Refactor: API contract for policy cache

See #801 (comment)

We'd like to support caching a few more lookups, but that's backwards-incompatible. By swapping out the cache, we could have a legacy cache (that only caches "the old way"), and a new cache that will cache _more_ things. This allows backwards-compatibility with the same API interface.

lib/pundit.rb

@@ -9,6 +9,8 @@
 require "active_support/dependencies/autoload"
 require "pundit/authorization"
 require "pundit/context"
+require "pundit/cache_store/null_store"
+require "pundit/cache_store/hash_store"
 
 # @api private
 # To avoid name clashes with common Error naming when mixing in Pundit,
@@ -66,8 +68,14 @@ def self.included(base)
 
   class << self
     # @see [Pundit::Context#authorize]
-    def authorize(user, record, query, policy_class: nil, cache: {})
-      Context.new(user: user, policy_cache: cache).authorize(record, query: query, policy_class: policy_class)
+    def authorize(user, record, query, policy_class: nil, cache: nil)
+      context = if cache
+        Context.new(user: user, policy_cache: cache)
+      else
+        Context.new(user: user)
+      end
+
+      context.authorize(record, query: query, policy_class: policy_class)
     end
 
     # @see [Pundit::Context#policy_scope]

lib/pundit/authorization.rb

@@ -19,7 +19,7 @@ module Authorization
     def pundit
       @pundit ||= Pundit::Context.new(
         user: pundit_user,
-        policy_cache: policies
+        policy_cache: Pundit::CacheStore::HashStore.new(policies)
       )
     end
 

lib/pundit/cache_store/hash_store.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+module Pundit
+  module CacheStore
+    # @api private
+    class HashStore
+      def initialize(hash = {})
+        @store = hash
+      end
+
+      def fetch(key)
+        @store[key] ||= yield
+      end
+    end
+  end
+end

lib/pundit/cache_store/null_store.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module Pundit
+  module CacheStore
+    # @api private
+    class NullStore
+      @instance = new
+
+      class << self
+        attr_reader :instance
+      end
+
+      def fetch(*, **)
+        yield
+      end
+    end
+  end
+end

lib/pundit/context.rb

@@ -2,7 +2,7 @@
 
 module Pundit
   class Context
-    def initialize(user:, policy_cache: {})
+    def initialize(user:, policy_cache: CacheStore::NullStore.instance)
       @user = user
       @policy_cache = policy_cache
     end
@@ -27,7 +27,9 @@ def authorize(possibly_namespaced_record, query:, policy_class:)
       policy = if policy_class
         policy_class.new(user, record)
       else
-        policy_cache[possibly_namespaced_record] ||= policy!(possibly_namespaced_record)
+        policy_cache.fetch(possibly_namespaced_record) do
+          policy!(possibly_namespaced_record)
+        end
       end
 
       raise NotAuthorizedError, query: query, record: record, policy: policy unless policy.public_send(query)