id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
26332	BaseCache.get_or_set() doesn't always return a value	Przemysław Suliga	nobody	"[https://github.com/django/django/blob/2109975e901440da70e29d0f330a600bc2d37e9a/django/core/cache/backends/base.py#L150-L169 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:
- [http://blog.regehr.org/archives/490 Race Condition vs. Data Race]
- [https://support.microsoft.com/en-us/kb/317723 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()`"	Bug	closed	Core (Cache system)	1.9	Normal	fixed			Accepted	1	0	0	0	0	0