BDB backend committed

[Colm Smyth <Colm Smyth ireland sun com>]

Hi,

I've just committed working source for a new backend for GConf, using
the Berkeley DB database toolkit. The README is attached.

I'd welcome feedback!

Colm.

A GConf backend is an adaptor to permit GConf to store configuration
information in some form of 'database'. The XML backend uses files
and a file-system directory structure to store key-value pairs,
schemas, and to represent the key hierarchy. The Berkeley DB
backend uses Sleepycat's distribution of the Berkeley DB
database toolkit to provide a relatively high performance GConf 
store.

If you decide not to build the Berkeley DB backend, simply remove
the reference to libgconfbackend-bdb.la in the definition of
backend_LTLIBRARIES in backends/Makefile.

See comments in bdb.h for an description of the databases and
their use.

Notes for this initial release:

- there are bound to be bugs, but the backend passes the GConf
  test suite (extended to include some schema-usage tests)
- some concurrency control has been built-in, but this has not
  been adequately tested
- by design there is no cache so that concurrent writers may
  be supported, but despite this performance is good and the
  testpersistence test (which synchronises each change individually
  to the GConfd database) is noticably faster than with the XML backend.
- Previous versions of Berkeley DB have altered the database structure in
  ways that are not backward compatible. If this occurs with future versions
  of Berkeley DB (after version 3.1.17), the major version number of the
  backend shared library will be incremented.
  

Known bugs:

- The backend uses a relatively new feature of Berkeley DB, the support
  for duplicate keys in a single database table. Currently Berkeley DB
  produces warning messages 'Duplicate data items are not supported with
  sorted data'. Sleepycat have said that this message is only to warn
  developers while the duplicate-support feature is new and will be removed
  in a future release.

- There is also an error that occasionally "corrupts" the key-hierarchy
  database which prevents the directory hierarchy from being traversed.
  Currently it is not clear if this is an error in the backend or in
  the database toolkit. This error can be seen in the testdirlist test
  program which initially works but after running all tests a few
  times, testdirlist produces an error message "*** FAILED: Did not get
  'testing' in listing of / after setting keys such as /testing/foo/bar/woo".
  This bug is under investigation, as it prevents serious use of
  the Berkeley DB backend, although applications which do not use the
  gconf_all_dirs() API should not encounter this problem.

You can obtain the source distribution for Berkeley DB from Sleepycat
Software at http://www.sleepycat.com/download.html.  The current
version is 3.1.17.

Colm Smyth (colm smyth sun com)