Handle tails in dependency cycles

lib/nanoc/base/entities/directed_graph.rb

@@ -159,17 +159,32 @@ def delete_vertex(v)
     # @group Querying the graph
 
     def any_cycle
-      path = [@vertices.keys.first]
-
-      loop do
-        nexts = direct_successors_of(path.last)
-        cycle_start_index = path.find_index { |node| nexts.include?(node) }
-        if cycle_start_index
-          break path[cycle_start_index..-1]
-        elsif nexts.empty?
-          break nil
-        else
-          path << nexts.sample
+      path_with_cycle = all_paths.lazy.find { |path| path.uniq != path }
+      if path_with_cycle
+        double_vertex = path_with_cycle.last
+        path_with_cycle[path_with_cycle.index(double_vertex)..-2]
+      end
+    end
+
+    def all_paths
+      Enumerator.new do |y|
+        @vertices.keys.each do |vertex|
+          dfs_from(vertex) do |path|
+            y << path
+          end
+        end
+      end
+    end
+
+    def dfs_from(vertex, path_so_far = [])
+      new_path = path_so_far + [vertex]
+      yield(new_path)
+
+      return if new_path.uniq != new_path
+
+      direct_successors_of(vertex).each do |next_vertex|
+        dfs_from(next_vertex, new_path) do |path|
+          yield(path)
         end
       end
     end

spec/nanoc/base/directed_graph_spec.rb

@@ -27,6 +27,18 @@
       it { is_expected.to eq([2, 3]) }
     end
 
+    context 'one cycle with tail' do
+      before do
+        graph.add_edge(1, 2)
+        graph.add_edge(2, 20)
+        graph.add_edge(20, 21)
+        graph.add_edge(2, 3)
+        graph.add_edge(3, 1)
+      end
+
+      it { is_expected.to eq([1, 2, 3]) }
+    end
+
     context 'large cycle' do
       before do
         graph.add_edge(1, 2)