search: fix user-dependent searches showing incorrect paginators.

Some searches, such as searches for private favorites or for the
status:unmoderated tag, return different results for different users.
These searches need to have their counts cached separately for each user
so that we don't return incorrect page counts when two different users
perform the same search.

This can also potentially leak private information, such as the number
of posts flagged, downvoted, or disapproved by a given user.

Partial fix for #4280.

test/unit/post_disapproval_test.rb

@@ -35,8 +35,8 @@ class PostDisapprovalTest < ActiveSupport::TestCase
           end
 
           should "remove the associated post from alice's moderation queue" do
-            assert(!Post.available_for_moderation(false).map(&:id).include?(@post_1.id))
-            assert(Post.available_for_moderation(false).map(&:id).include?(@post_2.id))
+            assert(!Post.available_for_moderation(CurrentUser.user, hidden: false).map(&:id).include?(@post_1.id))
+            assert(Post.available_for_moderation(CurrentUser.user, hidden: false).map(&:id).include?(@post_2.id))
           end
         end
 
@@ -47,8 +47,8 @@ class PostDisapprovalTest < ActiveSupport::TestCase
           end
 
           should "not remove the associated post from brittony's moderation queue" do
-            assert(Post.available_for_moderation(false).map(&:id).include?(@post_1.id))
-            assert(Post.available_for_moderation(false).map(&:id).include?(@post_2.id))
+            assert(Post.available_for_moderation(CurrentUser.user, hidden: false).map(&:id).include?(@post_1.id))
+            assert(Post.available_for_moderation(CurrentUser.user, hidden: false).map(&:id).include?(@post_2.id))
           end
         end
       end

test/unit/post_query_builder_test.rb

@@ -1153,5 +1153,15 @@ class PostQueryBuilderTest < ActiveSupport::TestCase
         end
       end
     end
+
+    context "for a user-dependent metatag" do
+      should "cache the count separately for different users" do
+        @user = create(:user, enable_private_favorites: true)
+        @post = as(@user) { create(:post, tag_string: "fav:#{@user.name}") }
+
+        assert_equal(1, PostQueryBuilder.new("fav:#{@user.name}", @user).fast_count)
+        assert_equal(0, PostQueryBuilder.new("fav:#{@user.name}").fast_count)
+      end
+    end
   end
 end