Deprecate initialize memcache store with dalli client

Why:
----

Following up on [rails#47323](rails#47323).
Many options are not forwarded to the Dalli client when it is
initialized from the `ActiveSupport::Cache::MemCacheStore`. This is to
support a broader set of features powered by the implementation. When an
instance of a client is passed on the initializer, it takes precedence,
and we have no control over which attributes will be overridden or
re-processed on the client side; this is by design and should remain as
such to allow both projects to progress independently. Having this
option introduces several potential bugs that are difficult to pinpoint
and get multiplied by which version of the tool is used and how each
evolves. During the conversation on the issue, the `Dalli` client
maintainer supports [deprecating](rails#47323 (comment))
this option.

How:
----

Removing this implicit dependency will ensure each library can evolve
separately and cements the usage of `Dalli::Client` as an [implementation
detail](rails#21595 (comment))

We can not remove a supported feature overnight, so I propose we add a
deprecation warning for the next minor release(7.2 at this time).

There was a constant on the `Cache` namespace only used to restrict
options passed to the `Dalli::Client` initializer that now lives on the
`MemCacheStore` class.

Co-authored-by: Eileen M. Uchitelle <[email protected]>

Author: aledustet

activesupport/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Deprecated initializing a `ActiveSupport::Cache::MemCacheStore` with an instance of `Dalli::Client`.
+
+    Deprecate the undocumented option of providing an already-initialized instance of `Dalli::Client` to `ActiveSupport::Cache::MemCacheStore`. Such clients could be configured with unrecognized options, which could lead to unexpected behavior. Instead, provide addresses as documented.
+
+    *aledustet*
+
 *   Stub `Time.new()` in `TimeHelpers#travel_to`
 
       ```ruby

activesupport/lib/active_support/cache/mem_cache_store.rb

@@ -28,6 +28,10 @@ module Cache
     # MemCacheStore implements the Strategy::LocalCache strategy which implements
     # an in-memory cache inside of a block.
     class MemCacheStore < Store
+      # These options represent behavior overridden by this implementation and should
+      # not be allowed to get down to the Dalli client
+      OVERRIDDEN_OPTIONS = UNIVERSAL_OPTIONS
+
       # Advertise cache versioning support.
       def self.supports_cache_versioning?
         true
@@ -108,6 +112,7 @@ def self.build_mem_cache(*addresses) # :nodoc:
       #
       # If no addresses are provided, but <tt>ENV['MEMCACHE_SERVERS']</tt> is defined, it will be used instead. Otherwise,
       # MemCacheStore will connect to localhost:11211 (the default memcached port).
+      # Passing a +Dalli::Client+ instance is deprecated and will be removed. Please pass an address instead.
       def initialize(*addresses)
         addresses = addresses.flatten
         options = addresses.extract_options!
@@ -117,15 +122,19 @@ def initialize(*addresses)
         super(options)
 
         unless [String, Dalli::Client, NilClass].include?(addresses.first.class)
-          raise ArgumentError, "First argument must be an empty array, an array of hosts or a Dalli::Client instance."
+          raise ArgumentError, "First argument must be an empty array, address, or array of addresses."
         end
         if addresses.first.is_a?(Dalli::Client)
+          ActiveSupport.deprecator.warn(<<~MSG)
+            Initializing MemCacheStore with a Dalli::Client is deprecated and will be removed in Rails 7.2.
+            Use memcached server addresses instead.
+          MSG
           @data = addresses.first
         else
           mem_cache_options = options.dup
           # The value "compress: false" prevents duplicate compression within Dalli.
           mem_cache_options[:compress] = false
-          (UNIVERSAL_OPTIONS - %i(compress)).each { |name| mem_cache_options.delete(name) }
+          (OVERRIDDEN_OPTIONS - %i(compress)).each { |name| mem_cache_options.delete(name) }
           @data = self.class.build_mem_cache(*(addresses + [mem_cache_options]))
         end
       end

activesupport/test/cache/stores/mem_cache_store_test.rb

@@ -271,9 +271,11 @@ def test_unless_exist_expires_when_configured
   end
 
   def test_uses_provided_dalli_client_if_present
-    cache = lookup_store(Dalli::Client.new("custom_host"))
-
-    assert_equal ["custom_host"], servers(cache)
+    assert_deprecated(ActiveSupport.deprecator) do
+      host = "custom_host"
+      cache = lookup_store(Dalli::Client.new(host))
+      assert_equal [host], servers(cache)
+    end
   end
 
   def test_forwards_string_addresses_if_present