Prevent infinite loops during symlink traversal

#8815 introduced a traversal
strategy that used recursion.
#9703 then fixed an issue with
this traversal which accounted for directories and symlinks.

When a symlink points to a parent directory that contains that symlink
it'll cause this to go into a loop until the filename is too long for
glob to handle.

We prevent this by checking for the inclusion of a symlink's real path
in the base directory's realpath. If the base directory's path starts
with the symlink's destination then we are in a loop and should skip
processing the directory

changelog/fix_prevent_infinite_loops_during_symlink.md

@@ -0,0 +1 @@
+* [#9748](https://github.com/rubocop/rubocop/pull/9748): Prevent infinite loops during symlink traversal. ([@Tonkpils][])

lib/rubocop/target_finder.rb

@@ -100,12 +100,19 @@ def wanted_dir_patterns(base_dir, exclude_pattern, flags)
                   next true if dir.end_with?('/./', '/../')
                   next true if File.fnmatch?(exclude_pattern, dir, flags)
 
-                  File.symlink?(dir.chomp('/')) && File.fnmatch?(exclude_pattern,
-                                                                 "#{File.realpath(dir)}/", flags)
+                  symlink_excluded_or_infinite_loop?(base_dir, dir, exclude_pattern, flags)
                 end
       dirs.flat_map { |dir| wanted_dir_patterns(dir, exclude_pattern, flags) }.unshift(base_dir)
     end
 
+    def symlink_excluded_or_infinite_loop?(base_dir, current_dir, exclude_pattern, flags)
+      dir_realpath = File.realpath(current_dir)
+      File.symlink?(current_dir.chomp('/')) && (
+        File.fnmatch?(exclude_pattern, "#{dir_realpath}/", flags) ||
+        File.realpath(base_dir).start_with?(dir_realpath)
+      )
+    end
+
     def combined_exclude_glob_patterns(base_dir)
       exclude = @config_store.for(base_dir).for_all_cops['Exclude']
       patterns = exclude.select { |pattern| pattern.is_a?(String) && pattern.end_with?('/**/*') }

spec/rubocop/target_finder_spec.rb

@@ -448,6 +448,12 @@
       end
     end
 
+    it 'prevents infinite loops when traversing symlinks' do
+      create_link('dir1/link/', File.expand_path('dir1'))
+
+      expect(found_basenames).to include('ruby1.rb').once
+    end
+
     it 'resolves symlinks when looking for excluded directories' do
       create_link('link', 'dir1')