pPager->needSync = !pPager->noSync;
    pPg->inJournal = 1;
    if( pPager->ckptInUse ){
      pPager->aInCkpt[pPg->pgno/8] |= 1<<(pPg->pgno&7);
      page_add_to_ckpt_list(pPg);
    }
  }

  /* If the checkpoint journal is open and the page is not in it,
  ** then write the current page to the checkpoint journal.
  */
  if( pPager->ckptInUse && !pPg->inCkpt && (int)pPg->pgno<=pPager->ckptSize ){
    assert( pPg->inJournal || (int)pPg->pgno>pPager->origDbSize );
    rc = write32bits(&pPager->cpfd, pPg->pgno);
    if( rc==SQLITE_OK ){
      rc = sqliteOsWrite(&pPager->cpfd, pData, SQLITE_PAGE_SIZE);
    }
    if( rc!=SQLITE_OK ){
      sqlitepager_rollback(pPager);
      pPager->errMask |= PAGER_ERR_FULL;
      return rc;
    }
    assert( pPager->aInCkpt!=0 );
    pPager->aInCkpt[pPg->pgno/8] |= 1<<(pPg->pgno&7);
    page_add_to_ckpt_list(pPg);
  }

  /* Update the database size and return.
  */
  if( pPager->dbSize<(int)pPg->pgno ){
    pPager->dbSize = pPg->pgno;
  }

  if( pPg->alwaysRollback || pPager->alwaysRollback ) return;
  if( !pPg->inJournal && (int)pPg->pgno <= pPager->origDbSize ){
    assert( pPager->aInJournal!=0 );
    pPager->aInJournal[pPg->pgno/8] |= 1<<(pPg->pgno&7);
    pPg->inJournal = 1;
    if( pPager->ckptInUse ){
      pPager->aInCkpt[pPg->pgno/8] |= 1<<(pPg->pgno&7);
      page_add_to_ckpt_list(pPg);
    }
  }
  if( pPager->ckptInUse && !pPg->inCkpt && (int)pPg->pgno<=pPager->ckptSize ){
    assert( pPg->inJournal || (int)pPg->pgno>pPager->origDbSize );
    assert( pPager->aInCkpt!=0 );
    pPager->aInCkpt[pPg->pgno/8] |= 1<<(pPg->pgno&7);
    page_add_to_ckpt_list(pPg);
  }
}

/*
** Commit all changes to the database and release the write lock.
**
** If the commit fails for any reason, a rollback attempt is made

}

/*
** Commit a checkpoint.
*/
int sqlitepager_ckpt_commit(Pager *pPager){
  if( pPager->ckptInUse ){
    PgHdr *pPg, *pNext;
    sqliteOsSeek(&pPager->cpfd, 0);
    sqliteOsTruncate(&pPager->cpfd, 0);
    pPager->ckptInUse = 0;
    sqliteFree( pPager->aInCkpt );
    pPager->aInCkpt = 0;
    for(pPg=pPager->pCkpt; pPg; pPg=pNext){
      pNext = pPg->pNextCkpt;
      assert( pPg->inCkpt );
      pPg->inCkpt = 0;
      pPg->pPrevCkpt = pPg->pNextCkpt = 0;
    }
    pPager->pCkpt = 0;
  }
  return SQLITE_OK;
}

/*
** Rollback a checkpoint.
*/