MemCacheStore should send cache keys with TTL of 0 to memcached

[andrejbl]

Memcached supports setting the TTL to 0 to cache the record permanently. However, trying to cache a value through MemCacheStore with exipres_in: 0 option instead causes the record to not be cached at all.

Steps to reproduce

(not providing a test for repro as it requires an externally running memcached server so it is not really self contained, hopefully this is ok as the repro steps are very basic)

Start a memcached server locally: memcached -p 11211

In a Rails console, run

cache = ActiveSupport::Cache.lookup_store(:mem_cache_store, "localhost:11211", {})
cache.write("key", "value", :expires_in => 0)
cache.read("key") # this will return `nil`

Expected behavior

Entry should be cached indefinitely.

Actual behavior

Entry is not cached at all.

System configuration

Rails version: 6.1.4.1

Ruby version: 2.6.5

[lfalcao]

Hi @andrejbl

When you pass the :expires_in it saves the cache entry with value + Time.now, so when you try to read it Rails deletes it as its already expired... (Source: https://github.com/rails/rails/blob/main/activesupport/lib/active_support/cache.rb#L923)

Possible fix:

@expires_in = expires_at&.to_f || expires_in && expires_in > 0 && (expires_in.to_f + Time.now.to_f)

[ghiculescu]

If you don't include the expires_in arg, the entry will never expire.

That said, I've also come across situations where that results in messy code. The fix above would work, but it would break existing code if anyone is depending on the behaviour described (though I dunno why you would...). My gut feel is this is a bug and we should fix it, but I've asked for a second opinion on if this would require a config option.

[ghiculescu]

#43727 treats it as a bug, but I'm happy to add a config option if we need backward compat.

[ghiculescu]

Chatted to @matthewd and his view was this is expected behaviour.

You can use expires_in: nil, or exclude the option entirely, to have non-expiring entries.

So given that, I am going to close this.