refactor: Use Set-based parent guard and deduplicate subgroups in DependencyGroupEngine

- Use @group_by_name_parent_names Set in matches_group_by_parent_group?
  for consistency with the recursion guard in
  create_dynamic_subgroups_for_dependency_name_groups
- Reuse existing subgroups instead of creating duplicates when
  assign_to_groups! is called multiple times (once per directory)

Author: markhallen

updater/lib/dependabot/dependency_group_engine.rb

@@ -145,7 +145,7 @@ def mark_ungrouped_if_no_matches(dependency, matched_groups)
     sig { params(dependency: Dependabot::Dependency).returns(T::Boolean) }
     def matches_group_by_parent_group?(dependency)
       @dependency_groups.any? do |group|
-        group.group_by_dependency_name? && group.contains?(dependency)
+        @group_by_name_parent_names.include?(group.name) && group.contains?(dependency)
       end
     end
 
@@ -226,16 +226,23 @@ def create_dynamic_subgroups_for_dependency_name_groups(dependencies)
         matching_deps = dependencies.select { |dep| parent_group.contains?(dep) }
 
         matching_deps.group_by(&:name).each do |dep_name, deps|
-          # Preserve "group-by" in subgroup rules so group_by_dependency_name? returns true,
-          # enabling correct handled-dependency tracking when semver rules reject an update.
-          subgroup_rules = parent_group.rules.merge("patterns" => [dep_name])
-          subgroup = Dependabot::DependencyGroup.new(
-            name: "#{parent_group.name}/#{dep_name}",
-            rules: subgroup_rules,
-            applies_to: parent_group.applies_to
-          )
-          subgroup.dependencies.concat(deps)
-          @dependency_groups << subgroup
+          subgroup_name = "#{parent_group.name}/#{dep_name}"
+          existing_subgroup = @dependency_groups.find { |g| g.name == subgroup_name }
+
+          if existing_subgroup
+            existing_subgroup.dependencies.concat(deps)
+          else
+            # Preserve "group-by" in subgroup rules so group_by_dependency_name? returns true,
+            # enabling correct handled-dependency tracking when semver rules reject an update.
+            subgroup_rules = parent_group.rules.merge("patterns" => [dep_name])
+            subgroup = Dependabot::DependencyGroup.new(
+              name: subgroup_name,
+              rules: subgroup_rules,
+              applies_to: parent_group.applies_to
+            )
+            subgroup.dependencies.concat(deps)
+            @dependency_groups << subgroup
+          end
         end
 
         parent_group.dependencies.clear

updater/spec/dependabot/dependency_group_engine_spec.rb

@@ -562,12 +562,12 @@
         )
         dependency_group_engine.assign_to_groups!(dependencies: [new_pkg_a])
 
-        # Should have exactly 2 subgroups for dummy-pkg-a (one per directory call),
-        # NOT 3 (which would happen if the first subgroup was treated as a parent)
+        # Should reuse the existing subgroup rather than creating a duplicate
         subgroups_a = dependency_group_engine.dependency_groups.select do |g|
           g.name == "per-dependency/dummy-pkg-a"
         end
-        expect(subgroups_a.length).to be(2)
+        expect(subgroups_a.length).to be(1)
+        expect(subgroups_a.first.dependencies).to contain_exactly(dummy_pkg_a, new_pkg_a)
 
         # Should have no sub-sub-groups
         nested = dependency_group_engine.dependency_groups.select do |g|
@@ -895,11 +895,14 @@
         # Simulate a second directory by calling assign_to_groups! again.
         dependency_group_engine.assign_to_groups!(dependencies: [dummy_pkg_a_dir2])
 
-        # Should have subgroups from both calls, but no sub-subgroups
+        # Should reuse the existing subgroup rather than creating a duplicate
         subgroups_a = dependency_group_engine.dependency_groups.select do |g|
           g.name == "monorepo-deps/dummy-pkg-a"
         end
-        expect(subgroups_a.length).to eq(2)
+        expect(subgroups_a.length).to eq(1)
+
+        # The existing subgroup should now contain deps from both calls
+        expect(subgroups_a.first.dependencies.count { |d| d.name == "dummy-pkg-a" }).to eq(3)
 
         nested = dependency_group_engine.dependency_groups.select do |g|
           g.name.start_with?("monorepo-deps/dummy-pkg-a/")