When using `Discourse.cache.fetch` with an expiry, there's a potential for a race condition due to how we read the data from redis.
The code used to be
```ruby
raw = redis.get(key) if !force
entry = read_entry(key) if raw
return entry if raw && !(entry == :__corrupt_cache__)
```
with `read_entry` defined as follow
```ruby
def read_entry(key)
if data = redis.get(key)
Marshal.load(data)
end
rescue => e
:__corrupt_cache__
end
```
If the value at "key" expired in redis between `raw = redis.get` and `entry = read_entry`, the `entry` variable would be `nil` despite `raw` having a value.
We would then proceed to return `entry` (which is `nil`) thinking it had a value, when it didn't.
The first `redis.get` can be skipped altogether and we can rely only on `read_entry` to read the data from redis. Thus avoiding the race condition and removing the double read operations.
Internal ref - t/132507
Currently when a cache entry is corrupt, we log the event without doing
anything else. It means the cache is still corrupt, and the proper value
isn’t computed again.
Normally, it’s very rare the cache becomes corrupt, but it can happen
when upgrading Rails for example and the cache format changes. This is
normally handled automatically by Rails but since we’re using a custom
cache class, we have to do it ourselves.
This patch takes the same approach the Rails team did, when a cache
entry is corrupt, we treat it as a miss, recomputing the proper value
and caching it in the new format.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
Loïc Guitaut
Régis Hanol
David Taylor
Phil Pirozhkov
Jarek Radosz