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 1.621 2009/07/25 11:40:08 danielk1977 Exp $
*/
#ifndef SQLITE_OMIT_DISKIO
#include "sqliteInt.h"

/*
** Macros for troubleshooting.  Normally turned off
*/

      pPager->state = (u8)locktype;
      IOTRACE(("LOCK %p %d\n", pPager, locktype))
    }
  }
  return rc;
}

/*
** Function assertTruncateConstraint(pPager) checks that one of the 
** following is true for all dirty pages currently in the page-cache:
**
**   a) The page number is less than or equal to the size of the 
**      current database image, in pages, OR
**
**   b) if the page content were written at this time, it would not
**      be necessary to write the current content out to the sub-journal
**      (as determined by function subjRequiresPage()).
**
** If the condition asserted by this function were not true, and the
** dirty page were to be discarded from the cache via the pagerStress()
** routine, pagerStress() would not write the current page content to
** the database file. If a savepoint transaction were rolled back after
** this happened, the correct behaviour would be to restore the current
** content of the page. However, since this content is not present in either
** the database file or the portion of the rollback journal and 
** sub-journal rolled back the content could not be restored and the
** database image would become corrupt. It is therefore fortunate that 
** this circumstance cannot arise.
*/
#if defined(SQLITE_DEBUG)
static void assertTruncateConstraintCb(PgHdr *pPg){
  assert( pPg->flags&PGHDR_DIRTY );
  assert( !subjRequiresPage(pPg) || pPg->pgno<=pPg->pPager->dbSize );
}
static void assertTruncateConstraint(Pager *pPager){
  sqlite3PcacheIterateDirty(pPager->pPCache, assertTruncateConstraintCb);
}
#else
# define assertTruncateConstraint(pPager)
#endif

/*
** Truncate the in-memory database file image to nPage pages. This 
** function does not actually modify the database file on disk. It 
** just sets the internal state of the pager object so that the 
** truncation will be done when the current transaction is committed.
*/
void sqlite3PagerTruncateImage(Pager *pPager, Pgno nPage){
  assert( pPager->dbSizeValid );
  assert( pPager->dbSize>=nPage );
  assert( pPager->state>=PAGER_RESERVED );
  pPager->dbSize = nPage;
  assertTruncateConstraint(pPager);
}

/*
** Shutdown the page cache.  Free all memory and close all files.
**
** If a transaction was in progress when this routine is called, that
** transaction is rolled back.  All outstanding pages are invalidated

      if( !aNew[ii].pInSavepoint ){
        return SQLITE_NOMEM;
      }
    }

    /* Open the sub-journal, if it is not already opened. */
    rc = openSubJournal(pPager);
    assertTruncateConstraint(pPager);
  }

  return rc;
}

/*
** This function is called to rollback or release (commit) a savepoint.