Remove empty Carrierwave directories when asset file is removed

Since #373 we've been deleting an asset's underlying file from the
filesystem once the file has been uploaded to S3. However, this was
leaving behind a bunch of empty directories. This change means that the
app now removes all empty parent directories when the file is removed.

The immediate motivation behind this change is that once
alphagov/govuk-puppet#7016 is applied in production, the Asset Manager
Carrierwave directory will no longer be sync-ed from the Asset Master to
the Asset Slaves and there will therefore be a danger that the
asset_master_and_slave_disk_space_similar_from_xxx Icinga alert [1]
might eventually be triggered by the non-zero disk space taken up by the
empty directories.

The implementation makes use of `FileUtils.rmdir` with the `parents`
option which is equivalent to the `rmdir -p` unix command and which
recursively removes empty directories from the supplied path upwards.

Note that I think this means that in some circumstances top-level
directories (e.g. `tmp/test_uploads/assets` in the test environment)
might be removed, but I'm pretty confident Carrierwave creates
directories using the equivalent of `mkdir -p` and so any missing
intermediate directories will be created.

[]1:
https://github.com/alphagov/govuk-puppet/blob/19837b4b351d97d84570bd8a7d01ef3420fbef94/modules/govuk/manifests/node/s_asset_slave.pp#L50-L63

app/models/asset.rb

@@ -58,7 +58,9 @@ class Asset
     end
 
     after_transition to: :uploaded do |asset, _|
+      path = asset.file.path
       asset.remove_file!
+      FileUtils.rmdir(File.dirname(path), parents: true)
     end
   end
 

spec/models/asset_spec.rb

@@ -525,6 +525,10 @@
     context 'when asset is clean' do
       let(:asset) { FactoryBot.create(:clean_asset) }
       let(:path) { asset.file.path }
+      let(:pathname) { Pathname.new(path) }
+      let(:dir) { pathname.parent }
+      let(:parent_dir) { pathname.parent.parent }
+      let(:grandparent_dir) { pathname.parent.parent.parent }
 
       it 'changes asset state to uploaded' do
         asset.upload_success!
@@ -543,6 +547,72 @@
 
         expect(File.exist?(path)).to be_falsey
       end
+
+      it 'removes the empty directory' do
+        asset.upload_success!
+
+        expect(Dir).not_to exist(dir), "#{dir} exists"
+      end
+
+      context 'when directory is not empty' do
+        before do
+          FileUtils.touch(dir.join('do-not-delete-me'))
+        end
+
+        after do
+          FileUtils.rm(dir.join('do-not-delete-me'), force: true)
+        end
+
+        it 'does not remove directory' do
+          asset.upload_success!
+
+          expect(Dir).to exist(dir), "#{dir} exists"
+        end
+      end
+
+      it 'removes the empty parent directory' do
+        asset.upload_success!
+
+        expect(Dir).not_to exist(parent_dir), "#{parent_dir} exists"
+      end
+
+      context 'when parent directory is not empty' do
+        before do
+          FileUtils.touch(parent_dir.join('do-not-delete-me'))
+        end
+
+        after do
+          FileUtils.rm(parent_dir.join('do-not-delete-me'), force: true)
+        end
+
+        it 'does not remove the parent directory' do
+          asset.upload_success!
+
+          expect(Dir).to exist(parent_dir), "#{parent_dir} exists"
+        end
+      end
+
+      it 'removes the empty grandparent directory' do
+        asset.upload_success!
+
+        expect(Dir).not_to exist(grandparent_dir), "#{grandparent_dir} exists"
+      end
+
+      context 'when grandparent directory is not empty' do
+        before do
+          FileUtils.touch(grandparent_dir.join('do-not-delete-me'))
+        end
+
+        after do
+          FileUtils.rm(grandparent_dir.join('do-not-delete-me'), force: true)
+        end
+
+        it 'does not remove the grandparent directory if not empty' do
+          asset.upload_success!
+
+          expect(Dir).to exist(grandparent_dir), "#{grandparent_dir} exists"
+        end
+      end
     end
 
     context 'when asset is infected' do