discourse/lib/cache.rb

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# frozen_string_literal: true
# Discourse specific cache, enforces 1 day expiry by default
# This is a bottom up implementation of ActiveSupport::Cache::Store
# this allows us to cleanly implement without using cache entries and version
# support which we do not use, in tern this makes the cache as fast as simply
# using `Discourse.redis.setex` with a more convenient API
#
# It only implements a subset of ActiveSupport::Cache::Store as we make no use
# of large parts of the interface.
#
# An additional advantage of this class is that all methods have named params
# Rails tends to use options hash for lots of stuff due to legacy reasons
# this makes it harder to reason about the API
class Cache
# nothing is cached for longer than 1 day EVER
# there is no reason to have data older than this clogging redis
# it is dangerous cause if we rename keys we will be stuck with
# pointless data
MAX_CACHE_AGE = 1.day unless defined?(MAX_CACHE_AGE)
attr_reader :namespace
# we don't need this feature, 1 day expiry is enough
# it makes lookups a tad cheaper
def self.supports_cache_versioning?
false
end
def initialize(namespace: "_CACHE")
@namespace = namespace
end
def redis
Discourse.redis
end
def reconnect
redis.reconnect
end
def keys(pattern = "*")
redis.scan_each(match: "#{@namespace}:#{pattern}").to_a
end
def clear
keys.each { |k| redis.del(k) }
end
def normalize_key(key)
"#{@namespace}:#{key}"
end
def exist?(name)
key = normalize_key(name)
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redis.exists?(key)
end
# this removes a bunch of stuff we do not need like instrumentation and versioning
def read(name)
key = normalize_key(name)
read_entry(key).tap { |entry| break if entry == :__corrupt_cache__ }
end
def write(name, value, expires_in: nil)
write_entry(normalize_key(name), value, expires_in: expires_in)
end
def delete(name)
redis.del(normalize_key(name))
end
def fetch(name, expires_in: nil, force: nil, &blk)
if !block_given?
if force
raise ArgumentError,
"Missing block: Calling `Cache#fetch` with `force: true` requires a block."
end
return read(name)
end
key = normalize_key(name)
if !force
if raw = redis.get(key)
entry = decode_entry(raw, key)
return entry if entry != :__corrupt_cache__
end
end
val = blk.call
write_entry(key, val, expires_in: expires_in)
val
end
protected
def log_first_exception(e, key)
return if defined?(@logged_a_warning)
@logged_a_warning = true
Discourse.warn_exception(e, message: "Corrupt cache... skipping entry for key #{key}")
end
def decode_entry(raw, key)
Marshal.load(raw) # rubocop:disable Security/MarshalLoad
rescue => e
# corrupt cache, this can happen if Marshal version
# changes. Log it once so we can tell it is happening.
# should not happen under any normal circumstances, but we
# do not want to flood logs
log_first_exception(e, key)
:__corrupt_cache__
end
def read_entry(key)
if data = redis.get(key)
decode_entry(data, key)
end
end
def write_entry(key, value, expires_in: nil)
dumped = Marshal.dump(value)
expiry = expires_in || MAX_CACHE_AGE
redis.setex(key, expiry, dumped)
true
end
end