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Changes to src/btree.c.

     5      5   ** a legal notice, here is a blessing:
     6      6   **
     7      7   **    May you do good and not evil.
     8      8   **    May you find forgiveness for yourself and forgive others.
     9      9   **    May you share freely, never taking more than you give.
    10     10   **
    11     11   *************************************************************************
    12         -** $Id: btree.c,v 1.339 2007/03/19 11:25:20 danielk1977 Exp $
           12  +** $Id: btree.c,v 1.340 2007/03/19 11:54:10 drh Exp $
    13     13   **
    14     14   ** This file implements a external (disk-based) database using BTrees.
    15     15   ** For a detailed discussion of BTrees, refer to
    16     16   **
    17     17   **     Donald E. Knuth, THE ART OF COMPUTER PROGRAMMING, Volume 3:
    18     18   **     "Sorting And Searching", pages 473-480. Addison-Wesley
    19     19   **     Publishing Company, Reading, Massachusetts.
................................................................................
  2645   2645   ** If wrFlag==1, then the cursor can be used for reading or for
  2646   2646   ** writing if other conditions for writing are also met.  These
  2647   2647   ** are the conditions that must be met in order for writing to
  2648   2648   ** be allowed:
  2649   2649   **
  2650   2650   ** 1:  The cursor must have been opened with wrFlag==1
  2651   2651   **
  2652         -** 2:  No other cursors may be open with wrFlag==0 on the same table
         2652  +** 2:  Other database connections that share the same pager cache
         2653  +**     but which are not in the READ_UNCOMMITTED state may not have
         2654  +**     cursors open with wrFlag==0 on the same table.  Otherwise
         2655  +**     the changes made by this write cursor would be visible to
         2656  +**     the read cursors in the other database connection.
  2653   2657   **
  2654   2658   ** 3:  The database must be writable (not on read-only media)
  2655   2659   **
  2656   2660   ** 4:  There must be an active transaction.
  2657   2661   **
  2658         -** Condition 2 warrants further discussion.  If any cursor is opened
  2659         -** on a table with wrFlag==0, that prevents all other cursors from
  2660         -** writing to that table.  This is a kind of "read-lock".  When a cursor
  2661         -** is opened with wrFlag==0 it is guaranteed that the table will not
  2662         -** change as long as the cursor is open.  This allows the cursor to
  2663         -** do a sequential scan of the table without having to worry about
  2664         -** entries being inserted or deleted during the scan.  Cursors should
  2665         -** be opened with wrFlag==0 only if this read-lock property is needed.
  2666         -** That is to say, cursors should be opened with wrFlag==0 only if they
  2667         -** intend to use the sqlite3BtreeNext() system call.  All other cursors
  2668         -** should be opened with wrFlag==1 even if they never really intend
  2669         -** to write.
  2670         -** 
  2671   2662   ** No checking is done to make sure that page iTable really is the
  2672   2663   ** root page of a b-tree.  If it is not, then the cursor acquired
  2673   2664   ** will not work correctly.
  2674   2665   **
  2675   2666   ** The comparison function must be logically the same for every cursor
  2676   2667   ** on a particular table.  Changing the comparison function will result
  2677   2668   ** in incorrect operations.  If the comparison function is NULL, a