FIX: ensure no infinite category loop

If there's ever a circular reference in categories, don't go into an infinite loop when generating the category slug.

Instead, keep track of parent ids, and bail out as soon as we're encountering one more than once.

app/models/category.rb

@@ -1091,11 +1091,13 @@ class Category < ActiveRecord::Base
       .find_each { |category| category.create_category_definition }
   end
 
-  def slug_path
+  def slug_path(parent_ids = Set.new)
     if self.parent_category_id.present?
-      slug_path = self.parent_category.slug_path
+      if parent_ids.add?(self.parent_category_id)
-      slug_path.push(self.slug_for_url)
+        self.parent_category.slug_path(parent_ids) << self.slug_for_url
-      slug_path
+      else
+        []
+      end
     else
       [self.slug_for_url]
     end

spec/models/category_spec.rb

@@ -47,8 +47,7 @@ RSpec.describe Category do
 
     it "should delete associated sidebar_section_links when category is destroyed" do
       category_sidebar_section_link = Fabricate(:category_sidebar_section_link)
-      category_sidebar_section_link_2 =
+      Fabricate(:category_sidebar_section_link, linkable: category_sidebar_section_link.linkable)
-        Fabricate(:category_sidebar_section_link, linkable: category_sidebar_section_link.linkable)
       tag_sidebar_section_link = Fabricate(:tag_sidebar_section_link)
 
       expect { category_sidebar_section_link.linkable.destroy! }.to change {
@@ -996,7 +995,7 @@ RSpec.describe Category do
         )
       category.clear_auto_bump_cache!
 
-      post1 = create_post(category: category, created_at: 15.seconds.ago)
+      create_post(category: category, created_at: 15.seconds.ago)
 
       # no limits on post creation or category creation please
       RateLimiter.enable
@@ -1419,7 +1418,7 @@ RSpec.describe Category do
 
     it 'returns nil when input is ["category:invalid-slug:sub-subcategory"] and maximum category nesting is 3' do
       SiteSetting.max_category_nesting = 3
-      sub_subcategory = Fabricate(:category, parent_category: subcategory, slug: "sub-subcategory")
+      Fabricate(:category, parent_category: subcategory, slug: "sub-subcategory")
 
       expect(Category.ids_from_slugs(%w[category:invalid-slug:sub-subcategory])).to eq([])
     end
@@ -1464,6 +1463,34 @@ RSpec.describe Category do
     end
   end
 
+  describe "#slug_path" do
+    before { SiteSetting.max_category_nesting = 3 }
+
+    fab!(:grandparent) { Fabricate(:category, slug: "foo") }
+    fab!(:parent) { Fabricate(:category, parent_category: grandparent, slug: "bar") }
+    let(:child) { Fabricate(:category, parent_category: parent, slug: "boo") }
+
+    it "returns the slug for categories without parents" do
+      expect(grandparent.slug_path).to eq [grandparent.slug]
+    end
+
+    it "returns the slug for categories with parent" do
+      expect(parent.slug_path).to eq [grandparent.slug, parent.slug]
+    end
+
+    it "returns the slug for categories with grand-parent" do
+      expect(child.slug_path).to eq [grandparent.slug, parent.slug, child.slug]
+    end
+
+    it "avoids infinite loops with circular references" do
+      grandparent.parent_category = parent
+      grandparent.save!(validate: false)
+
+      expect(grandparent.slug_path).to eq [parent.slug, grandparent.slug]
+      expect(parent.slug_path).to eq [grandparent.slug, parent.slug]
+    end
+  end
+
   describe "#slug_ref" do
     fab!(:category) { Fabricate(:category, slug: "foo") }