Only insert cache item into the database when there is no item already i...

[bobkarreman]

I made a change to the cache db backend to only do an insert when it's needed.

When there is already an result and its not expired the insert is going to fail so it could be skipped.

Or am I missing something?

[timgraham]

There are some failing tests with this change:

======================================================================
FAIL: test_add (cache.tests.DBCacheTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tim/code/django/tests/cache/tests.py", line 248, in test_add
    self.assertFalse(result)
AssertionError: True is not false

======================================================================
FAIL: test_forever_timeout (cache.tests.DBCacheTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/tim/code/django/tests/cache/tests.py", line 500, in test_forever_timeout
    self.assertEqual(added, False)
AssertionError: True != False

[bobkarreman]

You are right sorry.
This should fix it!

[timgraham]

buildbot, add to whitelist.

[timgraham]

I think the code could be written this way to avoid a race condition where the key is deleted after the "SELECT cache_key, expires..." query and before the "INSERT" query.