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 1.189 2005/02/15 02:54:15 danielk1977 Exp $
*/
#include "sqliteInt.h"
#include "os.h"
#include "pager.h"
#include <assert.h>
#include <string.h>

  u8 inJournal;                  /* TRUE if has been written to journal */
  u8 inStmt;                     /* TRUE if in the statement subjournal */
  u8 dirty;                      /* TRUE if we need to write back changes */
  u8 needSync;                   /* Sync journal before writing this page */
  u8 alwaysRollback;             /* Disable dont_rollback() for this page */
  short int nRef;                /* Number of users of this page */
  PgHdr *pDirty;                 /* Dirty pages sorted by PgHdr.pgno */
#ifdef SQLITE_CHECK_PAGES
  u32 pageHash;
#endif
  /* pPager->psAligned bytes of page data follow this header */
  /* Pager.nExtra bytes of local data follow the page data */
};

/*
** For an in-memory only database, some extra information is recorded about
** each page so that changes can be rolled back.  (Journal files are not

  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;
}

#ifdef SQLITE_CHECK_PAGES
/*
** Return a 32-bit hash of the page data for pPage.
*/
static u32 pager_pagehash(PgHdr *pPage){
  u32 hash = 0;
  int i;
  unsigned char *pData = (unsigned char *)PGHDR_TO_DATA(pPage);
  for(i=0; i<pPage->pPager->pageSize; i++){
    hash = (hash+i)^pData[i];
  }
  return hash;
}

/*
** The CHECK_PAGE macro takes a PgHdr* as an argument. If SQLITE_CHECK_PAGES
** is defined, and NDEBUG is not defined, an assert() statement checks
** that the page is either dirty or still matches the calculated page-hash.
*/
#define CHECK_PAGE(x) checkPage(x)
static void checkPage(PgHdr *pPg){
  Pager *pPager = pPg->pPager;
  assert( !pPg->pageHash || pPager->errMask || MEMDB || pPg->dirty || 
      pPg->pageHash==pager_pagehash(pPg) );
}

#else
#define CHECK_PAGE(x)
#endif

/*
** When this is called the journal file for pager pPager must be open.
** The master journal file name is read from the end of the file and 
** written into memory obtained from sqliteMalloc(). *pzMaster is
** set to point at the memory and SQLITE_OK returned. The caller must
** sqliteFree() *pzMaster.

    sqlite3OsDelete(pPager->zJournal);
    sqliteFree( pPager->aInJournal );
    pPager->aInJournal = 0;
    for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
      pPg->inJournal = 0;
      pPg->dirty = 0;
      pPg->needSync = 0;
#ifdef SQLITE_CHECK_PAGES
      pPg->pageHash = pager_pagehash(pPg);
#endif
    }
    pPager->dirtyCache = 0;
    pPager->nRec = 0;
  }else{
    assert( pPager->dirtyCache==0 || pPager->useJournal==0 );
  }
  rc = sqlite3OsUnlock(&pPager->fd, SHARED_LOCK);

    memcpy(pData, aData, pPager->pageSize);
    if( pPager->xDestructor ){  /*** FIX ME:  Should this be xReinit? ***/
      pPager->xDestructor(pData, pPager->pageSize);
    }
    if( pPager->state>=PAGER_EXCLUSIVE ){
      pPg->dirty = 0;
      pPg->needSync = 0;
#ifdef SQLITE_CHECK_PAGES
      pPg->pageHash = pager_pagehash(pPg);
#endif
    }
    CODEC(pPager, pData, pPg->pgno, 3);
  }
  return rc;
}

/*

        pPager->xReiniter(PGHDR_TO_DATA(pPg), pPager->pageSize);
      }else{
        memset(PGHDR_TO_EXTRA(pPg, pPager), 0, pPager->nExtra);
      }
    }
    pPg->needSync = 0;
    pPg->dirty = 0;
#ifdef SQLITE_CHECK_PAGES
    pPg->pageHash = pager_pagehash(pPg);
#endif
  }
  return rc;
}

/*
** Truncate the main file of the given pager to the number of pages
** indicated.

#ifndef NDEBUG
    else{
      TRACE3("NOSTORE %d page %d\n", PAGERID(pPager), pList->pgno);
    }
#endif
    if( rc ) return rc;
    pList->dirty = 0;
#ifdef SQLITE_CHECK_PAGES
    pList->pageHash = pager_pagehash(pList);
#endif
    pList = pList->pDirty;
  }
  return SQLITE_OK;
}

/*
** Collect every dirty page into a dirty list and

        }else{
          memset(PGHDR_TO_DATA(pPg), 0, pPager->pageSize);
        }
      }else{
        pPager->nRead++;
      }
    }
#ifdef SQLITE_CHECK_PAGES
    pPg->pageHash = pager_pagehash(pPg);
#endif
  }else{
    /* The requested page is in the page cache. */
    pPager->nHit++;
    page_ref(pPg);
  }
  *ppPage = PGHDR_TO_DATA(pPg);
  return SQLITE_OK;

  /* Decrement the reference count for this page
  */
  pPg = DATA_TO_PGHDR(pData);
  assert( pPg->nRef>0 );
  pPg->nRef--;
  REFINFO(pPg);

  CHECK_PAGE(pPg);

  /* When the number of references to a page reach 0, call the
  ** destructor and add the page to the freelist.
  */
  if( pPg->nRef==0 ){
    Pager *pPager;
    pPager = pPg->pPager;

    return pager_errcode(pPager);
  }
  if( pPager->readOnly ){
    return SQLITE_PERM;
  }

  assert( !pPager->setMaster );

  CHECK_PAGE(pPg);

  /* Mark the page as dirty.  If the page has already been written
  ** to the journal then we can return right away.
  */
  pPg->dirty = 1;
  if( pPg->inJournal && (pPg->inStmt || pPager->stmtInUse==0) ){
    pPager->dirtyCache = 1;

      ** size. If you do not write this page and the size of the file
      ** on the disk ends up being too small, that can lead to database
      ** corruption during the next transaction.
      */
    }else{
      TRACE3("DONT_WRITE page %d of %d\n", pgno, PAGERID(pPager));
      pPg->dirty = 0;
#ifdef SQLITE_CHECK_PAGES
      pPg->pageHash = pager_pagehash(pPg);
#endif
    }
  }
}

/*
** A call to this routine tells the pager that if a rollback occurs,
** it is not necessary to restore the data on the given page.  This