BaseCache.get_or_set() doesn't always return a value

[Przemysław Suliga]

​BaseCache.get_or_set() returns False if the key was set in cache between the first .get() and .add().

The reason for using .add() instead of .set() in there is a "big race condition" as stated in #12982. However, it is very unlikely that the caller will pass a default callable that reads the key from the cache and returns a result that depends on the value currently stored in cache which would be a prerequisite for a race condition to occur. Using set() as originally stated in the description of #12982 means that there will be a data race which doesn't affect the correctness of the program, because the result is that multiple writes happened, but all of them are correct.

Following texts contain a nice explanation of what is a race condition and how it relates to a data race:

• ​Race Condition vs. Data Race
• ​Description of race conditions and deadlocks

As a user of get_or_set() I am expecting it to always return a value, regardless if a write to the cache happened and it is inefficient for me to handle the False return scenario by calling my default() callback again to obtain the value I need (it was already executed inside .get_or_set, but the result was discarded).

I can see 2 possible fixes:

• always use .set() and always return the value of default
• ignore the result of .add() and always return to the user with the result of .get(key, default) after the call to .add()

[Przemysław Suliga]

​https://github.com/django/django/pull/6250 takes the second proposed approach to fix this.

[Tim Graham]

This issue was also ​raised on django-developers.

[Tim Graham <timograham@…>]

In 96ec67a:

Fixed #26332 -- Fixed a race condition in BaseCache.get_or_set().

[Tim Graham <timograham@…>]

In 76926f3:

[1.9.x] Fixed #26332 -- Fixed a race condition in BaseCache.get_or_set().

Backport of 96ec67a7cf89a136e793305343c5bba8521cdb47 from master