Caching: MemCache and Redis stores use local cache for multi-reads

Fixes #31909.
Closes #31911.

activesupport/CHANGELOG.md

@@ -1,5 +1,10 @@
 ## Rails 6.0.0.alpha (Unreleased) ##
 
+*   Caching: MemCache and Redis `read_multi` and `fetch_multi` speedup.
+    Read from the local in-memory cache before consulting the backend.
+
+    *Gabriel Sobrinho*
+
 *   Return all mappings for a timezone identifier in `country_zones`
 
     Some timezones like `Europe/London` have multiple mappings in

activesupport/lib/active_support/cache/strategy/local_cache.rb

@@ -54,6 +54,10 @@ module ActiveSupport
             @data[key]
           end
 
+          def read_multi_entries(keys, options)
+            Hash[keys.map { |name| [name, read_entry(name, options)] }.keep_if { |_name, value| value }]
+          end
+
           def write_entry(key, value, options)
             @data[key] = value
             true
@@ -116,6 +120,19 @@ module ActiveSupport
             end
           end
 
+          def read_multi_entries(keys, options)
+            return super unless local_cache
+
+            local_entries = local_cache.read_multi_entries(keys, options)
+            missed_keys = keys - local_entries.keys
+
+            if missed_keys.any?
+              local_entries.merge!(super(missed_keys, options))
+            else
+              local_entries
+            end
+          end
+
           def write_entry(key, entry, options)
             if options[:unless_exist]
               local_cache.delete_entry(key, options) if local_cache

activesupport/test/cache/behaviors/local_cache_behavior.rb

@@ -119,6 +119,16 @@ module LocalCacheBehavior
     end
   end
 
+  def test_local_cache_of_fetch_multi
+    @cache.with_local_cache do
+      @cache.fetch_multi("foo", "bar") { |_key| true }
+      @peek.delete("foo")
+      @peek.delete("bar")
+      assert_equal true, @cache.read("foo")
+      assert_equal true, @cache.read("bar")
+    end
+  end
+
   def test_middleware
     app = lambda { |env|
       result = @cache.write("foo", "bar")