Add rake task to prune hashed projects

This adds `gitlab:storage:prune_hashed_projects`, which removes
directories not associated with any known project.

This is a development-only task, and must be run with `FORCE=1` to
actually perform the deletions.

lib/gitlab/hashed_storage/rake_helper.rb

 # frozen_string_literal: true
 
+require 'find'
+
 module Gitlab
   module HashedStorage
     module RakeHelper
@@ -65,6 +67,7 @@ module Gitlab
       def self.projects_list(relation_name, relation)
         listing(relation_name, relation.with_route) do |project|
           $stdout.puts "  - #{project.full_path} (id: #{project.id})".color(:red)
+          $stdout.puts "    #{project.repository.disk_path}"
         end
       end
 
@@ -92,6 +95,43 @@ module Gitlab
         end
       end
       # rubocop: enable CodeReuse/ActiveRecord
+
+      def self.prune(relation_name, relation)
+        root = ENV['GDK_REPOSITORY_ROOT'].presence || '../repositories'
+        dry_run = !ENV['FORCE'].present?
+
+        known_paths = Set.new
+        listing(name, relation) { |p| known_paths << "#{root}/#{p.repository.disk_path}" }
+
+        marked_for_deletion = Set.new
+        prefix_length = Pathname.new(root).ascend.count
+
+        Find.find("#{root}/@hashed") do |path|
+          path = Pathname.new(path)
+          next unless path.directory?
+
+          path.ascend do |p|
+            base = p.to_s.gsub(/\.(\w+\.)?git$/, '')
+            Find.prune if known_paths.include?(base)
+          end
+
+          if path.ascend.count == prefix_length + 4
+            marked_for_deletion << path
+            Find.prune
+          end
+        end
+
+        $stdout.puts "Dry run. We would have deleted:" if dry_run
+
+        marked_for_deletion.each do |p|
+          if dry_run
+            $stdout.puts " - #{p}"
+          else
+            $stdout.puts "Removing #{p}"
+            p.rmtree
+          end
+        end
+      end
     end
   end
 end

lib/tasks/gitlab/storage.rake

+require 'find'
+
 namespace :gitlab do
   namespace :storage do
     desc 'GitLab | Storage | Migrate existing projects to Hashed Storage'
@@ -116,6 +118,19 @@ namespace :gitlab do
       helper.projects_list('projects using Hashed Storage', Project.with_storage_feature(:repository))
     end
 
+    desc 'Gitlab | Storage | Prune projects using Hashed Storage. Remove all hashed directories that do not have a project associated'
+    task prune_hashed_projects: :environment do
+      if Rails.env.production?
+        abort('This destructive action may only be run in development')
+      end
+
+      helper = Gitlab::HashedStorage::RakeHelper
+      name = 'projects using Hashed Storage'
+      relation = Project.with_storage_feature(:repository)
+
+      helper.prune(name, relation)
+    end
+
     desc 'Gitlab | Storage | Summary of project attachments using Legacy Storage'
     task legacy_attachments: :environment do
       helper = Gitlab::HashedStorage::RakeHelper